J. L. Valentín, D. López, R. Hernández, C. Mijangos, K. Saalwächter.
Structure of Poly(vinyl alcohol) Cryo-Hydrogels as studied by Proton Low Field NMR.
Macromolecules 42, 263–272 (2009). DOI
The network structure of Poly(vinyl alcohol) (PVA) hydrogels obtained by
freezing-thawing cycles was investigated by solid state 1H low field NMR spectroscopy. By application of
multiple-quantum NMR experiments, we obtain information about the segmental
order parameter, which is directly related to the restrictions to chain motion
(crosslinks) formed upon gelation. These measurements indicate that the network
mesh size, as well as the relative amount of non-elastic defects (i.e.
non-crosslinked chains, dangling chains, loops) decreases with the number of
freezing-thawing cycles, but it is independent of the polymer concentration. The
formation of the PVA network is accompanied by an increasing fraction of polymer
with fast magnetization decay (~20μs). The quantitative study of this rigid
phase with a specific refocusing pulse sequence shows that it is composed of a
primary crystalline polymer phase (around 5%), which constitutes the main
support of the network structure and determines the mesh size, and a secondary
population of more imperfect crystallites, which increase the number of elastic
chain segments in the polymer gel but does not affect the average network mesh
size appreciably. Correspondingly, progressive melting of the secondary
crystallites with increasing temperature does not affect the network mesh size
but only the amount of network defects, and melting of the main PVA crystallites
at around 80 °C leads to destruction of the network gel and formation of an
isotropic PVA solution.