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Polymer International 39 (1996) 113-119
Characterization of
N.N- Dimethylacrylamide/2- Methoxyethylacrylate Copolymers and Phase
Behaviour of their Thermotropic Aqueous
Solutions
Ali A. S. El-Ejmi & Malcolm B. Huglin*
Department of Chemistry and Applied Chemistry, University of Salford, Salford M5 4WT, UK
(Received 5 July 1995; revised version received 23 August 1995; accepted 9 September 1995)
Abstract: Dimethylacrylamide (DMA) has been copolymerized with 2methoxyethylacrylate (MOEA) and solutions of the products were analysed
by FTIR to yield derived reactivity ratios rDMA= 1.11 0.13 and rMOEA=
0.63 f 0.10. The measured glass transition temperatures T, of PDMA and
PMOEA were 395 K and 242 K, respectively. These and the values of T, for the
copolymers accorded well with the Fox relationship. Cloud point curves for
copolymers in water were established over a wide range of concentration, solubility decreasing with increase in temperature. For these reversibly thermotropic
solutions, the lower critical solution temperature (LCST) increased from 9°C to
80°C with decrease in content of MOEA in the copolymer from 91.1 mol% to
38.6 mol%.
Key words: dimethylacrylamide, 2-methoxyethylacrylate, copolymer, reactivity
ratios, lower critical solution temperature.
I NT R 0 DUCTI0 N
the LCST has been effected by the use of linear copolymers of NIPAM with acrylamide or N-alkyl substituted
a ~ r y l a m i d e s .The
~ . ~ latter comonomers have also been
copolymerized with alkyl or alkoxy alkylacrylates and
met ha cry late^.^.^ A related effect of deswelling of thermally reversible crosslinked polymers on heating to the
LCST has been studied in detail, particularly for hydrogels of PNIPAM.*-'O
Of particular interest are the linear copolymers of
dimethylacrylamide (DMA) with 2-methoxyethylacrylate (MOEA) for which some LCSTs have been
reported, but only from observations at one particular
copolymer c o n c e n t r a t i ~ n In
. ~ ~the
~ present communication the aims are (a) to establish complete cloud point
curves for such copolymers in water; (b) to extend the
range of copolymer composition; (c) to investigate the
general solubility characteristics of these polymers in
liquids other than water; and (d) to determine the
In addition to the common upper critical solution temperature as demarcation between two phases and one
phase on heating, there is also an entropically controlled phenomenon of solubility decreasing with
increase in temperature, the boundary being at the
lower critical solution temperature, LCST. A rationale
for this phenomenon has been given clearly by Cowie,'
and some potential applications exploiting the LCST
for aqueous polymer solutions have been indicated in a
patent by Mueller.'
Aqueous solutions of poly(N-isopropyl acrylamide)
(PNIPAM) comprise the most widely investigated thermally reversible
and flexibility in altering
* To whom correspondence should be addressed.
113
Polymer International 0959-8103/96/$09.00 0 1996 SCI. Printed in Great Britain
114
A. A. S . El-Ejmi, M . B. Huglin
TABLE 1. Copolymer data for copolymerization of D M A (1) with MOEA (2)
Copolymer
fl
F,
c1
c2
c3
c4
c5
C6
c7
C8
c9
c10
0.074
0.1 29
0.248
0.363
0.467
0,568
0.663
0.754
0,840
0.922
0.089
0.187
0.320
0.447
0.551
0.61 4
0.71 5
0.787
0.859
0.923
A,83S/(A,635
+A,,,,)
hitherto unreported monomer reactivity ratios as a
guide to those wishing to prepare copolymers of desired
composition.
0.1 11
0.227
0.377
0.51 2
0.61 4
0.674
0.764
0.825
0.884
0.933
Conversion
(wtY0)
Time
(min)
13.3
10-4
16.5
12.2
17.9
13.5
20.6
18.9
14.5
14.4
25
20
28
23
20
15
13
15
15
8
dried to constant weight in uucuo at ambient temperature.
