Modes selection in polymer mixtures undergoing phase separation by photochemical reactions

Phase separation kinetics and morphology of binary polymer mixtures (A/B) i n the presence of photochemical reactions were investigated by using phase- contrast optical microscopy combined with digital image analysis. The polym ers were chemically designed in such a way that two types of chemical react ions, intermolecular photodimerization and intramolecular photoisomerizatio n, of polymer segments can be induced and controled by irradiation with ult raviolet light. Unlike the conventional case, the phase separation in the p resence of these reactions is spontaneously frozen due to the suppression o f the long-wavelength instabilities, resulting in stationary spatial struct ures with intrinsic periodicities. These characteristic length scales are d etermined by the competition between the two antagonistic interactions: pha se separation as a relatively short-range activation and the photochemical reaction as a long- range inhibition. Furthermore, it was found that the sp atial symmetry breaking of concentration fluctuations can emerge from the e lastic stress associated with the nonhomogeneous kinetics of the reactions. Experimental data obtained with three types of reactions: A-A only cross-l ink, A-A and B-B simultaneous cross-links and the reversible A double left right arrow B photoisomerization are described. These results do not only i ndicate that combination of chemical reactions and phase separation could p rovide a novel method to control the morphology of multiphase polymer mater ials, but also suggest that photoreactive polymers can be used as a chemica l system to study the mode-selection process in polymers far from thermodyn amic equilibrium. (C) 1999 American Institute of Physics. [S1054-1500(99)00 302-X].