ZIGZAG MORPHOLOGY OF A POLY(STYRENE-B-HEXYL ISOCYANATE) ROD COIL BLOCK-COPOLYMER

The solid state morphology of an anionically synthesized P(S-b-HIC) ro d-coil block copolymer was studied using a number of techniques and ca sting solvents. Liquid crystalline ordering was seen in concentrated s olutions using optical microscopy(OM). Bulk and thin film samples cast from solutions in toluene and studied using transmission electron mic roscopy (TEM) revealed a new zigzag morphology. Electron diffraction ( ED) experiments were able to show that the PHIC rods were tilted with respect to the interface separating the PS and PHIC domains. In additi on, the PHIC rods were found to be highly crystalline, having an 8(3) or 8(5) helical conformation and packing in a two chain monoclinic or triclinic (pseudohexagonal) unit cell with a = b = 15.1 Angstrom c = 1 5.6 Angstrom, gamma = 120 degrees, and a crystal density of 1.10 g/cm( 3). A model for the zigzag morphology which allows interdigitation of the rods is consistent with TEM and ED results as well as domain spaci ng predictions based on molecular weight information. The formation of such a morphology is also consistent with thermodynamic arguments bas ed on a theory developed by Halperin for rod-coil block copolymers if, in addition, quantization of the allowed tilt angle by crystallizatio n is taken into account. Solvent quality was found to profoundly affec t the morphology formed from solution cast samples. In addition to the zigzag morphology, morphologies consisting of fragmented PS, zigzags and micelle-like regions were also observed. The choice of solvent mos t likely determines what phases macrophase separate from the isotropic solution before microphase separation of the rod-coil and crystalliza tion of the PHIC take place and also whether chain stretching or inter facial energy is more dominant in the thermodynamics of microphase sep aration.