Thermal diffusivity measurement of polymeric thin films using the photothermal displacement technique. I. Free-standing film case

The photothermal displacement technique has been used to measure the out-of-plane thermal diffusivity in free-standing polymer thin films. The technique can be applied to a single sample as well as a collection of samples of different film thickness. Polymers are well suited for this method because they usually possess a large vertical coefficient of thermal expansion and readily absorb UV laser radiation. In particular, this method yields a value of the thermal diffusivity for polymer films with thickness ranging from 125 to about 10 mu m. Different polymers have been studied with particular attention paid to UPILEX-S (biphenyl tetracarboxylic dianhydride-p-phenyl diamine) polyimide whose thermal diffusivity has been determined to be 0.0025 +/- 0.0005 cm(2)/s. Our study shows that the measurements are not strongly affected by uncertainties in the optical absorption coefficient of the polymer as well as photothermal displacement contributions from the backside of the free-standing film. Furthermore, neither the effect of heat dissipation into the surrounding media of the polymer film nor miraging of the probe beam should significantly affect the results. Finally, analysis shows that the value of the thermal diffusivity determined is that of the out-of-plane value as long as the in-plane value is not orders of magnitude larger. This article is the first part of a two-part examination of the thermal properties of polymers using this nondestructive method. (C) 1999 American Institute of Physics. [S0021-8979(99)09823-0].