Ultra-small-angle neutron scattering studies on phase separation of poly(vinyl alcohol) gels

Time-resolved light scattering measurements during the gelation process of a poly(vinyl alcohol) (PVA) solution in a mixture of dimethyl sulfoxide and water (H2O) have shown that a spinodal decomposition (SD) type phase separ ation takes place in the early stage of gelation. In this case the kinetics of SD is valid only before macroscopic gelation occurs because the growth rate is slowed down by gelation. Such SD type phase separation makes the so lution opaque as it proceeds, and hence the structural change can no longer be followed by light scattering. To investigate the structure of the opaqu e PVA gel as well, we have employed an ultrasmall-angle neutron scattering technique using a Bonse-Hart camera. These observations reveal that even af ter the macroscopic gelation the structure due to the microphase separation on a spatial scale of several mu m continues to grow against the elasticit y. This may be because at first the gel structure is too soft to suppress t he growth of the microphase separation, but within 24 h after the quenching the growth terminates. On the basis of the results, we will discuss a poss ible mechanism of the microphase separation after gelation.