Curing Kinetics of the Synthesis of Poly(2-hydroxyethyl
methacrylate) (PHEMA) with Ethylene Glycol
Dimethacrylate (EGDMA) as a Crosslinking Agent
CHEN-WEI HUANG, YI-MING SUN, WEI-FUNG HUANG
Department of Chemical Engineering, Yuan-Ze Institute of Technology, Chung-Li, Taiwan 320, Republic of China
Received 19 August 1996; accepted 3 December 1996
ABSTRACT: An experimental study was carried out to investigate the effect of ethylene
glycol dimethacrylate (EGDMA, as a crosslinking agent) content on the curing kinetics
of the polymerization of 2-hydroxyethyl methacrylate (HEMA), using differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy (FTIR). EGDMA
may cause a crosslinking-facilitated gel effect which reduces the termination rate of
living free radicals and enhances the overall reaction rate, but it may also induce a
diffusional resistance for the reactants so that some free monomers are trapped and
pendant vinyl groups are prohibited from reaction by the crosslinked structure. At
higher content of EGDMA, the later effect becomes predominant, and the reaction rate
and the final conversion are limited. The exothermic peak of the curing reaction tends
to carry a shoulder and then split into two peaks as the amount of EGDMA is increased,
possibly due to a later reaction of the trapped monomers and pendant vinyls. The heat
of reaction measured by DSC in the scanning mode is 61.2 kJ/mol C|C. The activation
energy (E) of the curing reaction ranges from 56.5 to 78.3 kJ/mol C|C depending on
the EGDMA content and the type of operation. The diffusion-limited reaction rate and
the different thermal history experienced in the nonisothermal and isothermal curing
can result in variations of the results in the activation energy measurement. q 1997
John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35: 1873–1889, 1997
Keywords: free-radical polymerization; kinetics; hydrogel; thermoset; crosslinking
INTRODUCTION
Hydrogels are crosslinked hydrophilic polymer
materials that can absorb a significant amount of
water while maintaining a distinct three-dimensional structure (insoluble). The preparation of
poly(2-hydroxyethyl methacrylate) (PHEMA)
hydrogels is a subject of great interest, especially
for biomedical and pharmaceutical applications.1–6
Correspondence to: Y.-M. Sun
Contract grant sponsor: National Science Council, Republic
of China; contract grant number: NSC-83-0425-B-155-001M08
Contract grant sponsor: Standard Chemical and Pharmaceutical Co.; contract grant number: NSC-83-0425-B-155-001M08
q 1997 John Wiley & Sons, Inc. CCC 0887-624X/97/101873-17
The hydrogels in a matrix form have been used
as soft contact lenses, 1,6 artificial organ, 2,6 implants, 3 – 4,6 and devices for controlled drug release.5,7 On the other hand, the hydrogels in a
spherical microparticle form (microspheres or
beads) are used in hemoperfusion, 8 enzyme immobilization, 9,10 endovascular occlusion, 11 and
also in controlled drug release.5,12 – 13 They have
the major advantages of good biocompatibility,
moderate degree of swelling in water, and biological inertness.
Several techniques have been used to prepare
these hydrogels: such as bulk polymerization (for
matrix of various shapes), 5,7 suspension polymerization 14,15 (for microspheres), and solution polymerization for a linear polymer then followed by
a crosslinking reaction.16 Many comonomers can
1873
/ 8G42$$0239
05-30-97 07:23:20
polca
W: Poly Chem
1874
HUANG, SUN, AND HUANG
be used to adjust the crosslinking density and hydrophilicity to modify the swelling properties, mechanical strength, and the permeation rate of solutes through the hydrogels. Ethylene glycol dimethacrylate (EGDMA) is a frequently used
comonomer in the preparation of PHEMA hydrogels, 5,17 and it functions as a crosslinking
agent in the thermosetting system. The reaction
kinetics of the thermosetting polymerization,
such as the curing of epoxy resins, 18 – 20 unsaturated polyester resins, 21 – 24 and some multifunctional vinyl or acetylenic systems, 25 – 28 has been
studied by many investigator; however, the manner in which the crosslinking agent influences the
reaction kinetics of PHEMA has seldom been reported. There is a need to better understand the
reaction kinetics during synthesis in order to
properly prepare the PHEMA hydrogels for a critical application.
Since the preparation of PHEMA is a free radical chain growth crosslinking copolymerization of
HEMA and EGDMA, as one may expect, initial
molar ratio of HEMA C|C to EGDMA C|C
bonds, concentrations of initiator and inhibitor,
reaction temperatures, and thermal histories of
curing would affect the reaction kinetics in different ways. It is well known that the gel effect
(Trommsdorff effect) can suppress the termination rate of a free radical polymerization so that
an autoacceleration generally results.29 It was
also reported that the free radical polymerization
was diffusion-controlled and the final conversion
was always incomplete.30,31 The presence of crosslinking structure will further complicate the diffusion behavior of the reactants and free radicals
during reaction. A kinetic expression with consideration of all those factors is essential to interpret
or predict the rate of polymerization at a given
temperature and conversion. However, detailed
mechanism of free radical crosslinking polymerization still remains obscure.28 Even if a kinetic
model based on mechanistic consideration is
available, 24,32 – 34 it is generally tedious in practical
application.
A simple phenomenological model, which took
the autoacceleration characteristics in to account,
was proposed by Kamal and co-workers 21,35 to correlate the kinetic data obtained from differential
scanning calorimetry (DSC) with good reproducibility. The model is expressed as
da
Å ka m (1 0 a ) n
dt
/ 8G42$$0239
05-30-97 07:23:20
(1)
where da /dt is reaction rate, k is rate constant,
and m and n are constants describing the order
of the reaction. The model can fit the bell-shape
reaction rate profiles which are typical for the curing of epoxy or unsaturated polyester resins and
its parameters (k, m, and n) can be determined
from the isothermal DSC results.20 – 24 The activation energy of the polymerization reaction can be
determined from the Arrhenius plot of the logarithm of rate constant (k) and the reciprocal of
the reaction temperature.
In the present study, the isothermal and nonisothermal (scanning mode; temperature raising
at predetermined constant rate) reaction kinetics
were determined by a differential scanning calorimeter (DSC) and the conversion of C|C bonds
were confirmed by a Fourier transform infrared
spectrophotometer (FTIR). The range of isothermal temperatures were selected according to
those commonly applied to decompose the initiator AIBN. The curing kinetics were analyzed according to the rate profiles of heat release recorded by DSC. The isothermal DSC results were
tested with the Kamal model mentioned above to
extract the kinetic parameters. In addition, the
activation energies of the polymerization reactions based on an arbitrary kinetic form36 were
determined from the nonisothermal DSC data
and were compared with those obtained from the
Kamal model. The reaction system mimics a bulk
polymerization with good heat transfer to its environment. The kinetic parameters obtained here
can be used to better correlate and design the
synthesis conditions of PHEMA in either bulk polymerization or suspension polymerization, in
which each monomer droplet of the suspended
phase can be considered to be a mini-bulk polymerization system.
EXPERIMENTAL
Materials
HEMA provided by Chung-Chun Chemical Co.
(Taiwan) as a gift was employed in this study.
There was a small amount of monomethyl ether
of hydroquinone in the monomer as an inhibitor.
