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Phase transition in swollen gels 24
Polymer Gels and Networks 6 (1998) 163—178
Phase transition in swollen gels 24: Effect of the concentration and structure of ionic comonomers on the collapse of poly(acrylamide) hydrogels Z. Sedla´kova´!, K. Bouchal!, M. Ilavsky´!,",* !Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, 162 06 Prague 6, Czech Republic "Faculty of Mathematics and Physics, Charles University, 180 00 Prague 8, Czech Republic Received 13 January 1998; received in revised form 18 March 1998; accepted 26 March 1998
Abstract The swelling and mechanical behaviour of ionized networks of copolymers of acrylamide (AAm), methylenebisacrylamide and sodium 2-[(3-carboxypyridine-4-carbonyl)oxy]ethyl methacrylate (PyNa, molar fraction of salt x "0—0.20) was investigated in water—acetone (w/a) S mixtures and in aqueous NaCl solutions. In the range x *0.01, a first-order phase transition S (collapse) was found in w/a mixtures; with increasing x , both the critical acetone concentration S a in the mixture at which collapse takes place and the extent of collapse (jump in the swelling # ratio X), Dlog X, increases. A comparison of these results with the results obtained for charged PAAm networks with sodium 2-[(2-carboxybenzoyl)oxy]ethyl methacrylate (BeNa) as ionic comonomer led to a conclusion that substituting benzoyl for pyridyl groups in the side chain slightly decreases Dlog X and the swelling degree in water, and does not change critical a values. On the other hand, a comparison with charged PAAm networks with sodium # methacrylate (MNa) as ionic comonomer showed that at constant x , the swelling degree in S water, the extent of collapse Dlog X and the concentration of elastically active network chains strongly decreased with the increased distance of the bound charge from the main chain (length of the side chain of the ionic comonomer). The expected decrease in the swelling degree of
*Correspondence address: Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, 162 06, Czech Republic. 0966-7822/98/$—see front matter ( 1998 Elsevier Science Ltd. All rights reserved. PII S 0 9 6 6 - 7 8 2 2 ( 9 8 ) 0 0 0 0 9 - 4
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ionized networks with increasing NaCl concentration was found. The experimental swelling data in w/a mixtures and in aqueous NaCl solutions could be described by a theory proposed previously if an effective degree of ionization i is introduced (i(x ). The mechanical behaviour S of gels in w/a mixtures is predominantly determined by the degree of swelling. On the other hand, different dependencies of the reduced modulus on the swelling ratio X found in w/a mixtures and in aqueous NaCl solutions in the high swelling region suggests that contracted gels with different microstructure were formed during gel deswelling in bad solvents or salt in water. ( 1998 Elsevier Science Ltd. All rights reserved.
1. Introduction Lightly crosslinked poly(acrylamide) (PAAm) gels with a low amount of charge on the chain (1—20 M%) prepared at high dilution show a first-order phase transition (collapse) when passing from a good (water) to a poor (acetone) solvent [1—4]. The appearance of this transition does not depend on the polarity of the charge on the chain introduced by copolymerization of AAm with variously charged comonomers [5,6]. On the other hand, it was found that while with the anionic comonomer sodium methacrylate (MNa), the collapse could be brought about by &1 M % of salt [3]. For the appearance of the collapse brought about by cationic comonomers, for example, by various quaternary ammonium salts, from three to ten times higher contents of ionic comonomers are necessary [7,8]. This drop in efficiency was mainly interpreted [7] by the large size of the ammonium group (which forms a weaker electrostatic field, because the hydration of the group is smaller) and by the influence of the counterion species on the hydration site (the hydration number of the anion Cl> is two and that of cation Na= is three). Furthermore, the positive charges in quaternary ammonium salts were localized at a greater distance from the main chain compared to MNa. The effect of the charge in an ionized crosslinker was also investigated [9,10]. In this case, the positive effect of increasing charge concentration on the collapse is compensated by the negative effect of the increasing crosslinking density. In this paper, the effect of concentration of the negative charge on the swelling and mechanical behaviour of PAAm networks, in which charge was introduced by copolymerization of AAm with sodium 2-[(3-carboxypyridine-4-carbonyl)oxy]ethyl methacrylate (PyNa), is investigated both in water/acetone mixtures and aqueous NaCl solutions. Moreover, by comparing these results and those obtained earlier [3,8] for ionized networks of AAm with MNa and of AAm with sodium 2-[(2carboxybenzoyl)oxy]ethyl methacrylate (BeNa), it is possible to specify the effect of distance of the charged group from the main chain, and the role of nitrogen in the pyridine ring (the distance of negative charge from the main chain in BeNa and PyNa is similar). As in copolymerization the proportion of participating comonomers changes with conversion, the chemical heterogeneity of the PyNa/AAm copolymer system was investigated by solution copolymerization in the absence of crosslinker. The results were compared with BeNa/AAm and MNa/AAm copolymer systems.
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2. Experimental 2.1. Preparation of 2-[(3-carboxypyridine-4-carbonyl)oxy]ethyl methacrylate To a mixture containing 0.1 mol of pyridine-3,4-dicarboxylic anhydride, 0.11 mol of 2-hydroxyethyl methacrylate and 0.01 wt% of 1,1-diphenyl-2-pikryl hydrazyl in 60 ml of benzene, 8 mmol of triethylamine was added with stirring. After 4 h, the reaction was terminated and the solvent was removed by distillation at 2.1 kPa. White crystals (m. p. 134°C) were obtained by recrystallization of the product from methanol. Yield: 89%. Elemental analysis gave the following results: calculated/found — %C 55.9/55.87; %H 4.69/4.62; %N 5.02/5.07. The structure was confirmed by 1H NMR and 13C NMR. 2.2. Copolymerization of acrylamide and sodium 2-[(3-carboxypyridine-4-carbonyl) oxy]ethyl methacrylate The sodium salt of 2-[(3-carboxypyridine-4-carbonyl)oxy]ethyl methacrylate (PyNa, ionic comonomer S) was prepared by hydrolysis with NaOH. The radical copolymerization of the binary system PyNa—acrylamide (AAm) was investigated in aqueous solution (volume fraction of both monomers was 0.1) with the aim of determining the monomer reactivity ratios. A series of copolymers was prepared by polymerization of monomer mixtures in which the molar fractions of PyNa, x (relative to both monomers) varied in the range 0.1—0.9. The PyNa content in the S copolymer, determined by elemental analysis, was compared with x . Using the S Kelen—Tu¨do¨s method [11,12] the monomer reactivity ratios r "0.30 (PyNa) and 1 r "0.37 (AAm) were determined. 2 2.3. Preparation of networks The ionic networks were prepared from 100 ml of an aqueous mixture which contained 5 g of acrylamide (AAm), 0.135 g of N,N@-methylenebisacrylamide (MBAAm, crosslinker), 0.02 g of ammonium peroxosulfate, 150 ll of N,N,N@,N@-tetramethylethylenediamine and various amounts of ionic comonomer, sodium 2-[(3carboxypyridine-4-carbonyl)oxy]ethyl methacrylate (PyNa, Scheme 1). Six networks were prepared with molar fractions of PyNa: x "0, 0.01, 0.02, 0.05, 0.10 and 0.20 S (Table 1). The polymerization proceeded at room temperature for 5 h in glass ampoules with diameter D*"10 mm. After polymerization, the samples were extracted in redistilled water.
