Download Effect of chemical structure on degree of conversion in light

Biomaterials 23 (2002) 1819–1829
Effect of chemical structure on degree of conversion in light-cured
dimethacrylate-based dental resins
I. Sideridou*, V. Tserki, G. Papanastasiou
Department of Chemistry, Aristotle University of Thessaloniki, GR-54006, Thessaloniki, Hellas, Greece
Received 16 March 2001; accepted 5 September 2001
Abstract
In this work the room-temperature photopolymerization of Bis-GMA, Bis-EMA, urethane dimethacrylate (UDMA) and
triethylene glycol dimethacrylate (TEGDMA) induced by camphoroquinone/N;N-dimethylaminoethyl methacrylate, as photoinitiator system, was followed by FT-IR. The results obtained were then fitted by a non-linear least square method to a rational
function, which permitted the accurate calculation of the limiting degree of conversion. The latter was found to increase in the order
Bis-GMAoBis-EMAoUDMAoTEGDMA. This trend is discussed in connection with the chemical structure of dimethacrylates.
The photopolymerization of mixtures of Bis-GMA/TEGDMA, Bis-GMA/UDMA and Bis-GMA/Bis-EMA showed a good linear
relationship of degree of conversion with the mole fraction of Bis-GMA and in the case of the first pair also with the Tg of the initial
monomer mixture. r 2002 Elsevier Science Ltd. All rights reserved.
Keywords: Dental materials; Composites; Copolymers; Dimethacrylates; Degree of conversion; Glass transition temperature; Bis-GMA; Bis-EMA;
UDMA; TEGDMA
1. Introduction
The polymerization of dimethacrylates produces
densely crosslinked networks, resins, which find wide
applications in dentistry such as dental composites, pit
and fissure sealants, dentine bonding agents and cements
[1–3], dental adhesives [4] dentures and elastomeric
impression materials [5].
Dental composites are used for the restoration of
teeth and consist of two principal components, an
organic matrix and inorganic filler. The organic matrix
is formed by free radical polymerization of dimethacrylates, which are non-toxic and capable of rapid
polymerization in the presence of oxygen and water,
because the restorations are polymerized in situ in a
tooth cavity. This matrix when used unfilled for the
restoration of teeth shows a poor wear resistance. This
can be improved by the inclusion of particulate fillers,
which are harder than the polymeric matrix. An
ambitious goal would be to match the remarkable
properties of dental enamel, which contains more than
95 vol% of hydroxyapatite crystallites tightly packed
*Corresponding author. Tel.: +30-3199-7825; fax: +30-3199-7769.
E-mail address: [email protected] (I. Sideridou).
into an intricate microstructure. In comparison the
current composite restorative materials have crude
microstructure with no more than 65 vol% of inorganic
filler [6]. A wide range of fillers of varying shapes and
sizes, ranging from colloidal dimensions to tens of
microns is being used in varying combinations. Vinyl
silane coupling agents are also used to promote matrixfiller adhesion. The current composite materials have
good color and translucency, but much lower wear
resistance than the silver amalgams, which they are
designed to replace. The lifetime for anterior polymeric
restorative materials is about 8 years, but for posterior
materials is often not longer than 2–4 years. In
comparison with traditional dental amalgams, which
have the time of use of about 10–20 years, this seems to
be a very short period [7]. Despite this deficiency there
are diverse cogent reasons, such as esthetics and
avoidance of mercury pollution of the environment,
which spur on their further development.
The most common dimethacrylate monomer in
current commercial dental composites is the so-called
Bis-GMA (Scheme 1), which is the reaction product of
bisphenol A and glycidyl ester methacrylate (GMA).
Advantages of using Bis-GMA over the first used smallsized dental monomers, such as methyl methacrylate,
0142-9612/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved.
PII: S 0 1 4 2 - 9 6 1 2 ( 0 1 ) 0 0 3 0 8 - 8
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CH3
CH3
CH2 = C
C=CH2
C=O
CH3
OCH2 CHCH 2 O
C
OH
C=O
OCH2 CHCH2 O
OH
CH3
Bis-GMA
2,2-bis-[4-(2-hydroxy-3-methacryloyloxyprop-1-oxy)phenyl]propane]
or
Bisphenol A glycol dimethacrylate
CH3
CH3
C=CH2
CH2=C
C=O
C=O
OCH2CH2OCH2CH2OCH2CH2O
TEGDMA
Triethyleneglycol dimethacrylate
CH3
CH3
CH2=C
C=O
C=CH2
C=O
CH3 CH3
OCH2CH2OCNHCH2CHCH2CCH2CH2NHCOCH2CH2O
CH3
O
O
UDMA
1,6-bis-(methacryloyloxy-2-ethoxycarbonylamino)-2,4,4-trimethylhexane
or
Urethane dimethacrylate
CH3
CH3
C=CH2
C=O
CH2=C
C=O
CH3
O(CH2CH2O)n
C
(OCH2CH2) nO
CH3
Bis-EMA
Bisphenol A ethoxylated dimethacrylate
(4 ethylene oxide groups/bisphenol A group)
Scheme 1. Chemical structures of the dimethacrylate monomers used.
