Effects of chain conformation on the viscoelastic properties of polyacrylonitrile gels under large amplitude oscillatory shear

Abstract

The viscoelastic properties of polyacrylonitrile (PAN) gels prepared from the 20 wt% solutions in N,N-dimethyl formamide (DMF) were investigated under large amplitude oscillatory shear (LAOS) in terms of aging temperature of the solutions from 30 to 100 degrees C. Aging of PAN solutions reduced intrinsic viscosity and hydrodynamic diameter with little degradation. In the UV-vis spectra, the absorption peak at 268 nm represented the dipole dipole pairs of physically associated nitrile groups, whose intensity increased with aging temperature. These results confirmed that aging of the solutions gave rise to the contracted chain conformation with anti-parallel orientation-dominant nitrile groups. In 20 wt%, aged solutions showed gelation behavior more rapid than fresh one during storage at 25 degrees C. At the imposed strain amplitude of 400% in the LAOS measurement, PAN gel prepared from fresh solution gave sinusoidal waveform signal of stress and ellipse-shape Lissajous curve. However, gels from the solutions aged above 60 degrees C showed non-sinusoidal stress waveform and characteristic Lissajous pattern, representing strain-stiffening behavior. In addition, the gel of the solution aged at 100 degrees C gave the greater strain-stiffening ratio than the initial value after the successive LAOS tests at 80 and 30 degrees C, indicating the thermo-irreversibility. (C) 2016 Elsevier Ltd. All rights reserved.