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

Abstract

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 (Tβ), at which localized molecular motion occurs. In contrast, smaller CVE values are observed for PIs exhibiting higher Tβs because their local molecular motions are suppressed due to the structural rigidity and dense molecular packing.

Effective Reduction of Volumetric Thermal Expansion of Aromatic Polyimide Films by Incorporating Interchain Crosslinking

“…Temperature variation over the thickness of the PI films was precisely measured using optical interferometry during the heating process [5,6]. The transmission spectra in the near-IR region were used, owing to the absence of absorptions.…”

“…However, the DC of approximately 42%-45% should be sufficiently high to fix and immobilize the PI main chains in the solid films. The suppression of local molecular motion, as represented by increased β-and γ-relaxation temperatures, is important to reduce the CVEs of polymers [5,6]. …”

“…In contrast, the MPD-PI films exhibited similar values of CTE// and CTE⊥ (32−46 ppm/K), which coincides with their small ∆n and |P200| values, indicating their highly isotropic character. We recently reported that substitution of m-phenylene for p-phenylene linkages in the PI main chains effectively reduced their CVEs [5]. This is because the local molecular motion of the former PIs are more restricted in the solid state than the latter PIs, even below their respective Tgs, because π-flip rotation at the m-phenylene linkage is sterically inhibited, while that at the p-phenylene linkage is allowed.…”

“…The relationship between the chemical structure of aromatic PIs and their anisotropic linear and volumetric thermal expansion (VTE) behaviors has been recently characterized [4][5][6]. PIs with bulky side groups, such as -CH 3 and -CF 3 , or those with flexible linkages, such as -O-and -CO-, in the main chain exhibit larger coefficients of volumetric thermal expansion (CVE) compared to PIs containing rigid and linear main chain structures without side groups [4].…”

“…PIs with bulky side groups, such as -CH 3 and -CF 3 , or those with flexible linkages, such as -O-and -CO-, in the main chain exhibit larger coefficients of volumetric thermal expansion (CVE) compared to PIs containing rigid and linear main chain structures without side groups [4]. In addition, poly(p-phenylene-3,3 ,4,4 -diphenyldiimide) (sBPDA/PPD) PI films with a rigid molecular structure and thermally imidized under very fast/slow heating rates exhibited different CVEs, owing to their varying degrees of main chain packing [5]. Moreover, films with dense molecular packing exhibited much smaller CVEs than those with loose molecular packing.…”