include less shrinkage, higher modulus and reduced
toxicity due to its lower volatility and diffusivity into
tissues. These desirable properties of Bis-GMA are
partially negated by a relatively high viscosity, which
does not permit the use of high amount of filler. The
increased filler content tends to improve mechanical
properties and to reduce curing shrinkage and the
thermal expansion coefficient. Since the viscosity of the
resultant past limits the amount of filler, which can be
incorporated, it is common practice to also use a less
viscous monomer as diluent comonomer, normally
triethylene glycol dimethacrylate (TEGDMA). However, TEGDMA has been shown to adversely affect the
properties of the matrix resin by increasing the water
sorption and curing shrinkage. Investigations are being
carried out in identifying new dimethacrylates, which
will have moderately low viscosities to eliminate or
minimize the use of the diluent monomer [8,9]. But the
only significant changes, which have found their way
into some commercial composites, were the alternative
employment of low viscosity structural analogous of
Bis-GMA [10] and the introduction of urethane
dimethacrylates [11].
The polymerization of dimethacrylates is chemical or
visible light-initiated with the latter being more preferable because of allowing a finer control of the entire
polymerization process. Thus initiation can be started
and stopped almost at will. The room-temperature
polymerization of dimethacrylates usually leads to
glassy resins in which only a part of the available
double bonds are reacted. Before the completion of
conversion the vitrification process decelerates the
reaction to a hardly perceptible rate. Only very flexible
monomers in which the reactive methacrylate groups are
relatively far apart can be completely reacted at ambient
temperature. The degree of conversion of resins is a
major factor influencing their bulk physical properties.
In general, the higher the conversion of double bonds,
the greater the mechanical strength. The unreacted
double bonds may either be present in free monomer or
as pendant groups on the network. The unreacted
monomer may leach from the polymerized material and
irritate the soft tissue. For example TEGDMA is
suspected to be propitious to bacterial growth around
the restoration [12]. Furthermore, monomer trapped in
the restoration may reduce the clinical serviceability of
composite through oxidation and hydrolytic degradation, which may be manifested in forms such as
discoloration of the fillings and accelerated wear [13].
The final degree of conversion of a resin depends on
the chemical structure of the dimethacrylate monomer
and the polymerization conditions i.e., atmosphere,
temperature, light intensity and photoinitiator concentration [14].
The aim of this work was to investigate the influence
of the chemical structure of dimethacrylates most
commonly used in the preparation of dental composites
on the degree of conversion. These are Bis-GMA,
TEGDMA, bisphenol A ethoxylated dimethacrylate
(Bis-EMA) and urethane dimethacrylate (UDMA)
(Scheme 1). The light-induced homopolymerization of
these monomers and copolymerization of two or more
monomers of varying compositions were carried out
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Table 1
Abbreviations and compositions of samples used in this study
Abbreviation
Composition (mol%) of sample
Bis-GMA
G
T
U
E
G–T (1)
G–T (2)
G–U (1)
G–U (2)
G–E (1)
G–E (2)
G–T–U–E (1)
G–T–U–E (2)
G–U–E
a
b
TEGDMA
UDMA
Bis-EMA
100
100
100
100
35.8a
56.6b
35.8
56.6
35.8
56.6
56.6
56.6
56.6
64.2a
43.4b
64.2
43.4
23.4
13.4
10
15
21.7
64.2
43.4
10
15
21.7
This molar composition corresponds to 50/50 (wt%).
Corresponds to 70/30 (wt%).
under exactly the same conditions in order to obtain
comparable results. A comparison of these results will
provide valuable information on the relationship between the chemical structure of dimethacrylates and
degree of conversion, which can help in the better
understanding of the behavior of composites contained
in these dimethacrylates and also in the development of
improved or new dental resins in a future work.
2. Experimental procedures
2.1. Materials
The dimethacrylates used were Bis-GMA (Polysciences Europe GmbH), Bis-EMA (Aldrich Chem.
Co.), UDMA (Ivoclar AG) and TEGDMA (Aldrich
Chem. Co.). They were used as received without further
purification. Nine mixtures of these monomers were
prepared, the composition of which is shown in Table 1.
In the commercial dental composites based on mixtures
of Bis-GMA and TEGDMA, they are used in a ratio
varied between 50 : 50 and 70 : 30 by weight, in order to
obtain viscosities of 1–2 Pa, suitable for the incorporation of the appropriate amounts of inorganic fillers [15].
