Anisotropic Linear and Volumetric Thermal-Expansion Behaviors of Self-Standing Polyimide Films Analyzed by Thermomechanical Analysis (TMA) and Optical Interferometry

摘要

The relationships between the molecular structure, polymer chain orientation, β-transition temperature, and the coefficients of in-plane and out-of-plane linear thermal expansions (CTE, α), as well as the coefficients of volumetric thermal expansion (CVE, β), are extensively examined for ten types of self-standing aromatic polyimide (PI) films via thermomechanical analysis, near-infrared optical interferometry, and dynamic mechanical analysis. The out-of-plane CTEs (α_⊥) of PI films having rigid-rod structures are 20-40 times larger than their in-plane CTEs (α_(//)), whereas the α_⊥ values of PI films having bent linkages in the main chains are comparable to their α_(//) values. In addition, the values of normalized anisotropy in CTE for PIs (η), defined as (α_⊥ -α_(//))/β, are proportional to the orientation coefficients (P200) estimated from the in-plane/out-of-plane birefringence (Δn) and molecular polarizability tensor. Furthermore, larger CVE values are observed for PIs exhibiting lower β-relaxation temperatures (T_β), at which localized molecular motion occurs. In contrast, smaller CVE values are observed for PIs exhibiting higher T_βs because their local molecular motions are suppressed due to the structural rigidity and dense molecular packing.