Linear thermal expansion, thermal ageing, relaxations and post-cure of thermoset polymer composites using modulated temperature thermomechanometry

The glass transition temperature (Tg) regions of polymers and composites were studied using static force thermomechanometry and modulated temperature thermomechanometry (mT-TM). Modulated temperature allowed measurement of linear thermal expansion coefficient and glass transition as reversing phenomena, independent of any residual cure and relaxations that are non-reversing in nature. The reversing dimension change curves were well defined with continuous expansion that increased after Tg, though sensitivity decreased with crosslinking and fibre content. The non-reversing dimension change curves showed the maximum variation and revealed complex changes, and the non-reversing characteristics were confirmed by repeated scans, both upon cooling or re-heating. Non-reversing curves showed contractions with increasing temperature. Lissajous figures demonstrated that temperature modulation deviated significantly from linear response in the temperature range below Tg, and during the Tg range, steady state was not maintained. Measurements made in mT-TM mode were compared with modulated force TM where Tg was revealed as a peak in loss modulus or tan(δ), whilst reversing events were consistent with changes in storage modulus.