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Fracture toughness of dental resins and composites modified with polymeric crosslinked nanoparticles

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Title:
Fracture toughness of dental resins and composites modified with polymeric crosslinked nanoparticles
Creator:
Lin, Winsean Ralph
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English
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ix, 64 leaves : illustrations ; 28 cm

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Subjects / Keywords:
Nanogels ( lcsh )
Nanoparticles ( lcsh )
Dental resins -- Fracture ( lcsh )
Polymeric composites ( lcsh )
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bibliography ( marcgt )
theses ( marcgt )
non-fiction ( marcgt )

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Bibliography:
Includes bibliographical references (leaves 63-64).
General Note:
Department of Mechanical Engineering
Statement of Responsibility:
by Winsean Ralph Lin.

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|University of Colorado Denver
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|Auraria Library
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All applicable rights reserved by the source institution and holding location.
Resource Identifier:
747707921 ( OCLC )
ocn747707921
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LD1193.E55 2011m L56 ( lcc )

Full Text
/
FRACTURE TOUGHNESS OF DENTAL RESINS AND COMPOSITES
MODIFIED WITH POLYMERIC CROSSLINKED NANOPARTICLES
by
Winsean Ralph Lin
B.S., University of Illinois at Urbana Champaign, 1997
M.S., Syracuse University, 2002
A thesis submitted to the
University of Colorado Denver
in partial fulfillment
of the requirements for the degree of
Master of Science
Mechanical Engineering
2011


This thesis for the Master of Science
degree by
Winsean Ralph Lin
has been approved
by
Atousa Plaseied
H/Z/il
Date
Jeffrey W. Stansbury


Lin, Winsean Ralph (M.S., Mechanical Engineering)
Fracture Toughness of Dental Resins and Composites Modified with Polymeric Crosslinked
Nanoparticles
Thesis directed by Assistant Research Professor Atousa Plaseied
ABSTRACT
Nanogels are novel new macromolecules that can improve the properties of methacrylate polymer
systems by reducing polymerization shrinkage and stress. Nanogels have been shown to improve the
flexural modulus of secondary monomers, but the effects of nanogels on fracture toughness have not
been reported. In this study, a 70/30 mol% ratio of BisEMA/TEGDMA monomer was loaded with
IBMA/UDMA and aromatic nanogels at loading concentrations of 25 wt% and 40 wt%. High and low
glass transition temperature nanogels at loading concentrations of 10 wt% and 50 wt% were also used.
Flexural and fracture toughness tests were performed. Flexural modulus was improved for all
specimens loaded with 25 wt% and 40 wt% IBMA/UDMA or aromatic nanogels loaded without using
solvent. No statistical difference was observed in fracture toughness, as compared to control at 25
wt% or 40 wt% nanogel concentrations, but a reduction in fracture toughness was seen when 70 wt%
0.4 micron glass particle filler was added. No statistical difference in flexural modulus or fracture
toughness was obtained with 10 wt% loading of high or low Tg nanogels. At 50 wt% loading there
was an increase in flexural modulus with the high Tg nanogel, a large decrease in flexural modulus
with low Tg nanogel, and a statistically significant reduction in fracture toughness with both high and
low Tg nanogels.
This abstract accurately represents the content of the candidates thesis. I recommend its publication.
Signed.
Atousa Plaseied


ACKNOWLEDGEMENTS
I would like to thank Dr. Carmem Pfeifer, Matthew Barros, and Steven Lewis at the
University of Colorado Denver, School of Dental Medicines Materials Lab, for all of their help with
the synthesis and characterization of nanogels. The help from Randy Ray at the University of
Colorado Denvers Mechanical Engineering Department Machine Shop for fabricating the PTFE
molds, and Parag Shah for taking SEM images of the fracture surfaces is truly appreciated. Finally, I
thank Dr. Atousa Plaseied and Dr. Jeffrey Stansbury for their guidance on this project, and also Confi-
Dental Products Company for providing the financial support.


TABLE OF CONTENTS
LIST OF FIGURES.........................................................................vii
LIST OF TABLES..........................................................................ix
Chapter
1. Introduction..........................................................................1
1.1 Background and Motivation........................................................1
1.2 Nanogels in Development..........................................................2
2. Literature Review.....................................................................6
2.1 F ormulation of N anogels........................................................6
2.2 Fracture Toughness Testing Methods for Brittle Materials.........................6
2.3 Fracture Toughness of Dental Materials...........................................9
2.4 Other Applications of Nanogels...................................................9
3. Experimental Program and Preliminary Results........................................11
3.1 Objectives......................................................................11
3.2 Plan of Work....................................................................11
3.3 Materials.......................................................................12
3.3.1 Nanogels..................................................................12
3.3.2 Secondary Monomers........................................................14
3.3.3 Secondary Monomer Polymerization Initiators...............................14
3.4 Testing Procedures..............................................................15
3.4.1 Nanogel Loading into Secondary Monomers...................................15
3.4.2 Thermal Curing of Test Specimens..........................................15
3.4.3 Photo Curing of Test Specimens............................................16
v


3.4.4 Degree of Conversion Tests.........................................16
3.4.5 Flexural Tests.....................................................17
3.4.6 Fracture Toughness Tests...........................................19
3.4.7 Scanning Electron Microscope Image Analysis........................21
3.5 Preliminary Results.....................................................21
3.5.1 Flexural Tests at Different Loading Rates..........................21
3.5.2 BisGMA/TEGDMA Flexural Test Results................................23
3.5.3 BisGMA/TEGDMA Fracture Toughness Test Results......................26
3.5.4 Thermal Curing Issues..............................................28
4. Experimental Results and Discussions........................................31
4.1 Loading Nanogels into BisEMA/TEGDMA Secondary Monomer...................31
4.2 IBMA/UDMA and Aromatic Nanogel Flexural Modulus Results.................32
4.3 IBMA/UDMA and Aromatic Nanogel Fracture Toughness Results...............35
4.4 Flexural Modulus Results for High and Low Tg Nanogels...................38
4.5 Fracture Toughness Results for High and Low Tg Nanogels.................41
4.6 Degree of Conversion Results............................................44
4.7 Scanning Electron Microscopy Image Analysis.............................46
5. Summary and Conclusions.....................................................53
Appendix
A. RESULTS OF INDIVIDUAL SPECIMEN TEST TRIALS...............................57
References......................................................................63
vi


LIST OF FIGURES
Figure 1.1. Structures of Monomers and Reagents Used in IBMA-UDMA Nanogel Synthesis..........3
Figure 1.2. Structures of Monomers Used to Prepare the Aromatic Nanogels.....................4
Figure 3.1. Flexural Test Specimen Molds: 3-Piece Steel and 1-Piece PTFE....................17
Figure 3.2. MTS Axial-Load Testing Machine and 3-point Bending Fixture......................18
Figure 3.3. 3-Point Bending Test Fixture with Fracture Toughness Specimen (Close-up)........18
Figure 3.4. Two Piece Mold with Blade Insert for Fracture Toughness Specimens (scale in cm).19
Figure 3.5. Fracture Toughness Specimens from Top to Bottom: 40 wt% Aromatic Nanogel, 40 wt%
Aromatic Nanogel with 70 wt% Ba Glass Filler, 40 wt% IBMA/UDMA Nanogel, and 50
wt% IBMA/UDMA Nanogel with 70 wt% Ba Glass Filler (scale in cm)......................20
Figure 3.6. Comparison of Flexural Modulus of BisGMA/TEGDMA at Different Loading Rates
(Means and Standard Deviations)...........................................22
Figure 3.7. Comparison of BisGMA/TEGDMA with 25 wt% Nanogel Loading Flexural Modulus
Results (Means and Standard Deviations)...................................24
Figure 3.8. Comparison of BisGMA/TEGDMA with 25 wt% Nanogel Loading Fracture Toughness
Results (Means and Standard Deviations)..................................26
Figure 3.9. Comparison of Flexural Modulus Results Using Different Curing Methods (Means and
Standard Deviations).....................................................29
Figure 4.1. Comparison of IBMA/UDMA and Aromatic Nanogel Flexural Modulus Results (Means
and Standard Deviations).................................................34
Figure 4.2.
Comparison of IBMA/UDMA and Aromatic Nanogel Fracture Toughness Results (Means
and Standard Deviations)............................................................37
Figure 4.3. Comparison of High and Low Tg Nanogel Flexural Modulus Results (Means and Standard
Deviations)....................................................................40
Figure 4.4. Comparison of High and Low Tg Nanogel Fracture Toughness Results (Means and
Standard Deviations)...........................................................43
Figure 4.5. Comparison of Mean Percent Degree of Conversion Results (Means and Standard
Deviations)....................................................................45
Vll


Figure 4.6. Fracture Surface of Control Specimen (xlOO)........................................47
Figure 4.7. Fracture Surface of Specimen with 10 wt% High Tg Nanogel at (a) x25 and (b) xlOO...48
Figure 4.8. Fracture Surface of Specimen with 50 wt% High Tg Nanogel (xlOO)....................49
Figure 4.9. Fracture Surface of Specimen with 10 wt% Low Tg Nanogel (xlOO)....................50
Figure 4.10. Fracture Surface of Specimen with 50 wt% Low Tg Nanogel at (a) x25 and (b) x5000... 52
viii


LIST OF TABLES
Table 3.1. BisGMA/TEGDMA flexural modulus at different loading rates.................22
Table 3.2. BisGMA/TEGDMA flexural modulus results....................................25
Table 3.3. BisGMA/TEGDMA fracture toughness results..................................27
Table 3.4. Flexural modulus results for BisEMA/TEGDMA using different curing methods.30
Table 3.5. Flexural modulus results for UDMA using different curing methods..........30
Table 4.1. Flexural modulus results for IBMA/UDMA and aromatic nanogels..............33
Table 4.2. Fracture toughness results for IBMA/UDMA and aromatic nanogels............36
Table 4.3. Flexural modulus results for high and low Tg nanogels.....................39
Table 4.4. Fracture toughness results for high and low Tg nanogels...................42
Table A. 1. Flexural test specimens with IBMA/UDMA and aromatic nanogels.............58
Table A.2. Fracture toughness test specimens with IBMA/UDMA and aromatic nanogels....60
Table A.3. Flexural test specimens with high and low Tg nanogels.....................62
Table A.4. Fracture toughness test specimens with high and low Tg nanogels...........62
IX


1. Introduction
1.1 Background and Motivation
The monomer Bisphenol A glycidyl methacrylate (BisGMA) has been available for decades,
as a primary component of the organic polymer matrix of composite restoratives. These composite
restoratives provide an alternative to opaque and mercury containing amalgam dental fillings [1], The
high viscosity of uncured BisGMA makes it difficult to handle and also limits the amount of
strengthening inorganic filler that can be added. The monomers Triethylene-glycol dimethacrylate
(TEGDMA) or Urethane dimethacrylate (UDMA) are often copolymerized with BisGMA to lower the
high viscosity of BisGMA in its unpolymerized state to make it much easier to handle. However, the
added monomer also increases the degree of conversion from monomer to polymer, which increases
polymerization shrinkage and in mm increases polymerization stress [2]. This shrinkage can strain the
bond at the bonding interface, which can cause gap formations, post-operative sensitivity, pulp
irritation, water sorption, and shortened filling life [3]. Another resin that has been investigated as a
dental material is ethoxylated Bisphenol A methacrylate (BisEMA), which has almost the same
structure as BisGMA but is much less viscous in its unpolymerized state due to the lack of hydroxyl
groups. These resins can be polymerized using free radical polymerization under visible or UV light
irradiation with photo-initiators such as Camphoroquinone or 2,2-Dimethoxy-2-phenylacetophenone
(DMPA), respectively.
The term composite is used when inorganic fillers, such as 0.4 micron silanized barium
borosilicate glass particles, are added to the resin to increase mechanical strength and reduce
polymerization shrinkage. However, the filler can reduce the adhesion of the resin to the base surface.
1


