MISCIBLE BLENDS OF CELLULOSE AND POLY(VINYLPYRROLIDONE)

Cellulose was dissolved in the solvent system dimethyl sulfoxide-paraformaldehyde (DMSO-PF) and blended with poly(vinylpyrrolidone) (PVP) dissolved in DMSO. The homopolymers and blend films were solution cast at 25-degrees-C under reduced pressure. To ascertain the state of miscibility of the blends, they were investigated by using wide-angle X-ray scattering (WAXS), Fourier transform infrared (FTIR) spectroscopy, differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), and solid-state magic-angle spinning (CP-MAS) NMR. X-ray analysis revealed that in the blends each polymer influences the structural order of the other. A detailed estimation of the glass transition temperature (T(g)) by DMA revealed that cast cellulose displays a T(g) at 208-degrees-C and that every blend shows a single T(g), which varies with composition. The increase in T(g) in going from the T(g) of PVP to the T(g) of cellulose is not monotonic but shows a discontinuity at a composition of ca. 60% (w/w) cellulose. This behavior is tentatively interpreted in relation to the molar ratio of the repeating units of the constituent polymers. The miscibility was shown, from FTIR and CP-MAS NMR, to be driven by the interaction between a portion of the ensemble of the carbonyl groups of PVP and the primary hydroxyl functionalities of cellulose. From solid-state CP-MAS NMR measurements of proton T1-rho relaxation times, it is suggested that below the discontinuity in the T(g) vs composition curve the two polymers mix on a scale of ca. 2.7 nm and that above the discontinuity they mix on a scale between 2.7 and 15 nm.