0 0.01 0.02 0.05 0.10 0.20
1 2 3 4 5 6
26.4 33.1 44.0 47.2 55.4 58.8
G 1 (g cm~2)
2.81 3.52 4.68 5.02 5.89 6.26
105 l $ (mol cm~3)
— 0.6 0.75 0.78 1.00 1.25
Dlog X
— 0.30 0.32 0.40 0.47 0.62
Dlog G
— 45 48 52 58 70
a # (vol%) — — 45 48 55 65
a #,5 (vol%) 8
40 52 61 82 110 152
Q N!C-
40.0 39.0 35.5 34.6 31.6 33.1
Q
— 0.150 0.130 0.082 0.065 0.047
/
0.490 0.488 0.490 0.491 0.494 0.487
s N!C-
— 0.094 0.135 0.155 0.225 0.247
D
— — 0.090 0.115 0.160 0.185
D 5
Note x is the molar fraction of ionic comonomer PyNa; G is the modulus after preparation; l is the concentration of elastically active network chains relaS 1 $ tive to the dry state; D log X and D log G, respectively, are collapse parameters; a and a , respectively, are the experimental and theoretical critical # #,5 acetone concentrations in w/a mixtures at collapse; Q and Q , respectively, are the swelling degrees in water and 1 M NaCl aqueous solutions; / is the 8 N!Ccorrection factor, s is the interaction parameter; D and D , respectively, are the experimental and theoretical differences in volumes of coexisting phases at N!C5 collapse.
x S
Sample
Table 1 Network characteristics and collapse parameters
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Experimental data were compared with those obtained earlier for ionic networks from AAm and sodium methacrylate [13] (MNa, Scheme 2) and for networks from AAm and sodium 2-(2-carboxybenzoyl)oxy]ethyl methacrylate [8] (BeNa, Scheme 3). 2.4. Swelling and mechanical measurements The samples were swollen in 200 ml of water/acetone (w/a) mixtures containing 0—100 vol% of acetone or in 100 ml of an aqueous NaCl salt solution (molar concentrations c "10~5—1 mol/l). Swelling proceeded for 28 days, after which the inverse N!Cswelling ratio X was determined: X"(D /D)3"» /» (1) 1 1 where D and D are the diameters of samples after preparation and at equilibrium, 1 respectively, and » and » are the respective sample volumes. Diameters were 1 measured using an Abbe’s comparator with an accuracy of $0.001 mm. Using X values, the volume fraction of the polymer in the swollen state v can be easily 2 calculated (v "v X, where v is the volume fraction of the polymer at network 2 1 1 formation, v "0.037). The degree of swelling Q"1/v . 1 2 Deformation measurements were carried out on cylindrical samples (height &1 cm) in an unidirectional compression using an apparatus described earlier [13]. The sample was compressed to a ratio j"l/l , l and l being the deformed and the 0 0 initial height of the sample, respectively, and the force f was determined after 30 s relaxation; ten values of j and f in the range 0.7(j (1 were determined. The i i i equilibrium shear modulus G was calculated from [13] G"f /[S (j2!j~1)] (2) 0 where S is the initial cross-section of the sample. The mechanical measurements were 0 carried out on samples just after their preparation (modulus G , Table 1) or on the 1 equilibrium swollen samples in w/a mixtures or aqueous NaCl solutions (modulus G). 3. Results and discussion 3.1. Structure of the linear copolymers Using the Skeist equation [14,15] with the determined monomer reactivity ratios r "0.3 (PyNa) and r "0.37 (AAm) the dependence of the changes in the molar 1 2
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Fig. 1. Dependence of the molar ratio of ionic comonomers in P (ionic comonomer/AAm) chains, F (calculated from the Skeist equation) on the molar conversion of both monomers, m: PyNa (1), BeNa (2) 1 and MNa (3). The horizontal line corresponds to hypothetical uniform chains (no chemical heterogeneity).
content of the ionic comonomer in PAAm chains, F , on molar conversion of 1 monomers, m, was evaluated for a constant molar ratio of PyNa; x "0.1 in the S monomer mixture (Fig. 1). In a similar way, the changes in the distribution of copolymers composition for both BeNa/AAm and MNa/AAm copolymers were calculated for molar ratios of ionic comonomers x "0.1 using the corresponding S monomer reactivity ratios given in the literature [16]; these dependencies are also shown in Fig. 1. As follows from Fig. 1, in all three systems, the polymer chains with different contents of ionized comonomers (chemically heterogeneous) are formed with increasing conversion. While at the beginning of the reaction (0(m(0.5), the participation of the salts is higher than the average value, at the end of the reaction it is lower. The largest chemical heterogeneity is predicted for PyNa/AAm copolymers where one can expect formation of almost pure PAAm chains at full conversion. On the other hand, BeNa/AAm copolymers show the lowest chemical heterogeneity. Similar differences in chemical heterogeneity can be expected in crosslinked systems. 3.2. Swelling and mechanical behaviour of networks in w/a mixtures As can be seen from Fig. 2, the network with the lowest ionic comonomer concentration (x "0.01) already shows a phase transition with a stepwise change in S the swelling ratio X characterized by the extent of collapse, Dlog X"log XA!log X@ (where X@ is the swelling ratio of the expanded state and XA is the ratio of the collapse state of the gel at the transition, respectively, Table 1) at a critical acetone concentration in w/a mixture a "45 vol%. Only for the uncharged PAAm network with # x "0, is the dependence of the ratio X on acetone concentration a continuous. S
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Fig. 2. Dependence of the swelling ratio, X, and modulus G on the acetone concentration, a, in w/a mixtures for gels with indicated PyNa concentrations x : LX; dG. S
Figure 2 shows that Dlog X increases with increasing anionic comonomer concentrations x ; a similar increase in a values where the collapse takes place with concentraS # tion x , can also be seen in Fig. 2 (see also Table 1). S As expected, the dependence of the modulus G of the uncharged PAAm network on acetone concentration, a, is continuous (Fig. 2). All charged networks undergo collapse, as shown by the stepwise change in G, characterized by the value of Dlog G (Table 1). The jumpwise change in modulus, Dlog G, gives a linear correlation with the corresponding change in the swelling ratio, Dlog X, with a slope &0.5 (Dlog G &0.5]Dlog X, Fig. 3). This slope is higher than the theoretical rubber elasticity value [3] (s"0.33), observed for networks of AAm with quaternary ammonium salts [17]. It is interesting to note that, within experimental scatter, the Dlog G vs. Dlog X dependence is the same for networks with positively charged comonomers PyNa and BeNa. Networks with MNa show a slightly higher slope, s"0.75. The dependence of the reduced modulus, log G (G "G/G ), on the swelling ratio, 3 3 1 log X, for all samples swollen in w/a mixtures is shown in Fig. 4. As in Fig. 3, the slopes found for these dependencies are in the range s"0.5—0.6 for X(10. A similar
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Fig. 3. Dependence of the stepwise change in the modulus, D log G, on the corresponding change in the volume of the gel, D log X, at the transition; data for BeNa/AAm hydrogels were taken from Ref. [8] and those for MNa/AAm hydrogels from Ref. [13].