Nanogels that are currently being developed can reduce the shrinkage and stress occurring in
dental resins during photo-polymerization, without compromising mechanical properties. Preliminary
results show a 25% to 30% reduction in shrinkage and a 25% to 40% reduction in polymerization
stress. Nanogels would ideally increase the strength and toughness of the resins without compromising
the potential stress reduction. Nanogels are polymeric particles with diameters as small as 10
nanometers. They are formed through the copolymerization of monovinyl and divinyl monomers, with
the lengths of the polymer chains being controlled with the use of a chain transfer agent. Keeping the
polymer chains short prevents macrogelation, in which a polymer network forms from a single
molecule extending throughout the polymerization vessel with ordinary covalent bond connections [4],
When polymer chain lengths are controlled, microgels with molecular weights in the millions and
nanogels with molecular weights in the low thousands can be created [5].
Nanogels have highly branched and crosslinked structures that can add strength without
covalent bonding through physical entanglement. Covalent attachment to matrix polymers can also be
added by adding functional groups to the surfaces of nanogel particles, to increase strength further.
Preliminary results show that nanogel modified dental resins and composites increase mechanical
strength, reduce polymerization shrinkage, and reduce polymerization stress. The lack of reported
investigation in literature on the effect of nanogels on fracture toughness of dental resins and
composites is the motivation for this project. Fracture toughness describes the resistance of a material
to crack propagation. Nanogels can be used to introduce well controlled nano-scale heterogeneity into
polymers, creating internal interfaces that may potentially act as sites for energy absorption.
1.2 Nanogels in Development
Three different nanogels being synthesized at the University of Colorados School of Dental
Medicine were evaluated in this study. One nanogel was a copolymeration of Isobomyl methacrylate
(IBMA), a monovinyl, and Urethane dimethacrylate (UDMA), a divinyl, at IBMA/UDMA molar ratios
2


of 70/30 or 80/20. The nanogel was thermally polymerized in a toluene solution, with 2,2'-
Azobisisobutyronitrile (AIBN) thermal initiator. 2-Mercaptoethanol (ME) was used as a chain transfer
agent to control nanogel size and to prevent macrogelation. Functional groups of 2-Isocyanoethyl
methacrylate (IEM) were added to make the nanogel particles reactive upon polymerization with a
secondary monomer. Structures of the monomers and reagents used to form IBMA-UDMA nanogels
are shown in Figure 1.1.
Figure 1.1. Structures of Monomers and Reagents Used in IBMA-UDMA Nanogel Synthesis
The reduced UDMA divinyl content in the 80/20 ratio decreases the number of crosslinks in the
nanogel, which reduces the interconnectivity, size, and molecular weight of the nanogel. The IBMA
monovinyl spaces out the crosslinks provided by the divinyl and helps to prevent macrogelation.
However, small groups attached to monovinyl structures can lower the glass transition temperature
(Tg) and decrease the mechanical strength. IBMA was chosen mainly for its high Tg as a linear
homopolymer, and the nanogel prepared with an 80/20 ratio of IBMA/UDMA had a Tg of 90C. Gel
3


permeation chromatography for IBMA/UDMA nanogels reported weight average molecular weights of
16,000 and a hydrodynamic radius of 3 nm, with a 2.4 polydispersity index.
The second nanogel was an aromatic nanogel. In this nanogel, Di-tert-butylphenol
methacrylate (DTBPUMA) takes the place of IBMA as the monovinyl, and BisGMA serves as the
divinyl in place of UDMA. Figure 2.1 shows the structures of monomers in the aromatic nanogel.
The aromatic nanogels have a large structure with a rotatable bond that can sweep out a large area and
fill up more of the empty space in the nanogel particles. This may reduce the swelling ability of the
nanogel and provide a higher Tg or mechanical strength without a high cross link density. Strong
intramolecular hydrogen bonding associated with the BisGMA structure promotes further non-covalent
reinforcement of the nanogel structure. The aromatic nanogel also gives a high refractive index, which
in addition to the primary nanogel project goals of decreasing polymerization shrinkage and stress,
may also increase the overall refractive index of dental resin composites containing inorganic filler.
The physical crosslinks without covalent attachments could also reduce the tendency of nanogels to
increase the viscosity of the secondary monomer being loaded with nanogel. Gel permeation
chromatography for aromatic nanogels reported weight average molecular weights of 400,000 and a
4


hydrodynamic radius of 6.8 nm, with a polydispersity index of 1.2.
The third nanogel was a low Tg nanogel, for the specific purpose of modifying the Tg of
secondary monomers being loaded with nanogel. The divinyl Isodecyl methacrylate (IDMA) and
monovinyl Bisphenol A ethoxylate diacrylate (BPAEDA) were copolymerized at a 70/30 mol ratio.
The low Tg of IDMA lowers the higher Tg of the BPAEDA, resulting in nanogels with more flexible
structures. When the nanogel Tg is lower than the room temperature during handling, the bonds in the
polymer chains of the nanogel are able to rotate, allowing the structure of its polymer chains to
reorganize. These bulky yet flexible nanogel structures can therefore locally lower the Tg of secondary
monomers being loaded with nanogel. Gel permeation chromatography for low Tg nanogels reported
weight average molecular weights of 214,000 and a hydrodynamic radius of 7.8 nm, with a
polydispersity index of 3.8. Low Tg nanogels in this study had glass transition temperatures of -45C
and -35C .
5


2. Literature Review
2.1 Formulation of Nanogels
Nanogels and microgels are soluble intramolecularly crosslinked macromolecules formed
from polymerizing polyfunctional precursors, to a point short of macrogelation. The molecular weight
(Mw) of the macromolecules produced can be controlled to produce microgels with Mw in the millions,
or nanogels with Mw in the low thousands. Nanogels and microgels have much lower viscosities in
solution than linear polymers of similar molecular weights. Nanogels in solution can have a radius of
gyration as low as 10 nm [5],
2.2 Fracture Toughness Testing Methods for Brittle Materials
The fracture toughness of a material describes its ability to resist crack propagation. Ki is the
stress intensity factor for the stresses seen at a crack tip under loading. A) is dependent on crack
length, load magnitude, and test specimen dimensions. When test specimens have a minimum
thickness relative to their width and length, plane strain condition will dominate and fracture will occur
suddenly with little crack growth [6], The fracture toughness of the material will then remain the same
with increasing relative thickness. This critical stress intensity factor, Klc, is then referred to as the
fracture toughness property of the material. The K/c property is independent of the loading magnitude
or specimen geometry, and can be determined experimentally through ASTM guidelines. A variety of
specimen configurations and methods have been used to determine fracture toughness:
SENB (single edge notched beam): A precrack notch is formed in specimens, according to
ASTM E-399 guidelines. The specimen is subjected to three-point bending, and the stress
intensity factor KIC is calculated from the specimen dimensions, precrack length, and
6


maximum load at break. The SENB method was used in this project, because small
specimens could be made from simple molds.
SEPB (single edge pre-cracked beam): A precrack is formed in the material by fatigue
loading. This provides a sharp crack tip, but producing the precrack on brittle materials can
be difficult. It can also be difficult to produce symmetric precracks.
SCF (surface crack in flexure): An indentation made on the surface of a specimen serves as a
precrack and the specimen is subjected to a 4 point bending test [7]. The surface of the
specimen must often be polished after indentation, to remove residual stresses from the
indentation process that can affect the fracture toughness results. Advantages include a small
precrack size on the order of natural ceramic flaws and the ability to control precrack size
through indentation load [8]. Disadvantages of this method include time consuming hand
polishing, difficulties in detecting the precrack in some materials, the requirement of
fractographic analysis to measure the precrack, and precracks not forming in soft or porous
ceramics [7], The formation of lateral cracks under loading can also interfere with the
precrack, affecting the results.
CNB (chevron notched beam): The chevron notched beam specimen modifies the SENB
specimen by forming the notch with two forty five degree cuts, resulting in a V-shaped cross
section in which the center of the specimen extends into the notch to become a point. This
modification is made to make the crack growth more symmetrical and stable, by having
cracks initiate in the center of the specimen, and by having the crack width increase during
crack propagation [9, 10]. The stable crack growth can also lead to slightly higher fracture
stress intensity values [11], but chevron notch beam specimens have the advantage of not
needing to create or measure precracks. Disadvantages of the chevron notched beam method
can include difficulty in reproducing consistent chevron notches, and requiring more material
to make a specimen. Wang et al. [10] found CNB to be comparable to SENB, in a study
7


performing fracture toughness testing on four dental ceramics.
NTP (notchless triangular prism): To use the chevron notched beam method with the limited
material available in individual teeth, Iwamato and Ruse [12] cut short beam triangular prisms
from human dentin, in order to replicate just the triangular cross-section being broken in
chevron notched beam specimens.
FA (fractographic analysis): In this method, the fracture surfaces of bar-shaped specimens
subjected to four-point flexural tests are examined with a scanning electron microscope. The
semi-elliptical fracture patterns are measured to determine the crack size, which is used with
the stress at fracture to calculate the fracture toughness value [13]. Advantages include not
requiring precracks to be created, while disadvantages are the requirement of training and
experience in evaluating the SEM images [14],
IF (indentation fracture): In this method, radial precracks are formed in materials by the
use of a material hardness testing machine. Loading is then continued on the material until
fracture. Wang et al. [15] found that the IF method produces results not in statistical
agreement to SENB or CNB.
SEVNB (single edge V-notched beam): Kubler [16] extended the SENB method by
sharpening saw cut notches into sharp V-notches, using a razor blade sprinkled with diamond
paste. A round robin was performed in which five ceramic materials were sent to more than
30 companies for fracture toughness testing, in order to determine the repeatability of the
method. The conclusions of the study were that the repeatability and reproducibility of the
method was equal to or better than other methods such as SEPB or SCF. The SEVNB method
was determined to be forgiving and robust to notch preparation and notch quality for
participants unfamiliar with the method. In a study on high strength zirconia ceramics,
Fischer et al. [17] described the importance of sharpening the crack tip to prevent
overestimated fracture toughness values. They concluded that the notch root radius had to be
8


less than a critical value, which would be the size of the major microstructural feature of the
material.
2.3 Fracture Toughness of Dental Materials
Bonilla et al. [18] performed a fracture toughness comparison of nine different commercial
dental flowable composite resins. Resin formulations included BisGMA, BisEMA/TEGDMA,
BisGMA/UDMA/TEGDMA, and UDMA. Filler particle sizes ranged from 0.7 to 3.7 pm, with
loadings of 50.0 wt% to 70.5 wt%. Dimensions of the test specimens were 2 x 4.2 x 20 mm, with a 3
mm preformed notch. Fracture toughness values ranged from 1.26 MN-m'372 to 1.65 MN-nf372. The
average K/c value for BisGMA based resins was 1.33 MN-m'372.
Mean fracture toughness values in human dentin using compact tension specimens were
found to be 2.07, 1.96, and 1.26 MPa-m172, for young (34-41 years), middle-aged (61-69 years), and
aged (85-99 years) samples, respectively [19]. Nalla et al. [20] found fracture toughness values to vary
from 1.44 to 2.65 MPam172, depending on the orientation of collagen fibers in the specimen.
2.4 Other Applications of Nanogels
Adding nanogels with a range of glass transition temperatures to secondary monomers could
produce a final polymer that mimics the behavior of a material with a gradient Tg. The Tg of a material
affects what frequencies of energy the material can absorb or dampen. Low Tg materials tend to
absorb low frequency energies, and high Tg materials tend to absorb high frequency energies. Block
copolymers tend to have two distinct Tgs, which could be synthesized to absorb high and low
frequencies, but not intermediary ones. Materials with a gradient Tg would be ideal at damping a wide
range of frequencies but can be difficult to make, requiring controlled radical polymerization, ring-
opening metathesis polymerization, or living cationic polymerization [21], Nanogels could greatly
simplify the synthesis of a gradient Tg material.
9


