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8/2/2019 Dewaele M. 2009 Dent. Mater. Influence of Curing Protocol on Selected Properties of Light Curing Polymers Degree
1/9
d e n t a l m a t e r i a l s 2 5 ( 2 0 0 9 ) 15761584
a v a i l a bl e a t w w w . s c i en c e d i r e c t . co m
j o u r n a l h o m e p a g e : w w w . i n t l . e l s e v i e r h e a l t h . c o m / j o u r n a l s / d e m a
Influence of curing protocol on selected properties of
light-curing polymers: Degree of conversion, volume
contraction, elastic modulus, and glass transition
temperature
Magali Dewaele a,b,, Erik Asmussen c, Anne Peutzfeldt c, E. Christian Munksgaard c,Ana R. Benetti d, Gauthier Finn a, Gatane Leloup e, Jacques Devaux a
a Laboratory of Chemistry and Physics of High Polymers, Universit catholique de Louvain, Louvain-la-Neuve, Belgiumb Department of Prosthodontics, School of Dentistry and Stomatology, Universit catholique de Louvain, Brussels, Belgiumc Department of Dental Materials, School of Dentistry, University of Copenhagen, Copenhagen, Denmarkd Department of Operative Dentistry, Endodontics and Dental Materials, Bauru School of Dentistry,
University of So Paulo, Bauru SP, Brazile Department of Operative Dentistry, School of Dentistry and Stomatology, Universit catholique de Louvain,
Brussels, Belgium
a r t i c l e i n f o
Article history:
Received 28 October 2008
Received in revised form
3 August 2009
Accepted 4 August 2009
Keywords:
Light-curing
Soft-start
Degree of conversion
Volume contraction
Elastic modulus
Glass transition temperature
Dental materials
Dental polymers
Resin composite
a b s t r a c t
Objectives. The purpose of this study was to investigate the effect of light-curing protocol on
degree of conversion (DC), volume contraction (C), elastic modulus (E), and glass transition
temperature (Tg) as measured on a model polymer. It was a further aim to correlate the
measured values with each other.
Methods. Different light-curing protocols were used in order to investigate the influence of
energy density (ED), power density (PD), and mode of cure on the properties. The modes
of cure were continuous, pulse-delay, and stepped irradiation. DC was measured by Raman
micro-spectroscopy. C was determined by pycnometry and a density column. E was mea-
sured by a dynamic mechanical analyzer (DMA), and Tg was measured by differential
scanning calorimetry (DSC). Data were submitted to two- and three-way ANOVA, and linear
regression analyses.
Results. ED, PD, and mode of cure influenced DC, C, E, and Tg of the polymer. A significant
positive correlation was found between ED and DC (r = 0.58), ED and E (r = 0.51), and ED and
Tg (r = 0.44). Taken together, ED and PD were significantlyrelated to DC and E. The regression
coefficient was positive for ED and negative for PD. Significant positive correlations were
detected between DC and C (r = 0.54),DC and E (r = 0.61),andDC and Tg (r = 0.53). Comparisons
between continuous andpulse-delay modes of cure showed significant influence of mode of
cure: pulse-delay curing resulted in decreased DC, decreased C, and decreased Tg. Influence
of mode of cure, when comparingcontinuous andstep modes of cure, wasmore ambiguous.
Corresponding author at: Cliniques universitaires St-Luc, Ecole de mdecine dentaire et de stomatologie, avenue Hippocrate 10/bte5732,1200 Bruxelles, Belgium. Fax: +32 2 764 90 62.
E-mail address: Magali.Dewaele@uclouvain.be(M. Dewaele).0109-5641/$ see front matter 2009 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
doi:10.1016/j.dental.2009.08.001
mailto:Magali.Dewaele@uclouvain.behttp://dx.doi.org/10.1016/j.dental.2009.08.001http://dx.doi.org/10.1016/j.dental.2009.08.001mailto:Magali.Dewaele@uclouvain.be8/2/2019 Dewaele M. 2009 Dent. Mater. Influence of Curing Protocol on Selected Properties of Light Curing Polymers Degree
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d e nt a l m a t e r i a l s 2 5 ( 2 0 0 9 ) 15761584 1577
A complex relationship exists between curing protocol, microstructure of the resin and the
investigated properties. The overall performance of a composite is thus indirectly affected
by the curing protocol adopted, and the desired reduction ofC may be in fact a consequence
of the decrease in DC.
