Thickness‐direction thermal expansion of polyimide films

The coefficient of thermal expansion (CTE) for thin polymer films is a property of considerable practical importance. In contrast to the abundance of CTE data in the in-plane or film direction, data in the thickness or z-direction is rarely available, in part due to the high sensitivity required for measurement of the small thickness changes involved. In this paper, we will describe both a capacitance change and a Fabry-Perot laser interferometric method for measuring z-direction CTE's. Results are presented for a variety of commercially available polyimide films of thickness 50 to 130 μm. Upilex and Kapton were found to have z-direction CTE's of about 55 and 81 ppm/°C, respectively. These CTE's for the thickness direction are many times higher than the corresponding values in the in-plane direction. This anisotropy is attributed to the preferential in-plane molecular orientation of the polymer chains.