Fig. 4. Dependence of the reduced modulus, G "G/G , on the swelling ratio, X, for variously ionized 3 1 PyNa/AAm gels swollen in w/a mixtures.
slope (s"0.65) was found earlier for ionized MNa/AAm gels in w/a mixtures [13]. Deviations from linearity in the range of low swelling (log X'1) are probably due to the influence of the main transition region (vitrification) at high acetone concentrations [13]. It should be mentioned that the recent theory of Rubinstein et al. [18] which describes the swelling and mechanical behaviour of polyelectrolyte gels, based on the scaling approach, predicts a value of 5/6 for the slope s. From Figs. 3 and 4 it also follows that the mechanical behaviour of gels in w/a mixtures is predominantly determined by their degree of swelling. The efficiency of a crosslinking reaction in our networks can be evaluated from comparison of the experimental modulus G measured after network formation and 1
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added amount of crosslinker MBAAm to the system. If network was formed at the volume fraction of polymer v and measured at the volume fraction of polymer in the 1 swollen state v then, according to the rubber elasticity theory, we can write for the 2 modulus G measured in the swollen state at v the equation [13] 2 2@3 1@3 G"l R¹v v Sr2T /Sr2T (3) $ 1 2 * where l is the concentration of elastically active network chains (EANCs) related to $ the dry state, R is the gas constant, ¹ is temperature and Sr2T and Sr2T , respectively, * are the mean-square end-to-end chain distances in the isotropic and reference states. Since G has been determined at the state of network formation it follows that 1 Sr2T "Sr2T and v "v . From Eq. (3) we have [3,13] * 2 1 l "G /R¹v (4) $ 1 1 The value of the ideal theoretical EANCs concentration l , calculated from the $,5 amount of crosslinker considering that each MBAAm molecule gives two EANCs, is l "4.6]10~4 mol cm~3. The much lower experimental l values (Table 1) indicate $,5 $ a low efficiency of crosslinking reaction (&6—14%), which is mainly due to a high cyclization caused by high dilution (&95 vol%) at network formation [13]. From Table 1 and Fig. 5 (in Fig. 5, data obtained earlier [3,8,13] for MNa/AAm and BeNa/AAm hydrogels are also included), it follows that the crosslinking efficiency increases with ionic comonomer concentration x in all three copolymer systems. As S one can expect that, with increasing charge concentrations, more expanded conformations of chains are preferred, such extended conformations will decrease the probability of loop formation in dilute systems and will increase the efficiency of crosslinking reaction. Fig. 5 also shows that, in the first approximation, the increase in the efficiency is the same for PyNa/AAm and BeNa/AAm systems; this increase is much lower than that found for MNa/AAm networks. The slower increase in l with $ x found for PyNa and BeNa salts is probably due to the longer side chains of these S comonomers compared to the side chain of MNa. The dependence of the extent of collapse, Dlog X, and the critical acetone concentration, a , on the salt concentration x for all three systems is shown in Fig. 5. It is # S obvious that while the dependence of the critical acetone concentration, a , on x is # S practically the same for all three ionic comonomers, the extent of the collapse, Dlog X, is dependent on the detailed structure of salts. It can be seen that increasing the length of the side chains has a strong negative effect on the extent of collapse. The introduction of a nitrogen atom into the benzoyl ring also leads to a decrease in the Dlog X values, but this effect is much lower than the effect of the side-chain length. As expected, the swelling degree in water Q increases with increasing PyNa 8 concentration x (Table 1). From Fig. 6, in which Q data obtained with BeNa and S 8 MNa are also included, it can be seen that the swelling Q strongly depends on the 8 ionic comonomer structure. As in the case of the extent of collapse, Dlog X, the largest negative effect on Q occurs with increasing side-chain length. The charged PyNa 8 networks show the lowest Q values at constant x . There are probably two reasons 8 S for the lowest swelling degree of PyNa networks: (a) as follows from Fig. 1, they have the highest non-uniform distribution of the charged comonomer in the network
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Fig. 5. Dependence of the concentration of elastically active network chains, l , of the critical acetone $ concentration in w/a mixture at the collapse, a , and of the extent of the collapse, D log X, on the molar # fraction of salts x ; data for BeNa/AAm hydrogels were taken from Ref. [8] and those for MNa/AAm S hydrogels from Ref. [13].
structure (highest chemical heterogeneity); and (b) the nitrogen atom in the pyridine ring can lead to a betain structure (inner salt) which is less polar than the PyNa structure. Both points (a) and (b) are reflected in a decrease in the swelling degree Q of 8 PyNa hydrogels. 3.3. Swelling and mechanical behaviour in aqueous NaCl solutions Fig. 7 shows the dependence of the degree of swelling, Q, on aqueous NaCl concentration of the external solution, c (data shown at c "10~7 M were N!CN!Cmeasured in pure water). As expected, the degree of swelling depends on the amount of PyNa and the ionic strength of the solution. With increasing salt concentration, c , N!Cthe ionized gels deswell and at c "1 M all gels have roughly the same swelling N!Cdegree Q (Table 1). The pronounced decrease in swelling occurs between 10~4 and N!C-
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Fig. 6. Dependence of the degree of swelling in water, Q , and of the correction factor, / [calculated from 8 Eq. (4)], on the molar fraction of ionic comonomers x ; data for BeNa/AAm hydrogels were taken from Ref. S [8] and those for MNa/AAm hydrogels from Ref. [13].
Fig. 7. Dependence of the swelling degree, Q, on the aqueous NaCl concentration, c , for networks with N!Cindicated x values. — — — theoretical dependence from Eq. (5). S
10~1 M salt concentrations; increasing PyNa content in gel shifts this decrease to higher c values. The well-known deswelling behaviour of polyelectrolyte hydrogels N!Cwith increasing c concentration is due to the increasing concentration of mobile N!Ccounterions (Na`) from surrounding liquid in the gel phase (screening effect of
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Fig. 8. Dependence of the reduced modulus, G "G/G , on the ratio, X, for variously ionized PyNa/AAm 3 1 gels swollen in aqueous NaCl solutions.
counterions). This concentration equals to the amount of co-ions (Cl~) inside the gel. The dependence of the reduced modulus, G "G/G , on the swelling ratio X is 3 1 shown in Fig. 8 together with the line of slope s"0.5. The expected decrease in the reduced modulus G with increasing swelling was observed only in the range of low 3 swelling (log X'!0.3) and departures from linearity at high swelling, were found for the two most ionized networks with x "0.1 and 0.2. It should be mentioned that S for the gels swollen in w/a mixtures, no such increase in G values was found in the 3 same range of swelling (see Fig. 4). From Fig. 8 it follows that the mechanical behaviour of gels in aqueous NaCl solutions in the high-swelling region differ from that in w/a mixtures. 3.4. Comparison between theory of swelling equilibria and experiment for gels swollen in water/acetone mixtures The introduction of the effect of electrostatic interactions between charges on the chain into the theory of swelling equilibria led to an expression for the swelling pressure P in the form [19,20] P"P #P #P #P (5) . 04 %%-4 where P corresponds to the mixing of the chain segment with diluent (Flory— . Huggins contribution with interaction s parameter), P corresponds to the mixing of 04 ions with diluent, P corresponds to a change in elastic energy with swelling, and %P corresponds to a change in the electrostatic interaction energy with swelling. In %-4 previous papers [19,20], the individual terms P were expressed using network * structure parameters (i.e. concentration of EANCs l , density of dry network $ o "1.35 g cm~3, degree of ionization a, molecular weight of monomeric unit M , $ 0 dilution at network formation v "0.037, dielectric constant e, and molar volume » , 1 1 of w/a mixtures) and in the swelling degree Q"1/v . Using Eq. (5), the dependence of 2
