Thickness-direction coefficient of thermal expansion measurement of thin polymer films

A highly sensitive capacitance-change technique, suitable for thin polymer films, has been developed for the measurement of the coefficient of thermal expansion (CTE) in the film-thickness direction. Using this technique, we have determined the thickness-direction CTE of Kapton polyimide to be about 81±4 ppm/°C, averaged over the temperature range 50–150 °C. This polymer was chosen because it is widely used in the electronics industry for the fabrication of flexible printed circuits. Two other techniques, a Fabry–Perot laser interferometric technique, and the more conventional but less sensitive thermomechanical analysis, were also investigated. The thickness-direction CTEs obtained from the three techniques were at least a factor of 2.4 greater than the in-plane CTE of about 34±2 ppm/°C. The in-plane CTE, found here via simple length measurements, was in good agreement with literature values. It is speculated that the CTE anisotropy observed might be due to the in-plane molecular orientation of the polymer chains.