These weight ratios correspond to molar ratios
35.8 : 64.2 and 56.6 : 43.4 and these molar ratios were
used in the preparation of all mixtures studied, in order
to keep the concentration of double bonds constant. To
make the samples light cured, 2 mol% of camphoroquinone (CQ) (Polysciences) used as photosensitizer, and
2 mol% of N,N-dimethylaminoethyl methacrylate
(DMAEMA) (Riedel-de Haen) used as reducing agent,
was added to each sample. This photoinitiator system is
the most common one used in the current photoactivated polymer-based dental materials. All samples
except TEGDMA were viscous liquid, so the CQ and
DMAEMA were first dissolved in dichloromethane,
then a certain amount of this solution was added to the
sample and the solvent was subsequently evaporated
under vacuum.
2.2. Degree of conversion
The degree of photopolymerization of a very thin film
formed from dimethacrylate monomer or a mixture of
monomers has been determined by using an FT-IR
spectrophotometer (Perkin–Elmer 1600). A small drop
of each sample contained in the photoinitiator system
was placed between two translucent polyethylene strips,
which were pressed between two NaCl crystals to
produce a very thin layer. Polyethylene film was used
mainly to avoid adhesion of the formed resin on NaCl
crystals and to also to prevent oxygen inhibition of
polymerization. The sample was irradiated for successive short periods of time, which gave cumulative
exposure times of 10, 20, 40, 60, 80, 120, 180 and
240 s. The FT-IR spectrum was recorded at zero time t0
and after each period of exposure to visible-light, over a
certain range of frequency for each monomer. This was
1660–1550 cm1 for Bis-GMA and Bis-EMA, 1860–
1600 cm1 for UDMA and 1800–1570 cm1 for TEGDMA. The spectra were recorded immediately after the
end of photoirradiation, in transmission with 16 scans at
a resolution of 1 cm1.
Photopolymerization was initiated with a XL 3000
dental photocuring source (3M, USA). This source
consisted of a 75 W tungsten halogen lamp, a series of
optical filters and lenses and a fused fiber optic light
guide with a 7 mm exit window. This unit emitted
radiation predominantly in the 420–500 nm range,
where also CQ absorbed (lmax ¼ 470 nm, e ¼ 3:8
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Table 2
Coefficients of the rational functions (Equation 1) used for the determination of the limiting degree of conversion of the neat dimethacrylates and
their mixtures
Sample
a1 1014
a2
a3
b1
b2
b3 102
Sy;x a
R2
Bis-GMA
TEGDMA
UDMA
Bis-EMA
G–T (1)
G–T (2)
G–U (1)
G–U (2)
G–E (1)
G–E (2)
G–T–U–E (1)
G–T–U–E (2)
G–U–E
9.51
5.19
1.29
51.1
16.9
12.3
37.5
36.4
10.3
2.74
12.6
2.85
14.6
22.67
0.4038
1.311
13.08
44.26
43.42
75.78
70.23
2.349
1.822
46.13
8.135
42.98
0.96027
0.77943
2.4564
2.5250
1.6691
1.5398
0.45165
0.93958
3.6127
1.6321
2.6871
4.4730
3.4067
2.10
3.48
0.259
19.0
3.60
2.47
8.28
8.01
2.65
0.671
2.58
0.592
3.17
0.9507
0.1458
0.1954
0.4636
0.9399
0.9852
1.566
1.613
0.7571
0.3892
1.421
0.8062
1.740
2.4633
1.0299
3.5318
4.7465
2.7397
2.8068
7.9046
1.8579
7.9542
3.8539
5.6415
9.9568
7.6584
0.2019
4.0581
0.1259
0.6985
0.3654
0.2611
0.2575
0.9306
0.2200
0.2322
0.1038
0.2894
0.1093
0.9999
0.9907
1.0000
0.9995
0.9999
0.9999
0.9999
0.9986
0.9999
0.9999
1.0000
0.9998
1.0000
a
b
b
Standard error of estimate.
Coefficient of determination.
104 cm2 mol1) [16]. The unit was used without the light
guide, at a distance of 6 mm from the sample. The curing
light and heat (infrared) intensity at the position of the
sample measured by a Hilux curing light meter, was 200
and 0 mW cm2 correspondingly.
The amount of double vinyl bonds remaining in the
sample exposed to irradiation is shown by the intensity
of the peak at 1637 cm1 referring to the C ¼ C
stretching of the vinyl group and 816 cm1 which refers
to the C ¼ C twisting. Both have been used in the study
of polymerization of acrylates and methacrylates [17].
