Effect of Chain Structure on the Rheological Properties of Vinyl Acetate−Vinyl Alcohol Copolymers in Solution and Bulkстатья
Статья опубликована в высокорейтинговом журнале
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Дата последнего поиска статьи во внешних источниках: 17 ноября 2014 г.
Аннотация:Bulk and solution studies revealed a strongly pronounced
effect of chain structure on the rheological and relaxation behavior of wellcharacterized
vinyl acetate−vinyl alcohol copolymers of similar composition
and polymerization degree. The frequency−temperature superposition
principle is fully applicable to the random copolymers, which demonstrate
all expected relaxation states, whereas a divergence of the reduced dynamic
moduli−frequency dependences is observed for the multiblock copolymers.
In the latter case, the terminal zone is sensitive to the self-assembling
of vinyl alcohol blocks into (depending on the copolymer composition)
crystalline or amorphous microstructures. The monomer unit distribution
particularly affects properties of the copolymer solutions in N,Ndimethylformamide
(DMF). 5% solutions behave as simple viscoelastic
liquids at 20 °C and show viscoplastic behavior at −20 °C, where more
blocky chains are characterized by up to 4 orders of magnitude higher yield
stress values. The multiblock copolymer solutions demonstrate a pronounced viscosity hysteresis in the heating−cooling cycle,
being absent in the random copolymers. 10% solutions of multiblock copolymers are practically gelatinous even at room
temperature. The observed effects are explained by examining the peculiarities of hydrogen bonding in vinyl acetate−vinyl
alcohol copolymers using FTIR spectroscopy. The multiblock copolymers are characterized by stronger hydroxyl−hydroxyl
H-bonds and greater fraction of interchain hydroxyl−acetyloxy H-bonds providing aggregation of chains and high viscosity of
the corresponding samples, whereas the random copolymers more strongly interact with the residual solvent. Dynamic light
scattering studies prove that the relaxation of concentration fluctuations is completely diffusive, being bimodal in the random
copolymers and trimodal in the multiblock ones. The fast mode in the latter case demonstrates anomalous concentration
behavior. In the dilute regime, up to very low concentrations, multiblock copolymer chains form stable aggregates, and this fact
correlates with an unusual growth of the reduced viscosity in the corresponding rheological experiments.