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the interaction parameter s on v (or Q) can be calculated by employing a procedure 2 described earlier [3,20] [as the data were obtained for the free swelling, P"0 in Eq. (5)]; as shown earlier [20], for the existence of collapse, the s vs. v dependence has to 2 show a van der Waals loop. The use of Eq. (5) for a pure PAAm network (x "0) swollen in water has led to the 4 expected value of the interaction parameter s"0.49 (Table 1). For water-swollen charged networks with x '0, Eq. (5) gives unrealistically high values of s, if we 4 assume that the degree of ionization a"x (for x "0.01, 0.02, 0.05, 0.1 and 0.2, S S s values of 0.550, 0.673, 0.958, 2.538 and 6.321, respectively, were found). As before [3,10,17], we can also require s"0.49 for ionic water-swollen networks, since the effect of charges in Eq. (5) is included in the P and P contributions. This 04 %-4 requirement can be met by assuming that the effective degree of ionization i is lower than a"x (the ionic comonomer concentration), i.e. i"a/"x /, where / is the S S semiempirical correction factor [this parameter / is related to the activity coefficient of counterions and may involve also other effects not considered in Eq. (5)]. While for the MNa comonomer, high / values were found [3,13] (/"0.95—1), for the two other ionic comonomers / values lie in the range 0.05—0.25 (Table 1, Fig. 6). From Fig. 6 it can be seen that lower / values at constant x are found for the PyNa hydrogels S compared to the BeNa ones. As the / values are calculated from Q values, the 8 molecular interpretation of the lower / values found for PyNa gels can be the same as for the interpretation of lower Q values given above. 8 Using known molecular parameters (l , M , o, v , » and e) and the effective $ 0 1 1 degrees of ionization i"x /, the s vs. v dependencies were calculated for individual S 2 charged networks using experimental v values (Fig. 9); the condition for free swelling, 2 P"0, was used in Eq. (5). As was shown previously [20], the critical value of the interaction parameter s and the compositions of coexisting phases (v@ and vA ) at the 2 2 # transition are given by equation lA 2
: (s!s ) dv "0 # 2
(6)
l@ 2
In Fig. 9, the van der Waals loop for network with x "0.1 is shown; the S application of Eq. (6) requires that the areas S and S are equal (Maxwell construc1 2 tion). As the v vs. a dependencies for ionic networks are known, the theoretical 2 critical acetone composition, a , can be determined from the s values. The a and #,5 # #,5 the jump in the gel volume at collapse, D "vA !v@ (where vA is the volume fraction of 2 2 2 5 the dry gel in the collapse state and v@ is the volume fraction of the dry gel in expanded 2 state, respectively) values are given in Table 1. Similar experimental values a and # D can be determined from data shown in Fig. 2 (Table 1). From Table 1 it can be seen that the theory underestimates the collapse parameters. 3.5. Comparison between theory of swelling equilibria and experiment for gels swollen in aqueous NaCl solutions Eq. (5) can be also used for the theoretical description of swelling in aqueous NaCl solutions [19,20]. This requires the knowledge of concentration of co-ions c ~ inside C-
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Fig. 9. Dependence of the interaction parameter s on the volume fraction of the dry polymer in the swollen state, v . The collapse parameters for network with x "0.1, s and D"vA !v@ are defined by the 2 S # 2 2 condition that the areas S "S [see Eq. (5)]. ——— course determined by Eq. (4). 1 2
the gel phase. The c ~ value can be calculated from the Donnan equilibrium [21—23] Cof a negatively charged PAAm gel with the molar amount of the bound charges x ~ and the gel volume » , immersed in aqueous NaCl solution bath with initial COO ' molar amount of salt x and volume » . After reaching equilibrium the amounts of N!C4 charges in the gel phase are: bound anions x ~, mobile anions x ~ and mobile COO Ccations x ` and those in the salt solution phase are: mobile anions y ~ and mobile N! Ccations y ` . From the neutrality condition of the both phases we can write: N! x `"x ~#x ~ and y `"y ~"x !x ~. From the Donnan equilibrium, N! COO CN! CN!CCassuming that the activity coefficients of the salt inside and outside the gel are equal, we have (7) x ~ (x ~#x ~)/»2"(x !x ~)2/»2 ' N!CC4 C- COO CFrom this equation the molar concentration of co-ions in the gel phase can be calculated as c ~"x ~/» . Due to the presence of the bound negative charges on the CC- ' chain, the c ~ concentrations are essentially lower than external c values at low CN!CNaCl concentrations and these differences increase with increasing amount of ionic comonomer in the gel and decrease with increasing amount of NaCl outside the gel. The calculated values c ~, together with other network molecular parameters (l , C$ M , o, v and e), s"0.49 and effective ionization i"/x , were used for the 0 1 S
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description of swelling in salt solutions. The dotted lines in Fig. 7 represent the theoretical dependencies and it can be concluded that the theory, at least in the first approximation, describes the experimental data. The requirement s"0.49 for all ionic networks seems to be the most serious approximation used in application of Eq. (5) as the ionic comonomer can also contribute to the value of the s parameter by its copolymerization effect [we should mention again that the effect of charges in used theory is included in P and P terms 04 -4 of Eq. (5)]. If the charge concentration is low, as in MNa/AAm hydrogels [13] (x )0.025), the constancy of s seems to be justified, but in the PyNa/AAm gels, the S charge concentrations are much higher (x )0.2). If we assume that at c "1 M, all S N!Cionized networks behave as uncharged systems, Eq. (5) can be applied to the swelling data obtained in 1 M solutions (and shown as Q values in Table 1) with P " N!C04 P "0, and the s parameters can be calculated. As can be seen from Table 1, the %-4 N!Cs &0.49 values are within experimental scatter, independent of x , and roughly the N!CS same as found for the pure PAAm network swollen in water. Thus, the copolymerization effect of ionic comonomer with fully suppressed electrostatic interactions does not change the value of the s parameter and cannot account for low values of the correction /.
4. Conclusions From swelling and mechanical measurements of the ionic networks of PyNa/AAm copolymers swollen in w/a mixtures or in aqueous NaCl solutions, the following conclusions can be made: (1) Gels with PyNa concentrations x *0.01 undergo a first-order transition in w/a S mixtures. With increasing ionic comonomer concentration, the discontinuous increase in the gel volume and the critical acetone concentration at this transition increase. The proposed theory describes the collapse phenomenon semiquantitatively if the effective degree of ionization i ((x ) is introduced. S (2) The swelling degree in water and aqueous NaCl solutions depends strongly on the amount of charges bound on the chain and the NaCl concentration in the external solution. At least as a first approximation experimental data can be described using the above-mentioned theory employing an effective degree of ionization i. (3) The stepwise change in the modulus at collapse in w/a mixtures correlates with a corresponding change in the gel volume. The reduced modulus, log G depends on 3 the swelling ratio log X in a different way for the cases of w/a mixtures and NaCl aqueous solutions at high degree of swelling. The results show that different reasons leading to gel contraction (presence of a bad solvent or a salt in water) lead to formation of contracted gels with different microstructures. (4) A comparison of the results obtained on PyNa/AAm gels with the results obtained for MNa/AAm and BeNa/AAm copolymers shows that increasing the length of the ionic comonomer side chain strongly decreases the swelling degree in water, the extent of collapse D log X and the concentration of elastically active network
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chains. The exchange of pyridyl for benzoyl group in the ionic comonomer slightly decreases the value of D log X and swelling degree in water.
Acknowledgement The financial support of the Grant Agency of the Czech Republic (Grant No. 203/95/1318) is gratefully acknowledged.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]
Shibayama M, Tanaka T. Adv Polym Sci 1993; 109 : 1. Khokhlov A, Starodubtsev S, Vasilevskaya VV. Adv Polymer Sci 1993; 109 : 123. Ilavsky´ M. Adv Polym Sci 1993; 109 : 173. Gehrke S. Adv Polym Sci 1993; 109 : 81. Ilavsky´ M, Hrouz J, Bouchal K. Polym Bull 1985; 14 : 301. Hirokawa Y, Tanaka T, Sato E. Macromolecules 1985; 18 : 2782. Ilavsky´ M, Bouchal K. In: Kramer O, editor. Biological and synthetic polymer networks. New York: Elsevier 1988 : 435. Sedla´kova´ Z, Bouchal K, Hrouz J, Ilavsky´ M. Polym Bull 1992; 27 : 577. Bouchal K, Sedla´kova´ Z, Ilavsky´ M. Polym Bull 1994; 32 : 331. Ilavsky´ M. Macromol Symp 1996; 109 : 169. Tu¨do¨s F, Kelen T, Turcsanyi B, Kennedy JP. J Polym Sci 1981; 19 : 119. Burda J, Pr\ a´dova´ O, Luka´s\ R. J Macromol Sci-Pure Appl Chem 1995; A32 : 251. Ilavsky´ M. Macromolecules 1982; 15 : 782. Skeist I. J Am Chem Soc 1946; 68 : 1781. Spinner IH, Lu BCY, Graydon WF. J Am Chem Soc 1955; 77 : 2798. Brandrup J, Immergut E. Polymer handbook 3rd ed. New York: Wiley, 1989. Ilavsky´ M, Sedla´kova´ Z, Bouchal K, Ples\ til J. Macromolecules 1995; 28 : 6885. Rubinstein M, Colby RH, Dobrynin AV, Joanny J-F. Macromolecules 1996; 29 : 398. Hasa J, Ilavsky´ M, Dus\ ek K. J Polym Sci Polym Phys Ed 1975; 13 : 253. Ilavsky´ M. Polymer 1981; 22 : 1687. Manning GS. J Chem Phys 1969; 51 : 924, 934. Mamytbekov G, Bouchal K, Ilavsky´ M, Bekturov EA. Polym J (submitted). Moe ST, Skjak-Braek G, Elgsaeter A, Smidsrod O. Macromolecules 1993; 26 : 3589.
