SIMULTANEOUS DETERMINATION OF THE BIAXIAL RELAXATION MODULUS AND THERMAL-EXPANSION COEFFICIENT OF RIGID-ROD POLYIMIDE FILMS USING A BENDING-BEAM TECHNIQUE

摘要

[[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, Ef/(1 - νf), and thermal expansion coefficient, αf, of a rigid-rod-like polyimide film, PMDA-B (pyromellitic dianhydride-benzidine), coated on a substrate. As measured, the two properties, especially αf, significantly increase with the increase of film thickness. At an observation temperature of 150°C, for example, the obtained Ef/(1 - νf) increases from 6.0 × 109 to 8.7 × 109 N/m2, and αf 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.