EGDMA was purchased from Aldrich Chemical
Co. (USA), and was also inhibited with monomethyl ether of hydroquinone. The monomers designated for kinetic measurements were vacuum
distilled, with small amounts of CuCl, so that hydroquinone could be removed. Azobisisobutyroni-
polca
W: Poly Chem
1875
SYNTHESIS OF PHEMA WITH EGDMA
Table I. The Compositions of HEMA, EGDMA, and Initiator (AIBN) in the Samples Prepared in this Study
Weight Ratio of EGDMA/HEMA
Total moles of C|C/g monomer
g AIBN/g monomer
mol AIBN/mol C|C
[AIBN]o,z /[AIBN]o,0/10a
a
0/10
1/9
2/8
3/7
7.69 1 1003
0.002
1.58 1 1003
(1)
7.93 1 1003
0.002
1.54 1 1003
(0.975)
8.17 1 1003
0.002
1.49 1 1003
(0.943)
8.42 1 1003
0.002
1.45 1 1003
(0.918)
z Å Weight ratio of EGDMA/HEMA (0/10, 1/9, 2/8, and 3/7).
trile (AIBN, Aldrich Chemical Co.) was used as
received.
Sample Preparation
The initiator (AIBN) concentration was fixed at
0.2% by weight with respect to the total amount
of monomers. In the sample preparation, HEMA
and EGDMA were weighed separately and mixed
together first at room temperature. The initiator
was added into the monomer solution and the solution was stirred until thoroughly mixed. Then
the solution was degassed by using an ultrasonic
cleaner (NEY, 300) before curing experiments.
Samples with the weight ratio of EGDMA/HEMA
(WR) equals to 0/10, 1/9, 2/8, and 3/10 were prepared (Table I). Although no EGDMA was added
to the sample with WR Å 0/10, the polymerization
still involves slight crosslinking reaction since the
diester EGDMA is always present in the HEMA
as a by-product during the preparation of raw
HEMA, and upon distillation the disproportionation of HEMA 1,37 increases the amount of EGDMA. In addition, some crosslinking of the polymer due to radical chain transfer mechanisms
was suggested.38 The hydrogels prepared in this
manner can be considered as thermosetting systems.
By convention, the addition of initiator was
based on the weight percentage of the monomers
used. To facilitate later discussion, the conversion
of the weight basis into the molar basis are listed
in Table I. It can be seen that the number of moles
of the C|C bonds, including the vinyl groups of
EGDMA and HEMA, per gram of monomers are
shown in row 1 increasing with the weight ratio
of EGDMA/HEMA. If we neglect the density variation of the sample solutions with different weight
ratio of EGDMA/HEMA, the values in row 3
would reflect the magnitudes of molar concentrations for AIBN. It can be seen that with the same
/ 8G42$$0239
05-30-97 07:23:20
weight percentage of AIBN relative to the monomers, the molar concentration, however, would
decrease with increasing molar concentration of
C|C bonds. The numbers in parenthesis shown
in rows 4 represent the relative values of the molar concentrations of AIBN by taking that of the
sample with weight ratio of 0/10 (EGDMA/
HEMA) as unity.
Because the rate of polymerization reaction
may be affected by the concentration of initiator
in the system, it should be corrected by taking this
effect into consideration. The propagation rate of
polymerization, Rp , is proportional to the square
root of the rate of initiation or free radical formation, 29 Ri , i.e.,
Rp Å kp[Mr][M] Å kp[M](Ri /2kt ) 1 / 2
(4)
where kp and kt are the rate constants for propagation and termination, respectively, [M • ] is the total concentration of all the chain radicals, and
[M] is the monomer concentration. The rate of
initiation depends on the initial concentration of
initiator, and
Ri } [AIBN]o
(5)
where [AIBN]o is the initial molar concentration
of initiator AIBN. Since the concentration of initiator is different for the sample with different composition of monomers, the following equation can
be used to correct the reaction rate later on.
Rpm Å Rp ,z /([AIBN]o ,z /[AIBN]o ,0 / 10 ) 1 / 2 ,
z Å 1/9, 2/8, 3/7 (6)
where the subscript z indicates the weight ratio
(WR) of EGDMA/HEMA in the sample and Rpm
is the modified polymerization rate.
polca
W: Poly Chem
1876
HUANG, SUN, AND HUANG
DSC Measurement
A DuPont 910 differential scanning calorimeter
was used to follow the overall reaction rate profiles in a close system form. The instrument was
calibrated with pure indium as standard. All the
reactions were conducted in hermetic aluminum
sample pans to prevent monomer evaporation.
The sample average weight was 6–10 mg. Nitrogen gas was feed at 30 ml/min into the reaction
environment as a purge gas during the reaction.
Profiles of the non-isothermal reaction rate versus
temperature (time) were determined in the scanning mode at raising rate of 5, 10, 15, and 207C/
min from room temperature to 2207C. Profiles of
the isothermal reaction rate versus time were
measured at 60, 70, 80, and 907C. Isothermal DSC
runs were ended when there was no further exotherm and samples were reheated to 2207C in the
scanning mode with a raising rate of 107C/min to
determine the residual reactivity left in the isothermally cured samples. For each testing condition, experiments were run for at least three
times. The total heat of reaction was calculated
from the area under the exothermic peak of a DSC
curve, where the heat release rate (dDHt /dt) was
plotted with time. The rate of heat release measured by DSC can then be directly converted into
the overall reaction rate (da /dt) and fractional
conversion ( a ) as a function of time and the formulae are shown as follows 39 :
1 d DH t
da
Å
dt
DHT dt
aÅ
1
D HT
(2)
* dDdtH dt Å DDHH
t
t
0
t
(3)
T
where DHt is the accumulated heat released before the time t, and DHT is the total heat released
at the end of the reaction. In a nonisothermal
scanning run, DHT equals to the accumulated
heat released from room temperature to 2207C
( DHS ), i.e., DHT Å DHS . In an isothermal run,
DHT may stand for the summation of the isothermal heat release, DHiso , and the residual scanning heat release, DHres , i.e., DHI Å DHiso / DHres
and DHT Å DHI .39 Intuitively, DHI should be the
same as DHS for the same sample if all the vinyl
groups in the system are converted, however, it
is usually not the case. The fractional conversion
( a ) of an isothermal run can be determined by
taking either one as reference. We denote aI and
aS as the conversion calculated based on DHI and
averaged DHS , respectively.
/ 8G42$$0239
05-30-97 07:23:20
Figure 1. Nonisothermal DSC rate profiles at a scanning rate of 57C/min for the curing reaction with various weight ratios of EGDMA/HEMA.
FTIR Measurement
In the present study, a Perkin-Elmer 1725X
Fourier transform infrared spectrophotometer
(FTIR) with a resolution of 4 cm01 in the transmission mode was used to confirm the consumption of C|C bonds in HEMA and EGDMA. The
reaction of HEMA–EGDMA cured in the DSC cell
was stopped by rapidly chilling the sample pan in
liquid nitrogen. The sample was then milled,
mixed with KBr, and pressed into a solid disk of
1 cm diam prior to the IR measurement. In the
analysis of IR spectra, change of the absorbance
of the peak at 1635 cm01 (C|C stretching) was
employed to estimate the conversion of vinyl
groups in the cured samples. The absorbance of
the peak at 1730 cm01 (C|O stretching) was
picked as an internal standard. Two straight lines
connected the points of a spectrum at 1600, 1660,
and 1850 cm01 were taken as the base lines for
peak intensity measurement to correct the extraneous background. The relative intensity for the
peaks 1635 and 1730 cm01 was determined by the
ratio of the intensities of these two peaks with the
base line correction.
RESULTS AND DISCUSSIONS
Reaction Rate Profiles
Figures 1–4 show the non-isothermal DSC profiles of heat release rates (dDHt /dt) for the samples with various weight ratio of EGDMA/HEMA
(WR Å 0/10, 1/9, 2/8, and 3/7) at the raising rate
of 5, 10, 15, and 207C/min, respectively, from room
polca
W: Poly Chem
SYNTHESIS OF PHEMA WITH EGDMA
1877
Figure 2. Nonisothermal DSC rate profiles at a scanning rate of 107C/min for the curing reaction with various weight ratios of EGDMA/HEMA.