We have chosen the 1637 cm1 absorption, because it is
stronger than the absorption at 816 cm1 and then will
provide less experimental deviation. The degree of
conversion was directly related to the decrease of
1637 cm1 absorption on the FT-IR spectra as follows:
Degree of conversion ¼
A0 At
100;
A0
where A0 is the absorption of the peak at 1637 cm1
when time is equal to zero and At is the absorption at
time t:
In the case of resins of Bis-GMA and Bis-EMA the
peak at 1608 cm1 assigned to aromatic C ¼ C bond was
used as an internal standard [18]. In the case of UDMA
and TEGDMA the sharp well-defined peak at
1720 cm1 assigned to the carbonyl stretching vibration
could not be used as an internal standard, because the
position and the intensity of this peak change during
polymerization [19]. In the uncured state the carbonyl
group is conjugated with the C ¼ C bond and on curing
this conjugation is lost. This results in a shift of the
carbonyl peak to a higher frequency in the cured state,
as the bond becomes stronger, owing to the fact that the
electrons are no longer delocalized. A significant loss of
intensity of this peak was also observed.
When mixtures of monomers were used, one of the
components was always the Bis-GMA, so the peak at
1608 cm1 of the aromatic C ¼ C bond was used as an
internal standard.
All experiments were carried out in triplicate and the
results were averaged. The mean values were then fitted
by a non-linear least square method to the following
rational function (i.e. the ratio of two polynomials):
Degree of conversion ¼
a1 þ a2 x þ a3 x2
:
b1 þ b2 x þ b3 x2
ð1Þ
The values of aI and bI ; are listed in Table 2 along with
the corresponding values of the standard error of
estimate (Sy;x ) and the coefficient of determination
(R2 ). The reported values of Sy;x and R2 show that
Eq. (1) closely represents the experimental data.
Eq. (1) then permitted the calculation with accuracy
of the limiting degree of conversion, from the ratio a3 =b3
for t-N:
2.3. Glass transition temperature (Tg )
The Tg values of monomers were determined by using
a differential scanning calorimeter (DSC-Pyris 1, Perkin–Elmer) at a scanning rate of 51C min1. The Tg
values were determined from the mid-point in the
thermogram, as measured from the extensions of the
pre- and post-transition baselines.
The Tg could not be readily discerned by DSC in the
case of cured dimethacrylates, due to the breadth of the
transition region [20]. So the Tg of polymer networks
was determined using a thermal mechanical analyzer
(TMS-2, Perkin–Elmer) at a heating rate of 51C min1
and a penetration probe loading with 150 g. Specimen
discs 5 mm in diameter and 1 mm in height were
fabricated in aluminum mold between two glass slides
covered with polyethylene film. They were irradiated for
200 s on each side with the XL 3000 dental photocuring
unit, without the light guide, at a distance B1.5 cm from
the sample, where the light intensity was 60 mW cm2.
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Table 3
Basic physical properties of dimethacrylate monomers studied
Monomer
MW
Conc. of double bonds (mol/kg)
Viscosity (Pa)
Refractive index (n20
D)
Bis-GMA
TEGDMA
UDMA
Bis-EMA
510.6
286.3
470.0
540
3.90
6.99
4.25
3.70
1200a,b
0.011b
23.1a/7.054c
0.9
1.5497
1.460
1.485d
1.5320
a
As cited in Ref. [21].
Cited in Ref. [15].
c
Cited in Ref. [22].
d
Cited in Ref. [9].
b
All specimens showed a very broad transition region
well illustrating the problem involving assignment Tg in
highly crosslinked polymers. The Tg values were
obtained by the use of the derivative curve. It is
noteworthy that generally the TMS Tg values agree
with those determined by DSC with the same heating
rate, because both are static methods (no movement of
the sample). This fact was also verified in this work with
the use of a poly(ethylene terephthalate) (PET) sample,
which showed by DSC a Tg ¼ 771C (H.R. 51C min1)
and by TMS with 150 g load penetration (H.R.
51C min1) a Tg ¼ 761C
3. Results and discussion
Table 3 shows some basic physical properties of the
four dimethacrylates used in this study. Bis-GMA and
Bis-EMA have about the same size in contrast to
UDMA and mainly of TEGDMA, which have smaller
size and therefore higher concentration of double bonds;
so the latter at equal degrees of conversion will exhibit
higher density of crosslinking and will form tighter
networks.
Bis-EMA is structurally analogous to Bis-GMA with
a stiff central phenyl ring core, without, however, the
two pendant hydroxyl groups, which are responsible for
the high water sorption [23] and mainly for the
extremely high viscosity of Bis-GMA due to the strong
hydrogen bonding [20,24,25]. UDMA shows a higher
viscosity than TEGDMA and Bis-EMA, due to the
hydrogen bonding between the aNHa and >C ¼ O
groups, which, however, is much lower than the
viscosity of Bis-GMA, because imino groups form
weaker hydrogen bonds compared to hydroxyl groups
[26]. The viscosity is a measure of the resistance of
molecules to flow and a high viscosity value is indicative
of the presence of intermolecular interactions. These
interactions can cause a decreased mobility of monomer
molecules during polymerization and also a decreased
flexibility of the corresponding polymeric network.
