Phase Behavior and Segmental Mobility in Binary Blends of Polystyrene and Poly(vinyl methyl ether)

Abstract

The phase behavior and segmental mobility in binary blends of polystyrene (PS) and poly(vinyl methyl ether) (PVME) were investigated. Two nearly monodisperse PSs having weight-average molecular weights (Mw) of 57 000 and 95 000 with polydispersity indexes (PDI) of 1.06 and 1.09, respectively, and PVME having Mw = 99 000 with a PDI of 2.13 were used for this study. Two sets of PS/PVME binary blends with varying compositions were prepared by solvent casting from toluene. Thermograms from differential scanning calorimetry showed that each blend has a single, yet very broad glass transition temperature. Cloud point measurements via He−Ne laser light scattering were taken to determine phase equilibria in each blend, which exhibited lower critical solution temperature (LCST) behavior. Solid-state nuclear magnetic resonance (NMR) spectroscopy was used to examine segmental mobility and component domain sizes. 1H T1ρ experiments were run at temperatures ranging from −40 to 140 °C. We observed only small differences in 1H T1ρ values of PS and PVME at temperatures below 45−80 °C (depending on blend composition) and a large divergence of 1H T1ρ values at higher temperatures. 13C T1ρ and wide-line separation (WISE) experiments were run at room temperature on untreated and heat-treated samples. WISE experiments revealed that heterogeneities from 3.5 nm to greater than 30 nm existed within the blends, depending on the temperature of heat treatment. Since it has been found that 1H T1ρ measurements can give ambiguous domain information, 1H-NOESY NMR was used to examine several blend compositions at 100 °C. We conclude from this study that nanoheterogeneities exist in these PS/PVME blends at temperatures below the binodal curve determined by cloud point measurements and that a broad, single glass transition should not be construed as evidence of miscibility at the molecular level. It has been shown that nanoheterogeneities exist on a segmental level and that there are large changes in mobility at temperatures above 45−80 °C. However, the blend does not phase separate until the critical temperature (LCST), determined by cloud point measurements, is reached.