Structure of Poly(vinyl alcohol) Gels Studied by Wide- and Small-Angle Neutron Scattering

Abstract

The structure of poly(vinyl alcohol) (PVA) gels formed in a mixture of deuterated dimethyl sulfoxide (DMSO-de) and heavy water at 23 °C has been investigated by wideand small-angle neutron scattering techniques. It was directly confirmed from the wide-angle neutron scattering measurements that cross-linking points or junction points in the gels are crystallites. In the small-angle scattering measurements, it was found that the scattering intensity I(Q) decreases with Q according to the -4th power law (Porod's law) in the Q range above 0.05 A-1 where Q = 4 sin 6/ (26 and are scattering angle and wavelength of neutron, respectively), suggesting that the surface of the crystallites has a clear boundary. On the other hand, the Q dependence of the scattering intensity 7(Q) can be described by 7(0)/[l + £2Q2] in the Q range below 0.035 A-1, where £ is a correlation length. The correlation length £ was assigned to the average distance between the neighboring crystallites. Distance distribution function P(r) which is defined by inverse Fourier transformation of scattering intensity 7(Q) was also calculated in order to see another aspect of the scattering intensity. Two peaks or shoulders at about 70 and 200 A were recognized in the calculated P(r). The former and the latter have been attributed to the intraand intercrystallite correlations, respectively. By separating the two correlations, the distribution of the crystallites is discussed.

Structure and dynamics of poly(vinyl alcohol) gels in mixtures of dimethyl sulfoxide and water

“…The overlap concentration could be calculated from the intrinsic viscosity, but this parameter was difficult to measure for the PVA in a mixture of DMSO and water (60/40 v/v) because of its aggregated nature. 19 In the temperature range between BÀ20 and B40 1C, each sample exhibits a C * value less than C Ã Rg . If the solution is homogeneous, gel formation is difficult at a concentration less than C Ã Rg because all the polymer chains cannot contact.…”

“…PVA is a typical crystalline polymer; crosslinking points in the gels are therefore believed to be crystallites, although this hypothesis has never been directly confirmed. 19 We conducted WANS measurements of deuterated PVA (PVA-d 4 ) gel at a PVA concentration of 10 g dl À1 in a mixture of DMSO-d 6 Figure 4a in the Q range of 0.8 to 10 Å À1 . Figure 4b was obtained when the scattering intensity of the solvent was subtracted from that of the gel after correcting for the volume of solvent molecules replaced by PVA molecules.…”

“…SANS measurements were performed on the PVA gels in DMSO-d 6 /D 2 O (60/40 v/v) at 23 1C in a Q range from 0.01 to 0.1 Å À1 using a small-angle scattering machine at KEK. 19 Figures 5a shows the scattering intensities I(Q)s of the PVA gels with C p ¼2, 5 and 10 g dl À1 , which were plotted in doublelogarithmic form after being normalized against C p . In the Q range above B0.05 Å À1 , the scattering intensity I(Q) decreased with increasing Q according to the 4th power law for all the concentrations:…”

“…For the past two decades, we have studied the structures and structure formation processes of PVA gels on a wide scale using techniques such as macroscopic observations, 12,13,17 wide-, small-and ultra-small-angle neutron scattering (WANS, SANS and USANS), [18][19][20][21][22][23] and light scattering. [24][25][26][27] In addition, neutron spin echo (NSE) measurements have also been performed to elucidate the dynamic properties of three kinds of PVA gels.…”

SANS Characterization of an Anisotropic Poly(vinyl alcohol) Hydrogel with Vascular Applications

“…18 The structure of PVA hydrogels has been probed by confocal microscopy, 19 transmission electron microscopy (TEM), solidstate nuclear magnetic resonance ( 13 C NMR) and small-angle X-ray scattering (SAXS), 20 light scattering, 21 and both smalland ultrasmall-angle neutron scattering (SANS and USANS, respectively). [22][23][24] On the basis of his study, Willcox et al 20 proposed a model for the formation of these gels whereby ice crystals formed during the low-temperature phase of the thermal cycles concentrate the polymer solution in regions between the ice crystals, promoting the formation of PVA crystallites. The crystallites act as cross-linking centers for dangling chains, which can bridge the gap between neighboring crystallites.…”