Figure 4. Nonisothermal DSC rate profiles at a scanning rate of 207C/min for the curing reaction with various weight ratios of EGDMA/HEMA.
temperature to 2207C. Figures 5–8 show the isothermal DSC rate profiles at several temperatures
(60, 70, 80, and 907C) for the samples with the
weight ratio of 0/10, 1/9, 2/8, and 3/7 (EGDMA/
HEMA), respectively. The rate shown in the figures
is expressed as kJ/mol C|C/s instead of kJ/g
monomer/s, since the number of moles of total
C|C bonds for every gram of monomer with different weight ratio differs from one another, 39 as
shown in Table I. However, the rate profiles shown
in Figures 1–8 were not corrected for the variation
of the molar concentration of the initiator in the
system. The overall reaction rate can be obtained
by dividing the instantaneous heat release rate by
the total heat released according to the eq. (2).
Table II summarizes the calculated results of
nonisothermal reactions including the total heat
of reaction ( DHT or DHS ), temperature at maximum rate (Tp ), fractional conversion at maximum
rate ( ap ), maximum rate (Rp ), and modified maximum rate (Rpm ). Tables III–VI summarizes the
results obtained from the isothermal reaction,
which includes the heat generated in isothermal
cure ( DHiso ), the residual heat ( DHres ) that was
released when the sample was heated to 2207C
from the isothermal temperature at 107C/min
upon completion of the isothermal cure, and the
total heat of cure ( DHI Å DHiso / DHres ). Also
listed in Tables III–VI are the final isothermal
conversions, aI and aS , based on DHI and DHS ,
respectively, induction time (td ), time at maximum rate (tm ), fractional conversion at maximum
Figure 3. Nonisothermal DSC rate profiles at a scanning rate of 157C/min for the curing reaction with various weight ratios of EGDMA/HEMA.
Figure 5. Isothermal DSC rate profiles for the curing
reaction with weight ratio of EGDMA/HEMA Å 0/10:
(
) experimental data, (rrrrr) simulated data.
/ 8G42$$0239
05-30-97 07:23:20
polca
W: Poly Chem
1878
HUANG, SUN, AND HUANG
Figure 6. Isothermal DSC rate profiles for the curing
reaction with weight ratio of EGDMA/HEMA Å 1/9:
(
) experimental data, (rrrrr) simulated data.
Figure 8. Isothermal DSC rate profiles for the curing
reaction with weight ratio of EGDMA/HEMA Å 3/7:
(
) experimental data, (rrrrr) simulated data.
rate ( am , based on DHI ), maximum rate (Rp ), and
modified maximum rate (Rpm ).
Although the isothermal and non-isothermal
operations are different in nature, their thermograms show some similarity in terms of the shape
of rate profiles. It can be seen that the exothermic
peak tends to carry a shoulder and then split into
two peaks in the extreme case, as the weight ratio
of EGDMA/HEMA increases under the same reaction condition. The peak splitting becomes more
pronounced when the reaction temperature is elevated in an isothermal operation or the scanning
rate is lowered in a non-isothermal one. The first
and the major peak is due to the reaction initiated
by the decomposition of AIBN and the propaga-
tion of the free radical polymerization. However,
the difunctional EGDMA may leave the main
chains unreacted vinyl pendant groups, which are
partially reacted because of their limited mobility
toward a free radical. Besides, more unreacted
free monomers may be trapped as the crosslinking
density is higher when higher fraction of EGDMA
is used. When the conversion of the polymerization increases, the density of the polymer matrix
increases, the polymer chains become close to
each other, and those unreacted vinyl groups and
trapped free monomers can line up if sufficient
energy is provided for a local rearrangement. A
zip propagation mechanism could result if a free
radical, which either has survived since the AIBN
initiation step or is produced due to thermal initiation, exists in the vicinity of these unreacted pendant vinyl groups or free monomers.29 A shoulder
or a second exothermic peak appears as a consequence. In an isothermal run, it can only be observed at higher reaction temperature, and a distinct second peak shows up at higher weight ratio
of EGDMA/HEMA. In a nonisothermal run, the
relative height of the second peak to the first one
increases with the weight ratio of EGDMA/
HEMA, but it decreases with the scanning rate.
The effect of the weight ratio has been explained
as above, and the effect of scanning rate needs a
further interpretation. At higher scanning rate, a
reaction system with the same weight ratio can
reach a higher temperature within shorter time
period so that most unreacted vinyl pendant
groups and free monomers will have better chance
to react with a living free radical, which is produced either by the AIBN initiation at beginning of
Figure 7. Isothermal DSC rate profiles for the curing
reaction with weight ratio of EGDMA/HEMA Å 2/8:
(
) experimental data, (rrrrr) simulated data.
/ 8G42$$0239
05-30-97 07:23:20
polca
W: Poly Chem
1879
SYNTHESIS OF PHEMA WITH EGDMA
Table II. Total Heat of Reaction (DHS ), Temperature at Maximum Rate (Tp), Conversion at Maximum Rate
(ap), Maximum Rate (Rp), and Modified Maximum Rate (Rpm) for EGDMA/HEMA Curing Reactions in the
Scanning Mode of DSC Study (Nonisothermal Operation)
Scanning Rate (7C/min)
EGDMA/HEMA Å 0/10
DHS (kJ/mol C|C)
Tp (K)
ap (Tp)
Rp (kJ/mol C|C/s)
Rpm (kJ/mol C|C/s)
EGDMA/HEMA Å 1/9
DHS (kJ/mol C|C)
Tp (K)
ap (Tp)
Rp (kJ/mol C|C/s)
Rpm (kJ/mol C|C/s)
EGDMA/HEMA Å 2/8
DHS (kJ/mol C|C)
Tp (K)
ap (Tp)
Rp (kJ/mol C|C/s)
Rpm (kJ/mol C|C/s)
EGDMA/HEMA Å 3/7
DHS (kJ/mol C|C)
Tp (K)
ap (Tp)
Rp (kJ/mol C|C/s)
Rpm (kJ/mol C|C/s)
5
10
15
20
61.5 { 0.3
378.8 { 0.1
0.52
0.59
0.59
61.3 { 0.2
391.1 { 0.2
0.53
1.02
1.02
61.8 { 0.5
399.0 { 0.1
0.54
1.26
1.26
61.6 { 0.2
405.4 { 0.2
0.54
1.43
1.43
60.8 { 0.2
376.4 { 0.3
0.41
0.76
0.78
60.8 { 0.8
385.4 { 0.7
0.43
1.18
1.21
61.2 { 0.7
393.4 { 0.9
0.40
1.70
1.74
61.4 { 0.9
398.9 { 0.1
0.41
2.03
2.08
61.9 { 0.2
374.2 { 0.1
0.36
0.88
0.93
60.9 { 0.2
383.6 { 0.1
0.37
1.33
1.41
60.8 { 0.2
390.8 { 0.3
0.37
1.94
2.06
60.5 { 0.2
395.6 { 0.4
0.34
2.17
2.30
61.3 { 0.4
370.6 { 0.1
0.32
0.57
0.62
58.7 { 0.2
379.8 { 0.2
0.35
1.10
1.20
57.5 { 0.6
385.9 { 0.2
0.35
1.39
1.52
57.5 { 0.4
391.0 { 0.4
0.35
1.71
1.86
the reaction or by the thermal initiation at the
higher temperature, because they have higher energy to cross the diffusion barrier formed by the
polymer networks. Therefore, the fraction of the remaining unreacted vinyl bonds (including pendant
and free ones) for the later zip reaction is reduced
and the relative height of the second peak to the
first one decreases with increasing scanning rate.