Another potential use of nanogels would be the rubber toughening of polymers. Rubber
toughening is the addition of soft and low Tg particles to brittle polymers, to increase their fracture
toughness by adding a second phase. This technique has been applied to high impact polystyrene,
automotive epoxies, and nylons. Theories for the increased toughness include crack bridging, crack
deflection, the initialization of matrix shear deformation, and the absorption of fracture energy by
deformation or cavitation of the low Tg particles [22]. Nanogels may be able to introduce even smaller
particles for rubber toughening, which would increase the probability of crack tip interaction.
10


3. Experimental Program and Preliminary Results
3.1 Objectives
1. Characterize the flexural modulus of dental resins loaded with two different kinds of nanogels
at two different loading concentrations, 25 wt% and 50 wt%. The 25 wt% concentration
represents a significant amount of nanogel that does not dramatically increase the
unpolymerized viscosity. The 50 wt% concentration represents an upper loading limit of 59
vol% with continuous nanogel contacts within the secondary monomer.
2. Characterize the flexural modulus of dental resins with 70 wt% filler particles (0.4 micron
silanized barium borosilicate glass), with and without nanogel loading. Two different
nanogels were loaded at two different loading concentrations (25 wt% and 50 wt%).
3. Determine if the two different nanogels, at two different loading concentrations, and with or
without 70 wt% barium glass filler particles, have effects on the fracture toughness properties
of common dental resins.
4. Characterize the effects of nanogels with high and low glass transition temperatures (Tg) on
the flexural modulus and fracture toughness of common dental resins. Loading
concentrations of 10 wt% and 50 wt% with high and low Tg nanogels were tested.
3.2 Plan of Work
Use the higher modulus BisGMA/TEGDMA at a 70/30 wt% ratio for the secondary monomer
to be loaded with nanogel. This monomer could exhibit greater changes in fracture toughness
in response to nanogel loading. It is also a good model for many commercial dental
composite materials. An alternative secondary monomer with a lower modulus and lower
11


unpolymerized viscosity is 70/30 BisEMA/TEGDMA.
Cure specimens with a thermal free radical polymerization initiator in order to cure more
specimens all at once in an oven, instead of individually exposing each specimen for 4
minutes with UV light polymerization. However, UV light curing has the advantage of being
a more practical curing system for dental materials, because thermal curing requires several
hours of exposure to 85C heat.
Load the secondary monomer with the following series of materials and form test specimens
according to ASTM D790-86 [23] and ASTM E-399 [24] test standards for 3-point bending
and fracture toughness tests, respectively:
1. control
2. control, with 70 wt% 0.4 /an glass particle filler
3. 25 wt% 80/20 IBMA/UDMA 5 mol% ME nanogel
4. 25 wt% 80/20 IBMA/UDMA 5 mol% ME nanogel, with 70 wt% 0.4 /an particle filler
5. 50 wt% 80/20 IBMA/UDMA 5 mol% ME nanogel
6. 50 wt% 80/20 IBMA/UDMA 5 mol% ME nanogel, with 70 wt% 0.4 fxm particle filler
7. 25 wt% aromatic nanogel
8. 25 wt% aromatic nanogel, with 70 wt% 0.4 fim glass particle filler
9. 50 wt% aromatic nanogel
10. 50 wt% aromatic nanogel, with 70 wt% 0.4 fim glass particle filler
11. 10 wt% high Tg nanogel
12. 50 wt% high Tg nanogel
13. 10 wt% low Tg nanogel
14. 50 wt% low Tg nanogel
3.3 Materials
3.3.1 Nanogels
Three different nanogel formulations were investigated in this study: a nanogel composed of
Isobomyl methacrylate and Urethane dimethacrylate (IBMA/UDMA), a nanogel with aromatic
groups (aromatic), and a nanogel with a low glass transition temperature (low Tg). The
IBMA/UDMA nanogel was also used as the high Tg nanogel in the comparison between low and
high Tg nanogels, with a higher chain transfer content of 15 mol%.
12


3.3.1.1 IBMA/UDMA Nanogel Synthesis
The IBMA/UDMA nanogels were synthesized with an 80/20 mol% ratio of Isobomyl
methacrylate (Esstech Inc., Essington PA) and Urethane dimethacrylate (Esstech Inc.). Free radical
polymerization took place in an 80 wt% toluene solution until 95% conversion was achieved. The
chain transfer agent 2-Mercaptoethanol (ME, Sigma-Aldrich Co., St. Louis MO) was added at 5 to 15
mol% to control the nanogel particle sizes, by limiting the nanogel chain lengths. 2,2'-
Azobisisobutyronitrile (AIBN, Sigma Aldrich Co.) thermal initiator at 1 wt% was used to start the
polymerization. The nanogel was purified and isolated by the dropwise addition of the reaction
mixture into an excess of hexane. The precipitated nanogel polymer was filtered and dried under high
vacuum to produce a dry powder. Functional groups of Isocyanatoethyl methacrylate (IEM, Sigma-
Aldrich Co.) at a 1:1 mol ratio to the ME used, were added to the nanogel surfaces by redissolving the
nanogel in methylene chloride or acetone, and stirring overnight with IEM and a catalyst (Dibutyltin
dilaurate, Sigma-Aldrich Co.). The nanogel was then re-precipitated and again dried under high
vacuum. Additional preliminary results were obtained from formulations modified with a 70/30
IBMA/UDMA mol% ratio, or modified with reduced amounts of chain transfer agent of 5 mol% or 10
mol% ME.
3.3.1.2 Aromatic Nanogel Synthesis
For the aromatic nanogel, di-tert-butyl phenol was mixed with urethane methacrylate. The
resulting DTBPUMA product was mixed with Bisphenol A glycidyl methacrylate (BisGMA) at a
70/30 mol% ratio. 2-Mercaptoethanol at 15 mol% was used to control nanogel particle size.
Polymerization took place in 80 wt% methyl ethyl ketone (MEK), with 1% AIBN thermal initiator.
Functional groups of IEM were added at a 1:1 mol ratio to the amount of ME used.
13


3.3.1.3 Low Tg Nanogel Synthesis
For the low Tg nanogel, Isodecyl methacrylate was copolymerized with Bisphenol A
ethoxylate diacrylate at a 70/30 mol% ratio, in a 80 wt% toluene solution. 2-Mercaptoethanol at 5
mol% was used to control nanogel size. Polymerization was initiated thermally with 1 wt% of AIBN.
Functional groups of Isocyanotoethyl methacrylate were added at a 1:1 mol ratio to the amount of 2-
Mercaptoethanol used. The nanogel was not precipitated after adding the functional groups, because
the nanogel remained in solution rather than forming a precipitate.
3.3.2 Secondary Monomers
Nanogels were loaded into secondary monomers to find out how the nanogels would modify
monomer and polymer properties. Important properties included polymerization stress, polymerization
shrinkage, flexural modulus, flexural strength, and fracture toughness. This study focused on the
flexural modulus and fracture toughness effects of nanogels already fulfilling the goals of decreased
shrinkage and stress. Among the secondary monomers tested were blends of Bisphenol A glycidyl
methacrylate (BisGMA, Esstech Inc.), ethoxylated Bisphenol A dimethacrylate (BisEMA, Esstech
Inc.), Triethylene glycol dimethacrylate (TEGDMA, Esstech Inc.), and Urethane dimethacrylate
(UDMA, Esstech Inc.). Secondary monomer formulas tested included a 70/30 wt% ratio of
BisGMA/TEGDMA, a 70/30 wt% ratio of BisEMA/TEGDMA, and 100% UDMA. Composite
materials were made by adding 70 wt% of 0.4 micron silanized barium borosilicate glass filler particles
(E-3000, Esstech Inc.) into the monomer formulas. Monomers were loaded with the nanogels before
the glass filler was added.
3.3.3 Secondary Monomer Polymerization Initiators
During initial experimental work, the free radical polymerization thermal initiators AIBN
(2,2'-Azobisisobutyronitrile, Aldrich Chemical Company, Inc., Milwaukee WI) and Benzoyl Peroxide
14


(Aldrich Chemical Company, Inc.) were used to cure BisGMA/TEGDMA specimens. After obtaining
low mechanical properties from thermal cured specimens (not previously seen with photo curing), a
change was made to BisEMA/TEGDMA specimens cured with UV light, using the photo-initiator
DMPA (2,2-Dimethosy-2 phenyl acetophenone, Aldrich Chemical Company, Inc.).
3.4 Testing Procedures
3.4.1 Nanogel Loading into Secondary Monomers
The loading of nanogels into secondary monomers often required adding a solvent (acetone)
to dissolve the dried nanogel particles, stirring the secondary monomer, nanogel, and solvent together
until the solution was homogenous, and finally using high vacuum to extract the solvent. The low
viscosity of the 70/30 BisEMA/TEGDMA secondary monomer sometimes allowed nanogel loading
without solvent, because it would allow the nanogel to dissolve into the monomer over several days of
constant stirring. Avoiding solvent would ideally prevent any residual solvent present in the secondary
monomer from affecting mechanical test results, but not using solvent could also compromise the
uniform dispersion of the nanogel into the monomer. Stirring was performed using magnetic bar
stirrers. Some of the monomer formulations became very viscous after being loaded with high nanogel
concentrations of 40 to 50 wt%. In order to get these monomers to flow freely into specimen molds,
some of these monomers were heated to temperatures of 60C or 80C. Monomers with 70 wt% glass
filler became hard pastes that had to be packed into specimen molds, instead of being poured.
3.4.2 Thermal Curing of Test Specimens
Specimens cured with AIBN or Benzoyl Peroxide thermal initiators were heated for up to 24
hours at 85C in an oven (Isotemp Vacuum Oven Model 282A, Fisher Scientific, Pittsburg PA).
Complete curing was expected to occur within 8 hours, so any additional time in the oven was not
expected to change specimen properties. Specimens were allowed to gradually cool to room
15


temperature before being tested. Specimens were also gradually warmed up from room temperature to
the 85C curing temperature by being present in the oven while the oven was warming up, instead of
inserting the specimens after the oven had been brought to curing temperature. This gradual heating
was an attempt to minimize the bubble formation occurring in the monomers upon exposure to heat.
Applying a vacuum to the oven chamber only made the bubble formation worse. One potential cause
of the bubble formation could be dissolved gasses or solvent in the resin expanding due to heat.
Placing the monomer formulations under vacuum to remove dissolved gasses before pouring the
monomer into molds seemed to reduce the bubble formation slightly, but also sometimes caused
macrogellation of the monomer. The macrogellation could have been due to the removal of oxygen,
which acts as a polymerization inhibitor.
3.4.3 Photo Curing of Test Specimens
Specimens with free radical polymerization photo initiators (CQA, DMPA) were cured using
a mercury arc UV light (Novacure, EXFO Photonic Solutions Inc., Mississauga ON). For specimens
with DMPA photo initiator, a 365 nm light filter was used, and the light distance was set to obtain an
irradiance of 45 mW/cm2. Specimens were cured for two minutes per side and were allowed to post-
polymerize for 24 hours before testing.
3.4.4 Degree of Conversion Tests
The percent degree of conversion for photo-cured specimens was measured immediately after
curing, and approximately 24 hours later before flexural modulus or fracture toughness testing. The
percent degree of conversion of composite specimens containing barium glass filler was not measured
because of the attenuation caused by the filler particles. An FTIR spectrophotometer (Nexus 670,
Nicolet Instrument Inc., Madison WI) was used to measure the area of the =CH2 peak (6163 cm'1)
before and after curing. Percent degree of conversion was calculated by:
16