2009 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
1. Introduction
It is widely admitted that the influence of photopolymeriza-
tion on the final properties of resin composites is of major
importance. Literature [15] has shown that the properties
of a light-curing resin composite are mainly influenced by
the amount of energy delivered during irradiation. The total
amount of energy per unit area, the so-called energy density
(ED) is the product of the power per unit area (power density:
PD) by the duration of irradiation (curing time). For a given ED,
different combinations of PD and curing time may be used to
cure the composite materials.
Contradictory data have been published about the effectof variation in the combination of PD and exposure duration.
Contrary to the total energy concept (referring to the reci-
procity between power density and exposure duration), recent
studies have demonstrated that for each ED, the combination
of PD and exposure duration had a significant influence on
degree of conversion,on extentof crosslinking andon physical
properties [3].
The light from dental photocuring devices can be deliv-
ered in different modes: continuous, pulse-delay or stepped
irradiation. The continuous mode delivers the same PD
uninterruptedly throughout the entire exposure period. The
pulse-delay mode initiates cure by a short flash of light fol-
lowed by a delay of one or more minutes before the finalpolymerization is performed. In the step-cure mode, a low PD
is used during thefirst part of the polymerization period and a
higherPD is used towards theend of theirradiation. Thepulse-
delay and the stepped mode of cure are so-called soft-start
modes of cure.
The soft-start modes of cure were introduced with the
purpose of slowing down the polymerization reaction, which
is inevitably accompanied by contraction of the material [6].
Soft-start curing modes result in significantly reduced gap
formation in cavity margins because of contraction stress
relief by flow [714]. However, few studies have evaluated the
effect of curing mode on the volume contraction itself. Some
authors found soft-start curingto combine increased marginalintegrity with identical [15] or superior physical properties
when compared to continuous curing [11]. Adversely, soft-
start techniques have resulted in either decreased degree of
conversion (DC) as compared to the one of the continuous
mode, or similar DC but at the same time in polymers of lower
crosslink density, and thus of decreased mechanical proper-
ties [1619].
As hypothesized in a previous study [17], it is conceivable,
that the different light-curing modes will lead to polymers of
different network structure, even though the DC is the same.
The DC does not give a complete characterization of polymer
structures because polymers of similar DC may have different
extent of crosslinking [1618]. This may occur as a result of
the soft-start modes of cure (pulse or step-cure mode) prob-ably due to the formation of relatively fewer growth centers,
which may result in a more linear polymer, with decreased
crosslinking. The increased concentration of crosslinks has
been associated with increased physical properties and sta-
bility of polymers [16,20].
It has been shown [20,21] that the extent of crosslinking
of a polymer may be assessed by measurements of the glass
transition temperature (Tg). Tg is an important parameter for
polymer characterization as it marks a region of dramatic
changes in the physical properties of the polymer. The Tg-
value represents the temperature region at which the polymer
is transformed from a glassy material into a rubberlike one.
Crosslinking reduces molecular mobility and thus gives riseto increased apparent Tg [22]. The curing procedure may influ-
ence the regularity of the network and the crosslink density,
and this may be reflected in the Tg.
To improve the overall performance of light-cured dental
polymers, a detailed understanding of the effects of mode of
irradiation on properties and how these properties are related
to each other is necessary. The present study represents an
effort in this direction. It was the aim of the study to deter-
mine, on the same experimental dental polymer, degree of
conversion, volume contraction, elastic modulus, and glass
transition temperature in relation with the curing protocol.
