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 d euterated 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) o f 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), respe ctively. In this work, we extended the Q range down to 3 x 10(-3) Angs trom(-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 struc ture due to phase separation based on the results of SANS and LS measu rements. In order to investigate the gelation process, time-resolved S ANS 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 An gstrom(-1) is dominated by concentration fluctuations in the early sta ge of the gelation before crystallization (t < 200 min) while, after t he 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 Fouri er transformation of the scattering intensity was calculated to see th e size and the distribution of the crystallites.