PHYSICAL AGING IN THE MECHANICAL-PROPERTIES OF MISCIBLE POLYMER BLENDS

Changes in mechanical properties during isothermal physical aging were investigated for three miscible blends: polystyrene(PS)/poly(2,6-dime thyl 1,4-phenylene oxide) (PPO), PS/poly(vinylmethylether) (PVME), and poly(methylmethacrylate) (PMMA)/poly(ethyleneoxide) (PEO). The kineti cs of stress relaxation was investigated for the blend, dilute in one component, and compared with that of the neat major component at equal temperature distances, T-g-T, from the midpoint glass transition temp erature. It is demonstrated that for all three blends, the mean stress relaxation time [tau] does not scale with T-g-T. For PS/PPO and PS/PV ME blends, the stress relaxation rates are faster compared to neat PS; for PMMA/PEO, they are slower than for neat PMMA. Two effects appear to be important in contributing to this discrepancy. First, addition o f the second component produces a change in the packing density of the blend: less dense for PS/PPO and PS/PVME; more dense for PMMA/PEO. Co mparison of average free volume hole sizes and fractional free volumes measured via orthopositronium annihilation lifetime measurements for all three blends versus the pure constituents are qualitatively consis tent with this interpretation. Second, because of the presence of conc entration fluctuations in the blend, it is expected that the initial s tress decay is dominated by regions enriched in the more mobile compon ent. From observations of the change in width of the stress relaxation time distribution, this effect appears to be particularly significant in the PS/PVME blend. (C) 1997 John Wiley & Sons, Inc.