Microstructure of poly(vinyl alcohol) hydrogels produced by freeze/thaw cycling

To understand the reversible gelation and subsequent aging of hydrogels pre pared by freeze/thaw processing of poly(vinyl alcohol) (PVOH) solutions, th e microstructures of gels prepared by different freeze/thaw protocols and a ged to varying extents are studied by cryogenic transmission electron micro scopy, solid-state nuclear magnetic resonance, X-ray scattering, and differ ential scanning calorimetry (DSC). As discussed in the literature, gelation by the freeze/thaw process occurs as a homogeneous aqueous poly(vinyl alco hol) solution is cycled, perhaps multiple times, between temperatures above 0 degrees C and well below 0 degrees C. The current investigation has dete rmined that a few percent of chain segments crystallize during the first cy cle, organizing themselves into 3-8 nm primary crystallite junctions separa ted on an irregular mesh by an average spacing of similar to 30 nm. Aging o r imposition of additional freeze/thaw cycles augments the level of crystal linity and transforms the as-formed liquid-like microstructure, characteriz ed in the electron microscope by rounded similar to 30 nm pores, into a fib rillar network. Observation that the transformation occurs at fixed mesh sp acing and approximately constant average crystallite size suggests the form ation of secondary crystallites that do not affect network connectivity. De ndritic ice crystallization and possibly spinodal decomposition superimpose on this nanoscale structure a matrix of much larger pores. (C) 1999 John W iley & Sons, Inc.