Effect of thermal lag on glass transition temperature of polymers measured by DMA

This paper investigates the influence of thermal lag on the glass transition temperature of polymers measured under different heating rates during dynamic mechanical analysis (DMA). The E-modulus–temperature curves of a two-component epoxy adhesive and a pultruded glass fiber-reinforced polyester were measured by a DMA machine. Experimental results showed that two kinds of thermal lag result when the heating rate exceeds 0.5 °C/min: one between the specimen temperature and the thermocouple of the machine and the other along the length of the specimen. A finite volume model was developed in order to simulate the thermal environment inside the DMA chamber and validate the measured thermal lags. A numerical procedure was adopted to calculate the influence of the thermal gradient along the length of the specimen on the E-modulus–temperature curve. It was found that when the thermal lag effects are excluded, the E-modulus of a specimen decreases with increasing heating rates. This result contradicts reported results obtained using the experimentally derived E-modulus vs. temperature curves, disregarding the influence of the thermal lags, and thus claiming an increase of the E-modulus with increasing heating rates.