Table 3 also shows the refractive indices of dimethacrylates, because an important consideration in the
formulation of esthetic dental composites with high
conversions and depths of cure is how well the refractive
indices of polymer matrices match those of reinforcing
fillers. Typical radiopaque fillers, such as those containing Ba, Sr and Zr have refractive indices of about 1.55.
It is noteworthy that the polymers often have a different
refractive index, slightly higher than their monomer
precursors [26,27]. From this point of view Bis-GMA
and Bis-EMA seem to be more suitable for use in dental
composites.
The monomers and the mixtures of monomers
(Table 1) were activated for visible light photopolymerization by the addition of CQ 2 mol% (varies between
0.62–1.16 wt% depending on sample) and DMAEMA
also 2 mol% (varies between 0.58–1.10 wt%). In several
commercially available visible-light-cured dental
composites, Taira et al detected CQ and DMAEMA
over a large concentration range of CQ (0.17–1.03 wt%)
and DMAEMA (0.09–1.39 wt%) [28]. At CQ concentrations of above 2 mol%, the initiator efficiency was
found to be independent of the concentration of
DMAEMA [29].
Photopolymerization of samples was followed by FTIR spectroscopy. The degree of conversion observed at a
certain polymerization time for the neat monomers is
shown in Fig. 1. These data are the mean values of three
experiments for each polymerization time. The solid
lines resulted from the fitting of the experimental data
by a non-linear least square method to the rational
Function (1) reported in the experimental part. The
advantage resulting from the use of rational functions
rather than linear polynomials for fitting experimental
data was discussed in the literature [30,31]. In accordance with this discussion, it was found in this
investigation that Eq. (1) provided a better fit to the
experimental data than the conventional polynomials
with the same number of adjustable coefficients. Eq. (1)
permitted the accurate calculation of the limiting degree
of conversion of dimethacrylate monomers (Table 4).
This equation could also be used for the calculation of
degree of conversion at any time of photopolymerization and vice-versa, without the performance of polymerization.
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Table 4
Dependence of limiting degree of conversion on Tg for neat dimethacrylate monomers
Monomer
Limiting degree of conversion
Tg (1C) of monomer
Tg (1C) of polymer
Bis-GMA
TEGDMA
UDMA
Bis-EMA
39.0
75.7
69.6
52.2
7.7
83.4
35.3
46.1
67
65
68
68
80
70
Degree of conversion (%)
60
50
40
30
Bis.GMA
20
TEGDMA
UDMA
Bis.EMA
10
0
0
50
100
150
200
250
Polymerization time (sec)
Fig. 1. Degree of conversion of C ¼ C double bonds (%) as a function
of irradiation time, for the neat dimethacrylates. The solid lines are
calculated fits of the experimental data to the Eq. (1).
The curves in Fig. 1 show considerable differences,
despite the similar initiation rates of polymerization due
to the constant initiator concentration and light
intensity used in all polymerizations.
UDMA and Bis-GMA showed much higher initial
polymerization reactivity than TEGDMA and BisEMA. After 10 s of polymerization 49.5% and 22.9%
of the double bonds were reacted in the case of UDMA
and Bis-GMA and only 13.7% and 13.5% in TEGDMA
and Bis-EMA correspondingly. However later TEGDMA and Bis-EMA continued to polymerize with much
higher rate than UDMA and Bis-GMA and finally the
TEGDMA showed the highest limiting degree of
conversion (Table 4). This quite different behavior of
monomers in reactivity during the polymerization
process is more clearly seen in Fig. 2, where the rates
of polymerization as a function of time are presented.
The photopolymerization of UDMA and Bis-GMA
under the experimental conditions used starts with a
maximum rate which is rapidly decreased, while the
photopolymerization of TEGDMA and Bis-EMA,
shows a maximum rate 1.9 and 1.6 s1 after 9.6 and
8.4 s when 13.1% and 11% of the double bonds were
reacted correspondingly. The differences in the polymerization behavior of the four dimethacrylates studied
must be due to the different chemical structure of the
spacer group connecting the methacrylate groups
(Scheme 1). For a better understanding of this effect, it
is worth mentioning the main characteristics of kinetics
of the free-radical bulk polymerization of dimethacrylates.
This polymerization generally exhibits certain complex features such as autoacceleration, autodeceleration
and a maximum limiting conversion, which is significantly less than unity, in spite of the presence of
unreacted double bonds and trapped radicals. This
complex behavior is due to the fact that the mobility of
the reacting medium decreases as the polymerization
proceeds. At very low conversions, where the reacting
medium is in liquid state, propagation and termination
step of polymerization are chemically controlled and the
polymerization proceeds with a constant rate. However,
as an insoluble infinitely large network forms (i.e.,
gelation) the movements of the macroradicals are
restricted and termination step, which involves the
reaction of two macroradicals, becomes diffusionlimited. A decrease in the termination rate leads to a
corresponding increase in the polymerization rate which
is known as autoacceleration or gel effect. This effect is
more pronounced in the case of viscous monomers.