Phase transition in swollen gels 24: Effect of the concentration and structure of ionic comonomers on the collapse of poly(acrylamide) hydrogels Z. Sedla´kova´!, K. Bouchal!, M. Ilavsky´!,",* !Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, 162 06 Prague 6, Czech Republic "Faculty of Mathematics and Physics, Charles University, 180 00 Prague 8, Czech Republic Received 13 January 1998; received in revised form 18 March 1998; accepted 26 March 1998
Abstract The swelling and mechanical behaviour of ionized networks of copolymers of acrylamide (AAm), methylenebisacrylamide and sodium 2-[(3-carboxypyridine-4-carbonyl)oxy]ethyl methacrylate (PyNa, molar fraction of salt x "0—0.20) was investigated in water—acetone (w/a) S mixtures and in aqueous NaCl solutions. In the range x *0.01, a first-order phase transition S (collapse) was found in w/a mixtures; with increasing x , both the critical acetone concentration S a in the mixture at which collapse takes place and the extent of collapse (jump in the swelling # ratio X), Dlog X, increases. A comparison of these results with the results obtained for charged PAAm networks with sodium 2-[(2-carboxybenzoyl)oxy]ethyl methacrylate (BeNa) as ionic comonomer led to a conclusion that substituting benzoyl for pyridyl groups in the side chain slightly decreases Dlog X and the swelling degree in water, and does not change critical a values. On the other hand, a comparison with charged PAAm networks with sodium # methacrylate (MNa) as ionic comonomer showed that at constant x , the swelling degree in S water, the extent of collapse Dlog X and the concentration of elastically active network chains strongly decreased with the increased distance of the bound charge from the main chain (length of the side chain of the ionic comonomer). The expected decrease in the swelling degree of
*Correspondence address: Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, 162 06, Czech Republic. 0966-7822/98/$—see front matter ( 1998 Elsevier Science Ltd. All rights reserved. PII S 0 9 6 6 - 7 8 2 2 ( 9 8 ) 0 0 0 0 9 - 4
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ionized networks with increasing NaCl concentration was found. The experimental swelling data in w/a mixtures and in aqueous NaCl solutions could be described by a theory proposed previously if an effective degree of ionization i is introduced (i(x ). The mechanical behaviour S of gels in w/a mixtures is predominantly determined by the degree of swelling. On the other hand, different dependencies of the reduced modulus on the swelling ratio X found in w/a mixtures and in aqueous NaCl solutions in the high swelling region suggests that contracted gels with different microstructure were formed during gel deswelling in bad solvents or salt in water. ( 1998 Elsevier Science Ltd. All rights reserved.
1. Introduction Lightly crosslinked poly(acrylamide) (PAAm) gels with a low amount of charge on the chain (1—20 M%) prepared at high dilution show a first-order phase transition (collapse) when passing from a good (water) to a poor (acetone) solvent [1—4]. The appearance of this transition does not depend on the polarity of the charge on the chain introduced by copolymerization of AAm with variously charged comonomers [5,6]. On the other hand, it was found that while with the anionic comonomer sodium methacrylate (MNa), the collapse could be brought about by &1 M % of salt [3]. For the appearance of the collapse brought about by cationic comonomers, for example, by various quaternary ammonium salts, from three to ten times higher contents of ionic comonomers are necessary [7,8]. This drop in efficiency was mainly interpreted [7] by the large size of the ammonium group (which forms a weaker electrostatic field, because the hydration of the group is smaller) and by the influence of the counterion species on the hydration site (the hydration number of the anion Cl> is two and that of cation Na= is three). Furthermore, the positive charges in quaternary ammonium salts were localized at a greater distance from the main chain compared to MNa. The effect of the charge in an ionized crosslinker was also investigated [9,10]. In this case, the positive effect of increasing charge concentration on the collapse is compensated by the negative effect of the increasing crosslinking density. In this paper, the effect of concentration of the negative charge on the swelling and mechanical behaviour of PAAm networks, in which charge was introduced by copolymerization of AAm with sodium 2-[(3-carboxypyridine-4-carbonyl)oxy]ethyl methacrylate (PyNa), is investigated both in water/acetone mixtures and aqueous NaCl solutions. Moreover, by comparing these results and those obtained earlier [3,8] for ionized networks of AAm with MNa and of AAm with sodium 2-[(2carboxybenzoyl)oxy]ethyl methacrylate (BeNa), it is possible to specify the effect of distance of the charged group from the main chain, and the role of nitrogen in the pyridine ring (the distance of negative charge from the main chain in BeNa and PyNa is similar). As in copolymerization the proportion of participating comonomers changes with conversion, the chemical heterogeneity of the PyNa/AAm copolymer system was investigated by solution copolymerization in the absence of crosslinker. The results were compared with BeNa/AAm and MNa/AAm copolymer systems.
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2. Experimental 2.1. Preparation of 2-[(3-carboxypyridine-4-carbonyl)oxy]ethyl methacrylate To a mixture containing 0.1 mol of pyridine-3,4-dicarboxylic anhydride, 0.11 mol of 2-hydroxyethyl methacrylate and 0.01 wt% of 1,1-diphenyl-2-pikryl hydrazyl in 60 ml of benzene, 8 mmol of triethylamine was added with stirring. After 4 h, the reaction was terminated and the solvent was removed by distillation at 2.1 kPa. White crystals (m. p. 134°C) were obtained by recrystallization of the product from methanol. Yield: 89%. Elemental analysis gave the following results: calculated/found — %C 55.9/55.87; %H 4.69/4.62; %N 5.02/5.07. The structure was confirmed by 1H NMR and 13C NMR. 2.2. Copolymerization of acrylamide and sodium 2-[(3-carboxypyridine-4-carbonyl) oxy]ethyl methacrylate The sodium salt of 2-[(3-carboxypyridine-4-carbonyl)oxy]ethyl methacrylate (PyNa, ionic comonomer S) was prepared by hydrolysis with NaOH. The radical copolymerization of the binary system PyNa—acrylamide (AAm) was investigated in aqueous solution (volume fraction of both monomers was 0.1) with the aim of determining the monomer reactivity ratios. A series of copolymers was prepared by polymerization of monomer mixtures in which the molar fractions of PyNa, x (relative to both monomers) varied in the range 0.1—0.9. The PyNa content in the S copolymer, determined by elemental analysis, was compared with x . Using the S Kelen—Tu¨do¨s method [11,12] the monomer reactivity ratios r "0.30 (PyNa) and 1 r "0.37 (AAm) were determined. 2 2.3. Preparation of networks The ionic networks were prepared from 100 ml of an aqueous mixture which contained 5 g of acrylamide (AAm), 0.135 g of N,N@-methylenebisacrylamide (MBAAm, crosslinker), 0.02 g of ammonium peroxosulfate, 150 ll of N,N,N@,N@-tetramethylethylenediamine and various amounts of ionic comonomer, sodium 2-[(3carboxypyridine-4-carbonyl)oxy]ethyl methacrylate (PyNa, Scheme 1). Six networks were prepared with molar fractions of PyNa: x "0, 0.01, 0.02, 0.05, 0.10 and 0.20 S (Table 1). The polymerization proceeded at room temperature for 5 h in glass ampoules with diameter D*"10 mm. After polymerization, the samples were extracted in redistilled water.
