Influence of cooling rate on stress relaxation response of strained PVDF film

Abstract

The influence of cooling rate on the stress relaxation response of prestrained polyvinylidene fluoride (PVDF) film cooled from 60°C, which is the upper service temperature limit, has been investigated. The film was heated to 60°C, strained at 0·5% and then cooled to ambient between 1 and 5°C min⁻¹. Subsequently, the heat treated films were subjected to stress relaxation tests at 0·5% strain (50 μm extension) and the relaxation curves were fitted using the Kohlraush–Williams–Watts (KWW) expression. Modulated differential scanning calorimetry (MDSC) showed that crystallinity changes were only detectable for the highest cooling rate. When the crystallinity level dropped by ∼2% for the sample cooled at 5°C min⁻¹, the relaxation time constant was reduced by nearly two orders of magnitude. The results indicate the film relaxation behavior is highly sensitive to thermomechanical loading near the onset of the α _c transition. In practical terms, small mechanical straining can dramatically alter PVDF film properties, in terms of material stiffness and viscoelastic response, when operating near the upper service temperature limit.

Keywords:

• STRESS RELAXATION
• POLYVINYLIDENE FLUORIDE
• DYNAMIC MECHANICAL ANALYSIS
• COOLING RATE EFFECT
• THERMOMECHANICAL LOADING
• MODULATED DIFFERENTIAL SCANNING CALORIMETRY
• DEGREE OF CRYSTALLINITY