A RHEO-INFRARED-DICHROISM STUDY OF SHEARED MOLTEN POLYSTYRENE FRACTIONS

The development of orientation in molten narrow molar mass fractions o f polystyrene during shear flow in the non-Newtonian shea. rate range (11-24 s(-1)) at temperatures between 225 and 275 degrees C has been a ssessed by measurement of the infrared dichroism. Deviation from linea rity between shear stress and orientation was found at high shear stre sses in the steady state region. A sudden change in the shear rate cau sed a change in the shear stress that was one to two orders of magnitu de faster than the response in the Hermans orientation function (f). T he steady-state level of the Hermans orientation function (f(ss)) was dependent on temperature, shear rate and molar mass (M): f(ss) = C-1-C -2/M (where C-1 and C-2 are constants). The time to reach steady state on the inception of shea. was proportional to M-1/2. The recovery of the isotropic state after the cessation of shear followed initially a simple exponential law: f proportional to e(-1), where t is the time a nd tau is the relaxation time. The relaxation time showed only a weak molar mass dependence (tau proportional to M-0.8), indicating that the decay of orientation was initially governed by processes other than r eptation.