Concentration Effects in Frozen Polyacrylonitrile Solutions During the Gelation Process as Studied by Viscoelastic Techniques.

Dynamic viscoelasticity measurements were carried out for N, N-dimethylacetamide solutions of polyacrylonitrile (PAN) frozen at −40 °C to characterize the effect of concentration in the frozen state in relation to the freezing and thawing method of the PAN gel. The storage (G′) and loss moduli (G″) were measured for samples obtained by separating the frozen and unfrozen regions during the thawing step. The G′ were also acquired for solutions without the separation (referred to as post-thawing solution), written as G ′ fro G ′ unfro and G ′ post − thaw for frozen region, unfrozen region, and post-thawing solution, respectively. The relationship, G ′ unfro < G ′ post − thaw < G ′ fro was observed, showing that regions of higher and lower concentrations were developed in the thawing step. ( G ′ fro / G ′ post − thaw ) was considered a relative index of the concentration increase, to be compared for those of the initial and later stages within the thawing step. ( G ′ fro / G ′ post − thaw ) was much greater than 1 in the initial stage, and then decreased and approached 1 in the later stage. The behavior of ( G ′ fro / G ′ post − thaw ) was discussed with reference to the effects of freezing time and concentration on the gelation time. The microscopic separation between the dilute and concentrated phases induced at the freezing step was proposed as the physical process to interpret the viscoelastic data. [ABSTRACT FROM AUTHOR]