Copolymer composition
EXPERIMENTAL
Materials
DMA (Aldrich Chemical Co., 99% purity grade) was
distilled at reduced pressure (54-55"C, 10.8 mm Hg).
MOEA (Lancaster Synthesis Ltd) was similarly purified
(21-22"C, 0.25-0.30 mm Hg). 2,2'-Azobisisobutyronitrile (AIBN) (Fluka) yielded a m.p. of 103-8°C after
recrystallization from ethanol. Toluene and chloroform
were dried over anhydrous magnesium sulphate and
distilled at atmospheric pressure.
Homopolymerization
DMA and MOEA were each polymerized in toluene
at 60°C using [AIBN] z 5 x 10-3moldm-3 and
[monomer] w 2 mol dm- '.
Polymerization was conducted under gaseous nitrogen to 15-20% conversion and polymers were precipitated in n-hexane or ethyl ether. Purification was
effected by dissolution in toluene followed by reprecipitation in diethyl ether for PDMA and in n-hexane for
PMOEA. Polymers were dried to constant weight in
uucuo at ambient temperature.
Compositions of copolymers were obtained from FTIR
measurements (Perkin-Elmer 1710) in solutions in
chloroform after previous calibrations with blends of
the homopolymers in the same solvent. For both calibration and measurements a fixed concentration of 3 g
per lOOg solution was used. Other details are as
reported previously."
Glass transition temperature
Glass transition temperatures Tg were measured for the
copolymers and the two homopolymers by DSC, using
a Mettler TA300 instrument at a heating rate of
lOKmin-' and subjecting all samples to the same
thermal history. T, was located by the mid-point
criterion.
Solubility
The miscibility of the liquid monomers and the solubility of polymers was assessed roughly by visual examination after addition of 2ml of a liquid to c. 0-02g of
monomer or polymer after leaving overnight at ambient
temperature. The 13 liquids examined covered a wide
range of solubility parameters 6 ranging from 6 =
14.9(MPa)'/' (n-hexane) to 6 = 47.9 (MPa)'l2 (water).
Copolymerization
Cloud points
Monomer mixtures in toluene covering a wide range of
accurately known composition were copolymerized
under gaseous nitrogen at 60°C, the concentration of
AIBN and the total concentration of monomers being c.
5 x
m ~ l d m - and
~
2 * 2 m 0 l d m - ~ respectively.
After precipitation in n-hexane, dissolution in toluene
and reprecipitation in n-hexane, the copolymers were
Cloud point curves were determined for solutions of
copolymers in water, using a previously adopted procedure." Onset of faint turbidity was noted visually on
heating and reversal to transparency on cooling, both
the solution in the tube and the external bath of water
being stirred magnetically. The highest initial copolyPOLYMER INTERNATIONAL VOL. 39, NO. 2, 1996
115
Characterization of N,N-dimethylacrylamide/2-methoxyethylacrylate
copolymers
RESULTS AND DISCUSSION
TABLE 2. Monomer reactivity ratios and methods
of determination
Copolymer composition
Method
rl
r2
1.1 0 f 0.13
1.11 f0.14
1.11 f0.13
K-T
EX. K-T
M-H
From the FTIR calibration a second-order relationship
of the following form was established between absorbance ratio and composition :
0.65f 0.10
0.63f 0.11
0.63f 0.10
+ A,,,,)
A1635/(A1635
= 0.00199
+ 1 . 2 5 ~ 1- 0.253~1
(1)
mer concentration was a weight fraction of c. 0.15, subsequent dilutions being made by weighed addition of
water. Heating and cooling were controlled to
0.2Kmin-’
In eqn (1) A1635 is the absorbance due to the carbonyl
group in DMA units, A,,,, is the absorbance due to
the carbonyl group in MOEA units and w 1 is the
0.8
0.75
0.75
0.70
0.7
r2
0.6:
r2
0.65
0.60
0.6
0.55
0.55
0 9
05
0.90
0.95
1.
