Blends of rigid and flexible macromolecules: Poly(benzimidazole) mixed with poly(vinyl acetate-star-vinyl alcohol) copolymers

Miscibility in a series of blends, derived from poly(benzimidazole) (PBI) and a range of poly(vinyl acetate-star-vinyl alcohol) [PV(Ac/OH)] copolymers with vinyl alcohol contents between 20 and 73 mol%, has been studied by differential scanning calorimetry, dynamic mechanical thermal analysis, Fourier transform infrared spectroscopy (FTi.r.), scanning electron microscopy and optical haze measurements. The results show that as the vinyl alcohol content of the copolymer component increases, the level of dispersion of the two components of the blend improves. While no one-phase blends, miscible at the molecular level, were obtained, the levels of dispersion in blends with PV(Ac/OH) compositions of 56.0 and 73.0 mol% vinyl alcohol were good enough to allow the formation of optically clear films. An enhanced storage modulus (E') for blend 5, which was larger than that for PBI alone, was also obtained. Evidence for significant hydrogen bonding between the blend components was found from FTi.r. measurements, but none of the blends exhibited a single glass transition temperature. Consequently it is estimated that the phase-domain sizes for the well-dispersed blends lie in the range: 20 nm <domain size <500 nm, which are larger than one would expect from single-phase miscible blends.