EFFECTS OF SHEAR ON MISCIBLE POLYMER BLENDS - INSITU FLUORESCENCE STUDIES

The effects of shear flow on the phase behavior of a miscible blend of polystyrene with poly(vinyl methyl ether) near the lower critical solution temperature have been investigated by fluorescence measurements. For this purpose approximately 1% of the polystyrene chains in the blend were labeled with anthracene and the fluorescence quenching was used as an indicator of miscibility. The modification of a Rheometrics RMS-800 rheometer for in situ fluorescence measurements along with simultaneous shear and normal stress studies under well-controlled temperature and deformation conditions is described. Following equilibration in the miscible state approximately 25 K below the coexistence temperature, slow heating to temperatures in the two-phase region shows that shearing at a constant rate increases the coexistence temperature. The elevation above the quiescent spinodal temperature is related to shear rate as DELTA-T/T(s) = (0.015 +/- 0.002) gamma-0.59 +/- 0.04 and this relation is independent of composition in the range of 20-60% (w/w) polystyrene, within the experimental error. Isothermal experiments in the two-phase region also show shear-induced mixing. The in situ fluorescence measurements provide a sensitive indicator of phase transitions that cannot be unambiguously identified by the measured shear stress and first normal stress difference alone.