0 0.01 0.02 0.05 0.10 0.20
1 2 3 4 5 6
26.4 33.1 44.0 47.2 55.4 58.8
G 1 (g cm~2)
2.81 3.52 4.68 5.02 5.89 6.26
105 l $ (mol cm~3)
— 0.6 0.75 0.78 1.00 1.25
Dlog X
— 0.30 0.32 0.40 0.47 0.62
Dlog G
— 45 48 52 58 70
a # (vol%) — — 45 48 55 65
a #,5 (vol%) 8
40 52 61 82 110 152
Q N!C-
40.0 39.0 35.5 34.6 31.6 33.1
Q
— 0.150 0.130 0.082 0.065 0.047
/
0.490 0.488 0.490 0.491 0.494 0.487
s N!C-
— 0.094 0.135 0.155 0.225 0.247
D
— — 0.090 0.115 0.160 0.185
D 5
Note x is the molar fraction of ionic comonomer PyNa; G is the modulus after preparation; l is the concentration of elastically active network chains relaS 1 $ tive to the dry state; D log X and D log G, respectively, are collapse parameters; a and a , respectively, are the experimental and theoretical critical # #,5 acetone concentrations in w/a mixtures at collapse; Q and Q , respectively, are the swelling degrees in water and 1 M NaCl aqueous solutions; / is the 8 N!Ccorrection factor, s is the interaction parameter; D and D , respectively, are the experimental and theoretical differences in volumes of coexisting phases at N!C5 collapse.
x S
Sample
Table 1 Network characteristics and collapse parameters
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Experimental data were compared with those obtained earlier for ionic networks from AAm and sodium methacrylate [13] (MNa, Scheme 2) and for networks from AAm and sodium 2-(2-carboxybenzoyl)oxy]ethyl methacrylate [8] (BeNa, Scheme 3). 2.4. Swelling and mechanical measurements The samples were swollen in 200 ml of water/acetone (w/a) mixtures containing 0—100 vol% of acetone or in 100 ml of an aqueous NaCl salt solution (molar concentrations c "10~5—1 mol/l). Swelling proceeded for 28 days, after which the inverse N!Cswelling ratio X was determined: X"(D /D)3"» /» (1) 1 1 where D and D are the diameters of samples after preparation and at equilibrium, 1 respectively, and » and » are the respective sample volumes. Diameters were 1 measured using an Abbe’s comparator with an accuracy of $0.001 mm. Using X values, the volume fraction of the polymer in the swollen state v can be easily 2 calculated (v "v X, where v is the volume fraction of the polymer at network 2 1 1 formation, v "0.037). The degree of swelling Q"1/v . 1 2 Deformation measurements were carried out on cylindrical samples (height &1 cm) in an unidirectional compression using an apparatus described earlier [13]. The sample was compressed to a ratio j"l/l , l and l being the deformed and the 0 0 initial height of the sample, respectively, and the force f was determined after 30 s relaxation; ten values of j and f in the range 0.7(j (1 were determined. The i i i equilibrium shear modulus G was calculated from [13] G"f /[S (j2!j~1)] (2) 0 where S is the initial cross-section of the sample. The mechanical measurements were 0 carried out on samples just after their preparation (modulus G , Table 1) or on the 1 equilibrium swollen samples in w/a mixtures or aqueous NaCl solutions (modulus G). 3. Results and discussion 3.1. Structure of the linear copolymers Using the Skeist equation [14,15] with the determined monomer reactivity ratios r "0.3 (PyNa) and r "0.37 (AAm) the dependence of the changes in the molar 1 2
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Fig. 1. Dependence of the molar ratio of ionic comonomers in P (ionic comonomer/AAm) chains, F (calculated from the Skeist equation) on the molar conversion of both monomers, m: PyNa (1), BeNa (2) 1 and MNa (3). The horizontal line corresponds to hypothetical uniform chains (no chemical heterogeneity).
content of the ionic comonomer in PAAm chains, F , on molar conversion of 1 monomers, m, was evaluated for a constant molar ratio of PyNa; x "0.1 in the S monomer mixture (Fig. 1). In a similar way, the changes in the distribution of copolymers composition for both BeNa/AAm and MNa/AAm copolymers were calculated for molar ratios of ionic comonomers x "0.1 using the corresponding S monomer reactivity ratios given in the literature [16]; these dependencies are also shown in Fig. 1. As follows from Fig. 1, in all three systems, the polymer chains with different contents of ionized comonomers (chemically heterogeneous) are formed with increasing conversion. While at the beginning of the reaction (0(m(0.5), the participation of the salts is higher than the average value, at the end of the reaction it is lower. The largest chemical heterogeneity is predicted for PyNa/AAm copolymers where one can expect formation of almost pure PAAm chains at full conversion. On the other hand, BeNa/AAm copolymers show the lowest chemical heterogeneity. Similar differences in chemical heterogeneity can be expected in crosslinked systems. 3.2. Swelling and mechanical behaviour of networks in w/a mixtures As can be seen from Fig. 2, the network with the lowest ionic comonomer concentration (x "0.01) already shows a phase transition with a stepwise change in S the swelling ratio X characterized by the extent of collapse, Dlog X"log XA!log X@ (where X@ is the swelling ratio of the expanded state and XA is the ratio of the collapse state of the gel at the transition, respectively, Table 1) at a critical acetone concentration in w/a mixture a "45 vol%. Only for the uncharged PAAm network with # x "0, is the dependence of the ratio X on acetone concentration a continuous. S
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169
Fig. 2. Dependence of the swelling ratio, X, and modulus G on the acetone concentration, a, in w/a mixtures for gels with indicated PyNa concentrations x : LX; dG. S
Figure 2 shows that Dlog X increases with increasing anionic comonomer concentrations x ; a similar increase in a values where the collapse takes place with concentraS # tion x , can also be seen in Fig. 2 (see also Table 1). S As expected, the dependence of the modulus G of the uncharged PAAm network on acetone concentration, a, is continuous (Fig. 2). All charged networks undergo collapse, as shown by the stepwise change in G, characterized by the value of Dlog G (Table 1). The jumpwise change in modulus, Dlog G, gives a linear correlation with the corresponding change in the swelling ratio, Dlog X, with a slope &0.5 (Dlog G &0.5]Dlog X, Fig. 3). This slope is higher than the theoretical rubber elasticity value [3] (s"0.33), observed for networks of AAm with quaternary ammonium salts [17]. It is interesting to note that, within experimental scatter, the Dlog G vs. Dlog X dependence is the same for networks with positively charged comonomers PyNa and BeNa. Networks with MNa show a slightly higher slope, s"0.75. The dependence of the reduced modulus, log G (G "G/G ), on the swelling ratio, 3 3 1 log X, for all samples swollen in w/a mixtures is shown in Fig. 4. As in Fig. 3, the slopes found for these dependencies are in the range s"0.5—0.6 for X(10. A similar
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Fig. 3. Dependence of the stepwise change in the modulus, D log G, on the corresponding change in the volume of the gel, D log X, at the transition; data for BeNa/AAm hydrogels were taken from Ref. [8] and those for MNa/AAm hydrogels from Ref. [13].
