Effect of endothermic reaction associated with glass transition on the breakdown strength of biodegradable polymer films

Effects of endothermic reaction associated with glass transition on the impulse, DC, and AC breakdown strengths of poly-L-lactic acid (PLLA) and polyethylene terephthalate succinate (PETS) were studied experimentally. Impulse breakdown strength at first decreases monotonically with an increase in temperature for both PLLA and PETS. However, it increases and shows a hump when the temperature approaches a certain temperature that is slightly lower than the glass transition temperature. Then it decreases again. Moreover, it is shown that the degree of increment in impulse breakdown strength at this hump depends upon the crystallinity in PLLA. In the case of DC and AC breakdown, a similar local maximum as seen in impulse breakdown is not observed. Two possibilities, namely energy absorption and structural change both due to glass transition are discussed to explain the experimental results.