The phase transition of gels containing N-isopropylacrylamide (NIPA) was investigated by differential scanning calorimetry (DSC) and swelling measurements. The enthalpy of dissociation of the hydrophobic interaction per molar unit of NIPA chains (ΔH N), the transition temperature (T c), and the number of water molecules associated with an NIPA monomer, (n) were evaluated as a function of polymer concentration. Significant differences in ΔH N and T c were found between two systems:  poly(NIPA-ran-acrylic acid) (NIPA/AAc; a weakly charged gel) and poly(NIPA-ran-dimethylacrylamide) (NIPA/DMAA; a neutral gel). ΔH N decreases with increasing comonomer concentration. However, a larger decrease in ΔH N was observed for NIPA/AAc than for NIPA/DMAA, which is accounted for by the strong hydrophilic effect of the charged AAc comonomers. No noticeable copolymer concentration dependence in n was observed in both systems. It is suggested that there are two types of water molecules, i.e., one associated with the phase transition, (n − n 0), and the other the lower limit for the hydrophobic hydration, n 0. The roles of these water molecules are discussed in relation to the volume phase transition.

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Dynamic light scattering studies have been carried out on poly(N-isopropylacrylamide) (NIPA) gels having different cross-link densities. Intensity−intensity time correlation functions obtained at 25 °C were successfully analyzed by taking account of the nonergodic nature of gels. By taking the ensemble average (〈I〉E) of the time average scattered intensity (〈I〉T), the dynamic component of the concentration fluctuations (〈I F〉T) and the spatial inhomogeneities (〈I〉E; the ensemble average) were evaluated as a function of cross-link density. While 〈I F〉T remained more or less constant, 〈I〉E increased with the increasing ratio of cross-linker concentration (C BIS) to the monomer concentration (C NIPA), r ≡ C BIS/C NIPA, where BIS is the abbreviation of the cross-linker, N,N‘-methylenebis(acrylamide). This indicates domination of the static inhomogeneity for large r. The correlation length, ξ, a measure of the range of dynamic fluctuations, was evaluated from the time correlation function. A linear relationship was obtained for ξ-1, i.e., 1/ξ = (1/ξsoln) + (1/ ξgel)r, with ξsoln = 144 Å and ξgel = 8.12 Å. The value of ξgel was found to be in good agreement with the literature value for the segment length of NIPA polymers, 8.12 Å.

Spatial inhomogeneities of poly(N-isopropylacrylamide-co-acrylic acid) (NIPA/AAc) gels were investigated by dynamic light scattering as a function of AAc comonomer concentration and temperature. The results showed the following: (1) The ensemble average scattered intensity, 〈I〉E, decreased with increasing AAc comonomer concentration and increased with temperature. (2) Dynamic fluctuations increased with temperature and diverged at the transition temperature, Tc. (3) Above the Θ temperature of NIPA homopolymer in an aqueous solution, the intensity-intensity time correlation function did not fit a single-exponential function, but fit a double exponential or a stretched exponential, indicating a heterogeneous structure, i.e., polymer-rich and polymer-poor phases at temperatures above Θ.

Thermally induced morphology transition from cylindrical to lamellar microdomains wan found for a poly(styrenablock-butediene-block-s~ene) (SBS) triblock copolymer having a 0.66 weight fraction of polystyrene blocka by small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). Polybutadiene (PB) cylinders with hexagonal c l w packing were formed in a polystyrene (PS) matrix when the SBS wan cast from a methyl ethyl ketone solution. It wan found that the PB cylinders were transformed into lamellae on annealing at 160 OC. This is a transition from thermodynamically quasi-stable to stable morphologies. On the baais of SAXS and TEM, the transition turned out to occur via coalescence of cylinders without their translational movementa. It ia propoeed that an undulation of the interface induced by the inatabiIity of the interface might play an important role for the coalescence of the cylinders. It wan also found by TEM that the coherency of lamellar microdomains thus formed became higher with further annealing;i.e., the persistence length of the undulating lamellae became longer.