Effect of Swelling and Deswelling on the Elasticity of Polyacrylonitrile Gels Prepared by Freezing and Thawing Techniques

Abstract

The repeating cycle of freezing and thawing brings about a physical gelation for the solutions of polyacrylonitrile (PAN) in N, N-dimethylacetamide (DMA) for appropriate concentrations. It was revealed from wide-angle-X-ray diffraction that the PAN molecules were cross-linked by the crystallization to form a network through the freezing and thawing steps. The profiles of small-angle-X-ray scattering for the gels showed the excess scattering from the network structure formed through gelation, which could not be observed for the pure PAN solution. A reversible swelling and shrinking were observable for the PAN gel as a function of the relative content of DMA and benzene by varying the mixing ratio. The degree of swelling measured by weight, for the swollen and shrunk gels, was in a range from 0.7 to 8. A scaling relation appeared for the gel swelling between the PAN volume fraction (ϕ) and the shear moduli (G) which was measured by tensile and indentation tests, as G = G_s ×(ϕ)^s; G_s= 1200 kPa and s = 2.26 ± 0.23. A discussion is given on the value of the scaling exponent, s, on the basis of the conformation of flexible chain molecules in solution and the analogy in the microscopic and macroscopic properties between semidilute polymer solutions and gels.

Keywords:

• elasticity
• freezing and thawing
• gelation
• N,N-dimethylacetamide
• polyacrylonitrile
• swelling–shrinking

Acknowledgement

The authors would like to thank Mitsubishi Chemical Corp. Japan for the PAN sample supply.