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 °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 °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 °C showed non-sinusoidal stress waveform and characteristic Lissajous pattern, representing strain-stiffening behavior. In addition, the gel of the solution aged at 100 °C gave the greater strain-stiffening ratio than the initial value after the successive LAOS tests at 80 and 30 °C, indicating the thermo-irreversibility.