The workinghypothesis was that the total energy density and,
for each level of ED, that the power density and the modeof cure, have an effect on the selected properties. A further
aim was to attempt to correlate the data obtained for these
properties with each other. In a further study [23] the data are
correlated with softening and elution of monomers in ethanol
as measured on identical polymers subjected to the same cur-
ing protocols.
2. Materials and methods
The experimental resin used in this study was com-
posed of Bis-GMA (bisphenol-A-glycidyl dimethacrylate,
Heraeus Kulzer, Germany) and TEGDMA (95% triethyleneg-lycol dimethacrylate, Aldrich, Belgium) at a molar ratio of
1:1, along with 0.5wt% CQ (97% camphorquinone, Aldrich) as
visible light initiator and 0.5 wt% DABE (97% N,N-dimethyl-p-
aminobenzoic acid ethylester, Aldrich) as co-initiator.
Cylindrical specimens were fabricated in a brass mold.
The specimens were covered on both sides with a transpar-
ent film (Mylar) and irradiated from one side only. Specimens
used for degree of conversion and contraction measurements
(height: 2 mm, diameter: 5 mm) and specimens used for elas-
tic modulus and glass transition temperature measurements
(height: 1 mm, diameter: 3 mm) were of different size as
dictated by the dimension requirement of the measuring
device.
http://dx.doi.org/10.1016/j.dental.2009.08.001http://dx.doi.org/10.1016/j.dental.2009.08.001http://dx.doi.org/10.1016/j.dental.2009.08.001http://dx.doi.org/10.1016/j.dental.2009.08.001http://dx.doi.org/10.1016/j.dental.2009.08.001http://dx.doi.org/10.1016/j.dental.2009.08.001http://dx.doi.org/10.1016/j.dental.2009.08.001http://dx.doi.org/10.1016/j.dental.2009.08.001http://dx.doi.org/10.1016/j.dental.2009.08.001http://dx.doi.org/10.1016/j.dental.2009.08.001http://dx.doi.org/10.1016/j.dental.2009.08.001http://dx.doi.org/10.1016/j.dental.2009.08.001http://dx.doi.org/10.1016/j.dental.2009.08.0018/2/2019 Dewaele M. 2009 Dent. Mater. Influence of Curing Protocol on Selected Properties of Light Curing Polymers Degree
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Table 1 Investigated curing protocols. Degree of conversion (DC), volume contraction (C), elastic modulus (E), and glass transition t
Irradiation (1st step)time @ power density
Delay (min) Irradiation (2nd step)time @ power density
Energy density(mJ/cm2)
DC (%) C (%)
Continuous
A 40 s @ 150 mW/cm2 6000 69 (2.9) 7.52 (0.04)
B 20 s @ 300 mW/cm2 6000 68 (2.6) 7.49 (0.02)
C 10 s @ 600 mW/cm2 6000 65 (0.5) 7.54 (0.15)
D 80 s @ 150 mW/cm2 12000 71 (2.0) 7.63 (0.15)
E 40 s @ 300 mW/cm2 12000 72 (3.3) 7.62 (0.15)
F 20 s @ 600 mW/cm2 12000 69 (7.4) 7.69 (0.06)
Pulse-delay
G 1 s @ 300 mW/cm2 1 19 s @ 300 mW/cm2 6000 62 (1.5) 7.35 (0.28)
H 1 s @ 300 mW/cm2 2 19 s @ 300 mW/cm2 6000 63 (0.4) 7.26 (0.27)
I 1 s @ 300 mW/cm2 3 19 s @ 300 mW/cm2 6000 61 (1.3) 7.30 (0.06)
J 1 s @ 300 mW/cm2
1 39 s @ 300 mW/cm2
12000 67 (1.8) 7.35 (0.24)K 1 s @ 300 mW/cm2 2 39 s @ 300 mW/cm2 12000 64 (3.2) 7.31 (0.34)
L 1 s @ 300 mW/cm2 3 39 s @ 300 mW/cm2 12000 66 (1.8) 7.47 (0.31)
M 1 s @ 600 mW/cm2 1 9 s @ 600 mW/cm2 6000 60 (4.0) 7.40 (0.17)
N 1 s @ 600 mW/cm2 2 9 s @ 600 mW/cm2 6000 61 (3.9) 7.29 (0.20)
O 1 s @ 600 mW/cm2 3 9 s @ 600 mW/cm2 6000 61 (1.1) 7.57 (0.08)
P 1 s @ 600 mW/cm2 1 19 s @ 600 mW/cm2 12000 65 (2.5) 7.43 (0.06)
Q 1 s @ 600 mW/cm2 2 19 s @ 600 mW/cm2 12000 65 (0.8) 7.42 (0.08)
