Iodine swelling of polyacrylonitrile I. Effect of orientation and evidence for a three-phase structure

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Abstract
The swelling of polyacrylonitrile (PAN) film in aqueous I2/KI (or I3 −) solutions of different concentrations up to 2.0 molal has been investigated, using both biaxially oriented and unoriented film. The absorption curve (equilibrium absorption vs concentration) for oriented film shows three distinct steps, at concentrations of 0.25, 0.65, and 1.6 molal, leading to weight increases of the sample of approximately 250%, 2000%, and 3200%, respectively. In unoriented film, smooth progressive increases of absorption are observed in place of the first and third steps; only the second abrupt step remains. A significant difference in desorption behavior is seen in extraction experiments carried out above and below the second step. It is hypothesized that the first and third steps of swelling correspond to the penetration of two different “amorphous” phases in the polymer, which together with a crystalline phase would indicate a three-phase structure for PAN. The second step of swelling may represent a conformational transition of the molecules in the phase penetrated in the first step. The effects of iodine swelling and extraction on the dynamic mechanical spectrum of PAN were also investigated, and it seems possible to associate the swelling in the region of the first and third steps with the two peaks (or transitions) in the tan δ curve of PAN, which might be regarded as glass transitions of the two amorphous phases. An unusual behavior of sample length vs temperature was observed during the dynamic mechanical measurements in the temperature region of these two transitions. Stress-strain curves were also measured on samples containing different amounts of absorbed iodine.

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