Conversion of Vinyl Bonds by FTIR Measurement
Figure 9 shows a typical FTIR spectra of a curing
system (a) before reaction, (b) after isothermal reaction, and (c) after rescanning. The band near
1730 cm01 , a characteristic band of C|O stretching, was chosen as an internal standard. It is shown
in Figure 9 that the intensity of band near 1635
cm01 , a characteristic band of C|C stretching, decreases after isothermal reaction and decreases
more after rescanning. The spectra confirm that the
vinyl bonds are consumed during the polymerization. The curve (c) in Figure 9 shows that there is
/ 8G42$$0239
05-30-97 07:23:20
still a small peak of C|C band after rescanning
reaction. It indicates that the system has a limited
conversion, there are still some residual C|C
bonds buried inside the network structure despite
the sample experiencing rescanning by DSC.
Figure 10 shows the relative intensities of C|C
band to C|O band for the samples that have been
cured at isothermal temperature. It can be seen
that the relative intensity increases with increasing
weight ratio of EGDMA/HEMA and with decreasing curing temperature. Because the higher the
crosslinking density is, the monomers will have
more difficulty to diffuse to the reactive site of a free
radical and remain unreacted. When the system is
cured at higher temperature, the monomers will
have higher mobility so that they are able to go
through the diffusion barrier and then the number
of the unreacted monomers become minimized.
Heat of Reaction and the Overall Conversion
The heat of reaction was determined from the
area under the peak(s) of a DSC thermogram.
polca
W: Poly Chem
1880
HUANG, SUN, AND HUANG
Table III. Summary of the Kinetic Information Obtained by Isothermal DSC Study for EGDMA/HEMA
Curing Reaction at 607C
Weight Ratio of EGDMA/HEMA
Heats of reaction of the isothermal
run (DHiso ), rescanning after
isothermal run (DHres ), and the
total (DHI ):
DHiso (kJ/mol C|C)
DHres (kJ/mol C|C)
DHI Å DHiso / DHres (kJ/mol C|C)
Total conversion of the isothermal run
based on DHI and averaged DHS
(61.2 kJ/mol C|C):
a1
aS
Induction time td (min)
Time at maximum rate tm (min)
Dt Å tm 0 td (min)
Conversion at max. rate, am
Rp (J/mol C|C/s)
Rpm (J/mol C|C/s)
0/10
1/9
2/8
3/7
51.6 { 1.3
4.7 { 0.1
56.3 { 1.4
48.7 { 1.5
7.2 { 0.1
55.9 { 1.6
42.8 { 0.2
10.0 { 0.2
52.8 { 0.4
42.0 { 1.4
10.8 { 0.2
52.8 { 1.6
0.92
0.84
68 { 10
129 { 2
61 { 12
0.59
39.0
39.0
0.87
0.79
81 { 1
122 { 7
41 { 8
0.55
40.2
41.2
0.81
0.70
128 { 5
167 { 3
39 { 8
0.51
44.4
47.0
0.80
0.69
83 { 2
123 { 1
40 { 3
0.47
47.0
51.2
Consistent results were obtained in the nonisothermal scanning operation ( DHS in Table II).
Except for the cases where the sample is with
EGDMA/HEMA ratio Å 3/7 and scanned at
higher heating rates, all the other cases give an
averaged reaction heat of 61.2 kJ/mol of C|C
with a standard deviation less than 2%. In comparison with the reaction heat of the polymeriza-
Table IV. Summary of the Kinetic Information Obtained by Isothermal DSC Study for EGDMA/HEMA
Curing Reaction at 707C
Weight Ratio of EGDMA/HEMA
Heats of reaction of the isothermal
run (DHiso), rescanning after
isothermal run (DHres), and the
total (DHI ):
DHiso (kJ/mol C|C)
DHres (kJ/mol C|C)
DHI Å DHiso / DHres (kJ/mol C|C)
Total conversion of the isothermal run
based on DHI and averaged DHS
(61.2 kJ/mol C|C):
aI
aS
Induction time td (min)
Time at maximum rate tm (min)
Dt Å tm 0 td (min)
Conversion of max. rate, am
Rp (J/mol C|C/s)
Rpm (J/mol C|C/s)
/ 8G42$$0239
0/10
1/9
2/8
3/7
51.7 { 0.8
4.0 { 0.03
55.7 { 0.9
49.0 { 1.0
4.8 { 0.1
53.8 { 1.1
44.2 { 1.5
7.1 { 0.1
51.3 { 1.6
44.1 { 0.5
8.4 { 0.1
52.4 { 0.6
0.93
0.84
14.6 { 1.6
41.5 { 0.5
27 { 2
0.53
72.1
72.1
0.91
0.80
11.4 { 0.5
28.6 { 1.3
17 { 2
0.47
88.8
91.1
0.86
0.72
28.0 { 0.8
44.0 { 1.1
16 { 2
0.45
74.6
79.1
0.84
0.72
20.5 { 1.1
38.1 { 1.0
17 { 1.1
0.44
84.6
92.1
05-30-97 07:23:20
polca
W: Poly Chem
SYNTHESIS OF PHEMA WITH EGDMA
1881
Table V. Summary of the Kinetic Information Obtained by Isothermal DSC Study for EGDMA/HEMA
Curing Reaction at 807C
Weight Ratio of EGDMA/HEMA
Heats of reaction of the isothermal
run (DHiso), rescanning after
isothermal run (DHres ), and the
total (DHI ):
DHiso (kJ/mol C|C)
DHres (kJ/mol C|C)
DHI Å DHiso / DHres (kJ/mol C|C)
Total conversion of the isothermal run
based on DHI and averaged DHS
(61.2 kJ/mol C|C):
aI
aS
Induction time td (min)
Time at maximum rate tm (min)
Dt Å tm 0 td (min)
Conversion at max. rate, am
Rp (J/mol C|C/s)
Rpm (J/mol C|C/s)
0/10
1/9
2/8
3/7
54.7 { 0.4
2.2 { 0.1
56.9 { 0.5
53.2 { 0.5
2.8 { 0.2
56.0 { 0.7
48.8 { 0.01
4.5 { 0.1
53.2 { 0.1
48.2 { 0.6
6.0 { 0.3
54.2 { 0.9
0.96
0.89
2.0 { 0.3
13.7 { 0.0
11.7 { 0.3
0.43
97.7
97.7
0.95
0.87
2.2 { 0.1
10.3 { 1.1
8.1 { 1.2
0.40
111.5
114.3
0.92
0.80
2.5 { 0.3
9.6 { 0.2
7.1 { 0.5
0.32
104.2
110.5
0.89
0.79
2.3 { 0.4
9.2 { 0.7
6.9 { 1.1
0.26
100.5
109.5
tion of methyl methacrylate, 56.5–61.0 kJ/mol
measured by DSC, 28 the obtained result shows a
good agreement among the monomers of similar
kind. The reaction heat was lowered in the pre-
viously mentioned exceptional cases (WR Å 3/7),
probably due to limited conversion of the pendant
vinyl groups or buried monomers in a highly
crosslinked network. If we consider 61.2 kJ/mol
Table VI. Summary of the Kinetic Information Obtained by Isothermal DSC Study for EGDMA/HEMA
Curing Reaction at 907C
Weight Ratio of EGDMA/HEMA
Heats of reaction of the isothermal
run (DHiso), rescanning after
isothermal run (DHres), and the
total (DHI ):
DHiso (kJ/mol C|C)
DHres (kJ/mol C|C)
DHI Å DHiso / DHres (kJ/mol C|C)
Total conversion of the isothermal run
based on DHI and averaged DHS
(61.2 kJ/mol C|C):
aI
aS
Induction time td (min)
Time at maximum rate tm (min)
Dt Å tm 0 td (min)
Conversion at max. rate, am
Rp (J/mol C|C/s)
Rpm (J/mol C|C/s)
/ 8G42$$0239
0/10
1/9
2/8
3/7
56.6 { 0.4
1.3 { 0.01
57.9 { 0.4
54.5 { 0.4
1.4 { 0.04
56.0 { 0.4
52.1 { 0.1
2.6 { 0.1
54.7 { 0.2
50.6 { 0.4
3.6 { 0.1
54.2 { 0.5
0.98
0.92
0.88 { 0.01
5.1 { 0.1
4.2 { 0.1
0.41
211.5
211.5
0.97
0.89
0.87 { 0.02
2.8 { 0.1
1.9 { 0.1
0.34
380.5
390.3
0.95
0.85
0.89 { 0.01
2.3 { 0.1
1.4 { 0.1
0.26
336.2
356.5
0.93
0.83
0.93 { 0.03
2.4 { 0.1
1.5 { 0.1
0.25
289.5
315.3
05-30-97 07:23:20
polca
W: Poly Chem
1882
HUANG, SUN, AND HUANG
Figure 9. FT-IR spectra of sample with EGDMA/HEMA Å 3/7: (a) before reaction,
(b) after isothermal curing at 907C, and (c) after rescanning to 2207C.
