Phase separation kinetics and morphology of binary polymer mixtures (A/B) in the presence of photochemical reactions were investigated by using phase-contrast optical microscopy combined with digital image analysis. The polymers were chemically designed in such a way that two types of chemical reactions, intermolecular photodimerization and intramolecular photoisomerization, of polymer segments can be induced and controled by irradiation with ultraviolet light. Unlike the conventional case, the phase separation in the presence of these reactions is spontaneously frozen due to the suppression of the long-wavelength instabilities, resulting in stationary spatial structures with intrinsic periodicities. These characteristic length scales are determined by the competition between the two antagonistic interactions: phase separation as a relatively short-range activation and the photochemical reaction as a long-range inhibition. Furthermore, it was found that the spatial symmetry breaking of concentration fluctuations can emerge from the elastic stress associated with the nonhomogeneous kinetics of the reactions. Experimental data obtained with three types of reactions: A-A only cross-link, A-A and B-B simultaneous cross-links and the reversible A<-->B photoisomerization are described. These results do not only indicate that combination of chemical reactions and phase separation could provide a novel method to control the morphology of multiphase polymer materials, but also suggest that photoreactive polymers can be used as a chemical system to study the mode-selection process in polymers far from thermodynamic equilibrium. (c) 1999 American Institute of Physics.
A ternary polymer blend with two components photo-cross-linked independently in its miscible region undergoes phase separation, exhibiting morphology with multiple length scales. Contrary to the case of thermally induced phase separation, the morphology exhibits a unimodal-->multimodal transition. It is shown that these multiple length scales are caused by the inhomogeneous freezing kinetics of the cross-linking process. This inhomogeneity arises from the autocatalytic feedback driven by the couplings between concentration fluctuations and the photo-cross-linking reactions.
The dc-electric-field dependence of the dielectric properties (complex permittivity and relaxation time) of the antiferroelectric CsH2PO4 was investigated at 4.5 kbar under various frequencies. The strong dielectric dispersion along the b axis associated with the field-enforced ferroelectric phase transition was observed. The slowing down of the relaxation time toward a critical electric field E", which induces the forced transition to the ferroelectric phase, was observed. Although the dielectric dispersion strength is zero without an applied dc field Eq, with an increase in Ez it increases gradually, rapidly near E" in the antiferroelectric phase and decreases monotonically in the ferroelectric phase. The relaxation time of the order of 10 s near E« in the antiferroelectric phase for the field-enforced transition is extremely 1ow in contrast to that of the order of 10 s near the transition temperature in the paraelectric phase for the paraelectricferroelectric phase transition.
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