R 1 s @ 600 mW/cm2 3 19 s @ 600 mW/cm2 12000 65 (4.2) 7.58 (0.09)
Step
S 10 s @ 300 mW/cm2 5 s @ 600mW/cm2 6000 61 (3.3) 7.54 (0.06)
T 10 s @ 150 mW/cm2 15s @ 300mW/cm2 6000 64 (0.7) 7.61 (0.02)
U 10 s @ 150 mW/cm2 7.5s @ 600 mW/cm2 6000 69 (5.0) 7.49 (0.11)
V 10 s @ 300 mW/cm2 15s @ 600mW/cm2 12000 71 (2.6) 7.63 (0.02)
X 10 s @ 150 mW/cm2 35s @ 300mW/cm2 12000 72 (1.6) 7.46 (0.26)
Y 10 s @ 150 mW/cm2 17.5s @ 600mW/cm2 12000 69 (2.7) 7.73 (0.03)
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The light-curing protocols shown in Table 1 were followed
in order to investigate the influence of the energy density, the
power density, and the mode of cure on the selected proper-
ties. For the pulse-delay groups, different delay periods were
used. Additionally, a short initial period of light exposure (1 s)
was adopted in the first cycle of light-activation because this
pulse-delay protocol demonstrated previously to reduce gaps
in the margins of restorations [14]. For the step-cured groups,the use of higher or lower power densities in the beginning
or in the end of the irradiation was also examined. The dif-
ferent power densities were obtained by changing the voltage
delivered to the quartztungstenhalogen curing unit (Optilux
501, Kerr Corporation, Danbury, CT, USA), by means of a vari-
able external resistor connected in series (Autralec Brussels,
Belgium). The desired power densities were monitored with
a dental curing radiometer (Optilux model 100, Demetron
Research Corporation/Kerr, Danbury, CT, USA).
2.1. Degree of conversion (DC)
TheDC was measured bymeansof Raman micro-spectroscopy
(Labram, Dilor Horiba-Jobin-Yvon, Lille, France) as previously
described [6]. In brief, this method allows the evaluation of the
DC by comparing the vibration bands of the residual unpoly-
merized methacrylate C C stretching mode at 1640 cm1 to
the aromatic C C stretching mode at 1610 cm1 used as inter-
nal standard. The unpolymerized resin was used as reference
(DC=0).
Three specimens were fabricated for each experimental
condition and were stored dry at ambient temperature for at
least 7 days. A notch was made to identify the side irradiated
by the light. For each specimen, the irradiated side was facing
the microscope objective. The degree of conversion was deter-
mined at three locations and the mean value calculated for
each of the three specimens. From these three mean values a
new mean value was calculated.
2.2. Volume contraction (C)
The same three samples of cured resin used to determine
the DC were used to measure the volume contraction. The
contraction (V/Vunpolymerized) was obtained by comparing
unpolymerized (dunpolymerized) to polymerized (dpolymerized)
specific masses and expressed as volume percent according
to the formula [6]:
V
Vunpolymerized(%) = 100
dpolymerized dunpolymerized
dpolymerized
The specific mass of uncured resin (liquid) was measured
by pycnometry. The specific mass of the cured resin (solid)
was determined by a density column (Daventest Instruments,
Fareham, UK). In the density column technique, samples were
immersed in a liquid having a densitygradient. The liquid was
prepared by mixing potassium bromide and water to reach
an appropriate density range (between 1.00 and 1.41), which
depends on the anticipated spread of densities in the samples
to be measured. The height at which samples stabilized was
observed and compared with calibrated marker floats [6].