Figure 10. (a) Relative intensity of C|C band to
C|O band in IR spectra for samples with different
weight percentage of EGDMA cured at 707C (h), (b)
Relative intensity of C|C band to C|O band in IR
spectra for samples with EGDMA/HEMA Å 3/7 cured
at different temperature ( n).
/ 8G42$$0239
05-30-97 07:23:20
as the reaction heat of all the reactable vinyl
groups in the system, then we can estimate that
those unreacted fraction of all the vinyl groups
may be up to 6% for the sample with EGDMA/
HEMA ratio Å 3 / 7 scanned at rate of 15 or
207C/min.
The reaction heat measured in an isothermal
run gives DHiso , the heat released during the rescanning to 2207C gives the residual reaction
heat DHres , and the sum of them gives DHI in
the Tables III – VI. The conversion in an isothermal run can be calculated based on DHI or DHS .
The estimated conversion ( aI or aS ) is always far
less than unity in the experimental conditions of
this study; therefore, there are a great number
of residual vinyl groups in the system, either in
form of unreacted free monomers or pendants.
Unreacted monomers can be easily extracted by
a good swelling agent. Nevertheless, a highly
crosslinked hydrogel may have a large number
of unreacted vinyl pendants if the curing reaction
is carried out at temperature similar to the range
of this study. One should be careful in using
these crosslinked gel for biomedical application
if the existence of unsaturated vinyl groups is
polca
W: Poly Chem
SYNTHESIS OF PHEMA WITH EGDMA
restricted. On the other hand, those unreacted
vinyl pendants can provide sites for grafting the
parent polymer with a second polymer for further
modification or a bioactive compound for controlled release.
There are two kinds of conversion in an isothermal run, aI and aS , calculated based on DHI and
DHS , respectively. By definition, aS is the isothermal conversion of all the reactable vinyl double
bonds in the system, and aI may represent the
apparent isothermal conversion of the vinyl bonds
which can be converted in this particular full operation (isothermal curing r rescanning). Because
all the initiators were completely consumed during the isothermal run and unreacted vinyl
groups were less reactive as they were buried between chains of formed polymer networks, further
scanning to a higher temperature is not effective
enough to react all the remaining vinyl groups so
that DHI is always lower than the DHS and thus
aI ú aS .
The isothermal conversion ( aI or aS ) decreases
with increasing EGDMA ratio but increases with
increasing reaction temperature. The free radical
polymerization is diffusion-controlled and the final conversion is always incomplete.30,31 Although
the formation of crosslinking structure reduces
the termination rate of free radicals, it also reduces the diffusion of reactants at later stage of
the reaction so the final conversion is limited.
Higher EGDMA ratio will give higher crosslinking density so the final conversion is lower. At
higher reaction temperature, reactants will have
higher energy to break the diffusion limitation so
the conversion is higher.
Effect of the EGDMA Content in the
Nonisothermal DSC Study
Both the temperature at maximum reaction rate,
Tp , and the conversion at maximum reaction rate,
ap , decrease with increasing weight ratio of EGDMA/HEMA as shown in Table II. It is wellknown that, for the diffusion-controlled free radical polymerization, the apparent termination rate
constant kt decreases much fast than the propagation rate constant kp does as the reaction goes on.
The reaction follows an autocatalytic kinetics and
will reach a maximum rate in the middle of the
reaction, and the so-called gel effect takes place.29
In the dynamic scanning study, the conversion at
the maximum point ( ap ) is relatively independent
of the scanning rate.36 The presence of a crosslinking agent further facilitates the gel effect because
/ 8G42$$0239
05-30-97 07:23:20
1883
Figure 11. The modified maximum reaction rate of
the first peak, Rpm , as a function of the weight percentage of EGDMA in a nonisothermal run. Scanning rate:
(l) 57C/min, (j) 107C/min, (l) 157C/min, (m) 207C/
min.
the crosslinked network creates a greater diffusional limitation for the termination of active free
radicals. The on-set of the gel effect will occur at
a lower conversion and lower temperature in a
consequence. The samples with higher EGDMA
content will let the diffusion-controlled propagation reaction begin at earlier time of the curing.
It is suggested that the generated crosslinking
structure plays a significant role in determining
the commencement of this liquid to solid transition at the gel point.
The maximum reaction rate is also affected by
the presence of a divinyl crosslinking agent. After
the correction for the variation of Ri by using WR
Å 0/10 as the basis (row 4 in Table I), the modified maximum reaction rate at peak, Rpm , is plotted with the weight percentage of EGDMA in the
Figure 11. The maximum reaction rate initially
increases with increasing EGDMA content up to
the point of 20% and then decreases as the EGDMA content further increases. It indicates that
the overall reaction rate is initially enhanced by
the crosslinking facilitated gel effect and then retarded due to limited mobility of unreacted monomers or pendants in a highly crosslinked structure. Therefore, more and more unreacted vinyl
groups (including pendant and free ones) are left
in the network so that a second peak, as we have
discussed previously, appears in the rate profile
at later time and higher temperature and becomes
more distinct with increasing EGDMA content. It
polca
W: Poly Chem
1884
HUANG, SUN, AND HUANG
can be seen from Figures 1–4 that the increasing
order of Rp for the second peak is that WR Å 1/9
õ 2/8 õ 3/7.
Effect of the EGDMA Content in the Isothermal
DSC Study
The degree of crosslinking density also affects the
isothermal reaction as it does in the nonisothermal operation due to the same reason. At any
curing temperature, aI or aS and DHiso decrease
with increasing weight ratio of EGDMA/HEMA
(Tables III–VI). The diffusional limitation
brought by the increasing degree of crosslinking
structure can leave more residual vinyl groups in
the network so the isothermal conversion and the
accompanying heat released will be reduced. The
overall conversion for the samples with the same
EGDMA content is elevated at higher reaction
temperature because both the monomers and
polymer network will have higher energy to relax
the limitation of diffusion.
The orders of the induction time (td ) and the
time at maximum rate (tm ) at any reaction temperature do not show any explicit dependence on
the EGDMA content, but the difference between
them ( Dt Å tm 0 td ) shows a regular pattern as
it is largely reduced to a similar value, which is
independent of the weight ratio, with the presence
of EGDMA (Tables III–VI). The reason for this
behavior is not clear at this moment, but it may
be also related to the diffusional limitation
brought by the presence of a crosslinked structure
and the suppressed termination of free radicals.