% DOC = (l areaftercurin3) x 100
V area before curing/
(3.1)
3.4.5 Flexural Tests
Flexural test specimens were made according to ASTM D790-86 guidelines, with dimensions
of 25 x 2 x 2 mm. Three piece steel molds sandwiched between microscope slides were used initially.
The brittle BisGMA/TEGDMA specimens had a tendency to stick to the mold surfaces and break upon
removal, even when petroleum jelly was used as a mold release lubricant. Therefore, one piece molds
were made from 2 mm thick PTFE sheet, which allowed for much easier specimen removal. Figure
3.1 shows examples of the steel and PTFE molds used.
Figure 3.1. Flexural Test Specimen Molds: 3-Piece Steel and 1-Piece PTFE
Flexural and fracture toughness tests were both performed on a hydraulic axial-load testing machine
(858 Mini-Bionix II, MTS Systems Corporation, Eden Prairie MN). A three-point bending fixture was
used for both flexural and fracture toughness tests. Figures 3.2 and 3.3 show the testing equipment and
3-point bending fixture, respectively.
17


Figure 3.2. MTS Axial-Load Testing Machine and 3-point Bending Fixture
", i
Figure 3.3. 3-Point Bending Test Fixture with Fracture Toughness Specimen (Close-up)
18


Loading conditions for flexural tests were: 1 mm/min crosshead speed, 20 mm span, 1 N break detect,
and 10 Hz sampling rate. Flexural modulus (Ej) was calculated by the following equation:
Fi3
E = r 1
f 4bh3d
(3-2)
where l is the span length, b is the specimen width, h is the specimen height, and d and F are deflection
and load, respectively, at a moment when the material is behaving as elastic material before yielding.
Flexural strengths could also be calculated from these tests, but they were highly variable (any
variation in a valid test specimen would affect its breaking point). Flexural modulus was the focus of
this study, because it was a much more repeatable mechanical property of test specimens.
3.4.6 Fracture Toughness Tests
Single edge notched beam fracture toughness specimens were formed in two-piece brass
molds with a razor blade insert. The blade inserts were machined from single edge razor blades and
bonded into place with UV light cured dental adhesive. Vacuum grease was used as a mold lubricant
and the mold was sandwiched between glass microscope slides to form fracture toughness specimens.
Figure 3.4 shows the fracture toughness mold. Some of the fracture toughness test specimens are
shown in Figure 3.5.
Figure 3.4. Two Piece Mold with Blade Insert for Fracture Toughness Specimens (scale in cm)
19


Figure 3.5. Fracture Toughness Specimens from Top to Bottom: 40 wt% Aromatic Nanogel, 40
wt% Aromatic Nanogel with 70 wt% Ba Glass Filler, 40 wt% IBMA/UDMA
Nanogel, and 50 wt% IBMA/UDMA Nanogel with 70 wt% Ba Glass Filler (scale in
cm)
Load conditions for fracture toughness testing were: 1 mm/min crosshead speed, 20 mm span, 2 N
break detect, and 100 Hz sampling. The 1 N break detect setting used for flexural tests sometimes
caused the load testing machine to stop the test before starting crack propagation, with a peak load
much lower than other fracture toughness test specimens. Repeating tests on these specimens would
produce crack propagation with peak loads similar to other specimens with the same formulation. The
2 N break detect setting also occasionally stopped the test before the crack had fully propagated
through the specimen. Fracture toughness was calculated by the following equation:
20


Kir =
PS
3(^)1/2[1.99-^(l-^)(2.15-3.94+2.7g)]
IC BW3/2
2(i+2S(1-^:
,3/2
(3.3)
where P is the peak load, S is the loading fixture span, B is the specimen thickness, W is the specimen
width, and a is the length of the precrack.
3.4.7 Scanning Electron Microscope Image Analysis
Fracture surfaces of fracture toughness specimens were analyzed using images from a field
emission scanning electron microscope (JSM-7401F, JEOL USA, Peabody, MA). Microstructural
features in the interface of nanogel particles and secondary matrix could not be observed by SEM, but
fracture surface features common to brittle materials were examined.
3.5 Preliminary Results
3.5.1 Flexural Tests at Different Loading Rates
All flexural tests were performed with a 1 mm/min loading rate. Samples made with
BisGMA/TEGDMA secondary monomer were also tested at 0.1 mm/min and 10 mm/min to observe
the effects of loading rate on bending tests. Samples with the higher loading rate had a slightly higher
mean flexural modulus, and samples with the lower loading rate had a slightly lower mean flexural
modulus (see Table 3.1 and Figure 3.6). The reason for this could be that specimens with a slower
loading rate engage in a greater degree of frequency dependent viscoelastic response.
21


Table 3.1. BisGMA/TEGDMA flexural modulus at different loading rates
Loading rate (mm/min) E mean (GPa) # Test samples StDev
0.1 (w/o 70 wt% filler) 3.40 6 0.14
1 (w/o 70 wt% filler) 3.55 24 0.22
10 (w/o 70 wt% filler) 3.78 10 0.12
0.1 (w/ 70 wt% filler) 8.22 6 0.50
1 (w/ 70 wt% filler) 8.69 7 0.66
10 (w/ 70 wt% filler) 8.92 6 0.43
Loading Rate (mm/min)
Figure 3.6. Comparison of Flexural Modulus of BisGMA/TEGDMA at Different Loading Rates
(Means and Standard Deviations)
22


3.5.2 BisGMA/TEGDMA Flexural Test Results
Figure 3.7 and Table 3.2 show flexural modulus results for BisGMA/TEGDMA secondary
monomer loaded with nanogels. All specimens were cured thermally with 1% Benzoyl Peroxide
initiator. Nanogels included a 70/30 mol% ratio of IBMA/UDMA nanogel synthesized with AIBN
thermal initiator, and a 70/30 mol% ratio of IBMA/UDMA nanogel synthesized with BAPO photo
initiator, with different mol% amounts of 2-Mercaptoethanol chain transfer agent. Solvent was used to
load the nanogels at 25 wt% concentrations. The addition of 0.4 micron barium glass filler at 70 wt%
increased the flexural modulus of the resins greatly, while nanogel caused a reduction in flexural
modulus in specimens with and without filler. When the monomer with nanogel synthesized with
BAPO and 10 mol% ME was subjected to additional vacuuming to try removing more residual solvent,
the flexural modulus of the resulting specimens actually decreased even more, as compared to the
control. Note that residual solvent can reduce the modulus as a plasticizer in the polymer, or increase
the modulus by facilitating higher conversion.
23


Flexural Modulus (GPa)
control AIBN5ME AIBN 10ME BAPO 5ME BAPO 10ME BAPO 10ME,
more vacuuming
Nanogel Type
Figure 3.7. Comparison of BisGMA/TEGDMA with 25 wt% Nanogel Loading Flexural Modulus
Results (Means and Standard Deviations)
24


Table 3.2. BisGMA/TEGDMA flexural modulus results
Nanogel E mean (GPa) # Test samples StDev DOC mean (% )
control 3.55 24 0.22 88.9
control with 70 wt% filler 8.69 7 0.66 -
25 wt% 70/30 IU AIBN 5% ME 2.10 1 0.00 -
25 wt% 70/30 IU AIBN 10% ME 2.20 3 0.10 -
25 wt% 70/30 IU AIBN 15% ME 0.00 0 0.00 -
25 wt% 70/30 IU AIBN 10% ME, 70 wt% filler 5.23 3 0.06 -
25 wt% 70/30 IU BAPO 5% ME 1.80 2 0.00 88.0
25 wt% 70/30 IU BAPO 10% ME 1.65 2 0.07 84.0
25 wt% 70/30 IU BAPO 10% ME, additional solvent vacuuming 0.95 2 0.07 76.0
25 wt% 70/30 IU BAPO 10% ME, additional solvent vacuuming, 70 wt% filler 2.00 3 0.61 -


3.5.3 BisGMA/TEGDMA Fracture Toughness Test Results
Fracture toughness results for thermally cured BisGMA/TEGDMA secondary monomer are
shown in Figure 3.8 and Table 3.3. For nanogel loaded specimens, there was an inverse correlation
between the fracture toughness and the amount of chain transfer agent used during nanogel synthesis.
However, the fracture toughness of specimens with BAPO light cured 10 mol% ME nanogel increased
when the monomer was subjected to additional vacuum treatment in order to remove any additional
residual solvent.
i without filler with filler
1.0
0.8
£ 06
0.4
0.2
0.0
114414
control AIBN5ME AIBN 10ME AIBN 15ME BAPO 5ME BAPO 10ME BAPO 10ME,
more
vacuuming
Nanogel Type
Figure 3.8. Comparison of BisGMA/TEGDMA with 25 wt% Nanogel Loading Fracture
Toughness Results (Means and Standard Deviations)
26


Table 3.3. BisGMA/TEGDMA fracture toughness results
Nanogel K,Cmean (MN-Itl'3'2) # Test samples StDev
control 0.80 6 0.15
control with 70 wt% filler 0.87 10 0.10
25 wt% 70/30IU AIBN 5% ME 0.72 3 0.03
25 wt% 70/30 IU AIBN 10% ME 0.65 2 0.04
25 wt% 70/30 IU AIBN 15% ME 0.57 3 0.04
25 wt% 70/30 IU AIBN 10% ME, 70 wt% filler 0.71 2 0.06
25 wt% 70/30 IU BAPO 5% ME 0.76 2 0.06
25 wt% 70/30 IU BAPO 10% ME 0.67 2 0.01
25 wt% 70/30 IU BAPO 10% ME, additional solvent vacuuming 0.70 2 0.05
25 wt% 70/30 IU BAPO 10% ME, additional solvent vacuuming, 70 wt% filler 0.53 1 0.00


