Effects of Structural Isomerism and Precursor Structures on Thermo-optic Coefficients of BPDA/PDA Polyimide Films

Abstract

The effects of the structural isomerism and the alkyl side-chain lengths in the precursors of BPDA/PDA polyimide (PI) on the temperature dependence of the refractive indices (i.e. thermo-optical coefficients) have been investigated. The PIs were formed on Si substrates by thermal curing of a) poly(amic acid)s (PAA) derived from three isomers (s-, a-, and i-) of biphenyltetracarboxylic acid dianhydride (BPDA) and p-diminobenzene (PDA) and b) poly(amide alkylester)s (PAEs) having methyl-, n-propyl-, n-pentyl-, and n-octyl side chains. The TO coefficients of PIs increase as the average refractive indices and thermal expansion coefficients increase, which can be well explained with the derivative form of the Lorentz-Lorenz formula. The absolute values of TO coefficients for s-BPDA/PDA are smaller than those expected from their refractive indices, which is due to the dense molecular packing and the small coefficient of thermal expansion. Significantly larger TO coefficients were observed for the PI films prepared from PAEs than that from PAA, indicating that the larger volatile components (alcohols) eliminated during thermal imidization cause loose molecular packing in the former PIs. The use of structural isomers and chemical modification of precursors are effective to control the TO coefficients and their anisotropies in BPDA/PDA films.