Crosslinking can restrict the relative movement of macromolecular chains in a polymer. In
a consequence, gelation in a polymerizing medium
can occur at a lower conversion. The conversion
at maximum reaction rate ( am ) decreases with
increasing weight ratio of EGDMA/HEMA at any
isothermal reaction temperature (Tables III–VI).
On the other hand, a higher curing temperature
will largely enhance the reaction rate, and the
propagation of the polymerization may become
diffusion limited at lower degree of gelation.
Therefore, the conversion at maximum reaction
rate ( am ) for the samples with the same EGDMA
content decreases with increasing reaction temperature.
The effect of EGDMA content on the modified
maximum rate Rpm is a complicated matter and
the effect also depends on the reaction temperature. There are two opposite effects which determine the reaction rate. The crosslinking facili-
/ 8G42$$0239
05-30-97 07:23:20
Figure 12. The modified maximum reaction rate of
the first peak, Rpm , as a function of the weight percentage of EGDMA in an isothermal run. Reaction temperature: ( l) 607C, (j) 707C, (l) 807C, (m) 907C.
tated gel effect can enhance the reaction rate due
to reduced termination reaction, and a highly
crosslinked structure will decrease the reaction
rate due to limited mobility of unreacted monomers or pendants. At 607C, the exothermic peaks
for all the samples of different EGDMA contents
are bell-shape without any sign of a shoulder or
second peak (Figures 5–8). The modified maximum rate Rpm increases monotonically as the EGDMA content increases (Fig. 12). Probably the
reaction rate is only enhanced by the crosslinking
facilitated gel effect and the formation of crosslinked structure is not ample enough, due to the
lower conversion at this temperature, to retard
the diffusion of reactants to the active sites of
living radicals. At 707C, the shape of the exothermic peaks for the samples of different EGDMA
contents are not consistently the same (Figures
5–8), and the dependence of the modified maximum rate Rpm on EGDMA content is not distinct
(Fig. 12). At 80 and 907C, the exothermic peak
tends to carry a shoulder and then split to two
peaks as the EGDMA content increases (Figs. 5–
8). The modified maximum rate Rpm first increases as the EGDMA content changed from 0 to
10% (wt.) then decreases as the EGDMA content
further increases (Fig. 12). The reaction rate may
first be enhanced by the gel effect then be retarded
by the slow diffusion of reactants. When the EGDMA content is higher than 10% (wt.), the reduction of the maximum reaction rate in the first
polca
W: Poly Chem
SYNTHESIS OF PHEMA WITH EGDMA
peak results in the formation of a second minor
peak in the rate profile. The second minor peak
becomes more distinct with increasing EGDMA
content (Figs. 5–8).
Kinetic Models and Activation Energies
The free radical polymerization of monovinyl
(HEMA) and divinly (EGDMA) monomers is
thermosetting and autoaccelerated so that it can
be described by eq. (1). The rate constant (k) follows the Arrhenius equation:
k Å A exp(0E/RT )
(7)
ln k Å ln A 0 E/RT
(8)
where A is pre-exponential factor, E is activation
energy, R is gas constant, and T is absolute curing
temperature. To a good approximation, the isothermal curing of a free radical polymerization
can be assumed to be a second order reaction.21–23
Mathematically, this is expressed as
m/nÅ2
(9)
Substituting eq. (9) into eq. (1) and rearranging
the equation, 23 one can obtain
ln
F
da /dt
a2
G
Å ln k / n ln
F
10a
a
G
(10)
Applying this equation to the isothermal rate data
of each run, we can obtain the rate constant k and
corresponding reaction orders. The results are
shown in Table VII. It was found that the reaction
orders changed with temperature, so the averaged
values of them were calculated. The activation
energy of the reaction for each formulation was
determined by eq. (8) in an Arrhenius plot. The
results along with the pre-exponential factor and
the square of the linear regression coefficient are
given in Table VIII. Simulated rate profiles by
using eq. (1) with the averaged m and n in Table
VII and the Arrhenius parameters in Table VIII
are plotted accompanying the experimental rate
profiles in Figures 5–8. Because the first-order
differential equation of eq. (1) needs an initial
condition to start the integration, an nonzero initial conversion ( a ) has to be defined at the td (induction time) for each run, otherwise the calculated reaction rate will be zero all the time. The
initial conversions were arbitrarily chosen as
/ 8G42$$0239
05-30-97 07:23:20
1885
0.001, 0.0015, 0.005, and 0.02 for reaction temperatures of 60, 70, 80, and 907C, respectively, and
they were the same for samples of different compositions. Variation of the initial conversion will
shift the predicted tm (time at maximum rate) to
a different time, but it does not affect the shape
and the maximum rate of the predicted profile for
each run (Fig. 13). Fairly good agreement, at
least in the same order of magnitude, between the
simulated and the experimental rate profiles can
be observed. Some deviation between them are
unavoidable due to the limitation of the model,
and other poor predicted results can be explained
by the large standard deviations of the averaged
reaction orders and the less satisfactory values of
the square of regression coefficients in fitting eqs.
(8) and (10). Nevertheless, the analysis demonstrates that the simple autoaccelerated kinetic
model can roughly follow the process of the curing
reaction but the model cannot describe the nonideal shapes (shoulder or the second peak) of the
experimental rate profiles and predict the induction time (td ), the time at maximum rate (tm ), or
the difference between them ( Dt) for each run.
On the other hand, the nonisothermal DSC
data can give the activation energy of the polymerization reaction without assuming an exact
rate expression in prior provided that the conversion at maximum rate ( ap ) is independent of heating rate ( f ). Based on the work of Ozawa 40,41 and
Doyle, 42,43 a simple relationship between activation energy, heating rate, and temperature at
maximum rate (Tp ) is given as 36 :
EÉ
0R d ln f
1.052 d(1/Tp )
(11)
where R is the gas constant. The activation energy
is for the rate constant of an arbitrary kinetic
form36 :
da
Å k f (a )
dt
(12)
where f ( a ) is an empirical function representing
the conversion-dependent part of the rate expression. By plotting ln f vs. (1/Tp ), the activation
energy (E) can be determined from the slope. The
results are shown in Table IX.