3.5.4 Thermal Curing Issues
Benzoyl peroxide thermal initiator caused the flexural modulus and fracture toughness of the
secondary monomer BisGMA/TEGDMA to decrease with the addition of nanogels. This was in
opposite to the desired increase in flexural modulus, which was previously obtained with light curing.
To verify this result, two other secondary monomers, 70/30 BisEMA/TEGDMA and UDMA, were
loaded with nanogel and cured both thermally and with UV light. All of the thermally cured secondary
monomers showed a reduction in flexural modulus with 25 wt% nanogel loading, as compared to
thermally cured control specimens. The thermally cured UDMA specimens loaded with nanogel could
only reach a 34% degree of conversion and could not be tested, as they could be folded in half without
breaking. All of the photocured specimens showed an increase in flexural modulus with nanogel
loading, as compared to photocured control specimens without nanogel. Percent degree of conversion
was significantly higher for thermally cured specimens. Figure 3.9 and Tables 3.4 and 3.5 show the
results of different curing methods for BisEMA/TEGDMA and UDMA. Table 3.4 also shows the
results of adding both photo and thermal initiators into BisGMA/TEGDMA specimens, curing them
first with UV light, and then curing them a second time thermally. For these dual-cured specimens,
percent degree of conversion was raised to the level of thermally cured specimens and flexural
modulus decreased with nanogel loading.
Based on the preliminary test results showing the effects of curing methods on flexural
modulus of BisEMA/TEGDMA and UDMA, UV light was used to cure all further test specimens.
BisEMA/TEGDMA was also chosen as the secondary monomer for this study, because its lower
viscosity could allow nanogel loading without solvent. The residual solvent in the nanogel loaded
monomer could otherwise affect the results.
28


Flexural Modulus (GPa)
3.5
without nanogel 25 wt% nanogel
3.0
2.5
2.0
1.5
1.0
0.5
0.0
BisEMA/TEGDMA BisEMA/TEGDMA BisEMA/TF.GDMA UDMA light UDMA thermal
light thermal light + thermal
Curing Method
Figure 3.9. Comparison of Flexural Modulus Results Using Different Curing Methods (Means
and Standard Deviations)
29


Table 3.4. Flexural modulus results for BisEMA/TEGDMA using different curing methods
Curing method E mean (GPa) # Test samples StDev DOC mean (% )
light curing 1.33 3 0.12 76.3
light curing with 25 wt% nanogel loading 1.70 3 0.10 75.0
thermal curing 2.83 3 0.06 94.7
thermal curing with 25 wt% nanogel loading 2.20 3 0.20 83.0
light and thermal curing 2.77 3 0.12 97.0
light and thermal curing with 25 wt% nanogel loading 2.53 3 0.06 86.0
Table 3.5. Flexural modulus results for UDMA using different curing methods
Curing method E mean (GPa) # Test samples StDev DOC mean (% )
light curing 0.77 3 0.12 64.0
light curing with 25 wt% nanogel loading 1.30 3 0.10 86.7
thermal curing 2.50 3 0.06 98.0
thermal curing with 25 wt% nanogel loading did not cure 4 - 34.0


4. Experimental Results and Discussions
4.1 Loading Nanogels into BisEMA/TEGDMA Secondary Monomer
The lower viscosity of unpolymerized BisEMA (in comparison to BisGMA) would allow the
loading of nanogels at 25 and 40 wt% concentrations, without requiring the use of solvent to dissolve
the secondary monomer and nanogel together. While solvent can be extracted with high vacuum,
some residual solvent could remain in the monomer and affect the subsequent polymerization and
polymer properties. Not using solvent would ideally prevent the presence of any residual solvent from
affecting the mechanical properties of the test specimens. There was, however, a slight decrease in
specimen optical clarity when solvent was not used, which could be due to incompletely dissociated
clusters of nanogel particles still being present. Attempts to incorporate nanogel into monomers at 50
wt% loading without using solvent resulted in monomers that appeared homogenous before curing, but
had undissolved clumps of nanogel appearing upon polymerization. Therefore, these specimens were
not considered to have full nanogel integration with the secondary monomer.
Alternatively, using solvent at 50 wt% loading resulted in a significant increase in optical
clarity in unpolymerized and polymerized specimens, but also caused a significant increase in viscosity
after removing the solvent. The monomer loaded with aromatic nanogel was too viscous to pour into
molds, and the monomer loaded with IBMA/UDMA nanogel had a viscosity similar to that of
unpolymerized BisGMA monomer. Redissolving these 50 wt% solvent loaded monomers and adding
enough monomer to reduce the nanogel loading to 40 wt% helped the monomer with aromatic nanogel
to have a viscosity similar to unpolymerized BisGMA. Making test specimens with this monomer
failed due to multiple air bubbles forming during polymerization, which caused specimen fracture
upon mold removal. The monomer with solvent-loaded 40 wt% IBMA/UDMA nanogel had a
31


viscosity that allowed it to be poured very slowly into the specimen molds, and this made some of the
most optically clear specimens to date. However, the specimens showed no increase in flexural
modulus, as compared to the control monomer. This was opposed to the increases in flexural modulus
obtained with nanogel concentrations of 25 and 40 wt%.
4.2 IBMA/UDMA and Aromatic Nanogel Flexural Modulus Results
BisEMA/TEGDMA monomer loaded with 25 wt% nanogel showed an increase in mean
flexural modulus values with both IBMA/UDMA and aromatic nanogels. The IBMA/UDMA nanogel
specimens showed a 17% increase, and the aromatic nanogel specimens showed a higher increase of
50%. When nanogel concentrations increased to 40 wt%, the mean flexural modulus of IBMA/UDMA
loaded specimens increased to 27%, and the mean flexural modulus of specimens with aromatic
nanogel increased to 50%. Specimens with 70 wt% filler showed similar increases in flexural
modulus, with aromatic nanogels increasing the flexural modulus by 24% for both 25 and 40 wt%
nanogel concentrations, and the IBMA/UDMA nanogels increasing the flexural modulus by 10% and
21% for 25 wt% and 40 wt% loading concentrations, respectively. These flexural modulus results as
well as the ones for 50 wt% nanogel loading are shown in Table 4.1 and Figure 4.1. Note that the
specimens with 50 wt% nanogel loading had undissolved clusters of nanogel appearing during curing,
since solvent was not used. Therefore, these specimens could not be considered to be homogenous
with complete nanogel integration with the monomer.
In Table 4.1, statistical t-tests were used to determine the effect of nanogel loading. Two-
tailed t-tests were performed to determine if the mean flexural modulus (or mean fracture toughness)
values were equal between sample sets, at a 5% significance level. Sample sets were assumed to have
equal but unknown variances. A t-test result of 1 rejects the null hypothesis that the means of the
compared sets are equal, while a result of 0 accepts the null hypothesis.
32


Table 4.1. Flexural modulus results for IBMA/UDMA and aromatic nanogels
Nanogel E mean (GPa) StDev # Test samples T-test\s control DOC mean (% )
control 1.7 0.15 6 78
control with 70 wt% filler 5.9 0.33 7 1 -
25 wt% IU 70/30 2.0 0.06 3 1 76
25 wt% IU 80/20 2.0 0.00 7 1 75
25 wt% IU 80/20, 70 wt% filler 6.4 0.35 6 1 (vs filler control) -
40 wt% IU 80/20, solvent 1.8 0.00 3 0 81
40 wt% IU 80/20 2.2 0.15 6 1 76
40 wt% IU 80/20, 70 wt% fiUer 7.1 0.43 6 1 (vs filler control) -
50 wt% IU 80/20, visible nanogel 2.5 0.21 3 1 73
25 wt% aromatic 2.6 0.21 6 1 81
25 wt% aromatic, 70 wt% filler 7.3 0.15 6 1 (vs filler control) -
40 wt% aromatic 2.6 0.31 6 1 80
40 wt% aromatic, 70 wt% filler 7.3 0.36 6 1 (vs filler control) -
50 wt% aromatic, visible nanogel 2.9 0.21 2 1 79


8
cs
P.
a
3
O
O
3
X
s
control 25 wt% IU 40 wt% IU 50 wt% IU, visible 25 wt% aromatic 40 wt% aromatic 50 wt% aromatic,
nanogel visible nanogel
Nanogel Loading
Figure 4.1. Comparison of IBMA/UDMA and Aromatic Nanogel Flexural Modulus Results (Means and Standard Deviations)


4.3 IBMA/UDMA and Aromatic Nanogel Fracture Toughness Results
The fracture toughness values shown in Table 4.2 had large standard deviations within sample
sets. Therefore, statistical t-tests were used to determine the effect of nanogel loading. According to
these tests, the IBMA/UDMA nanogels had no significant effect on mean fracture toughness at
concentrations of 25 wt% and 40 wt%, or when 70 wt% filler was added to the 25 wt% nanogel
concentration. There was a statistically significant drop in fracture toughness when 40 wt% nanogel
concentration was loaded using solvent, when 70 wt% filler was added to the 40 wt% nanogel
concentration (no solvent), and in the 50 wt% nanogel concentration that had visible nanogel clumps.
For the specimens with aromatic nanogels, only the specimens with 70 wt% filler showed a statistically
significant drop in fracture toughness (see Figure 4.2).
35


Table 4.2. Fracture toughness results for IBMA/UDMA and aromatic nanogels
Nanogel K,Cmean (MN*m 3 2> StDev # Test samples T-test vs control DOC mean (% )
control 0.93 0.16 14 79
control with 70 wt% filler 0.93 0.14 9 0 -
25 wt% IU 70/30 0.84 0.12 3 0 83
25 wt% IU 80/20 0.90 0.19 6 0 76
25 wt% IU 80/20, 70 wt% filler 0.86 0.06 6 0 (vs filler control) -
40 wt% IU 80/20, solvent 0.60 0.33 3 1 86
40 wt% IU 80/20 0.88 0.22 6 0 81
40 wt% IU 80/20, 70 wt% filler 0.68 0.08 7 1 (vs filler control) -
50 wt% IU 80/20, visible nanogel 0.71 0.05 3 1 71
25 wt% aromatic 1.04 0.12 6 0 82
25 wt% aromatic, 70 wt% filler 0.75 0.06 5 1 (vs filler control) -
40 wt% aromatic 0.91 0.14 6 0 79
40 wt% aromatic, 70 wt% filler 0.77 0.05 6 1 (vs filler control) -
50 wt% aromatic, visible nanogel 0.74 0.09 3 0 78


K,c (MN/m'W)
1.4
without filler with filler
Nanogel Loading
Figure 4.2. Comparison of IBMA/UDMA and Aromatic Nanogel Fracture Toughness Results (Means and Standard Deviations)


4.4 Flexural Modulus Results for High and Low Tg Nanogels
At nanogel loading concentrations of 10 wt%, there was no statistically significant change in
mean flexural modulus with high or low Tg nanogels. At nanogel loading concentrations of 50 wt%,
there was a slight increase in mean flexural modulus of specimens with high Tg nanogels, and a
significant decrease in flexural modulus of specimens with low Tg nanogels. Specimens with low Tg
nanogels at 50 wt% concentration also reached a percent degree of conversion of over 90%, a level not
seen with any other photo-cured nanogel specimens. Two sets of low Tg nanogels were used that had a
10C difference in glass transition temperature, but the two sets still showed similar results (see Table
4.3 and Figure 4.3).
38


Table 4.3. Flexural modulus results for high and low Tg nanogels
Nanogel E mean (GPa) StDev # Test samples T-testvs control DOC mean (% ) StDev
10 wt% High Tg (90C), solvent 1.7 0.12 3 0 77 1.15
50 wt% High Tg (90C), solvent 2.1 0.10 3 1 69 1.53
10 wt% Low Tg (-45C) 1.5 0.06 3 0 81 1.00
50 wt% Low Tg (-35C), solvent 0.5 0.09 4 1 92 2.16
50 wt% LowTg (-45C) 0.5 0.03 2 1 95 0.00