During the phase of gelation the reaction system
transforms from a liquid to a rubber and consists of
two species: the sol component consisting primarily of
residual monomer and the gel (insoluble but swellable in
good solvents fraction) consisting of branched and
mainly of crosslinked polymer chains. As the polymerization progresses, the system becomes even more
crosslinked and restricted, so the propagation step,
which involves the reaction of the smaller monomer
molecules with macroradicals, also becomes diffusionlimited. A balance between diffusion-controlled propagation (which decreases the rate of polymerization) and
diffusion-controlled termination (which increases the
I. Sideridou et al. / Biomaterials 23 (2002) 1819–1829
1825
system changes as a function of conversion
[32,33,37,38].
The Tg of the polymerizing system can be estimated
from the Tg ’s of its separate components using the
known Fox equation applied to plasticizers [20]:
3.0
Bis-GMA
2.5
1=TgðnetÞ ¼ wsol =TgðmonomerÞ þ wgel =TgðgelÞ ;
Polymerization rate R p (s-1 )
UDMA
2.0
vitrification
1.5
TEGDMA
1.0
Bis-EMA
gelation
0.5
0.0
0
10
20
30
40
50
60
Polymerization time (sec)
Fig. 2. Rate of polymerization Rp (expressed in s1) as a function of
time for the neat monomers.
rate of polymerization) results to a maximum in the rate
max
of polymerization (Rmax
is observed when the
p ). The Rp
glass transition temperature (Tg ) of the reacting system
becomes coincident with the polymerization temperature. Shortly after the polymerization reaches the Rmax
p ;
solidification (i.e. vitrification) starts with transformation of the network from the rubbery to the glassy state
[5,20,32–36].
During isothermal vitrification the polymerization
rate is controlled by diffusion, i.e. by free volume
activation and the overall rate constant can be described
by a WLF-type equation of the form [20,33,34]:
lnk ¼ c1 ðT Tg Þ=ðT Tg þ c2 Þ;
ð2Þ
where c1 and c2 are constants, T the curing temperature
and Tg the glass transition temperature of the reacting
system. When Tg approaches the value of T þ c2 the rate
goes to zero, leaving residual unreacted monomer,
pendant double bonds and trapped radicals frozen in
the glass. So, the final degree of conversion of the double
bonds in an isothermal polymerization depends exclusively on the polymerization temperature and the
relationship of degree of conversion with the Tg of the
reacting system, which expresses its mobility. Studies of
the bulk polymerization of various dimethacrylates
showed that and the shape of the rate curve also
depends on how rapidly the mobility the of reacting
where wsol and wgel are the weight fractions of sol
(monomer, acting as plasticizer) and gel; TgðgelÞ is the Tg
of the gel fraction, which depends on the flexibility of
the monomer units consisted and the crosslinking
density. It is obvious that the lower the Tg of a
dimethacrylate monomer and the lower the concentration of the vinyl bonds per unit mass, the lower the Tg of
the monomer units of the corresponding formed
polymer.
In Table 4 the Tg of dimethacrylate monomers are
shown. This is a measure of chain flexibility of
monomer, which depends on the nature and the size of
the groups of the chain. Large and polar groups, which
are responsible for intra- and inter-molecular interactions, decrease the flexibility of the chain and increase
the Tg value.
Bis-GMA exhibits the highest Tg (7.71C) due to the
presence of the rigid aromatic nuclei and mainly due to
the strong hydrogen bonding, if we take into account the
much lower Tg value (46.11C) of its structural analog
Bis-EMA. The hydroxyl groups, positioned diametrically across the rigid bisphenol core structure of the
molecule, cannot form intramolecular hydrogen bonds
but only intermolecular, resulting in a quasi-network
hydrogen bonded structure [21]. This structure results in
an extremely viscous and hindered reaction environment, which must be responsible for the immediate
autoacceleration region observed in the polymerization
of Bis-GMA (Fig. 2). The newly formed radicals attach
to a rigid network; it is difficult to terminate them by
combination or disproportionation, leading to a pronounced maximum in the polymerization rate. Nevertheless the polymerization was carried out at room
temperature (about 231C) and the polymerization
window up to Rmax
was very low, from TgðmonomerÞ ¼
p
7:71C up to TgðnetÞ ¼ Tcure ¼ 231C: The restricted
mobility of the network then also rapidly suppressed
the propagation reaction, by hindering the diffusion of
the free monomer and mainly of the pendant bonds. The
polymerization stopped when the formed polymer
network showed a Tg ¼ 671C (Table 4) obtained with a
very low degree of conversion (39%). At this stage
unreacted pendant double bonds, macroradicals and
free monomer became trapped among network units
and completely immobilized.