1.05
1.1
1.15
13
135
13
0.95
1
1-05
1.1
1.15
1.2
135
ri
‘1
Fig. 1. Joint confidence intervals for the reactivity ratios in DMA (1)-MOEA (2) copolymerization calculated according to (a) the
K-T method, (b) the Ex. K-T method, and (c) the M-H iterative procedure.
POLYMER INTERNATIONAL VOL. 39, NO. 2, 1996
A. A . S. El-Ejmi, M . B. Huglin
116
weight fraction of DMA units in the blend. From the
measured absorbance ratio for a copolymer, w , (and
hence the corresponding mole fraction F , ) was
obtained. Values of F , together with the mole fractions
fl of DMA in the initial feed mixture are listed in Table
1, which also contains details of the corresponding
copolymerization conditions at 60°C.
for this copolymerization or even for copolymerization
involving similar monomers.
Glass transition temperature
The plot of reciprocal of T, versus the weight fraction of
DMA (Fig. 2) shows good compliance with that predicted by the Fox e q ~ a t i 0 n . The
l ~ points for the two homopolymers fall well on the line. The values of w1 were of
course obtained by FTIR. However, for this system the
difference between T values of the homopolymers is
very large; this, coupled with the good linearity, indicates that for samples of unknown composition, measurement of T could provide a convenient alternative
method of obtaining the copolymer composition. The
values found here for the T, of the homopolymers
PMOEA and PDMA are 242 K and 395 K respectively.
With regard to the former, the only value available
elsewherela for comparison is the lower one of 224K,
which was also measured by DSC but at various
heating rates and derived finally by extrapolation to
zero heating rate. Neither the present authors nor the
majority of workers adopt this device and we do not
wish to comment on whether it is an improved procedure. In general, however, the heating rate does not
have a large effect on the T,. It is possible that the low
value of 224K is a consequence of the low molecular
weight of the PMOEA sample used; however, no information on molecular weight was reported.18 The
Monomer reactivity ratios
The feed and copolymer composition data were treated
according to the Kelen-Tudos (K-T) method,' the
extended Kelen-Tiidos (Ex. K-T) method,14 and the
more recent iterative procedure devised by Mao &
Huglin (M-H).l5*I6 The values of the resultant reactivity ratios r , (DMA) and r2 (MOEA) are listed in Table
2, and the joint confidence intervals for the three procedures are shown in Fig. 1. The M-H procedure has
the advantage of being applicable to any conversion.
However, for the present systems this potential advantage is not fully exploited since all the conversions were
small-medium, the highest conversion being 20.6% for
sample C7.
Although the Ex. K-T and M-H methods yield
similar values of r , and r 2 , the uncertainty limits are
rather smaller for the M-H procedure. Since rlrz =
0-697 there is no azeotropic composition for this
copolymerization. As far as the authors can ascertain
there have been no previously reported reactivity ratios
4.5
4
4
3.5
0
<" 3
;
\
d
w
2.5
2
1.5
0
0.1
0.2
0.3
0.5
0.4
0.6
0.7 0.8 0.9
1
wI
Fig. 2. Reciprocal glass transition temperatures as a function of weight fraction w1 of DMA in DMA-MOEA copolymers.
POLYMER INTERNATIONAL VOL. 39, NO. 2, 1996
117
Characterization of N,N-dimethylacrylamide/2-methoxyethylacryZate
copolymers
present value for the T, of PDMA lies in good accord
with the value of 391-394K obtained by Mohajer et
a l l 9 for a range of samples of low-medium isotactic
contents. Only for a sample of high (41%) isotacticity
was
found to be lower (385K). It is not uncommon
for the value of T, to be influenced by the technique
utilized. Thus even lower values of 362K and 383K
were obtained via dilatometry by Krause et aLZ0 Possible causes for the difference between these two values
were suggested without firm substantiation.
guide to workers wishing to conduct homopolymerization in homogeneous solution. Thus, for
polymerization of MOEA, toluene, THF, chloroform,
dioxan and DMF are suitable solvents, whereas for the
polymerization of DMA the number of suitable,
common liquids is somewhat greater and includes such
highly polar liquids as water, methanol and ethanol.