2.3. Elastic modulus (E)
Elastic modulus of the dental resins may be measured in ten-
sion, in flexion, or in shear. In this work, due to the very low
dimensions of the samples, shear modulus was measured by
use of a dynamic mechanical analyzer (DMA/SDTA861e, Met-
tler Toledo, Greifensee, Switzerland). Tensile and shear elastic
modulus may be designated E and G, respectively. E and G arelinked by the Poisson coefficient according to E =2(1+)G.
For most isotropic materials, is between 0.2 and 0.5. For such
materials, E is equal to 2.4 to 3 times G. In the present study,
the measured values ofG were multiplied by 3 to give E.
Six specimens were fabricated for each experimental con-
dition and were stored dry at ambient temperature for at least
7 days before testing. Two identical specimens were placed
symmetrically between shear clamps. Samples were analyzed
in shear mode isothermally at 25 C with a preloading force of
6 N, a displacement amplitude limit of 0.5m, and a frequency
of 1Hz.
2.4. Glass transition temperature (Tg)
The glass transition temperature (Tg) was measured by Differ-
ential Scanning Calorimetry (DSC822e/HSS7, Mettler Toledo,
Greifensee, Switzerland). Four of the six samples of cured
resin used to determine the modulus were used to measure
the glass transition temperature. Samples were analyzed at a
heating rate of 10 C/min from 20 C to 70 C under nitrogen.
The Tg-value represents the temperature region at which
(the amorphous phase of) a polymer is transformed from a
brittle, glassy material into a tough rubberlike one. This effect
is accompanied by a step-wise increase of the DSC heat
flow/temperature or specific heat/temperature curve. Thus, Tgwas determined from an endothermic parallel transition of
the baseline [22].
An exothermic reaction occurred within the Tg region and
superimposes on the heat flow shift. This exothermic reac-
tion is due to post-polymerization occurring due to increased
chain-segment mobility in the polymer at temperaturesabove
Tg. The temperature at the intersection of the extrapolated
heat flow curve, at the low temperature end, and the tangent
of the ascending curve, at the inflection point, was taken as
the Tg (Fig. 1).
2.5. Statistical methods
Bartletts testwas used to check the homogeneity of variances.
The data were submitted to analysis of variance (ANOVA), to
Newman Keuls multiple comparison test and to linear regres-
sion analysis. The level of significance was defined as = 0.05.
3. Results
The degree of conversion (DC), the volume contraction (C), the
elastic modulus (E) and the glass transition temperature (Tg)
for each of the 24 irradiation conditions are shown in Table 1.
For an energy density (ED) of 6000mJ/cm2, one-way ANOVA
showed statistically significant differences between the 12
mean values of DC, E and Tg, respectively. However, when
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Fig. 1 Heat flow shift of the experimental dental polymer.
Endothermic parallel transition of the baseline and glass
transition temperature. Superimposed exothermic reaction.
ED was 12000mJ/cm2, a significant difference was detected
only for DC. Regarding the volume contraction, no statisti-
cally significant differences were found with one-way ANOVA
between the 12 mean values neither at ED = 6000mJ/cm2 nor
at ED=12000mJ/cm2.
Linear two- and three-dimensional regression analyses
were used to search for significant relationships between
energy density, power density (PD), DC, C, E and Tg. Two-
dimensional analysis showed significant although weak
relationships between DC and C (r = 0.54), DC and E (r = 0.61),
and DC and Tg (r = 0.53), between C and E (r = 0.50), and C and
Tg (r = 0.34). Furthermore, significant relationships were found
between ED and DC (r = 0.58), ED and E (r = 0.51), and ED and Tg(r = 0.44). Three-dimensional regression analysis showed a sig-
nificant positive correlation between ED and DC and between
ED and E and a concomitant significant negative correlation
between PD and DC and between PD and E.
