MEASUREMENT OF THERMAL-DIFFUSIVITY AND SPECIFIC-HEAT CAPACITY OF POLYMERS BY LASER FLASH METHOD

We measured thermal diffusivity and heat capacity of polymers by laser flash method, and the effects of measurement condition and sample size on the accuracy of the measurement are discussed. Thermal diffusivities of PTFE films with thickness 200-500 mum were the same as those data that have been reported. But, the data for film thickness less than 200 mum have to be corrected by an equation to cancel thermal resistance between sample film and graphite layers for receiving light and detecting temperature. Thermal diffusivity was almost unaffected by the size of area vertical to the direction of laser pulse, because heat flow for the direction could be negligible. Specific heat capacity of polymer film was exactly measured at room temperature, provided that low absorbed energy (< 0.3 J) and enough sample mass (> 25 mg) were satisfied as measuring conditions. Thermal diffusivity curve of PS or PC versus temperature had a terrace around T(g), whereas that of PE decreased monotonously with increasing in temperature until T(m). Further, we estimated relative specific heat capacity (RCp) by calculating ratios of heat capacities at various temperatures to the one at 299 K. RCp for PS obtained by laser flash method was larger than that obtained by DSC method, whereas the RCps for PE obtained by the both methods agreed with one another until T(m) (305 K). RCp for PS decreased linearly, with increase in temperature after it increased linearly until T(g) (389 K), showing similarity to temperature dependency of thermal conductivity. RCp for PE also decreased until T(m), similar to thermal conductivity. (C) 1995 John Wiley & Sons, Inc.