The effect of the EGDMA content on the activation energies obtained from the isothermal and
non-isothermal studies was not consistent (Table
VIII and IX). In the nonisothermal curing, the
polca
W: Poly Chem
1886
HUANG, SUN, AND HUANG
Table VII. Rate Constants (k), Reaction Orders (m and n), and the Square of Regression Coefficient (r 2) for the
EGDMA/HEMA Curing Reaction at Different Isothermal Temperature
Isothermal
Temperature
(7C)
k
(s01)
m
r2
n
EGDMA/HEMA Å 0/10
60
70
80
90
average
2.41
4.28
7.57
1.32
{
{
{
{
0.04
0.06
0.58
0.03
1
1
1
1
1003
1003
1003
1002
1.20
1.08
0.96
0.87
1.03
{
{
{
{
{
0.05
0.00
0.03
0.01
0.12
0.80
0.92
1.04
1.13
0.97
{
{
{
{
{
0.05
0.00
0.03
0.01
0.12
0.989
0.987
0.986
0.987
{
{
{
{
0.001
0.001
0.003
0.002
0.97
1.05
1.11
1.32
1.11
{
{
{
{
{
0.01
0.04
0.05
0.02
0.13
0.982
0.988
0.996
0.994
{
{
{
{
0.001
0.001
0.003
0.001
1.00
1.07
1.19
1.44
1.17
{
{
{
{
{
0.05
0.02
0.04
0.01
0.17
0.980
0.985
0.997
0.989
{
{
{
{
0.005
0.012
0.001
0.000
1.04
1.06
1.21
1.40
1.18
{
{
{
{
{
0.02
0.00
0.02
0.04
0.14
0.986
0.986
0.996
0.989
{
{
{
{
0.011
0.002
0.001
0.003
EGDMA/HEMA Å 1/9
60
70
80
90
average
2.47
5.30
8.59
1.89
{
{
{
{
0.02
0.28
0.50
0.13
1
1
1
1
1003
1003
1003
1002
1.03
0.95
0.89
0.68
0.89
{
{
{
{
{
0.01
0.04
0.05
0.02
0.13
EGDMA/HEMA Å 2/8
60
70
80
90
average
2.89
5.43
7.44
1.70
{
{
{
{
0.09
0.38
0.09
0.17
1
1
1
1
1003
1003
1003
1002
1
1
1
1
03
1.00
0.93
0.81
0.56
0.83
{
{
{
{
{
0.05
0.02
0.04
0.01
0.17
EGDMA/HEMA Å 3/7
60
70
80
90
average
2.67
5.60
7.65
1.57
{
{
{
{
0.06
0.13
0.19
0.10
10
1003
1003
1002
activation energy increased as the EGDMA content increased. In the isothermal curing, the activation energy increased first as the EGDMA content increased from 0 to 10% (WR Å 0/10 to 1/
9), and then dropped to a similar lower value
as the amount of EGDMA further increased. The
results of nonisothermal curing suggest that the
0.96
0.94
0.79
0.60
0.82
{
{
{
{
{
0.02
0.00
0.02
0.04
0.14
presence of a crosslinking agent facilitates the gel
effect so that the reaction becomes more sensitive
to temperature and the activation energy increases. In the isothermal curing, the results of
samples with lower EGDMA content (WR Å 0/10
and 1/9) also agreed with this suggestion. The
increment of the activation energy was about the
Table VIII. Arrhenius Parameters for the Rate Constant Based on eq. (1): Activation Energy (E),
Pre-exponential Factor (A), and the Square of Linear Regression Coefficient (r 2) of the Analysis
EGDMA/HEMA
E (kJ/mol C|C)
A (s01)
r2
/ 8G42$$0239
0/10
1/9
2/8
3/7
57.2
2.09 1 106
0.9997
66.2
5.99 1 107
0.991
56.5
2.04 1 106
0.969
56.6
2.05 1 106
0.978
05-30-97 07:23:20
polca
W: Poly Chem
SYNTHESIS OF PHEMA WITH EGDMA
Figure 13. Effect of the initial conversion on the simulated heat release profile for a sample of EGDMA/
HEMA Å 1/9 cured at 607C. Curves from left to right:
initial conversion Å 1 1 10 02 , 1 1 10 03 , 1 1 10 04 , 1
1 10 05 , and 1 1 10 07 , respectively, while the induction
time td is assumed to be 0 min.
same as the weight ratio changed from 0/10 to 1/
9 in both of the isothermal and non-isothermal
operations. However, the results of samples with
higher EGDMA content (WR Å 2/8 and 3/7) disagreed with it.
The disagreement is caused by the highly crosslinked network structure, which limits the mobility of reactants toward living free radicals. In an
isothermal curing, the rate data were used for the
determination of reaction rate constant [eq. (11)].
However, the rate is limited by the diffusion-controlled propagation step in the highly crosslinked
structure as we discussed previously. The rate
constants obtained in the cases of WR Å 2/8 and
3/7 were smaller than those obtained in the case
of WR Å 1/9 at 80 and 907C (Table VII). In a
consequence, the activation energies obtained in
the former two cases were smaller than the one
1887
obtained in the later one. Furthermore, the activation energies obtained in the former two cases
might not be adequate because the regression coefficients was not good enough (r 2 õ 0.98). The
reaction may encounter different level of diffusion
limitation at different isothermal reaction temperature in these two cases. On the contrary, the
calculation of activation energy from non-isothermal data only requires the peak temperature and
heating rate, and it is performed under a criteria
that the conversion at the peak is the same for
different heating rate. The same conversion guarantees that the level of diffusion limitation of
monomers is the same when the activation energy
is determined. High quality linear regression was
obtained in all the cases with various EGDMA/
HEMA ratio as the square of the regression coefficient was higher than 0.99.
The activation energy is determined by a combination effect of the intrinsic chemical reaction
rate and the mass transfer resistance of reacting
species in the system. The activation energies for
the cases of WR Å 0/10 and 1/9 measured in nonisothermal curing is about 10% higher than those
obtained in isothermal curing. These results are
similar to the results reported by Prime 19 who
studied the curing kinetics of epoxy resins by
DSC. The unknown kinetic form in eq. 12 may be
different from that in eq. (1) possibly due to a
different level of diffusional resistance in these
two types of curing operations. The diffusion of
reactants within the system may become the rate
limiting step as the reaction temperature goes
higher. All the temperatures at maximum rate
were higher than 957C in a nonisothermal operation, and the curing temperature was between 60
and 907C in an isothermal operation. The diffusion limitation may not be the same for the same
sample at different reaction temperature; therefore, difference in the results of activation energy
is observed due to the different thermal history
experienced in each curing operation.
Table IX. Arrhenius Parameters for the Rate Constant Based on eq. (12): Activation Energy (E) and the
Square of Linear Regression Coefficient (r 2) of the Analysis
EGDMA/HEMA
E (kJ/mol C|C)
r2
/ 8G42$$0239
0/10
1/9
2/8
3/7
63.6
0.9993
72.4
0.991
75.3
0.997
78.3
0.998
05-30-97 07:23:20
polca
W: Poly Chem
1888
HUANG, SUN, AND HUANG
CONCLUSIONS
The effect of EGDMA content on the curing kinetics of the synthesis of PHEMA hydrogels has been
studied by DSC over the entire conversion range
and FTIR for the cured product. The presence of
EGDMA may promote the reaction rate initially
due to a crosslinking-facilitated gel effect but may
retard the diffusion of reactants and leave unreacted vinyl groups (trapped free monomers or
pendants) within the system at later stage. The
exothermic peaks in the rate profiles of the curing
tend to carry a shoulder and then split into two
peaks as the amount of EGDMA increases. Slower
heating rate in the nonisothermal curing and
higher reaction temperature in the isothermal
curing will promote the formation of two distinct
peaks. The reaction of those trapped free monomers and pendant vinyl groups on the main chain,
possibly due to a zip propagation mechanism, is
responsible for the formation of the shoulder or
second peak in the rate profiles. The heat of reaction measured by DSC in the scanning mode is
61.2 kJ/mol C|C, which is almost independent
of the heating rate, and it shifts to a lower value
only when the weight ratio of EGDMA/HEMA is
3/7 due to limited final conversion. The heat released of a sample cured in an isothermal run is
always lower than that in a non-isothermal scanning run indicates that the reaction is never complete during an isothermal curing at temperature
of 907C or less. Both the DSC and FTIR measurements demonstrate that the conversion ( aI or aS )
of vinyl groups decreases as the weight ratio of
EGDMA/HEMA increases and the reaction temperature decreases in an isothermal run. The temperature at maximum rate (Tp ) and the conversion at maximum rate ( ap ) in a nonisothermal
curing and the conversion at maximum rate ( am )
in a isothermal curing decrease with increasing
EGDMA content due to the crosslinking facilitated gel effect. In the nonisothermal curing and
the isothermal curing at higher temperature (80
and 907C), the maximum rate (Rp or Rpm ) can be
enhanced first due to the same facilitated gel effect and then be reduced due to the diffusional
limitation generated by the crosslinking structure
as the amount of EGDMA increases. In an isothermal run, the induction time (td ) and the time at
maximum rate (tm ) do not show any regular pattern with the weight ratio of EGDMA, but the
difference between them ( Dt Å tm 0 td ) reduces
to a relatively constant value with the presence
of EGDMA. The Kamal model can generally follow
/ 8G42$$0239
05-30-97 07:23:20
the isothermal curing reaction; however, it cannot
predict the time events (td , tm , and Dt) and describe the nonideal rate profiles in detail. As the
content of EGDMA increases from 0 to 30%, the
activation energy (E) of the curing reaction in the
non-isothermal operation increases monotonically
from 63.6 to 78.3 kJ/mol C|C, and that in the
isothermal operation changes from 57.2 (WR Å 0/
10) to 66.2 (WR Å 1/9) kJ/mol C|C and drops
to about 56.5 kJ/mol C|C (WR Å 2/8 or 3/7).