UDMA contains an aliphatic spacer group between
the methacrylates, but exhibits a relatively high Tg ; due
to the presence of the urethane groups aNHCOOa
which contain rigid quasi-conjugated double bonds [39]
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I. Sideridou et al. / Biomaterials 23 (2002) 1819–1829
and also form hydrogen bonds. Although the viscosity
and the Tg of UDMA is much lower than that of BisGMA, the shape of the rate versus time curve is
analogous to that of Bis-GMA (Fig. 2). The immediate
auto-acceleration region occurred in the polymerization
of UDMA most probably due to the labile hydrogen
atoms of aNHa groups, which greatly favor chaintransfer reactions [40]:
*
BMd þaNHa-BMH þ aNa;
where BMd is a macroradical. The aNHa group may
be part of a monomer or a polymer molecule, so the
*
newly formed aNa
radical can correspondingly cause
the initiation of polymerization or crosslinking. Thus in
the polymerization of UDMA the initiation rate is
higher than that for all the other monomers and the
formed network is more dense and rigid than is
predicted by its structure, considering as crosslinking
sites only those of the two vinyl groups.
Bis-EMA and TEGDMA are non-viscous liquids with
a very low Tg which must be responsible for a
suppressed diffusion effect on the termination reaction
during gelation and on propagation during vitrification,
resulting in the broadness of the shape of the rate versus
time curves of these monomers (Fig. 2). TEGDMA
showed the highest limiting degree of conversion, in
spite of the highest concentration of the double bonds
per unit mass (Table 3) because of the very low value of
Tg ¼ 83:41C: It is also worth mentioning that UDMA
with a higher Tg value (35.31C) than Bis-EMA
(Tg ¼ 46:11C) gave polymer network with much higher
degree of conversion. This high value (69.6%) may be
due to chain transfer reactions caused by the aNHa
groups with the polymer, which increase the mobility of
radical sites on the network and thereby offer an
alternative path of continuation of the polymerization
until the sample is deeply in the glassy state.
In Table 4 the Tg values of the prepared polymer
networks are also shown. They are all about the same
(65–681C), which is 42–451C above the curing temperature. This result is in accordance with the general
observation that typically: Tg ¼ Tcure þ 202401C [35].
In Table 5 the results obtained from the study of
photo-polymerization of mixtures of the four dimethacrylates are presented, the composition of which are
shown in Table 1. The values of limiting degree of
conversion of mixtures, calculated on the basis of Eq. (1)
as in the case of neat monomers, showed a good linear
relationship between the conversion and the mole
fraction of Bis-GMA in the mixture (Fig. 3).
As far as the rate behavior during the polymerization
of mixtures is concerned, the mixtures of Bis-GMA/
UDMA and Bis-GMA/Bis-EMA showed an intermediate behavior between those of the corresponding neat
monomers. Especially the mixtures of Bis-GMA/BisEMA with 35.8 and 56.6 mol% content of Bis-GMA,
correspondingly showed a maximum rate 2.8 and 3.3 s1
after 2.5 and 1 s, when 5.6 and 2.9% of the double bonds
had been reacted. These results show an approximate
additive effect of the mixture components on the
polymerization rate. On the contrary, the mixtures of
Bis-GMA/TEGDMA showed a maximum rate from the
onset of the polymerization and a generally higher
initial polymerization reactivity than that of neat BisGMA. This result show a synergistic effect of components of this mixture on the polymerization rate, which
may be most probably be due to the better plasticizing
effect of TEGDMA on Bis-GMA than UDMA and BisEMA.
In Table 5 the Tg values of the monomer
mixtures experimentally determined by a DSC and
the theoretically values calculated from the Fox
equation are shown. It is observed that in all mixtures
the experimental value is lower than the calculated
value and this difference is higher in the case of BisGMA/TEGDMA mixtures. Bis-GMA as has already
been mentioned has a structure of a quasi-network
formed by strong hydrogen bonding. In the mixtures
Table 5
Limiting degree of conversion of mixtures of Bis-GMA with various dimethacrylates
Monomer mixture
G–T (1)
G–T (2)
G–U (1)
G–U (2)
G–E (1)
G–E (2)
G–T–U–E (1)
G–T–U–E (2)
G–U–E
a
b
Limiting degree of conversion
60.9
54.9
57.1
50.6
45.4
42.3
47.6 (51.8)b
44.9 (50.4)b
44.5 (48.5)b
Tg (1C) of monomer mixture
a
Tg (1C) of polymer network
Exp.
Fox
Exp.
Foxa
61.1
47.0
29.4
24.6
36.4
30.4
51.8
36.1
25.6
19.8
34.1
26.4
75
76
73
73
67
68
66.0
66.4
67.6
67.4
67.6
67.4
Fox equation is applied for Tg in K.
These are theoretical values of degree of conversion calculated on the basis of the law of mixtures.