Three-way ANOVA and Newman Keuls multiple compar-
ison test were used to analyze the influence of ED, PD, and
mode of cure on DC, C, E and Tg.
3.1. Degree of conversion (DC)
Comparisons of the continuous and the pulse-delay mode of
cure showed significant influence of ED (p 0.05).
Comparisons of the continuous and the stepped mode
of cure showed an increased DC with an increasing ED,
although not significant when step 1 = 150mW/cm2 and step
2=600mW/cm2.
3.2. Volume contraction (C)
Comparisons of the continuous and the pulse-delay mode of
cure showed significant influence of mode of cure (p < 0.05).
The pulse-delay mode resulted in decreased C. The length of
the delay was not significant (p >0.05).
Comparisons of the continuous and the stepped mode
of cure showed a significant increase in C with an increas-
ing ED (p < 0.05), except when step 1 = 150mW/cm2 and step
2=300mW/cm2.
3.3. Elastic modulus (E)
Comparisons of the continuous and the pulse-delay mode ofcure showedsignificant influence of ED (p < 0.05). Increased ED
resulted in increased E.
Comparisons of the continuous and the stepped mode of
cure showed significant influence of ED (p 0.05).
4. Discussion
In the present study, resin composites were polymerized
according to three different curing modes: continuous, pulse-delay, and stepped. These modes included variations in ED,
andpower density(PD),and forthe pulse-delaymode different
lengths of delay.
The foregoing has shown that ED, PD, and mode of cure
influenced the investigated properties: degree of conversion
(DC), volume contraction (C), elastic modulus (E) and glass
transition temperature (Tg). Thus, the working hypothesis was
validated.
The results confirm that the properties of a light-curing
polymer-based material are mainly influenced by the amount
of energy delivered to the material during irradiation. The
higher ED applied to the material, the higher were DC, E and
Tg. This is in agreement with the results of Halvorson et al.[2] and Peutzfeldt and Asmussen [3] who showed that DC
and E increased with increasing ED. The increase in E and
Tg with energy density is caused by the increase in DC and
the significant relationship between DC and E, and between
DC and Tg. However, in this study, the correlation between
ED and C was found to be not significant, although DC and C
were significantly correlated. This point does not agree with
other studies [4,5] and may be due to a type II statistical
error.
The measured values of DC, E and Tg were statistically dif-
ferent at 6000mJ/cm2 although only the values of DC were
statistically different at 12000 mJ/cm2. A tentative explana-
tion could be that DC, E and Tg increase with increasing ED
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but tend towards a maximum. E and Tg may have a tendency
to level off earlier than DC. At 6000 mJ/cm2, the material has
not reached its maximum properties for DC, E and Tg, and
the polymerization rate has an influence on these proper-
ties. At 12000mJ/cm2, E and Tg have reached their maximum,
and the polymerization rate has less influence on these prop-
erties. As mentioned in the introduction, soft-start favors
reduced gap formation [714]. However, the influence of cur-ing modes on the mechanical and physical properties remains
conflicting. For a given monomer composition, the mechani-
cal and physical properties of the resulting polymer are to a
large extent governed by the twointerrelated but independent
fundamental properties of network structures: degree of con-
version and degree of crosslinking. In this study, in order to
understand how the curing modes influence the properties,
the underlying kinetics and mechanisms of the reaction and
the structural characteristics must be understood. Therefore,
this paragraph discusses the most relevant aspects related
to the polymerization reaction and speculates the possible
outcomes for the different light-curing protocols investigated.
Owing to the high similarity of the samples (nature, volume,shape) it is first assumed that the T reached by all samples
at the end of polymerization (vitrification T) does not differ
significantly from one sample to another. The effect of heat-
ing by the light source is thus never taken into account in
this work.
The characterization of the crosslinked network structure
is extremely difficult due to the structural heterogeneity that
develops in the polymer during polymerization.
