Effect of ionization on the temperature- and pressure-induced phase transitions of poly(N-isopropylacrylamide) gels

Abstract

The ionization effects on the pressure-induced phase transition of weakly charged poly(N-isopropylacrylamide-co-acrylic acid) (PNIPA-AAc) gels have been investigated by small-angle neutron scattering. At low temperature, T, and pressure, P, the structure factor of PNIPA-AAc gels was well represented by a Lorentzian (L) function, which was similar to noncharged PNIPA gels. However, at high Ps, the contribution of inhomogeneities became large and a squared-Lorentzian term had to be added in addition to the L term. At high Ts, on the other hand, a scattering maximum appeared, indicating microphase separation. This scattering maximum was suppressed by increasing P up to P approximately 100 MPa and then reincreased at higher Ps. The following facts were disclosed: (1) The peak position and height were very sensitive to P, which is mainly ascribed to strong pressure dependence of hydrophobic interaction, (2) ionization leads to microphase separation at elevated temperatures, (3) the re-entrant phase behavior is commonly observed in the P-T plane due to the parabolic variation of the polymer-solvent interaction with P, and (4) the pressure and temperature dependence of the structure factor was reproduced with the Rabin-Panyukov theory and was interpreted with a convexity of hydrophobic interaction with respect to pressure.

Small-angle neutron scattering on polymer gels: phase behavior, inhomogeneities and deformation mechanisms

“…The pressure and temperature dependence of the structure factor of PNIPA/AAc gels was reproduced with the Panyukov-Rabin theory and interpreted with a convexity of hydrophobic interaction with respect to pressure. 19 Figure 7 Two-dimensional (2D) small-angle neutron scattering (SANS) patterns of poly(N-isopropylacrylamide)/acrylic acid gels before (l¼1.07) and after deformation (l¼1.25).…”

“…We performed a series of high-pressure SANS studies on polymer-aqueous systems to elucidate the pressure dependence of the hydrophobic interactions in PNIPA neutral gels, 17 PNIPA/AAc charged gels, 18,19 hydrophilic-hydrophobic block copolymer aqueous solutions [20][21][22] and globular proteins. 56 Figure 8 shows a pressure cell for SANS (PCI-400-SANS, Teramex, Kyoto, Japan).…”

“…61 We have discussed the reason for the convexity of the phase diagram in previous work. 18,19 According to the Clapeyron-Clausius equation, the derivative of pressure with respect to temperature is…”

“…Third, we review the applications of SANS to harsh environments, that is, phase behavior of polymeric systems under very high hydrostatic pressures. [17][18][19][20][21][22] Here, the roles of hydrophobic interactions in aqueous systems are discussed as a function of pressure. Fourth, the structural characterization of various types of super-tough gels will be shown by focusing on deformation mechanisms.…”

Surprising shrinkage of expanding gels under an external load

“…Externally applied stress has also been shown to change the EWC of gels [2][3][4] and to shift the transition temperature of thermally induced discontinuous phase transitions [14][15][16][17] , such as the classical gel collapse that occurs in poly(N-isopropylacrylamide) at 33 • C. Practical actuator devices will always be subjected to external loads, so it is important to characterize their response under load.…”