Appropriate precipitants are also evident to which can
be added petroleum ether (of ill-defined value of solubility parameter). The liquids capable of dissolving both
monomers are useful for copolymerization studies, but
these may not necessarily apply to the resultant copolymer as it is formed. For this purpose the results of
rough solubility tests on copolymers encompassing a
wide range of composition are given in Table 4, which
Solubility
The solubilities of the two monomers and their homopolymers are given in Table 3 as a rough but useful
TABLE 3. Solubility" of monomers and homopolymers in various
liquids at room temperature
Liquid
d(MPa)''*
n- Hexane
Diethyl ether
Cyclo hexane
Toluene
TH F
Chloroform
Acetone
Dioxan
Propan-2-01
DMF
Ethanol
Methanol
Water
" +, Soluble;
DMA
MOEA
+
+
+
+
+
+
+
+
+
+
+
+
+
+
14.9
15.1
16.8
18.2
18.6
19.0
20.3
20.5
23.5
24.8
26.0
29.7
47.9
+
+
+
+
+
+
+
+
+
+
PDMA
PMOEA
+
ss
+
+
+
ss
+
+
+
+
+
-, insoluble; ss, slightly soluble.
TABLE 4. Solubility" of DMA-MOEA copolymers in liquids having solubility
parameters given in Table 3. For compositions of copolymers C1-C10 refer t o
Table 1
Liquid
n-Hexane
Diethyl ether
Cyclo hexane
Toluene
TH F
Chloroform
Acetone
Dioxan
Propan-2-01
DMF
Ethanol
Methanol
Water
" +, Soluble;
C1
C2
C3
C4
C5
C6
C7
C8
C9
C10
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
ss
+
ss
ss
ss
ss
ss
ss
ss
ss
ss
ss
+
ss
ss
ss
ss
ss
ss
ss
ss
ss
ss
+
ss
ss
ss
ss
ss
-
-
+
+
+
+
+
+
+
+
+
+
-, insoluble; ss, slightly soluble.
POLYMER INTERNATIONAL VOL. 39, NO. 2, 1996
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
A. A . S . El-Ejmi, M . B. Huglin
118
heating (T,(h)) is c. 0.5K higher than that obtained on
cooling, T,(C). The latter is probably slightly more reliable due to the better control of slow cooling. However,
the average of T,(h) and T,(c) was used, an example of
its dependence on concentration being shown in Fig. 3
for copolymer C4. The general form of the cloud point
curve was similar for the other samples. The decrease of
T, with dilution is followed by quite a wide span of
further dilution within which T, remains constant. On
further dilution T, increases markedly. The minimum,
acts as a guide also to precipitants and liquids useful for
copolymer characterization (e.g. by NMR or FTIR) in
solution.
Cloud point curves
Cloud point curves were established for the six copolymers Cl-C6 of composition given in Table 1 in water.
In general the phase separation temperature noted on
49
u
.
+=
0
47
4s
8
4
0
wt %
12
16
copolymer
Fig. 3. Cloud point curve for copolymer C4 in water.
60
.
-
V
0
+v
30
0
l
35
t
4
~
l
8
,
,
l
75
55
mol%
,
,
,
l
’
95
MOEA
Fig. 4. Dependence of LCST on copolymer composition for poly(DMA-co-MOEA). Data reported in Ref. 7 are denoted by 0.
POLYMER INTERNATIONAL VOL. 39, NO. 2, 1996
Characterization of N,N-dimethyEacrylamide/2-methoxyethylacrylate
copolymers
constant value of T, was taken as the lower critical solution temperature T,. The dependence of T, on copolymer composition is shown in Fig. 4. We have succeeded
in covering what is probably the maximum practical
range of copolymer composition and T , , since PDMA
does not exhibit a T, in water (additional tests were conducted up to 225°C in sealed tubes) and PMOEA is
insoluble in water. The change in T, is very sensitive to
hydrophobicity and T, decreases most markedly with
MOEA content in the region of low-medium mol%
MOEA in copolymer. Although Mueller7 has represented the change as a linear one, this can only be the
situation within a restricted range of composition. The
data of Mueller have been included in Fig. 4, where it is
seen that, at corresponding copolymer compositions, his
values of T, are rather higher than those obtained here.
