Phase behavior and segmental mobility in binary blends of polystyrene and poly(vinyl methyl ether)

The phase behavior and segmental mobility in binary blends of polystyrene ( PS) and poly-(vinyl methyl ether) (PVME) were investigated. Two nearly mono disperse PSs having weight-average molecular weights (M-w) of 57 000 and 95 000 with polydispersity indexes (PDI) of 1.06 and 1.09, respectively, and PVME having M-w = 99 000 with a PDI of 2.13 were used for this study. Two s ets of PS/PVME binary blends with varying compositions were prepared by sol vent casting from toluene. Thermograms from differential scanning calorimet ry showed that each blend has a single, yet very broad glass transition tem perature. Cloud point measurements via He-Ne laser light scattering were ta ken to determine phase equilibria in each blend, which exhibited lower crit ical solution temperature (LCST) behavior. Solid-state nuclear magnetic res onance (NMR) spectroscopy was used to examine segmental mobility and compon ent domain sizes. H-1 T-1 rho experiments were run at temperatures ranging from -40 to 140 degrees C. We observed only small differences in H-1 T-1 rh o values of PS and PVME at temperatures below 45 -80 degrees C (depending o n blend composition) and a large divergence of H-1 T-1 rho, Values at highe r temperatures. C-13 T-1 rho and wide-line separation (WISE) experiments we re run at room temperature on untreated and heat-treated samples. WISE expe riments revealed that heterogeneities from 3.5 nm to greater than 30 nm exi sted within the blends, depending on the temperature of heat treatment. Sin ce it has been found that H-1 T-1 rho measurements can give ambiguous domai n information, H-1-NOESY NMR was used to examine several blend compositions at 100 degrees C. We conclude from this study that nanoheterogeneities exi st in these PS/PVME blends at temperatures below the binodal curve determin ed by cloud point measurements and that a broad, single glass transition sh ould not be construed as evidence of miscibility at the molecular level. it has been shown that nanoheterogeneities exist on a segmental level and tha t there are large changes in mobility at temperatures above 45-80 degrees C . However, the blend does not phase separate until the critical temperature (LCST), determined by cloud point measurements, is reached.