Flexural Modulus (GPa)
2.5
control 10wt%highTg 50wt%highTg 10wt%-45CTg 50wt%-35CTg 50wt%-45CTg
Nanogel Loading
Figure 4.3. Comparison of High and Low Tg Nanogel Flexural Modulus Results (Means and Standard Deviations)


4.5 Fracture Toughness Results for High and Low Tg Nanogels
For fracture toughness tests (see Table 4.4 and Figure 4.4), there was no statistically
significant change in mean fracture toughness at 10 wt% concentrations of high or low Tg nanogels,
but there were large decreases in fracture toughness at 50 wt% concentrations. The low Tg nanogel at
10 wt% concentration could have been statistically different, because one of the three specimens in that
set had a higher fracture toughness than the other two, which could have affected the t-test. Because of
these adverse results, no additional effort was spent on adding 70 wt% filler to monomers with high
and low Tg nanogels.
41


Table 4.4. Fracture toughness results for high and low Tg nanogels
Nanogel K,Cmean (MN-m'3'2) StDev # Test samples T-test vs control DOC mean (% )
10 wt% High Tg (90C), solvent 1.11 0.18 3 0 76
50 wt% High Tg (90C), solvent 0.40 0.07 3 1 68
10 wt% Low Tg (-45C) 0.79 0.15 3 0 84
50 wt% Low Tg (-35C), solvent 0.45 0.05 3 1 93
50 wt% Low Tg (-45C) 0.49 - 1 - 93


K[c (MN/rrr3/2)
1.4
U
U)
1.2
1.0
0.8
0.6
0.4
0.2
0.0
control 10wt%highTg 50wt%highTg 10wt%lowTg 50wt%-35CTg 50wt%-45CTg
Nanogel Loading
Figure 4.4. Comparison of High and Low Tg Nanogel Fracture Toughness Results (Means and Standard Deviations)


4.6 Degree of Conversion Results
For IBMA/UDMA nanogels, the percent degree of conversion, as compared to control
specimens, was unchanged at 10 wt% loading (high Tg nanogel), slightly reduced at 25 wt% and 40
wt% loadings, and reduced more at 50 wt% loading (high Tg nanogel). Specimens with aromatic
nanogels had slightly higher percent degree of conversion values than control. The percent degree of
conversion of specimens with low Tg nanogels increased with the nanogel concentration and reached to
95% at 50 wt% nanogel concentration. This high level of conversion was not observed with any other
nanogels with photo polymerization at any wt% concentration. There were more variations in the
percent degree of conversion measurements with fracture toughness specimens than with the flexural
test specimens, which could be due to the razor insert in the fracture toughness molds interfering with
FTIR readings. The percent degree of conversion results with respect to nanogel loading are shown in
Figure 4.5.
44


100

i Bending Samples
i Fracture Toughness Samples
90
control 25wt%IU 40wt%IU 25 wt% 40wt% 10wt%high 50wt%high 10wt%-45C 50wt%-35C 50wt%-45C
aromatic aromatic Tg Tg Tg Tg Tg
Nanogel Loading
Figure 4.5. Comparison of Mean Percent Degree of Conversion Results (Means and Standard Deviations)


4.7 Scanning Electron Microscopy Image Analysis
Fracture surfaces of fracture toughness test specimens were analyzed using a field emission
scanning electron microscope (JSM-7401F, JEOL USA, Peabody, MA). Control, 10 wt% high Tg
nanogel, 50 wt% high Tg nanogel, 10 wt% low Tg nanogel, and 50 wt% low Tg nanogel test specimens
were examined. Microstructural nanogel features could not be observed under SEM, so the interface
between nanogel particles and the secondary monomer matrix could not be examined. Images were
obtained of fracture surface features such as hackles, twist hackles, tide marks, and river lines.
Hackles are lines in fracture surfaces aligned in the local direction of crack propagation, that separate
parallel but non-coplanar portions of the crack surface [25]. Twist hackles are hackles separating
portions of the crack surface that have rotated from the original crack plane, due to specimen rotation
during fracture failure and/or twisting of the axis of principle tension [25]. Tide marks are lines in the
fracture surface perpendicular to the direction of crack propagation and hackles, which can indicate
increased material toughness causing changes in the crack propagation speed or even momentary crack
front arrest [26]. River-lines are marks in the direction of crack propagation that are much more faint
than hackles [26]. For fracture toughness specimens in this study as seen in Figure 3.3, the origin of
the crack was known to be at the precrack tip, and the direction of crack propagation was known to be
parallel to the precrack and opposite to the direction of loading.
Figure 4.6 shows the precrack tip interface between the precrack surface and the fracture
testing crack propagation surface. The rougher area on the left side is the precrack surface and the
smoother area on the right side is the fracture testing crack propagation surface. The line at which the
two areas meet used to be the precrack tip spanning the specimen thickness. Some hackles and twist
hackles can be seen emanating from the precrack tip. Faint river lines in the direction of crack
propagation can be seen further away from the precrack tip.
46


Figure 4.6. Fracture Surface of Control Specimen (xlOO)
Figure 4.7 shows the fracture surface of a fracture toughness specimen with 10 wt% high Tg
nanogel at two different magnifications. In the lower magnification image, twist hackles or river-line
marks further away from the precrack tip are in the direction of crack propagation. In the higher
magnification image, twist hackles near the precrack tip appear almost perpendicular to the direction of
crack propagation, which might indicate an initial lateral crack propagation spanning the width of the
precrack before propagating through the rest of the specimen. Alternatively these twist hackles might
be tide-lines, indicating crack front arrest or changes in crack propagation speed at the onset of crack
propagation. This could be due to nanogel toughening, and the mean fracture toughness of the 10 wt%
high Tg specimens was in fact slightly higher than control (as seen in Figure 4.4), although not found
statistically different due to the broad range of control specimen results.
47


(a)
(b)
Figure 4.7. Fracture Surface of Specimen with 10 wt% High Tg Nanogel at (a) x25 and (b) xlOO
48


The precrack/crack propagation interface for a 50 wt% high Tg nanogel specimen is shown in
Figure 4.8. In this figure, the precrack surface is on the lower right side of the image and the direction
of crack propagation is towards the upper left comer of the image. The lines perpendicular to the
direction of crack propagation are likely to be tide marks indicating crack front arrest, which are more
pronounced and more parallel to the precrack tip than the lateral lines seen in Figure 4.7(b) of the 10
wt% high Tg nanogel specimen. Twist hackles and river lines in the direction of crack propagation
cannot be observed, which can be the case in toughened brittle polymers. However, 50 wt% high Tg
nanogel specimens had a worse fracture toughness than control. Therefore other factors such as
residual solvent could still be making the fracture toughness results worse in spite of fracture surface
features that suggest toughening.
49


The fracture surface of a fracture toughness test specimen with 10 wt% low Tg nanogel is
shown in Figure 4.9. The precrack surface is shown in the lower left comer of the image. The bright
islands on the crack propagation surface could be excess gold sputtering applied during SEM
specimen preparation. Twist hackles in the direction of crack propagation can be seen emanating from
the precrack tip, and none of the lateral twist hackles or tide marks seen in the high Tg nanogel
specimens are present.
Figure 4.9. Fracture Surface of Specimen with 10 wt% Low Tg Nanogel (xlOO)
The fracture surface of a fracture toughness specimen with 50 wt% low Tg nanogel is shown
in Figure 4.10(a). Huge hackle marks are seen emanating from the precrack tip, with the precrack
region being in the lower left comer of the image. Twist hackles and river marks are also seen further
away from the precrack tip, in the direction of crack propagation. Lateral hackles or tide lines are not
observed in this figure. In the upper right comer of the image, an air bubble can also be seen.
At a higher magnification of x5000 in Figure 4.10(b), round clusters can be observed
(approximately 0.8 micrometers in diameter) that were not seen in the control specimen, the high Tg
50


nanogel specimens, or in the 10 wt% low Tg nanogel specimen. Voids or microcracks in the direction
of crack propagation can also be seen. Features similar to these could not be seen at this magnification
in the other specimens, which indicates that the 50 wt% low Tg nanogel specimens could be much
more heterogeneous in composition. This may explain the severely reduced fracture toughness values
for these specimens. The clusters in Figure 4.10(b) might indicate that local concentrations of this low
Tg nanogel are polymerizing first in areas with more free radical polymerization initiator, leading to
this heterogeneous composition and reduced nanogel integration to the secondary matrix. Another
possibility could be a phase separation occurring between the nanogel and secondary matrix. The
presence of residual solvent could also be the cause of these clusters or voids, but solvent was not used
in the incorporation of low Tg nanogels with the secondary monomer.
51


(a)
(b)
Figure 4.10. Fracture Surface of Specimen with 50 wt% Low Tg Nanogel at (a) x25 and (b)
x5000
52


5. Summary and Conclusions
In this study, the effect of nanogels on the flexural modulus and fracture toughness of
methacrylate polymer systems was investigated. Four nanogel formulations were used in this study: a
nanogel with an 80/20 mol% ratio of IBMA/UDMA and 5 mol% of chain transfer agent, a nanogel
with aromatic groups, a low Tg nanogel, and a high Tg nanogel. A 70/30 wt% mixture of
BisEMA/TEGDMA was the secondary monomer loaded with nanogels, which was polymerized using
ultraviolet light. The high Tg nanogel was another 80/20 mol% ratio of IBMA/UDMA with a higher
15 mol% of chain transfer agent.
The high and low Tg nanogels had no statistically significant effect on flexural modulus or
fracture toughness at 10 wt% nanogel concentrations. At 50 wt% nanogel concentrations, the high and
low Tg nanogels lowered the fracture toughness by over 50%. These results imply that modifications
to the nanogels or nanogel loading techniques will be required if fracture toughness is to stay
unaffected at high wt% concentrations of nanogels High nanogel wt% concentration is desired
because it has lead to the highest reductions in polymerization shrinkage and stress.
The low Tg nanogel also increased the percent degree of conversion of photocured
BisEMA/TEGDMA more than any other nanogel, and at 50 wt% concentration it had the largest drop
in flexural modulus of 70%. An increase in conversion could be advantageous if it reduced the
amount of unreacted monomer that could leach out and be replaced with water in the environment.
However, the increase in conversion is normally accompanied with increased shrinkage, which
nanogels were intended to reduce. Therefore, the low Tg nanogel used in this study does not appear
useful in improving or at least unaffecting the mechanical properties of methacrylate polymer systems
loaded with nanogels. It may, however, still be useful for other applications explained in Section 2.4:
for rubber toughening or loading multiple nanogels into a monomer system.
53