A first explanation for the heterogeneity of the network
may be related to the rate of photopolymerization. The rate
of polymerization is the speed at which DC is growing with
time. In classical free radical processes (assuming conditions
of radical stationarity), the polymerization rate scales as thesquare root of the rate of initiation. At low PD, a quasi-linear
relationship can be assumed between PD and the rate of ini-
tiation. Thus, in classical photopolymerization, at low PD, the
polymerization rate scales as the square root of PD. How-
ever, conditions of radical stationarity are almost never met in
the present bulk photopolymerization, hence the square-root
law is not valid anymore. Moreover, at higher PD a prema-
ture termination of radicals can occur if radicals just created
react with each other, without initiating polymerization. This
leads to a net loss of radicals. Kinetically, it can be demon-
strated that the rate of this premature termination reaction
depends on the square of the concentration of radicals. At
higher PD, the relationship between PD and the rate of ini-tiation becomes thus less than linear. Thus, at high PD, the
relationship between PD and rate of polymerization becomes
highly nonlinear, with a power law smaller than the square-
root law. In simpler words, at high PD, for a given time of
irradiation, increasing PD will lead to a much lower increase
of DC.
The case of dimethacrylate monomers with two equiva-
lent functional (vinyl) groups is still more complicated as the
functional group reactivity apparently changes with conver-
sion in the system. This is also linked to the microstructureof the network with a large amount of pendant functional
groups.
To understand the polymer network, reference is made
to the definitions of cyclic structures proposed by Dotson
et al. [24]. When a radical involving a pendant double bond
on a polymer chain propagates (i.e. a radical formed at a
double bond from a unit having one double bond already
reacted), a primary cycle, secondary cycle or crosslink, can
be formed. When the radical reacts with a pendant dou-
ble bond on its own chain, due to the vicinity in the close
neighborhood, a primary cycle results. Primary cycles do not
contribute to the network but trap a few pendant vinyl
groups leading to microgel zones. If the radical reacts witha pendant double bond on a different chain but with which
it is already (cross)linked, a secondary cycle forms. Finally, a
crosslink forms when the radical reacts with a pendant dou-
ble bond on a different chain [21]. Secondary cycles and
crosslinks appear consecutively and are rather undistin-
guishable from each other. Only the crosslinks connecting
different chains leading to network formation will enhance
the mechanical properties of the material. However, primary
and secondary cycles, as well as crosslinks, contribute to the
degree of conversion. As only crosslinks influence Tg, this
explains that polymers with the same DC can exhibit different
Tgs.
The apparent unequal reactivity of the functional groupsand the ensuing competition between primary cyclization on
the one hand, and secondary cyclization and crosslinking on
the other hand, are a second explanation forstructural hetero-
geneity of the network. Indeed, in case of divinyl monomers,
with two equivalent functional (vinyl) groups, the functional
group reactivity apparently changes with conversion in the
system. At low conversions, the few pendant functional
groups neighboring an active radical chain end exhibit an
apparent enhancement of their reaction rate, due to their
spatial proximity with the propagating radicals, leadingto pri-
mary cycles (Fig. 2).
Therefore, initially, primary cyclization dominates
crosslinking and secondary cyclization. This behavioraccounts for the formation of a first type of microgel regions
and a first kind of heterogeneity of the network.
Fig. 2 Schematic picture showing how primary cycles are formed and how they trap vinyl groups in microgel zones (in
grey).
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will give rise to relatively few growth centers, and a relatively
linear polymer is formed.
Comparisons of the continuous and the pulse-delay mode
of cure did not show a significant influence on E, despite
a decreased DC with the pulse-delay mode and a positive
correlation between DC and E. Actually, from Table 1, the
pulse-delay mode of cure shows a decreased E compared to
the continuous mode when ED = 6000mJ/cm2. No differencesare noticeable at ED=12000mJ/cm2 because, as previously
explained, E has the tendency to level off earlier than DC and
has already reached its maximum.
