The Role of Density and Temperature in the Dynamics of Polymer Blends

The relative effect of volume and thermal energy on the local segmental dynamics, as reflected in the ratio of the isochoric, E V, and isobaric, E P, activation enthalpies, is determined for blends of polystyrene (PS) with poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) and with poly(vinyl methyl ether) (PVME). We find that neat PPO near T g has the lowest value of E V / E P = 0.25 ± 0.02 reported for any polymer, indicating volume-dominated dynamics. Addition of the lower T g PS alleviates constraints on local motion, resulting in a weaker volume effect. The opposite situation prevails with PS/PVME blends. PS has a higher T g, and in blends segmental relaxation of the PVME becomes more controlled by volume than for neat PVME. We also show herein that the relaxation times for the PVME/PS blends measured at various T and P superpose when plotted vs T υ γ, where υ is the specific volume and γ is a material constant. This scaling, which has previously been demonstrated for various neat glass-formers and is reported herein for the first time for a blend, enables E V / E P to be determined at T g in the absence of actual measurements near T g .