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 binary 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 of 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 higher or lower than that of the PVA). In both series of blends, development of the molecular orientation in the drawn films became gradually suppressed with increasing PVP content. PVA/PVP-H blends acquired a higher orientation than the corresponding PVA/PVP-L blends with the same composition. The optical birefringence of the oriented films, when compared at a given draw ratio for a series of blends, decreased drastically with an increase in the PVP fraction and eventually changed from positive to negative values at a certain fraction. The critical PVP fraction 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 contributions of oriented PVA and PVP, respectively. The molecular weight dependence 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.