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Langmuir 1999, 15, 4056-4061
Effects of Ionic Additives and Ionic Comonomers on the Temperature and Pressure Responsive Behavior of Thermoresponsive Polymers in Aqueous Solutions† S. Kunugi,* Y. Yamazaki, K. Takano, and N. Tanaka Department of Polymer Science & Engineering, Kyoto Institute of Technology, Matsugasaki, Sakyo, Kyoto, 606, Japan
M. Akashi Department of Applied Chemistry, Kagoshima University, Koorimoto, Kagoshima, 890, Japan Received September 8, 1998. In Final Form: December 8, 1998 The cloud points of the aqueous solutions of poly(N-vinylisobutyramide) (PNVIBA) and poly(Nisopropylacrylamide) (PNIPAM) were measured down to a subzero temperature under elevated pressures, and the effects of ionic additives and the ionic comonomer were studied. The polymers with higher molecular weight showed lower transition pressure (Pt) and smaller ellipsoids, due to larger absolute values of ∆Cp and ∆β. Addition of 0.02% sodium dodecyl sulfate (SDS) into aqueous solutions of these polymers apparently extinguished the clouding phenomenon, but even under these conditions the dynamic light scattering measurement showed the chain collapse of the random coil preceding the interchain aggregations. Pt and transition temperature (Tt) were highly dependent on the addition of inert anions. Less lyotropic ions showed lower Pt and Tt, producing smaller ellipses as a result, while additions of SCN- and I- caused a slight increase in both Pt and Tt and therefore larger ellipses. Introduction of ionic comonomer (4-pentenoic acid) into PNIPAM (less than 20%) made the transition P-T curves pH dependent and Pt was much more influenced by the charged comonomer than Tt. Anionic residues less than 5% were sufficient to screen the cloud point (20 °C and 0. Smeller and Heremans19 indicated that eq 2 can be directly derived by a second-order Tayler’s development of the free energy equation and that taking higher order derivatives into account (i.e., compressibility, expansibility, and/or heat capacity are temperature- and/or pressure-dependent) resulted in distinct deviations from ellipsoidal features. We have shown that the data for aqueous solutions of PNVIBA and PNIPAM can be fitted to ellipsoid curves.7 This produces a set of thermodynamic parameters that are relative to ∆G0. When the center of the ellipse is in the first quadrant, both the apparent extremum of ∆V ) 0 (dP/dT of transition is ∞) with increasing pressure and that of ∆S ) 0 (dP/dT of transition is 0) with increasing temperature can be observed; in other words, dP/dT > 0 near P ∼ P0 and near T ∼ T0 (antagonism of pressure and temperature) but dP/dT < 0 in the intermediate region. The data for the PNVIBA and PNIPAM solutions obtained in the range of 10-45 ° and 0.1-400 MPa showed distinctly apparent extremum of ∆V ) 0 with increasing pressure.7 However, the extremum of ∆S ) 0 with increasing temperature was not clearly observed, although the data could be fitted to ellipsoid curves. Taking advantage of positive ∆V of fusion of water near atmospheric pressure (the freezing point of water becomes lower with increasing pressure up to 210 MPa; where water freezes at ca. -22 °C), the T-P diagrams of transition were drawn down to nearly -20 °C. Effects of the Molecular Weight of Homopolymer. Figure 1 shows the results for three PNVIBA (a) and three PNIPAM (b) samples with different molecular weights. The ellipses, which adequately explain the obtained data,
4058 Langmuir, Vol. 15, No. 12, 1999
Kunugi et al.