Although the FTIR method employed here is probably
more accurate than elemental analysis used by Mueller
for determining copolymer composition in this particular system, it would require significant changes in
copolymer composition to force accord between the two
sets of data. It seems most likely that the rather higher
values of T, obtained by Mueller can be attributed to
the fact that the cloud point curves were not established, the value of T, being identified with the value of
Tp observed at a single copolymer concentration of
1wt%. As is evident from Fig. 3, this low concentration
affords a higher T',, than the true T, manifested at higher
concentrations. The possible influence of molecular
weight is an aspect to be investigated.
ACKNOWLEDGEMENTS
A.A.S.E-E would like to thank the Libyan Secretary of
Education and the University of El-Fatah for their
POLYMER INTERNATIONAL VOL. 39, NO. 2, 1996
119
financial support. The authors are also indebted to Mr
R. Mao for kindly providing a computer program to
calculate the reactivity ratios.
REFERENCES
1 Cowie, J. M. G., Polymers; Chemistry and Physics of Materials.
Blackie and Son Ltd, Glasgow, 1991, p. 174.
2 Mueller, K. F., US Patent 5,057,560 (1991).
3 Dong, L. C. & Hoffman, A. S., J . Controlled Release, 4 (1986) 223.
4 Bae, Y. M., Okano, T. & Kim, S. W., J . Polym. Sci., Polym. Phys.
Ed., 28 (1990) 923.
5 Cole, C. A., Schreiner, S. M., Priest, J. H., Monji, N. & Hoffman,
A. S., in Reversible Polymer Gels and Related Systems, ed. P. S .
Russo. ACS Symposium Series 350, Washington, DC, 1987,
Chapter 17.
6 Priest, J. H., Murray, S. L., Nelson, R. J. & Hoffman, A. S., in
Reversible Polymer Gels and Related Systems, ed. P. S. Russo. ACS
Symposium Series 350, Washington, DC, 1987, Chapter 18.
7 Mueller, K. F., Polymer, 33 (1992) 3470.
8 Dong, L. C. & Hoffman, A. S., in Reversible Polymer Gels and
Related Systems, ed. P. S . Russo. ACS Symposium Series 350,
Washington, DC, 1987, Chapter 16.
9 Wu, X. S., Hoffman, A. S. & Yader, P., J . Polym. Sci., Part A,
Polym. Chem., 30 (1991) 2121.
10 Tanaka,T., Sci. Am.,244(1981) 110.
11 Mao, R., H u g h , M. B. & Davis, T. P., Eur. Polym. J., 29 (1993)
475.
12 Evans, J. M. & Huglin, M. B., Makromol. Chem., 141 (1969) 127.
13 Kelen, T. & Tiidos, F., J . Macromol. Sci.-Chem. A, 9 (1975) 1.
14 Tiidos, F., Kelen, T., Foldea-Berezsnich, T. & Turcsayni, B., J .
Macromol. Sci.-Chem. A, 10 (1976) 1513.
15 Mao, R., Huglin, M. B., Davis, T. P. & Overend, A. S., Polym. Int.,
31 (1993) 375.
16 Mao, R. & Huglin, M. B., Polymer, 34 (1993) 1709.
17 Fox,T. G., Proc. Am. Phys. SOC.,l(1956) 123.
18 Fischer, K. & Eisenbach, C. D., Makromol. Chem., Rapid Commun.,
9 (1988) 503.
19 Mohajer, Y., Wilkes, G. L. &. McGrath, J. E., J. Appl. Polym. Sci.,
26 (1981) 2827.
20 Krause, S., Gormley, J. J., Romans, N., Shetter, J. A. & Watanabe,
W. H., J . Polym. Sci. Part A, 3 (1965) 3573.
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