In this study, it was assumed that Tg was a good parameter
to determine the crosslink density. The pulse-delay technique
led to polymers of decreased Tg. The decreased Tg may be
interpretedas the manifestationof a polymer structure having
fewer crosslinks. In the pulse-delay curing mode, as already
explained, after the short flash of initial light, only few growth
centers remain active during the delay. Consequently, the
propagation of polymerization will predominantly add one
molecule of monomer after the other to the growing polymer
chain. Also, the concentration of pendant monomers accessi-ble to reaction [MP] decreases. The result is that a relatively
linear polymer is formed before the final cure is carried out.
The final irradiation will give rise to a multitude of growth
centers. Many growth centers will increase the tendency to
form a branched polymer. At the end, the pulse-delay mode
results in more linear chains, with proportionally decreased
crosslinking. The finding that the polymers that were pre-
cured with a pulse at 600 mW/cm2 exhibited a lower Tg than
the corresponding polymers precured at 300 mW/cm2 is prob-
ably linked to the propagation of the polymerization. At a
pulse PD of 300mW/cm2 relatively few growth centers are
formed, with the result that a larger part of the polymeriza-
tion occurs at the final cure. At a pulse PD of 600 mW/cm2,many more growth centers are formed, with the result that
a larger part of the polymer structure will be linear before
final polymerization. Indeed, the pulse of light initiates a con-
centration of radicals [R] increasing very fast, then decaying
(exponentially) during the delay. On the average, during the
[pulse+ delay] duration, a low [R] is active during the period
of time that depends on the PD duringthe pulse.These results
are in accordance with the ones of Asmussen and Peutzfeldt
[17,18]. In their studies, the pulse-delay curing resulted in
less crosslinked polymers compared to continuously cured
polymers. The extent of crosslinking was estimated by the
Wallace hardness after storage in ethanol. The softening
effect also increased with the power density of the initialexposure.
To conclude, the pulse-delay mode of cure has earlier been
found to result in reduced gap formation [9,14]. Concerning
the structure of the polymers, the pulse-delay mode resulted
in decreased degree of conversion and in decreased degree
of crosslinking, since it was assumed that Tg was a good
parameter of the crosslink density. A significantly lower DC
and crosslink density may have possible consequences for the
stability of the polymer (for example elution and softening
by food components). To confirm the results obtained in this
study, determination of crosslink density by measurements of
softening after storage in ethanol and elution were addressed
in a further study [23].
Comparisons of the continuous and the stepped mode of
cure are more ambiguous. A possible explanation is that, in
the step-cure mode, PD of the first step will have effects on
degree of conversion and on density of crosslinking in the
opposite direction. A high PD will produce higher concentra-
tions of radicals that will rapidly react with each other, thus
yielding premature termination and resulting in lower DC. But,
high PD will give rise to a multitude of growth centers, increas-ing the tendency to form a branched polymer (higher crosslink
density). On theother side, a lower PD in thefirst step will pro-
portionally yield a higher efficiency of the initiated radicals.
This results in a delayed termination of polymerization and in
higher DC. But lower PD will give rise to relatively few growth
centers, and a relatively linear polymer is formed. This expla-
nation is in accordance with other studies [11,17,18], which
found that, depending of the PD and the duration of the first
step, step-cured composites resulted in either lower, or higher
crosslinking or modulus of elasticity. Thus, the difficulty lies
in determining the optimal duration and PD of the two steps.
In conclusion, a step-cure mode has also been found to result
in reduced contraction gaps [13] although not significant in allstudies [26]. Further investigations are necessary to find the
optimal power density and duration of the steps to allow, on
the one hand, enough flow to increase marginal integrity, and,
on the other hand, to activate the optimal amount of radicals
to form a highly crosslinked polymer, with a high degree of
conversion, and consequently to obtain similar or even better
physical properties. It must be remembered, though, that the
optimal power density actually depends on the concentration
and also on the specific chemistry of the initiator system [11].
Acknowledgements
This study was supported by a FIRST-Europe research grant
of the Walloon Region (Belgium) and by the European social
funds. The authors also gratefully acknowledge Thrse Glo-
rieux, Pascal Van Velthem and Jean-Jacques Biebuyck for
technical assistance.
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