Internal stresses in hypercrosslinked polystyrene: Polarized optical studies

Polarized optical microscopy is used for studying the hypercrosslinked PS network prepared by the crosslinking of DC swollen spherical beads of styrene copolymer containing 0.5% of divinylbenzene by monochlorodimethyl ether up to 100% crosslinking. The removal of the solvent decreases the volume of the sample and leads to the development of internal stresses, which manifest themselves as strong optical anisotropy of dry beads. On the contrary, upon swelling in toluene, the network is expanded and shows anisotropy. When the network swells in any thermodynamically good solvent, existing stresses invert their sign and this network appears to be free of any internal stresses. This state of the network should be treated as the I state, in which beads are optically isotropic. This state can also be achieved by swelling of a hypercrosslinked polymer in methanol, which eliminates compressive stresses in the hypercrosslinked network via increase in volume but cannot extend it like toluene. Conventional gel copolymers of styrene with divinylbenzene prepared without any toluene or in the presence of minor amounts of toluene are fully isotropic only in their dry state. These copolymers are inert to methanol but swell in hexane and cyclohexane, and their swelling is accompanied by the development of internal stresses.