Fig. 4. Dependence of the reduced modulus, G "G/G , on the swelling ratio, X, for variously ionized 3 1 PyNa/AAm gels swollen in w/a mixtures.
slope (s"0.65) was found earlier for ionized MNa/AAm gels in w/a mixtures [13]. Deviations from linearity in the range of low swelling (log X'1) are probably due to the influence of the main transition region (vitrification) at high acetone concentrations [13]. It should be mentioned that the recent theory of Rubinstein et al. [18] which describes the swelling and mechanical behaviour of polyelectrolyte gels, based on the scaling approach, predicts a value of 5/6 for the slope s. From Figs. 3 and 4 it also follows that the mechanical behaviour of gels in w/a mixtures is predominantly determined by their degree of swelling. The efficiency of a crosslinking reaction in our networks can be evaluated from comparison of the experimental modulus G measured after network formation and 1
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added amount of crosslinker MBAAm to the system. If network was formed at the volume fraction of polymer v and measured at the volume fraction of polymer in the 1 swollen state v then, according to the rubber elasticity theory, we can write for the 2 modulus G measured in the swollen state at v the equation [13] 2 2@3 1@3 G"l R¹v v Sr2T /Sr2T (3) $ 1 2 * where l is the concentration of elastically active network chains (EANCs) related to $ the dry state, R is the gas constant, ¹ is temperature and Sr2T and Sr2T , respectively, * are the mean-square end-to-end chain distances in the isotropic and reference states. Since G has been determined at the state of network formation it follows that 1 Sr2T "Sr2T and v "v . From Eq. (3) we have [3,13] * 2 1 l "G /R¹v (4) $ 1 1 The value of the ideal theoretical EANCs concentration l , calculated from the $,5 amount of crosslinker considering that each MBAAm molecule gives two EANCs, is l "4.6]10~4 mol cm~3. The much lower experimental l values (Table 1) indicate $,5 $ a low efficiency of crosslinking reaction (&6—14%), which is mainly due to a high cyclization caused by high dilution (&95 vol%) at network formation [13]. From Table 1 and Fig. 5 (in Fig. 5, data obtained earlier [3,8,13] for MNa/AAm and BeNa/AAm hydrogels are also included), it follows that the crosslinking efficiency increases with ionic comonomer concentration x in all three copolymer systems. As S one can expect that, with increasing charge concentrations, more expanded conformations of chains are preferred, such extended conformations will decrease the probability of loop formation in dilute systems and will increase the efficiency of crosslinking reaction. Fig. 5 also shows that, in the first approximation, the increase in the efficiency is the same for PyNa/AAm and BeNa/AAm systems; this increase is much lower than that found for MNa/AAm networks. The slower increase in l with $ x found for PyNa and BeNa salts is probably due to the longer side chains of these S comonomers compared to the side chain of MNa. The dependence of the extent of collapse, Dlog X, and the critical acetone concentration, a , on the salt concentration x for all three systems is shown in Fig. 5. It is # S obvious that while the dependence of the critical acetone concentration, a , on x is # S practically the same for all three ionic comonomers, the extent of the collapse, Dlog X, is dependent on the detailed structure of salts. It can be seen that increasing the length of the side chains has a strong negative effect on the extent of collapse. The introduction of a nitrogen atom into the benzoyl ring also leads to a decrease in the Dlog X values, but this effect is much lower than the effect of the side-chain length. As expected, the swelling degree in water Q increases with increasing PyNa 8 concentration x (Table 1). From Fig. 6, in which Q data obtained with BeNa and S 8 MNa are also included, it can be seen that the swelling Q strongly depends on the 8 ionic comonomer structure. As in the case of the extent of collapse, Dlog X, the largest negative effect on Q occurs with increasing side-chain length. The charged PyNa 8 networks show the lowest Q values at constant x . There are probably two reasons 8 S for the lowest swelling degree of PyNa networks: (a) as follows from Fig. 1, they have the highest non-uniform distribution of the charged comonomer in the network
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Fig. 5. Dependence of the concentration of elastically active network chains, l , of the critical acetone $ concentration in w/a mixture at the collapse, a , and of the extent of the collapse, D log X, on the molar # fraction of salts x ; data for BeNa/AAm hydrogels were taken from Ref. [8] and those for MNa/AAm S hydrogels from Ref. [13].
structure (highest chemical heterogeneity); and (b) the nitrogen atom in the pyridine ring can lead to a betain structure (inner salt) which is less polar than the PyNa structure. Both points (a) and (b) are reflected in a decrease in the swelling degree Q of 8 PyNa hydrogels. 3.3. Swelling and mechanical behaviour in aqueous NaCl solutions Fig. 7 shows the dependence of the degree of swelling, Q, on aqueous NaCl concentration of the external solution, c (data shown at c "10~7 M were N!CN!Cmeasured in pure water). As expected, the degree of swelling depends on the amount of PyNa and the ionic strength of the solution. With increasing salt concentration, c , N!Cthe ionized gels deswell and at c "1 M all gels have roughly the same swelling N!Cdegree Q (Table 1). The pronounced decrease in swelling occurs between 10~4 and N!C-
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173
Fig. 6. Dependence of the degree of swelling in water, Q , and of the correction factor, / [calculated from 8 Eq. (4)], on the molar fraction of ionic comonomers x ; data for BeNa/AAm hydrogels were taken from Ref. S [8] and those for MNa/AAm hydrogels from Ref. [13].
Fig. 7. Dependence of the swelling degree, Q, on the aqueous NaCl concentration, c , for networks with N!Cindicated x values. — — — theoretical dependence from Eq. (5). S
10~1 M salt concentrations; increasing PyNa content in gel shifts this decrease to higher c values. The well-known deswelling behaviour of polyelectrolyte hydrogels N!Cwith increasing c concentration is due to the increasing concentration of mobile N!Ccounterions (Na`) from surrounding liquid in the gel phase (screening effect of
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Fig. 8. Dependence of the reduced modulus, G "G/G , on the ratio, X, for variously ionized PyNa/AAm 3 1 gels swollen in aqueous NaCl solutions.
