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

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 - nu(f)), and thermal expansion coefficient, alpha(f), of a rigid-rod-like polyimide film, PMDA-B (pyromellitic dianhydride-benzidine), coated on a substrate. As measured, the two properties, especially alpha(f), significantly increase with the increase of film thickness. At an observation temperature of 150-degrees-C, for example, the obtained E(f)/(1 - nu(f)) increases from 6.0 x 10(9) to 8.7 x 10(9) N/m2, and alpha(f) from -0.33 x 10(-6) to + 2.89 x 10(-6)/degrees-C for thicknesses ranging from 10 to 35-mu-m. 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.