1827
I. Sideridou et al. / Biomaterials 23 (2002) 1819–1829
80
80
2
Bis-GMA/TEGDMA R = 0.997
Bis-GMA/UDMA R2= 0.995
2
Bis-GMA/Bis-EMA R2= 0.950
R = 0.982
70
Degree of conversion (%)
Degree of conversion (%)
70
60
50
40
60
50
40
0.0
0.2
0.4
0.6
0.8
1.0
Mole fraction of Bis-GMA
-80
-60
-40
-20
0
o
Monomer Tg ( C)
Fig. 3. Limiting degree of conversion of various mixtures of BisGMA, versus mole fraction of Bis-GMA.
Fig. 4. Variation of limiting degree of conversion with Tg of the initial
monomer mixture Bis-GMA/TEGDMA.
many of these bonds are destroyed, the flexibility
of the Bis-GMA molecules increases and so the Tg
value of Bis-GMA in the mixtures is lower than
that in neat Bis-GMA. This effect, which is not predicted
by the Fox equation, is probably responsible for the
lower experimental values than the calculated values.
Also, the higher difference in these values in the
Bis-GMA/TEGDMA mixtures must be attributed
to the much smaller size of TEGDMA (Table 3)
resulting in a better compatibility of Bis-GMA and
TEGDMA.
In Fig. 4 the dependence of limiting degree of
conversion as a function of the Tg of monomer mixture
Bis-GMA/TEGDMA is shown. A good linear relationship is observed (R2 ¼ 0:982) which shows that in this
mixture the network mobility depends mainly on the Tg
of the initial monomer mixture. In the case of monomer
mixture of Bis-GMA/UDMA and of Bis-GMA/BisEMA, the above coefficient of linear correlation was
correspondingly R2 ¼ 0:935 and R2 ¼ 0:834:
In Table 5 the Tg values, experimental and theoretical,
of the prepared copolymer networks are also presented.
For the calculation of these Tg from the Fox equation,
the Tg values of the homopolymer networks of the
corresponding monomers presented in Table 4 were
used. It is observed that while in mixtures of Bis-GMA/
Bis-EMA the experimental and theoretical values are
about the same, in the mixtures of Bis-GMA/UDMA
and mainly in Bis-GMA/TEGDMA the experimental
values are higher. These higher values could not
attributed to a higher degree of conversion of these
copolymers than the expected, since this was found to
follow the additive law of mixtures (Fig. 3). However,
this result could be explained if it is accepted that the
polymerization rate goes to zero when the temperature
Tb and not the Tg of the polymerizing system coincides
with the curing temperature, as has already been
suggested by Kloosterboer and Lijten [36]. Tg (atransition) is characteristic of the onset of segmental
motion of the main chain of polymer, while Tb (btransition) is usually ascribed to localized group
motions, in polymethacrylates with the rotation of
aCOOR group and in polydimethacrylates with polymer segments or domains containing pendant unreacted
end groups [41]. The higher Tg values of copolymers BisGMA/UDMA and Bis-GMA/TEGDMA is probably
due to the restriction of the flexibility of the UDMA and
TEGDMA monomer units by the rigid and high polar
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I. Sideridou et al. / Biomaterials 23 (2002) 1819–1829
Bis-GMA monomer units. So, the Tg values of the
UDMA and TEGDMA monomer units seem to be
higher in the copolymers than those in the corresponding homopolymers.
In this work the mixtures of Bis-GMA with more than
one comonomer were also studied. The limiting degree
of conversion was slightly less than that predicted by the
additive law of mixtures (Table 5).
4. Conclusions
The room-temperature photopolymerization of the
most widely used dimethacrylates in dentistry was
studied by FT-IR. The results obtained were then fitted
by a non-linear least square method to a rational
function, which permitted the accurate calculation of the
limiting degree of conversion. The latter was found to
increase in the order:
Bis-GMAoBis-EMAoUDMAoTEGDMA:
This trend is connected with the mobility of the
polymerizing system, which depends on Tg of the
formed network and mainly on the Tg of the unreacted
monomer. Especially in the case of UDMA, the degree
of conversion is higher than that expected, most
probably due to chain transfer reactions caused by the
aNHa groups, which increase the mobility of radical
sites on the network. These chain-transfer reactions may
also be responsible for the high polymerization reactivity of UDMA.
The final degree of conversion is not affected by the
polymerization reactivity of monomers. The maximum
rate of polymerization of UDMA and Bis-GMA was
higher than that of TEGDMA and Bis-EMA.
The limiting degree of conversion of mixtures of BisGMA with the other dimethacrylates showed a good
linear relationship with the mole fraction of Bis-GMA
and in the case of Bis-GMA/TEGDMA mixture, also
with the Tg of the initial monomer mixture.
TEGDMA was found to have a better plasticizing effect on Bis-GMA than UDMA and Bis-EMA,
which is responsible for the synergistic effect on the
polymerization rate observed in the mixture Bis-GMA/
TEGDMA.
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