The aromatic nanogel increased the mean flexural modulus by 50% for both nanogel
concentrations of 25 wt% and 40 wt%. Specimens with 70 wt% glass particle filler also showed a 24%
increase in flexural modulus at both loading levels. Specimens with aromatic nanogel did not have
statistically different mean fracture toughness values versus control specimens at either loading levels
when there was no filler, but the fracture toughness decreased by about 18% at both nanogel
concentrations when 70 wt% filler was added. This implies that aromatic nanogels will favorably
improve the mechanical properties of methacrylate polymer systems when inorganic filler is not added.
Using IBMA/UDMA nanogel at loading concentrations of 25 wt% and 40 wt%, the mean
flexural modulus of BisEMA/TEGDMA increased by 18% and 29%, respectively. Fracture toughness
was not affected at both wt% loading concentrations when particle filler was not added. A statistically
significant drop in fracture toughness was observed when filler was used with a 40 wt% nanogel
concentration. These results imply that similar to the aromatic nanogel, the IBMA/UDMA nanogel will
favorably improve the mechanical properties of methacrylate monomers when filler is not added.
An investigation is required to know how these results could be affected by the use of solvent
to dissolve the nanogels into the secondary monomer. Solvent was not used with the IBMA/UDMA
and aromatic nanogels at 25 wt% and 40 wt% concentrations, and these monomers were slightly
translucent before and after polymerization. This could be due to non-uniformly dispersed nanogel
aggregates failing to integrate fully with the monomer matrix. These specimens could, therefore, be
acting like specimens with inorganic filler, which also increased the flexural modulus of control
specimens without affecting the fracture toughness. Monomers using solvent to load nanogels were
more optically clear before and after polymerization, with an increased unpolymerized viscosity that
also implied better nanogel integration with the monomer. On the other hand, when the IBMA/UDMA
nanogel was loaded at 40 wt% with solvent, the flexural modulus did not improve and the fracture
toughness decreased by 35%. The high Tg nanogels with significant 50% reductions in fracture
toughness at 50 wt% nanogel concentration were also loaded with solvent. This could mean that while
54


using solvent provides improved optical clarity and nanogel integration, it still somehow weakens the
mechanical properties. A possible reason for this could be related to any residual solvent remaining in
the monomer. Unfortunately, high nanogel loading concentrations of 40 wt% and 50 wt% were nearly
impossible to achieve without solvent. Alternative techniques of nanogel loading need to be
investigated, perhaps a no-solvent method, a different solvent from the acetone used in this study, or an
improved method of extracting the solvent after nanogel loading.
One benefit of nanogel loading is network reinforcement, by physically filling up the polymer
with a preformed precursor to a secondary network that integrates with the matrix through
entanglement and covalent bonds. Carbon molecule double bond concentrations drop as nanogel
concentration is increased, such that at 50 wt% nanogel loading, there are approximately half as many
reactive groups present in the control resin. By reducing crosslink density in this manner, the viscosity
of the monomer can be reduced while maintaining flexural modulus.
The lack of significant fracture toughness improvement at high nanogel concentrations imply
that cracks are not being deflected, or that crack energy is not being dissipated. This could be due to
crack propagation through the nanogel particles themselves, if the particles are worse at absorbing
fracture energy than the monomer. If the cracks are in fact propagating around the particles, it could
mean that less energy is required to propagate through the interface between the secondary monomer
matrix and the nanogel particles, despite the increase in crack path length.
The nanogel loading concentrations of 10 wt% and 50 wt% should represent the extremes of
loading levels. Nanogel loading at 10 wt% corresponds to a dispersed state, in which particles are
not interconnected and some aggregate clumps of particles exist. As the wt% of nanogel is increased a
percolation state is reached, in which continuous paths of nanogel particles are in contact
throughout the matrix while some regions of base matrix remain particle free. Further increasing the
nanogel concentration eventually results in the dense random packing or overlapping state, where
the particles form a secondary network in contact with every random space. A theoretical loading limit
55


of approximately 59 vol% could be achieved if the particles were assumed to be solid spheres of
uniform size. The nanogels in this study have a relative density very close to that of the bulk monomer
(about 1.14 g/ml for both BisEMA and 70/30 mol% IBMA/UDMA), so a nanogel concentration of 50
wt% is nearly equivalent to 50 vol%. The nanogels in this study are also swellable and become
infused with some of the bulk monomer, which further reduces the volume of the free monomer.
Therefore, a 50 wt% concentration of nanogel might correspond to a highly dense, near-continuous
nanogel configuration.
This study was a part of an overall project in which the primary objectives were
polymerization shrinkage and polymerization stress reductions of methacrylate polymer systems by
using nanogels. Polymerization shrinkage reductions of 50% have been attained at 50 wt% nanogel
loading concentrations, from the reduction in double bond concentration. The reduced shrinkage leads
to reduced polymerization stress. This is very important, because polymerization stress causes internal
defects in the final polymer and external warping that induces stress at polymer-substrate interfaces.
The reduced shrinkage could also promote non-dental applications such as injection molding
processes, in which mold designers must account for part shrinkage. Solid steel injection molds could
be heated to lower the viscosity of monomers with high nanogel wt% concentrations to make the
handling and application of the monomer easier. This study has shown that nanogels loaded at 25 and
40 wt% concentrations without using solvent can increase flexural modulus without affecting the
fracture toughness. It has also shown that 50 wt% concentrations of high and low Tg nanogels may
significantly lower fracture toughness, but this may be resolved by improving the nanogel loading
technique.
56


APPENDIX
A. RESULTS OF INDIVIDUAL SPECIMEN TEST TRIALS
57


Table A.l. Flexural test specimens with IBMA/UDMA and aromatic nanogels
Nanogel (wt%) Date mmddyv Specimen Filler Solvent E (GPa) Peak bad (N) Stress at peak (MPa) Strain at peak m Note Degree of conversion (%>
0 (control) 91010 B1 1.6 19.2 66.8 7.5 mold & slide surfaces slightly wet 76.6
91010 B2 1.5 17.0 61.6 6.0 78.7
91010 B3 1.7 20.4 54.5 3.0 76.5
91510 B1 1.8 19.8 70.7 6.2 77.0
91510 B2 1.8 29.1 73.4 9.8 76.7
91510 B3 1.9 20.6 76.8 10.3 77.9
0 (control) 112310 cfl X 6.1 25.3 82.8 1.6 could not measure
70 Ba glass filler 112310 cf2 X 6.3 32.9 117.9 2.7 "
112310 cf3 X 6.2 21.0 72.7 1.4
112310 cf5 X 5.8 27.7 96.3 2.3 slight peel break N
91510 BF1 X 5.7 26.2 83.0 1.8 "
91510 BF2 X 5.5 26.9 87.2 1.9 "
91510 BF3 X 5.5 26.5 84.6 2.0 1st trial stooped with 12937 MPa modulus, .936 N peak bad "
25 IBMA/UDMA 80/20 5ME 100110 25IB1 2.0 11.7 43.9 2.4 76.3
100110 25IB2 2.0 16.0 59.2 3.8 73.9
100110 25IB3 2.0 17.9 63.5 4.5 76.0
100110 25IB4 2.0 11.4 0.9 75.7
102210 25IB1 2.1 17.6 65.0 3.9 320-390uv filter, 5.3cm 73.0
102210 25IB2 2.0 19.0 88.7 4.6 320-390uv fiber, 5.3cm 72.0
102210 25IB3 2.0 9.4 37.1 2.0 320-390uv filter, 5.3cm 76.2
25 IBMA/UDMA 80/20 5ME 100110 25IbFl X 6.0 16.6 56.3 1.1 could not measure
70 Ba glass filler 100110 25IbF2 X 6.6 12.3 42.2 0.6
100110 25IbF3 X 6.0 19.7 61.7 1.2 peel break "
112310 251 fl X 6.8 17.5 55.8 0.8 "
112310 25if2 X 6.7 20.5 60.1 1.0 "
112310 25if3 X 6.5 17.6 54.8 0.9 "
40 IBMA/UDMA 80/20 5ME 102910 4IBls X 1.8 16.4 61.9 5.7 went perfectly plastic 80.3
solvent loading 102910 4IB2s X 1.8 16.3 61.9 6.1 went perfectly plastic 81.3
102910 4IB3s X 1.8 16.1 60.4 6.3 82.0
40 IBMA/UDMA 80/20 5ME 110410 4IB1 2.2 8.6 29.0 1.4 75.0
110410 4IB2 2.3 9.1 30.6 1.5 75.6
110410 4IB3 2.2 9.1 30.0 1.6 75.7
110410 41B4 2.3 11.3 35.9 1.7 74.0
110410 4IB5 2.2 7.4 24.3 1.2 75.3
111910 4tbl 1.9 7.5 24.5 1.3 slight peel 77.0


Table A.l (contd)
Nanogel (wt%) Date mmddyy Specimen Filler Solvent E (GPa) Peak load (N) Stress at peak (MPa) Strain at peak (%> Note Degree of conversion (%)
40 IBMA/UDMA 80/20 5ME 111210 4ibfl X 7.0 14.3 41.3 0.6 could not measure
70 Ba glass filler 111210 4ibf2 X 7.7 16.6 49.2 0.7 3mm of top edge missing "
111210 4ibf3 X 7.2 17.7 47.5 0.7 "
111210 4ibf4 X 7.0 18.5 48.8 0.8 lot of top edge missing
111910 4ibfl X 7.3 18.7 52.3 0.8 "
111910 4ibf2 X 6.4 13.5 39.0 0.6 big chips on top edge M
50 1BMA UDMA 5ME 101410 5IB1 2.4 7.5 23.8 1.1 undissolved nanogel visible, barely 21 mm long 76.0
undissolved nanogel visible 101410 5IB2 2.7 7.1 22.2 0.9 undissolved nanogel visible 70.1
25 aromatic 100110 25Ab4 2.4 21.8 79.1 4.0 1 wk postcure .14W 78.6
100110 25Ab5 2.5 19.6 72.2 3.3 1 wk postcure .14W 80.2
100110 25Ab6 3.0 8.1 30.2 1.1 I wk postcure .14W 83.8
102210 25AB1 2.6 15.5 54.3 2.2 4 days post curing, peel break 81.8
102210 25AB3 2.6 14.8 51.4 2.1 4 days post curing, peel break 79.7
102610 25AB3 3.0 7.3 23.0 0.8 78.2
102910 25ab4 2.5 19.4 70.1 3.3 peel break 80.5
25 aromatic 100110 25AbFl X 7.4 12.7 38.6 0.6 could not measure
70 Ba glass filler 100110 25AbF2 X 7.2 19.9 67.7 1.0 "
100110 25AbF3 X 13 13.7 45.5 0.7 barely 21 mm long "
100110 25AbF4 X 7.1 18.9 53.3 0.8 "
102910 25abfl X 7.4 29.9 88.6 1.4 "
102910 25abf2 X 7.5 28.2 92.7 1.4 short peel break "
40 aromatic, solvent 110410 4AB1S X 2.7 15.8 54.4 2.2 l week postcure, peel break
40 aromatic 110410 4AB1 2.8 7.9 24.5 0.9 81.4
110410 4AB2 2.4 9.0 27.6 1.2 80.2
110410 4AB3 2.5 6.5 20.7 0.8 79.6
110410 4AB4 2.6 11.9 38.7 1.7 80.3
110410 4AB5 2.9 8.4 27.2 0.9 78.3
111910 4abl 2.1 6.7 23.8 1.1 peel break off center 79.4
40 aromatic 111210 4abfl X 7.3 21.7 61.3 0.9 could not measure
70 Ba glass filler 111210 4abf2 X 7.0 19.0 61.6 1.0 H
111210 4abf3 X 6.7 20.1 59.2 0.9 "
111210 4abf4 X 7.7 18.4 58.8 0.3 "
111910 4abfl X 7.5 20.7 66.2 1.0 slight peel "
111910 4abf2 X 7.3 16.7 51.2 0.7 "
50 aromatic 101410 5IB4 2.3 7.6 24.6 1.0 undissolved nanogel visible 73.2
undissolved nanogel visible 101410 5AB1 2.7 6.3 18.5 0.8 undissolved nanogel visible 78.7