counterions). This concentration equals to the amount of co-ions (Cl~) inside the gel. The dependence of the reduced modulus, G "G/G , on the swelling ratio X is 3 1 shown in Fig. 8 together with the line of slope s"0.5. The expected decrease in the reduced modulus G with increasing swelling was observed only in the range of low 3 swelling (log X'!0.3) and departures from linearity at high swelling, were found for the two most ionized networks with x "0.1 and 0.2. It should be mentioned that S for the gels swollen in w/a mixtures, no such increase in G values was found in the 3 same range of swelling (see Fig. 4). From Fig. 8 it follows that the mechanical behaviour of gels in aqueous NaCl solutions in the high-swelling region differ from that in w/a mixtures. 3.4. Comparison between theory of swelling equilibria and experiment for gels swollen in water/acetone mixtures The introduction of the effect of electrostatic interactions between charges on the chain into the theory of swelling equilibria led to an expression for the swelling pressure P in the form [19,20] P"P #P #P #P (5) . 04 %%-4 where P corresponds to the mixing of the chain segment with diluent (Flory— . Huggins contribution with interaction s parameter), P corresponds to the mixing of 04 ions with diluent, P corresponds to a change in elastic energy with swelling, and %P corresponds to a change in the electrostatic interaction energy with swelling. In %-4 previous papers [19,20], the individual terms P were expressed using network * structure parameters (i.e. concentration of EANCs l , density of dry network $ o "1.35 g cm~3, degree of ionization a, molecular weight of monomeric unit M , $ 0 dilution at network formation v "0.037, dielectric constant e, and molar volume » , 1 1 of w/a mixtures) and in the swelling degree Q"1/v . Using Eq. (5), the dependence of 2
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175
the interaction parameter s on v (or Q) can be calculated by employing a procedure 2 described earlier [3,20] [as the data were obtained for the free swelling, P"0 in Eq. (5)]; as shown earlier [20], for the existence of collapse, the s vs. v dependence has to 2 show a van der Waals loop. The use of Eq. (5) for a pure PAAm network (x "0) swollen in water has led to the 4 expected value of the interaction parameter s"0.49 (Table 1). For water-swollen charged networks with x '0, Eq. (5) gives unrealistically high values of s, if we 4 assume that the degree of ionization a"x (for x "0.01, 0.02, 0.05, 0.1 and 0.2, S S s values of 0.550, 0.673, 0.958, 2.538 and 6.321, respectively, were found). As before [3,10,17], we can also require s"0.49 for ionic water-swollen networks, since the effect of charges in Eq. (5) is included in the P and P contributions. This 04 %-4 requirement can be met by assuming that the effective degree of ionization i is lower than a"x (the ionic comonomer concentration), i.e. i"a/"x /, where / is the S S semiempirical correction factor [this parameter / is related to the activity coefficient of counterions and may involve also other effects not considered in Eq. (5)]. While for the MNa comonomer, high / values were found [3,13] (/"0.95—1), for the two other ionic comonomers / values lie in the range 0.05—0.25 (Table 1, Fig. 6). From Fig. 6 it can be seen that lower / values at constant x are found for the PyNa hydrogels S compared to the BeNa ones. As the / values are calculated from Q values, the 8 molecular interpretation of the lower / values found for PyNa gels can be the same as for the interpretation of lower Q values given above. 8 Using known molecular parameters (l , M , o, v , » and e) and the effective $ 0 1 1 degrees of ionization i"x /, the s vs. v dependencies were calculated for individual S 2 charged networks using experimental v values (Fig. 9); the condition for free swelling, 2 P"0, was used in Eq. (5). As was shown previously [20], the critical value of the interaction parameter s and the compositions of coexisting phases (v@ and vA ) at the 2 2 # transition are given by equation lA 2
: (s!s ) dv "0 # 2
(6)
l@ 2
In Fig. 9, the van der Waals loop for network with x "0.1 is shown; the S application of Eq. (6) requires that the areas S and S are equal (Maxwell construc1 2 tion). As the v vs. a dependencies for ionic networks are known, the theoretical 2 critical acetone composition, a , can be determined from the s values. The a and #,5 # #,5 the jump in the gel volume at collapse, D "vA !v@ (where vA is the volume fraction of 2 2 2 5 the dry gel in the collapse state and v@ is the volume fraction of the dry gel in expanded 2 state, respectively) values are given in Table 1. Similar experimental values a and # D can be determined from data shown in Fig. 2 (Table 1). From Table 1 it can be seen that the theory underestimates the collapse parameters. 3.5. Comparison between theory of swelling equilibria and experiment for gels swollen in aqueous NaCl solutions Eq. (5) can be also used for the theoretical description of swelling in aqueous NaCl solutions [19,20]. This requires the knowledge of concentration of co-ions c ~ inside C-
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Fig. 9. Dependence of the interaction parameter s on the volume fraction of the dry polymer in the swollen state, v . The collapse parameters for network with x "0.1, s and D"vA !v@ are defined by the 2 S # 2 2 condition that the areas S "S [see Eq. (5)]. ——— course determined by Eq. (4). 1 2
the gel phase. The c ~ value can be calculated from the Donnan equilibrium [21—23] Cof a negatively charged PAAm gel with the molar amount of the bound charges x ~ and the gel volume » , immersed in aqueous NaCl solution bath with initial COO ' molar amount of salt x and volume » . After reaching equilibrium the amounts of N!C4 charges in the gel phase are: bound anions x ~, mobile anions x ~ and mobile COO Ccations x ` and those in the salt solution phase are: mobile anions y ~ and mobile N! Ccations y ` . From the neutrality condition of the both phases we can write: N! x `"x ~#x ~ and y `"y ~"x !x ~. From the Donnan equilibrium, N! COO CN! CN!CCassuming that the activity coefficients of the salt inside and outside the gel are equal, we have (7) x ~ (x ~#x ~)/»2"(x !x ~)2/»2 ' N!CC4 C- COO CFrom this equation the molar concentration of co-ions in the gel phase can be calculated as c ~"x ~/» . Due to the presence of the bound negative charges on the CC- ' chain, the c ~ concentrations are essentially lower than external c values at low CN!CNaCl concentrations and these differences increase with increasing amount of ionic comonomer in the gel and decrease with increasing amount of NaCl outside the gel. The calculated values c ~, together with other network molecular parameters (l , C$ M , o, v and e), s"0.49 and effective ionization i"/x , were used for the 0 1 S
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description of swelling in salt solutions. The dotted lines in Fig. 7 represent the theoretical dependencies and it can be concluded that the theory, at least in the first approximation, describes the experimental data. The requirement s"0.49 for all ionic networks seems to be the most serious approximation used in application of Eq. (5) as the ionic comonomer can also contribute to the value of the s parameter by its copolymerization effect [we should mention again that the effect of charges in used theory is included in P and P terms 04 -4 of Eq. (5)]. If the charge concentration is low, as in MNa/AAm hydrogels [13] (x )0.025), the constancy of s seems to be justified, but in the PyNa/AAm gels, the S charge concentrations are much higher (x )0.2). If we assume that at c "1 M, all S N!Cionized networks behave as uncharged systems, Eq. (5) can be applied to the swelling data obtained in 1 M solutions (and shown as Q values in Table 1) with P " N!C04 P "0, and the s parameters can be calculated. As can be seen from Table 1, the %-4 N!Cs &0.49 values are within experimental scatter, independent of x , and roughly the N!CS same as found for the pure PAAm network swollen in water. Thus, the copolymerization effect of ionic comonomer with fully suppressed electrostatic interactions does not change the value of the s parameter and cannot account for low values of the correction /.
4. Conclusions From swelling and mechanical measurements of the ionic networks of PyNa/AAm copolymers swollen in w/a mixtures or in aqueous NaCl solutions, the following conclusions can be made: (1) Gels with PyNa concentrations x *0.01 undergo a first-order transition in w/a S mixtures. With increasing ionic comonomer concentration, the discontinuous increase in the gel volume and the critical acetone concentration at this transition increase. The proposed theory describes the collapse phenomenon semiquantitatively if the effective degree of ionization i ((x ) is introduced. S (2) The swelling degree in water and aqueous NaCl solutions depends strongly on the amount of charges bound on the chain and the NaCl concentration in the external solution. At least as a first approximation experimental data can be described using the above-mentioned theory employing an effective degree of ionization i. (3) The stepwise change in the modulus at collapse in w/a mixtures correlates with a corresponding change in the gel volume. The reduced modulus, log G depends on 3 the swelling ratio log X in a different way for the cases of w/a mixtures and NaCl aqueous solutions at high degree of swelling. The results show that different reasons leading to gel contraction (presence of a bad solvent or a salt in water) lead to formation of contracted gels with different microstructures. (4) A comparison of the results obtained on PyNa/AAm gels with the results obtained for MNa/AAm and BeNa/AAm copolymers shows that increasing the length of the ionic comonomer side chain strongly decreases the swelling degree in water, the extent of collapse D log X and the concentration of elastically active network
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chains. The exchange of pyridyl for benzoyl group in the ionic comonomer slightly decreases the value of D log X and swelling degree in water.
Acknowledgement The financial support of the Grant Agency of the Czech Republic (Grant No. 203/95/1318) is gratefully acknowledged.
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