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. 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