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

Abstract

To understand the reversible gelation and subsequent aging of hydrogels prepared by freeze/thaw processing of poly(vinyl alcohol) (PVOH) solutions, the microstructures of gels prepared by different freeze/thaw protocols and aged to varying extents are studied by cryogenic transmission electron microscopy, solid‐state nuclear magnetic resonance, X‐ray scattering, and differential scanning calorimetry (DSC). As discussed in the literature, gelation by the freeze/thaw process occurs as a homogeneous aqueous poly(vinyl alcohol) solution is cycled, perhaps multiple times, between temperatures above 0 °C and well below 0 °C. The current investigation has determined that a few percent of chain segments crystallize during the first cycle, organizing themselves into 3–8 nm primary crystallite junctions separated on an irregular mesh by an average spacing of ∼ 30 nm. Aging or imposition of additional freeze/thaw cycles augments the level of crystallinity and transforms the as‐formed liquid‐like microstructure, characterized in the electron microscope by rounded ∼ 30 nm pores, into a fibrillar network. Observation that the transformation occurs at fixed mesh spacing and approximately constant average crystallite size suggests the formation of secondary crystallites that do not affect network connectivity. Dendritic ice crystallization and possibly spinodal decomposition superimpose on this nanoscale structure a matrix of much larger pores. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 3438–3454, 1999

A real time study on drying and the mechano-optical behavior of polyvinyl alcohol films in solid and swollen state

“…Crystallinities were determined by dividing the area above the melting endotherm peak with heat of fusion of 100% crystalline PVA as shown in the following equation. The heat of fusion for 100% crystalline PVA employed was 150 J/g [27,28].…”

The effect of the annealing on the poly(vinyl alcohol) obtained by freezing–thawing

“…When PVA films are annealed at constant temperature (between the glass transition and the melting temperatures), the macromolecular chains have enough mobility to align themselves and fold to form crystals [13]. It is well known that the increasing in the polymer crystallinity degree, resulting in a more resistant material [20]. The swelling and mechanical properties of PVA hydrogels were also examined.…”

Poly(vinyl alcohol)/Clay‐Based Nanocomposite Hydrogels: Swelling Behavior and Characterization

“…[6] This interesting characteristic of PVA solutions has been used to produce PVA hydrogels without chemical crosslinkers to design non-toxic and biocompatible biomedical devices. [7][8][9] Montmorillonite (MOM) and bentonite are the main…”