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Autori: Bercea M., Morariu S., Brunchi C.E
Editorial: Springer Netherlands, International Journal of Thermophysics, 30, p.1411-1422, 2009.
Thermal-induced gelation of polyacrylonitrile (PAN) solutions in dimethylformamide was studied through rheological measurements. The entangled and non-entangled states were delimited for an overlap parameter of macromolecular coils of 2.7. In temperature sweep experiments, two types of gelation processes were identified for PAN entangled solutions at: (1) a reversible gelation in the region of low temperatures (below 15°C) due to the formation of a crosslinked structure with a small region of order that dissolves completely in an excess of solvent, and (2) an irreversible gelation at high temperatures (above 60°C) and this gel is insoluble in an excess of solvent, due to the formation of a chemical three-dimensional network. Physical PAN gels with good elastic properties were also obtained by the freezing and thawing method. The gel properties depend on the thermal history of the solution (freezing time, aging time, and aging temperature), its composition (concentration and molar mass of the polymer), and gelation conditions (freezing rate). The gelation was attributed to the formation of junction zones via an aggregation process between a certain number of chain segments due to attractive dipole-dipole interactions.
Cuvinte cheie: Entangled Solutions, Gel, Polyacrylonitrile (PAN), Viscoelastic Parameters