Table A.2. Fracture toughness test specimens with IBMA/UDMA and aromatic nanogels
Nanogel (wt%) Date mmddw specimen Filler Solvent E (GPa) Peak load (N) Stress at peak (MPa) Stran at peak (%> Note Degree of conversion ey Kk: (MN m *)
0 (control) 91010 1 0.6 14.3 6.4 1.1 pieces actually flying, bisgma never did 80.8 0.77
91010 2 0.8 16.4 7.3 1.0 76.9 0.86
91010 3 0.7 22.7 9.9 1.5 77.3 1.20
91010 4 0.6 24.6 10.5 1.8 messy IR 75.2 1.22
91010 5 0.5 20.9 8.9 1.0 75.7 1.05
91510 1 0.6 13.4 6.0 1.1 76.4 0.73
91510 2 0.7 17.0 7.4 1.0 77.3 0.85
91510 3 0.6 20.2 8.7 1.4 messy IR 77.1 1.02
91510 4 0.7 20.5 9.0 1.4 79.0 1.09
91510 5 0.6 17.5 7.6 1.3 78.5 0.85
91510 6 0.7 14.5 6.4 0.9 77.4 0.76
112310 ckl 0.6 17.3 8.2 1.4 broke fully 81.7 0.90
112310 ck2 0.6 18.2 8.1 1.3 broke fully 84.5 0.92
112310 ck3 0.7 13.6 6.4 1.0 broke fullv 93.5 0.74
0 (control) 91510 FI X 1.2 13.3 5.9 0.6 could not measure 0.72
70 Ba glass filler 91510 F2 X 1.8 18.7 7.8 0.5 " 1.16
91510 F3 X 1.4 14.6 6.4 0.5 0.76
91510 F4 X 1.7 16.9 7.5 0.4 " 0.89
91510 F5 X 1.5 17.7 7.8 0.5 " 0.95
91510 F6 X 0.8 2.6 1.1 0.0 didn't break fully " 0.13
91510 F6 X 1.1 17.8 7.6 0.7 redo " 0.89
112310 ckfl X 1.9 17.7 8.2 0.4 " 0.95
112310 ckf2 X 1.5 19.7 9.1 0.6 didn't stay together " 1.04
112310 ckf3 X 1.4 18.9 8.6 0.5 " 1.00
25 IBMA/UDMA 80/20 5ME 100110 25Ikl 0.5 14.1 6.6 1.3 75.8 0.76
100110 25Ik2 0.6 19.9 9.1 1.4 76.1 1.04
100110 25Ik3 0.7 22.3 10.2 1.4 75.2 1.16
102210 25IK1 0.5 14.2 6.5 1.2 75.4 0.65
102210 25IK2 0.6 20.0 9.2 1.5 75.9 0.96
111910 25ikl 0.5 16.3 7.7 1.7 75.3 0.85
25 IBMA/UDMA 80/20 5ME 100110 25IkFl X 2.0 17.9 8.1 0.4 didn't break fully could not measure 0.95
70 Ba glass filler 100110 25IkF2 X 1.8 15.4 6.9 0.4 didn't break fully " 0.83
100110 25IkF3 X 1.7 14.5 6.5 0.4 didn't break fully " 0.80
112310 25ikfl X 1.9 17.4 7.8 0.4 " 0.92
112310 25ikf2 X 1.8 16.6 7.3 0.4 " 0.84
112310 25ikf3 X 1.9 16.6 7.4 0.4 M 0.84
40 IBMA/UDMA 80/20 5ME 102910 43d X 0.5 10.6 5.1 1.0 87.8 0.62
solvent loading 102910 43c2 X 0.5 5.4 2.4 0.5 didn't break fuSy 85.9 0.26
102910 43c3 X 0.5 17.7 8.2 1.5 84.2 0.92


Table A.2 (contd)
Nanogel (wt%) Date mmddw Specimen Filler Solvent E (GPa) Peak load (N) Stress at peak (MPa) Strain at peak (%) Note Degree of conversion (%> Ktc (MNm15)
40 IBMA/UDMA 80/20 5ME 110410 41K1 0.7 16.7 7.6 1.2 chunk missing next to precrack 88.4 0.99
110410 41K2 0.8 15.9 7.1 1.0 76.0 0.95
110410 41 K3 0.8 15.6 7.0 1.0 77.4 0.90
110410 4IK4 0.7 16.6 7.1 1.1 89.9 1.02
110410 4IK5 0.7 15.6 7.2 1.2 sliver missing over one support 77.9 0.96
111910 4ikl 0.7 8.1 3.7 0.5 75.4 0.43
40 IBMA/UDMA 80/20 5ME 111210 4ikf2 X 1.8 15.3 6.0 0.3 could not measure 0.70
70 Ba glass filler 111210 4flcf3 X 1.8 13.8 5.9 0.3 " 0.69
111210 40cf4 X 1.5 14.5 5.7 0.4 " 0.73
111910 4flcfl X 1.8 12.4 5.2 0.3 " 0.58
111910 4ikf2 X 1.8 13.4 5.5 0.3 didn't break trial 10, redo triat 11 " 0.63
111910 4ikf3 X 2.0 13.2 5.8 0.3 0.64
112310 4ikfl X 1.9 17.0 7.1 0.3 didn't stav together " 0.81
50 IBMA UDMA 5ME 101410 5IK1 0.7 13.5 6.0 1.0 comer bubble over one support 65.0 0.69
undissolved nanogel visible 101410 51K2 0.6 13.9 5.8 1.1 72.1 0.68
101410 51 K.3 0.8 15.5 6.8 0.9 76.8 0.77
25 aromatic 100110 25Akl 0.6 18.7 9.0 1.5 on 1001 lObending.mss 81.3 0.96
100110 25Ak2 0.8 19.4 8.9 1.1 82.3 1.01
100110 25Ak3 1.0 22.1 10.0 1.1 on 1001 {Okie.mss 81.2 1.15
102210 25AK1 0.8 21.0 9.5 1.2 4 days postcure 82.5 1.18
102210 25AK2 0.8 18.4 8.5 1.1 4 days postcure 82.5 1.09
102210 25AK3 0.6 16.3 7.3 1.3 4 days postcure 81.4 0.85
25 aromatic 100110 25AkFl X 1.5 13.9 6.1 0.3 didn't break fully could not measure 0.70
70 Ba glass filler 100110 25AkF2 X 2.3 17.3 7.6 0.3 didn't break fuDy " 0.85
102910 25akfl X 1.8 14.6 6.4 0.4 didn't break fuDy " 0.74
102910 25akf2 X 2.6 14.3 6.0 0.3 didn't break fuDy " 0.71
102910 25akf3 X 1.5 15.9 6.6 0.4 " 0.77
40 aromatic 110410 4AK1 0.8 17.3 7.7 1.2 79.2 1.04
110410 4AK2 0.8 16.2 7.2 0.9 surface bubbles over erne support 80.7 1.00
110410 4AK3 0.8 16.1 7.2 1.0 surface bubble on top comer over one support 81.2 0.94
110410 4AK4 0.7 16.2 7.4 1.1 79.1 1.00
111910 4akl 0.6 12.4 5.8 0.9 79.1 0.71
111910 4ak2 0.5 14.7 6.8 1.2 81.5 0.77
40 aromatic 111210 4akfl X 1.3 16.7 6.8 0.5 could not measure 0.82
70 Ba glass filler 111210 4akf2 X 1.7 16.4 6.7 0.4 " 0.82
111210 4akO X 1.5 16.6 6.2 0.4 0.71
111910 4akfl X 2.3 17.2 7.1 0.3 " 0.77
111910 4akf2 X 1.5 14.7 6.1 0.4 didn't break trial 6, redo trial 7 0.71
111910 4akf3 X 1.6 16.6 6.6 0.4 " 0.78
50 aromatic 101410 5AK1 0.9 14.3 6.2 0.7 didn't break fuDy 82.3 0.69
undissotved nanogel visible 101410 5AK2 0.7 16.4 7.1 1.0 bubble over one support 79.7 0.84
101410 5AK3 1.4 13.7 5.9 0.5 didn't break fuDv 72.4 0.64


Table A.3. Flexural test specimens with high and low Tg nanogels
Nanogel (wt%) Date mddyy Specimen Solvent E (GPa) Peak load (N) Stress at peak (MPa) Strain at peak (%) Note Degree of conversion (%)
10 high Tg (90C) 12011 HlObl X 1.8 19.6 70.5 8.6 plastic 78.3
12011 H10b2 X 1.6 18.2 62.9 6.2 77.7
12011 H10b3 X 1.6 17.9 65.0 7.3 75.6
50 high Tg (90C) 12011 H50b3 X 2.0 20.9 70.5 6.0 bubble right in the middle 69.8
12511 H50bl X 2.1 21.7 72.3 5.4 heated to 60*C for molds, bubbles after cure, 21 hr post cure 67.4
12511 H50b2 X 2.2 19.1 71.2 4.2 " 69.1
10 low Tg (-45C) 12011 LlObl 1.6 19.6 69.5 9.1 80.2
12011 L10b2 1.5 18.6 66.0 10.8 plastic 81.2
12011 L10b3 1.5 18.8 66.3 12.1 plastic 81.8
50 low Tg (-35C) 20111 L50b 1 x, gelled 0.4 5.2 18.7 14.2 stopped at 18% strain, plastic, didn't break fully 95.4
20111 L50b2 x, gelled 0.5 7.1 25.6 11.4 stopped at 28% strain, didn't break fully 91.1
20111 L50b3 x, gelled 0.6 7.6 21.2 10.7 broke 89.6
20111 L50b4 x, gelled 0.6 8.0 28.8 11.0 broke 92.2
50 low Tg (-45C) 20411 L50b 1 0.5 6.3 24.3 10.8 plastic & yielded 94.7
20411 L50b2 0.4 5.7 22.8 12.6 bubble, plastic & yielded 94.5
C\
to
Table A.4. Fracture toughness test specimens with high and low Tg nanogels
Nanogel (wt%) Date irmddyy Specimen Solvent E (GPa) Peak load (N) Stress at peak (MPa) Strain at peak (%) Note Degree ofconversion (%) Kic (MN m'3/2)
10 high Tg (90C) 12011 HlOkl x 0.5 24.3 10.9 2.2 800 1.31
12011 H10k2 x 0.5 21.5 9.2 1.8 72.7 1.06
12011 H10k3 X 0.5 18.5 8.2 1.7 76.3 0.96
50 high Tg (90C) 12011 H50k3 X 0.8 6.7 3.0 0.4 67.2 0.38
12511 H50kl X 0.5 7.2 3.4 0.6 heated to 60*C for molds, bubbles after cure 71.0 0.48
20111 H50kl X 0.8 5.9 2.8 0.3 chunk missing from comer, didn't use N2 on postcure IRscan 66.7 0.34
10 low Tg (-4?C) 12011 l.lOkl 0.4 14.1 6.2 1.5 85.3 0.70
12011 L10k2 0.5 18 1 8.1 1.6 86 1 0.96
12011 L10k3 0.5 13.8 6.1 1.2 didn't break fully 80 7 0.71
50 low Tg (-35O 20111 l.50kl X 0.1 8.5 4.0 3.3 bubbles, didn't break fully 92.8 0.48
20111 I50k2 X 0.1 7.3 3.3 33 bubbles, stress peaked and fell, didn't break fully 93 l 0.39
20111 L50k3 X 0.1 86 4.0 3.0 bubbles, didn't break fully 92.5 0.48
50 low Tg (-45C) 20411 L50kl 0.1 9 1 4.3 3.2 1 mm bubble in one comer, didn't break fully 93.4 0.49


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