Abstract
A bending-beam technique with the use of two different substrates, silicon and gallium arsenide, has been successfully employed to simultaneously determine the biaxial relaxation modulus, E f /(1-v f ), and thermal expansion coefficient, α f , of a rigid-rod-like polyimide film, PMDA-B (pyromellitic dianhydride-benzidine), coated on a substrate. As measured, the two properties, especially α f , significantly increase with the increase of film thickness. At an observation temperature of 150°C, for example, the obtained E f /(1-v f ) increases from 6.0 × 10 9 to 8.7 ×10 9 N/m 2 , and α f from -0.33 ×10 -6 to +2.89 × 10 -6 /°C for thicknesses ranging from 10 to 35 μm. The increase of the thermal expansion coefficient with respect to film thickness can be attributed to the decrease in the in-plane orientation of the film, according to an X-ray diffraction experiment. The increase of the biaxial relaxation modulus with respect to thickness may presumably be attributed to the decrease in the void size and/or void fraction in the film. © 1992, American Chemical Society. All rights reserved.