No abstract

We present first results of studies of the volume phase transition in poly(N-isopropylacrylamide) (NIPA) hydrogels by high-sensitive differential scanning calorimetry (HS−DSC). We examine the behavior of the low-concentrated gels at different scanning rates and discuss the dependencies of the transition parameters upon heating rate. It is shown that HS−DSC measurements at heating rate of 0.125 K/min provide results closely approximating the equilibrium ones in every respect except for the value of transition temperature. At higher heating rates, the transition temperature appears to be practically rate-independent, and network exhibits signs of spatially nonuniform structures developing in the course of collapse, with some parts of the system collapsing under the restrictions of others being almost unperturbed. The apparent enthalpy of this constrained transition is higher than that observed at equilibrium. We discuss the hypothesis that the transition occurs through the spinodal decomposition.

Proteinlike copolymers were first predicted by the computer-aided biomimetic design (Physica A 1998, 249, 253-261). These copolymers consist of comonomer units of differing hydrophilicity/ hydrophobicity. Heterogeneous blockiness, characteristic for such copolymers, facilitates chain folding with the formation of specific spatial packing: a dense core consisting of hydrophobic units and a polar shell formed by hydrophilic units. This paper describes the synthesis of N-vinylcaprolactam/Nvinylimidazole copolymers via the redox-initiated radical copolymerization in the medium of 10% aqueous DMSO at the temperatures both below and above the phase separation threshold. The synthesized macromolecular products were separated into thermally precipitating and nonprecipitating fractions. Their molecular weight characteristics were evaluated using size-exclusion chromatography; their comonomer composition was determined from 1 H NMR spectra of copolymers dissolved in DMSO-d6. The temperature-dependent behavior of copolymer macromolecules in water was investigated by thermonephelometry, high-sensitivity differential scanning calorimetry, and 1 H NMR spectroscopy of the copolymers dissolved in D2O. It was shown that thermally nonprecipitating copolymer fractions obtained at initial comonomer molar ratios of 85:15 and 90:10 can be identified as proteinlike copolymers.