Shunsuke Hirotsu

Softening of bulk modulus and negative Poisson’s ratio near the volume phase transition of polymer gels

The static bulk and shear modulus of poly (N‐isopropylacrylamide)/water gel (NIPA gel) have been obtained as a function of temperature by measuring the change of sample size induced by an applied tension. It is deduced that the method gives the static elastic moduli of gels to a good approximation except just above the discontinuous transition temperature T0, where the stress‐induced shift of T0 made the estimation of elastic strain impossible. Two kinds of NIPA gels with different polymer concentration and crosslinking density were used as samples, of which one undergoes a discontinuous volume phase transition and the other a continuous one. In both gels, the bulk modulus softens drastically towards the transition showing that the soft mode of this transition is a uniform and isotropic volume‐deformation mode. The shear modulus, on the other hand, shows only a small anomaly at the transition. The anomaly exhibited by the Poisson’s ratio σ is quite peculiar, i.e., it becomes negative and approaches −1 tow...

Critical points of the volume phase transition in N‐isopropylacrylamide gels

Two critical points of the gel–gel transition in N‐isopropylacrylamide gels were determined on the basis of the measurement of the coexistence curve. One of them was realized by changing the composition of solvent (water–methanol mixture) and the other by applying the osmotic pressure to a gel network, which was generated by adding large molecules to the outer solution. From the result the phase diagram in the xm(solvent composition)–T(temperature)–π(osmotic pressure) space was constructed. The order parameter of the gel–gel transition was determined for the first time as functions of temperature and solvent composition. The existence of the critical points was discussed in terms of recent theoretical studies.

Phase transition of a polymer gel in pure and mixed solvent media

A volume phase transition of poly-N-isopropylacrylamide (NIPA) gel immersed in some pure and mixed solvents has been investigated experimentally. A slightly discontinuous transition of NIPA get in water changed to a more distinctly first-order transition on adding a small amount (<20%) of alcohols to water. As the concentration of alcohols in the mixed solvent increased further, the volume transition blurred and eventually disappeared. Analyzing the experimental results on the basis of the mean field theory of polymer solution, it has been shown that the polymer-solvent interaction parameter χ exhibits a drastic change at the volume transitions. This indicates that the enthalpy and entropy of solution depend strongly on polymer concentration.

Critical Phenomena and Anomalous Thermal Hysteresis Accompanying the Normal-Incommensurate-Commensurate Phase Transitions in Rb2ZnCl4

Measurements were made of the dielectric constant along the ferroelectric axis e a , the spontaneous polarization P s and the heat capacity c p of Rb 2 ZnCl 4 crystals in the temperature range from -150^° C to 150°C. The normal-incommensurate transition point determined as the peak position of c p lies at 30.1°C, which is about 6°C lower than the temperature for the maximum of e a . It was found that e a and P s show a characteristic thermal hysteresis over a wide temperature range including the incommensurate-commensurate trarasition point (-80^° C ). The thermal hysteresis occurs even when heating and cooling runs are cycled within the incommensurate phase. The observed thermal hysteresis is understood if we assume that defects such as impurities act as obtacles to diffusion of discommensurations and prevent the crystal from reaching thermal equilibrium.

Static and time-dependent properties of polymer gels around the volume phase transition

Abstract Recent progress in the study of the volume phase transition of polymer gels is reviewed. The phenomenological theories of swelling equilibrium and phase transition of gels are summarized, and some basic experimental results on poly-N-isopropylacrylamide (NIPA) gels are compared with the prediction from these theories. Special attention is paid to the elastic properties of the gel network near the volume phase transition. The effect of external stresses on the swelling and the phase transition is analyzed. Some anomalous and unique characteristics revealed in NIPA gels such as shape- and size-dependent swelling and phase transition properties, curious phase coexistence, and domain structure are presented. Experimental results on some time-dependent phenomena such as phase separation, spinodal decomposition, and pattern formation are also presented and discussed. Some problems inherent to gels from biological bodies are briefly discussed.

Experimental Studies of Structural Phase Transitions in CsPbCl3

Some experimental investigations have been made in order to understand the nature of phase transitions in the perovskite-type crystal CsPbCl 3 . Measurements of the birefringence and conoscopic observations confirm three phase transitions occurring at 37°C, 42°C and 47°C, respectively. The crystal system in each phase and the superstructure below 37°C are determined from the results of these measurements and an X-ray work. A dielectric measurement shows that CsPbCl 3 is neither ferroelectric nor antiferroelectric in any of the phases. The transition entropies estimated from the result of a specific heat measurement are considerably smaller than that reported by Moller. The elastic compliance measured by the method of composite oscillator shows anomaly at each transition point. Temperature-dependent Raman spectra demonstrate the existence of “soft” modes which are overdamped near the transition temperatures. The nature of these phase transitions is discussed on the basis of the experimental results.

Kinetics of volume phase transition in poly(N-isopropylacrylamide) gels

Kinetics of volume phase transition in poly(N-isopropylacrylamide) (NIPA) gels jumped from a low-temperature swollen phase to a high-temperature shrunken phase was studied as functions of NIPA monomer and crosslinker concentrations. We found for the first time a clear kinematical boundary at which the shrinking relaxation time of gels changes discontinuously by 102–104 times, and that the profile of the boundary correlates with the sol-gel transition line and the contour line of turbidity of gels. A “morphological” boundary which characterizes the emergence of the bubble formation on gel surface was also determined. The theoretical calculation of the phase diagram on the basis of the mean field theory shows qualitatively that the shrinking speed of gels could be connected with the depth of the thermodynamic region of the spinodal instability (K+4μ/3=0) into which they are transferred where K and μ are the bulk and the shear moduli, respectively. A mechanism of discontinuous change of the shrinking speed i...

Effect of impurities on the incommensurate-commensurate phase transitions in Rb2ZnCl4, K2ZnCl4 and Rb2ZnBr4

Abstract The thermal hysteresis accompanying the incommensurate-commensurate phase transitions in Rb2ZnCl4, K2ZnCl4 and Rb2ZnBr4 is assumed to be caused by pinning of the incommensurate wave by defects in the crystal. The results obtained for (Rb1-xKx)2ZnCl4 mixed crystal system are also well explained in terms of such pinning effect of defects.

Light scattering study of condensation of poly(N-isopropylacrylamide) chain

The spectrum of light scattered from poly (N-isopropylacrylamide) (NIPA) chains (molecular weight M W ∼1.3×10 6 ) in water has been investigated as a function of temperature by using photon correlation spectroscopy. In a dilute solution (27 µg/cm 3 ) we observed a sharp change in polymer size that corresponds to a coil-globule transition of a single polymer chain. The transition occurred at 34.1°C, accompanied by a sharp decrease in hydrodynamic radius from 1000 to 600 A as the temperature was increased. In a semi-dilute solution (2.7 mg/cm 3 ) a phenomenon of critical slowing down in the polymer concentration fluctuations was observed around the phase separation temperature, which is very similar to the previous observation on NIPA/water gel.

Volume phase transition of gels under uniaxial tension

We present the first experimental evidence of the effect of uniaxial tension on a volume phase transition (or gel-gel transition) of polymer gels. With an increase in the magnitude of tension, the first-order transition temperature T 0 of poly-N-isopropylacrylamide/water gel increases and the size discontinuity at the transition becomes larger. These results are consistent with a phenomenological theory of volume phase transition of gels under anisotropic stress. The magnitude of the shift of T 0 with tension can be correctly predicted using empirical parameters determined in previous studies.