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. Copyright © 1993 Society of Plastics Engineers