Diffusion-controlled reaction rate may cause a deviation in activation energy measurement in the
isothermal curing of the sample with WR Å 2/8
or 3/7. The different thermal history experienced
in the isothermal and nonisothermal curing can
cause a discrepancy in the values of the activation
energy measured in these two operations. This
study provides us with ample information about
the kinetic behaviors of the PHEMA synthesis either qualitatively or quantitatively, which will be
useful for the design of a fabrication process for
the production of hydrogels in many different
forms for biomedical applications. Although this
study avoids thorough analyses of the microgel
formation, 28,44 mechanistic kinetic modeling, 24,32–34
and quantifying the diffusion limitation, 32,45 it
will benefit the future studies of those subjects.
This work was supported by a joint research grant
NSC-83-0425-B-155-001-M08 from the National Science Council of the Republic of China and the Standard
Chemical and Pharmaceutical Co.
REFERENCES AND NOTES
1. O. Wichterle, in Encyclopedia of Polymer Science
and Technology, Vol. 15, H. F. Mark, N. G. Gaylord,
and N. M. Bikales, Eds., Interscience, New York,
1971, p. 273.
2. B. D. Ratner and A. S. Hoffman, in Hydrogels for
Medical and Related Applications, J. D. Andrade,
Ed., ACS Symposium Series 31, American Chemical Society, Washington, DC, 1976, p. 1.
3. N. A. Peppas, Ed., Hydrogels in Medicine and Pharmacy, Vols. I–III, CRC Press, Boca Raton, FL,
1986–1987.
4. V. Kudela, in Encyclopedia of Polymer Science and
Engineering, Vol. 7, 2nd ed., H. F. Mark, N. M. Bikales, C. G. Overberger, G. Menges, and J. I.
Kroschwitz (eds.), Wiley, New York, 1987, p. 783.
5. S. H. Gehrke and P. I. Lee, in Specialized Drug
Delivery Systems, Manufacturing and Production
Technology, P. Tyle, Ed., Dekker, New York, 1989,
p. 333.
polca
W: Poly Chem
SYNTHESIS OF PHEMA WITH EGDMA
6. J.-P. Montheard, M. Chatzopoulos, and D. Chappard, J. M. S.-Rev. Macromol. Chem. Phys., C32,
1 (1992).
7. N. R. Vyavahare, M. G. Kulkarni, and R. A. Mashelkar, J. Membrane Sci., 54, 205 (1990).
8. P. J. Skelly and B. J. Tighe, Polym. Commun., 20,
1051 (1979).
9. M. Chang, M. Colvin, and A. J. Rembaum, J.
Polym. Sci. Polym. Lett. Ed., 24, 603 (1986).
10. A. Denizli, A. Y. Rad, and E. Piskin, J. Chromatography, B: Biomedical Appl., 668, 13 (1995).
11. D. Horak, F. Svec, J. Kalal, K. Gumargalieva, A.
Adamyan, N. Skuba, M. Titova, and N. Trostenyuk,
Biomaterials, 7, 188 (1986).
12. P. I. Lee and C.-J. Kim, J. Controlled Rel., 16, 229
(1991).
13. I. Orienti, V. Bertasi, and V. Zecchi, J. Controlled
Rel., 22, 159 (1992).
14. K. F. Mueller, S. J. Heiber, and W. L. Plankl, U.S.
Pat. 4,224,427 (1980).
15. A. B. Scranton, A. G. Mikos, L. C. Scranton, and
N. A. Peppas, J. Appl. Polym. Sci., 40, 997 (1990).
16. A. G. Andreopoulos, J. Appl. Polym. Sci., 45, 1005
(1992).
17. Y.-M. Sun and J.-N. Chang, J. Polym. Res., 2, 71
(1995).
18. K. Horie, H. Hiura, M. Sawada, I. Mita, and H.
Kambe, J. Polym. Sci. A-1, 8, 1357 (1970).
19. B. Prime, Polym. Eng. Sci., 13, 365 (1973).
20. M. R. Keenan, J. Appl. Polym. Sci., 33, 1725
(1987).
21. M. R. Kamal and S. Sourour, Polym. Eng. Sci., 13,
59 (1973).
22. K.-W. Lem and C. D. Han, Polym. Eng. Sci., 24,
175 (1984).
23. P. W. K. Lam, Polym. Composites, 8, 427 (1987).
24. Y. J. Huang, J. D. Fan, and L. J. Lee, Polym. Eng.
Sci., 30, 684 (1990).
25. P. E. Willard, Polym. Eng. Sci., 12, 120 (1972).
/ 8G42$$0239
05-30-97 07:23:20
1889
26. A. Thakur, A. K. Banthia, and B. R. Maiti, J. Appl.
Polym. Sci., 58, 959 (1995).
27. S. B. Sastri, J. P. Armistead, and T. M. Keller, Polymer, 36, 1449 (1995).
28. Y. Y. Chiu and L. J. Lee, J. Polym. Sci. Part A:
Polym. Chem., 33, 257 (1995).
29. G. Odian, Principles of Polymerization, 3rd ed., Wiley, New York, 1991, chapter 3, p. 198.
30. K. Horie, I. Mita, and H. J. Kambe, J. Polym. Sci.
A-1, 7, 2561 (1969).
31. K. Horie, I. Mita, and H. J. Kambe, J. Polym. Sci.
A-1, 8, 2839 (1970).
32. G. L. Batch and C. W. Macosko, J. Appl. Polym.
Sci., 44, 1711 (1992).
33. J. F. Stevenson, Polym. Eng. Sci., 26, 746 (1986).
34. H. Ng and I. Manas-Zloczower, Polym. Eng. Sci.,
29, 1097 (1989).
35. M. R. Kamal, S. Sourour, and M. E. Ryan, SPE
ANTEC Tech. Papers, 19, 187 (1973).
36. B. R. Prime, in Thermal Characterization of Polymeric Materials, E. A. Turi, Ed., Academic, New
York, 1981, chapter 5, p. 435.
37. E. H. Schacht, in Recent Advances in Drug Delivery
Systems, J. M. Anderson and S. W. Kim, Eds., Plenum, New York, 1984, p. 259.
38. D. E. Gregonis, C. M. Chen, and J. D. Andrade, in
Hydrogels for Medical and Related Applications,
J. D. Andrade, Ed., ACS Symposium Series 31,
American Chemical Society, Washington, DC,
1976, p. 88.
39. Y. J. Huang and C. J. Chen, J. Appl. Polym. Sci.,
46, 1573 (1992).
40. T. Ozawa, Bull. Chem. Soc. Jpn., 38, 1881 (1965).
41. T. Ozawa, J. Therm. Anal., 2, 301 (1970).
42. C. D. Doyle, J. Appl. Polym. Sci., 5, 285 (1961).
43. C. D. Doyle, J. Appl. Polym. Sci., 6, 639 (1961).
44. Y. Y. Chiu and L. J. Lee, J. Polym. Sci. Part A:
Polym. Chem., 33, 269 (1995).
45. Y. J. Huang and L. J. Lee, AIChE J., 31, 1585
(1985).
polca
W: Poly Chem