Figure 1. T-P diagrams for the cloud point of PNVIBA (a) and PNIPAM (b) solutions down to subzero temperatures: (a) (O) Mn ) 460 × 103, (0) Mn ) 66 × 103 (4) Mn ) 11 × 103, 0.1% (wt/v); (b) (O) Mn ) 600 × 103, (0) Mn ) 49 × 103, (4) Mn ) 12 × 103, 0.5% (wt/v); open symbols, temperature scanning; closed symbols, pressure scanning; - - -, freezing point of water at each pressure. Table 1. Thermodynamic Parameters Explaining Ellipsoid Curves in Figures 1, 3, and 4a polymer PNVIBA
PNIPAM
a
Mn (103)
added salt (M)
∆V0/∆G0 (10-2 cm3 cal-1)
∆S0/∆G0 (10-3 K-1)
∆β/∆G0 (10-4 cm6 cal-2)
∆R/∆G0 (10-5 cm3 cal-1 K-1)
∆Cp/∆G0 (10-6 K-1)
11 66 460 66 66 66 66 12 49 600 49 49
none none none KCl (1 M) Na2SO4 (0.125 M) KI (1 M) NaSCN (1 M) none none none KI (1 M) NaSCN (1 M)
2.1 3.0 6.0 8.3 6.1 2.7 1.5 2.1 1.8 1.8 1.1 1.1
-1.7 -5.8 -25 -9.7 -5.9 -7.0 -6.0 0.92 -0.29 3.1 17 16
-2.2 -3.4 -6.3 -7.6 -7.5 -2.4 -1.6 -2.7 -2.9 -3.5 -2.2 -2.0
14 0.73 -10 -26 2.3 0.63 0.66 -5.0 -4.8 -10 -1.6 -1.9
1.1 1.5 2.7 3.6 2.7 1.4 1.2 1.6 1.6 1.7 1.0 1.0
The change was expressed in the direction from coil to collapse. The approximate deviations are about (5%.
are drawn by curve-fitting. The broken curves show the freezing point of pure water. Extremum of ∆S ) 0 with increasing temperature was observed at 0 to 10 °C for PNVIBA and at -10 to 0 °C for PNIPAM. Generally, the cloud point temperatures for these kinds of thermoresponsive polymers are believed to be rather independent of the molecular weight; some researchers found no dependence.9 The transition temperature (Tm ) at 0.1 MPa of the present polymer samples with different molecular weights differed by only 2 to 3 °C; a slightly lower Tm was observed for higher molecular weights. On the contrary, the transition pressure varied more sensitively with molecular weight. Polymers with a higher molecular weight showed a lower transition pressure and, hence, a smaller ellipse. The parameters calculated from these fitted ellipses are summarized in Table 1. They are given in values relative to the standard free energy of transition (∆G0). An ellipse is uniquely determined by these relative values only. By decreasing the measured temperature down to below 0 °C and observing the extremum of ∆S ) 0, we can now more easily evaluate the parameters. Although the axes of the ellipses are somehow inclined from the T-P axes, the axial lengths of the ellipses are primarily determined by the coefficients of the (T - T0)2 and (P - P0)2 terms; i.e., ∆Cp and ∆β, respectively. Hence, these two parameters indicate larger absolute values with increasing molecular weight, although the changes in ∆Cp for PNIPAM were very small. Since the reference (standard) state of these thermodynamic analyses (0 °C and
0.1 MPa) is well inside the inducted ellipses, the values of ∆G0 should be positive in all examples. Thus, each value of the parameters in Table 1 has the same sign as the relative value. Effects of the SDS Addition in the Homopolymer Solutions. The premise of cloud point observation is that the aggregation of collapsed polymer chains increases to a certain observable “particle” size. In regard to the calorimetric or dynamic light scattering measurements of PNIPAM solutions, the addition of surfactant of low concentrations (6) even under atmospheric pressure. In the case of copoly(NIPAM85-PA15), the apparent extremum of ∆V ) 0 (dP/dT of transition is ∞) with increasing pressure at pH 5.1 was well outside of the measuring range and the LCSP was not observed at higher pressures (up to 400 MPa).
Discussion The dehydration of amide groups and/or the liberation of the hydrophobic hydration in the polymers through a collapse transition will increase the amount of water in the bulk state and thus make the system more compress-
Kunugi et al.
