|Title||Investigation of the relationships between the chain organization and rheological properties of atactic poly(vinyl alcohol) hydrogels|
|Publication Type||Articolo su Rivista peer-reviewed|
|Year of Publication||2003|
|Authors||Ricciardi, Rosa, Gaillet C., Ducouret G., Lafuma F., and Lauprêtre F.|
|Keywords||article, carbon nuclear magnetic resonance, chemical structure, concentration response, Crystalline materials, Crystallinity, Crystallization, Deformation, flow kinetics, freezing, hydrogel, Hydrogels, hydrogen bond, Hydrogen bonds, molecular interaction, Molecular Structure, Nuclear magnetic resonance, Phase separation, polyvinyl alcohol, Polyvinyl alcohols, proton nuclear magnetic resonance, Rheology, Storage, Strain rate, thawing, Young modulus|
The structure and rheological behavior of atactic poly(vinyl alcohol) (a-PVA) hydrogels prepared by freeze/thaw cycles were investigated as a function of polymer concentration and number of freeze/thaw cycles. The presence of phases with different mobilities was observed using 13C CP/MAS and DP/MAS NMR experiments. The degree of crystallinity of the a-PVA-rich phase was determined by 1H NMR free decay experiments. Measurements of the shear storage and loss modulus were performed at a fixed frequency of 1 Hz and a strain value of 0.1%, i.e. under conditions where the deformation imposed on the gel structure is entirely reversible. Results thus obtained showed that an increase in the number of freeze/thaw cycles induces an increase in the degree of crystallinity in the polymer-rich phase together with an increase in the storage modulus. The a-PVA hydrogels became more fragile as the number of freeze/thaw cycles was increased. Moreover, both the percentage of protons in a rigid environment measured by 1H NMR and the storage modulus values tended to a limiting value after six freeze/thaw cycles. These results show that the first five or six freeze/thaw cycles play a very important role in determining the hydrogel structure and rheological properties. A more detailed comparison of NMR and rheological data led to the conclusion that the storage modulus is mainly controlled by the a-PVA crystallinity while the hydrogen bond interactions have a much smaller contribution. © 2003 Elsevier Science Ltd. All rights reserved.
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