GELATION PROCESS OF POLY(VINYL ALCOHOL) AS STUDIED BY SMALL-ANGLE NEUTRON AND LIGHT-SCATTERING

We report small-angle neutron scattering (SANS) and light scattering (LS) studies on poly(vinyl alcohol) (PVA) gels formed in a mixture of deuterated dimethyl sulfoxide (DMSO-d(6)) and heavy water at 23 degrees C. It was reported in a previous paper that the SANS intensity I(Q) of the PVA gels is well described by the Ornstein-Zernike (OZ) formula I(0)/(1 + xi(2)Q(2)) and Porod's law Q(4) for 0.01 Angstrom(-1) < Q < 0.035 Angstrom(-1) and 0.05 Angstrom(-1) < Q < 0.1 Angstrom(-1), respectively. In this work, we extended the Q range down to 3 x 10(-3) Angstrom(-1) and found that I(Q) turns up for Q < 8 x 10(-3) Angstrom(-1) to deviate from the OZ formula. This upturn has been assigned to structure due to phase separation based on the results of SANS and LS measurements. In order to investigate the gelation process, time-resolved SANS measurements were carried out on the PVA solutions after quenching to 23 degrees C from 100 degrees C. It was found that the correlation length xi evaluated by the OZ formula in a and range of 0.01-0.035 Angstrom(-1) is dominated by concentration fluctuations in the early stage of the gelation before crystallization (t < 200 min) while, after the crystallization initiates, the average correlation distance between the nearest-neighboring crystallites becomes a dominant factor in xi. Distance distribution function P(r) which is defined by inverse Fourier transformation of the scattering intensity was calculated to see the size and the distribution of the crystallites.