ible,23 which is expressed as the negative absolute compressibility as observed here. Experimental observation of the extrema of ∆V ) 0 and ∆S ) 0 is related to the coordinates in the center of the ellipse: (Tc, Pc). If Pc has a positive sign (or the center is in the meaningful pressure range; -∆V0∆Cp/T0 - ∆S0∆R > 0), then both extrema are observable experimentally. The position of the extremum of ∆S ) 0 is, however, determined by the Tc value, and this extremum is practically impossible to observe when Tc is very negative, because water is not liquid anymore. In the case that Pc < 0 (the center is in the meaningless pressure range; -∆V0 ∆Cp/T0 - ∆S0 ∆R < 0), extremum of ∆V ) 0 is difficult to be observed. Since the reference (standard) state of this thermodynamic analysis (0 °C, 0.1 MPa) is well inside the inducted ellipses, the values of ∆G0 should have positive signs. Therefore each value of the parameters in Table 1 has the same sign as the relative value to ∆G0. In all the examples determined here, the values for Pc are positive and therefore the sign of the ∆S0 values is correlated with the tangential line of the free energy contour ellipse at 0 °C and 0.1 MPa; when this line has a negative slope, ∆S0 has a negative sign and vice versa, since ∆S0 ) -(∂∆G/∂T)p at (0 °C, 0.1 MPa). Even with a negative ∆S0, a positive ∆Cp makes ∆S positive at higher temperatures in order to draw the thermal transition. ∆V0 values were positive in all examples. Since the P coordinate of the center of the ellipse has positive values and the T coordinate of it is close to 0 °C, the difference in the partial molar volumes of the two states of the polymers (coil and collapse) increases first with increasing pressure and then decreases with the contribution from negative ∆β values in order to draw pressure-induced transitions. The molecular weight dependence of the T-P diagrams and the effects of SDS addition on the transition curve are related to the critical molecular size of these polymers in order to draw an all-or-none (two-state) transition. Tiktopulo et al.16,17 found that such a size for PNIPAM is around 10 kDa and that the PNVIBA will have a size of a similar number. Shorter polymers have a single collapse point in a single chain, and these collapsed chains may aggregate easily, even with the assistance of the surfactant. Polymers that are long enough to possess several collapse centers within a single chain can hold a molecularly dispersed state with the help of the surfactant and can avoid aggregation with other polymers. Furthermore, the concentrations of the polymers in our experiments were controlled by weight, which introduced a larger number of polymer chains for shorter polymers to increase the encounter frequency between different collapsed chains. The transitions of these thermoresponsive polymers occur due to the dehydration process and the strengthening of hydrophobic interactions among side chains, as well as the structural deformation of water around the hydrophobic groups. In many different examples (from oligopeptides or proteins to synthetic water-soluble polymers), the effectiveness of salt addition (especially that of anions) follows the Hofmeister or the lyotropic series.24-26 In a study of the higher-order structure of proteins, such as ribonuclease A, it was shown that anions with higher lyotropic numbers (salting-in ions) lead to the denaturation of proteins into coillike structures, and those with lower numbers (salting-out ions) induced native and ordered (23) Gekko, K.; Hasegawa, Y. Biochemistry 1986, 25, 6563. Gekko, K.; Hasegawa, Y. J. Phys. Chem., 1989, 93, 426. (24) von Hippel, P. H.; Schleich, T. Acc. Chem. Res. 1969, 2, 257. (25) Robinson, D. R.; Jencks, W. P. J. Am. Chem. Soc. 1965, 87, 2470. (26) Ataman, M. Colloid Polym. Sci. 1987, 265, 19.
Behavior of Thermoresponsive Polymers in Aqueous Solutions
structures (globule state).27 Our results can be explained in a similar context. Salting-in anions such as SCN- or I- may break the structure of bulk water and stabilize hydrophobic hydration, and then the coillike structures will be favored. On the contrary, salting-out anions act as structure makers for water in order to strengthen the hydrophobic interactions that promote the collapse state in polymers. Hydrophobic interactions among alkyl chains have been found to show slightly positive volume changes,28 and thus the antagonism of temperature and pressure effects was observed. This type of antagonism was first observed in a PNIPAM solution by Otake et al.29 and has also been observed in a gel formed by the same monomer.30 The compressibility of bulk water is larger than that of the water that surrounds the hydrophobic groups; thus, increasing the pressure further can invert the situation. Above 200 MPa, hydrophobic interactions among alkyl chains are favored, and the extremum in the T and P dependence of the cloud point was observed for both polymer solutions. In ionic solutions, however, the relative volumetric situations of hydrating and bulk water are different. Salting-out ions will promote this inversion, and the transition ellipses will become smaller with the addition of anions with lower lyotropic numbers while salting-in ions will not change the situation. In addition, the presence of SO42- higher than 0.25 M diminishes the apparent antagonism of pressure and temperature, which means the center of the ellipse is no longer in the first (or second) quadrant. These characteristics are