MOLECULAR-ORIENTATION AND OPTICAL ANISOTROPY INDUCED BY THE STRETCHING OF POLY(VINYL ALCOHOL) POLY(N-VINYL PYRROLIDONE) BLENDS

The polymer pair, poly(vinyl alcohol)/(N-vinyl pyrrolidone) (PVA/PVP), is thermodynamically miscible in the amorphous state, as judged from the detection of a single T(g) which is situated between the T(g)s of the two homopolymers, over the whole range of compositions of the bina ry blend. The molecular orientation and optical anisotropy induced by uniaxial deformation of the polymer blends were characterized through fluorescence polarization and birefringence measurements. Two series o f samples, designated as PVA/PVP-H and PVA/PVP-L, were examined; these were cast as films from aqueous polymer solutions containing a slight amount of a stilbene derivative as a fluorescent probe (the notations H and L indicate whether the molecular weight of the PVP is much high er or lower than that of the PVA). In both series of blends, developme nt of the molecular orientation in the drawn films became gradually su ppressed with increasing PVP content. PVA/PVP-H blends acquired a high er orientation than the corresponding PVA/PVP-L blends with the same c omposition. The optical birefringence of the oriented films, when comp ared at a given draw ratio for a series of blends, decreased drastical ly with an increase in the PVP fraction and eventually changed from po sitive to negative values at a certain fraction. The critical PVP frac tion was much lower in the PVA/PVP-H series than in the corresponding series. An explanation for this is presented in terms of an effect of birefringence compensation, due to the positive and negative contribut ions of oriented PVA and PVP, respectively. The molecular weight depen dence of the segmental relaxation of PVP chains in the drawing process and that of the frequency of interpolymer interactions are taken into account in the discussion.