VARIATION OF DYNAMIC MECHANICAL PROPERTIES OF POLYACRYLONITRILE WITH HEAT TREATMENT

Abstract

The effect of heat treatment on the dynamic mechanical properties of polyacrylonitrile (PAN) was investigated in connection with whether the change of the mechanical loss tangent (tan δ) peak at 160°C at the earlier stages of heat treatment can be attributed to thermal degradation proceeding preferentially in accordance with the ordering in polymer structure. The temperature dependence of the dynamic mechanical properties of the PAN films and fibers heat-treated at 220°C and 260°C for various periods, respectively, were measured at a constant frequency of 110Hz. The infrared absorbances at specified wave numbers and x-ray scatterings of those samples were also measured to characterize the chemical and physical changes of the polymer structure with heat treatment. The tan δ peak at 160°C decreased its peak height eminently by heat treatments for shorter periods, and simultaneously the x-ray scattering peaks at the Bragg angle 2θ=17.0° and 2θ=29.5° sharpened, whereas neither the infrared absorbance due to a carbon-carbon double bond nor the absorbance due to a carbon-nitrogen triple bond made a significant change. This indicates that the thermal degradation in PAN will not be a major cause of the decrease of the tan δ peak at 160°C. It is considered that the decrease of the tan δ peak at 160°C with heat treatment will be due to the change of the less ordered region to the ordered region, and that in thermo-degradative treatment under the proper conditions PAN will go through a step where the chain-to-chain lateral order augments, before the cyclization due to polymerization of the nitrile side groups proceeds substantially.