related with the thermodynamic parameters. The salting-out ions made the absolute compressibility more negative and the heat capacity slightly larger and hence shifted both the transition pressure and temperature to the lower values. On the contrary, the salting-in ions made the compressibility less negative and shifted the transition pressure to the larger values. These changes in compressibility conquered the counteracting change in ∆V. The rotational angle (θ) of the ellipse around T-P axes is determined by 2θ )tan-1{-2∆R/(∆Cp/T0 + ∆β)}. This means that the ellipse is rotated when ∆R * 0 (nonzero P-T cross term) and that θ becomes larger with larger |∆R| or smaller |∆Cp/T0 + ∆β|. Thus the rotation of the ellipse in the case of KCl addition is mainly related with the larger (absolute) value of ∆R for this system. The results of ionic copolymers will be primarily explained by considering the electrostatic repulsive force in the dehydration process and the weakening of hydrophobic interactions among the side chains. From the pH titration of the aqueous solutions of these copolymers (data not shown), the apparent degrees of dissociation of PA units in these pH ranges were roughly estimated as 0.8 at pH 6.3 for copoly(NIPAM90-PA10) and 0.05 at pH 4.3 and 0.4 at pH 5.1 for copoly(NIPAM85-PA15). On the basis of these values, it is known that the introduction of anionic groups less than 1% is sufficient to shift the transition profile and that the introduction of about 10% made the cloud point practically invisible. This means that a charge content as low as ionomer level is sufficient to prevent the apparent cloud point phenomenon. These titration data (27) von Hippel, P. H.; Wong, K. Y. J. Biol. Chem. 1965, 240, 3909. (28) Suzuki, K.; Taniguchi, Y.; Watanabe, T. J. Phys. Chem. 1973, 77, 1918. (29) Otake, K.; Karaki, R.; Ebina, T.; Yokoyama, C.; Takahashi, S. Macromolecules 1993, 26, 2194. (30) Kato, E.; Kitada, T.; Nakamoto, C. Macromolecules 1993, 26, 1758.
Langmuir, Vol. 15, No. 12, 1999 4061
are only for those under atmospheric pressure. As for the pressure effects on the weak acid dissociation, it is known that carboxylic acids such as acetic acid show rather large and negative ∆V (i.e., -11 mL/mol for acetic acid at 25 °C);31 dissociation is facilitated under high pressure. This means that the shifts of transition curves in these ionic copolymers are enhanced under higher pressure, which might be a part of the reason of the larger effects of ionic residues on the transition pressure than the transition temperature. The apparent disappearance of cloud point in both the SDS-containing medium and the anionic copolymer at higher pH can be commonly explained by assuming that the ionic intermolecular repulsive interactions made it difficult to associate the collapsed molecules in order to form larger aggregates which strongly scatter the white light. In this sense, the ionic copolymer under higher pH conditions more resembles the isolated (independent) protein molecules, but the presently observed transition is a change from a random coil to a globule (collapsed chain). The generally mentioned change (denaturation) in protein structure is, however, a process of losing a globular structure and performing a more random coillike shape. Therefore, the transition in the present synthetic polymers resembles the inverse process of the so-called cold denaturation or transition from a random coillike structure to a disordered globule.17,32,33 This coil-globule transition is believed to be a precursor of the formation of the molten globule structure and recently its general importance in protein structure formation processes has been recognized strongly. Conclusion Extremum of ∆S ) 0 was observed for the coil-globule transition of the PNVIBA and PNIPAM in aqueous solutions at elevated pressures and subzero temperatures. This supported further the applicability of Hawley’s equation to these systems. The transition pressure varied more sensitively with molecular weight, and polymers with a higher molecular weight showed a lower transition pressure and, hence, a smaller ellipse, with larger ∆Cp and ∆β. P-T diagram of transition was strongly influenced by addition of salt, and the results were principally explained by terms of lyotropic numbers (salting-in or salting-out). The salting-out ions made ∆β more negative and ∆Cp slightly larger and hence shifted both the transition pressure and temperature to the lower values, while the salting-in ions made ∆β less negative and shifted the transition pressure to the higher values. When pentenoic acid was introduce to PNIPAM, both LCST and LCSP became higher at higher pH and the results were explained by considering the electrostatic repulsive force in the dehydration process and the weakening of hydrophobic interactions among the side chains. The charge content as low as ionomer level was sufficient to prevent the cloud point phenomenon. The apparent disappearance of cloud point at much higher pH can be related with the ionic (intermolecular) repulsive interactions preventing the association of the collapsed molecules in order to form larger aggregates, which is common to the effect of (anionic) surfactant. LA981184M (31) Kunugi, S. Prog. Polym. Sci. 1993, 18, 805. (32) Karplus, M.; Shakhnovich, E. In Protein Folding; Creighton, T. E., Ed.; W. H. Freeman: New York, 1992; pp 127-195. (33) Ptitsyn, O. B. In Protein Folding; Creighton, T. E., Ed.; W. H. Freeman: New York, 1992; pp 243-300.
