Insights into Entropy Change of β₂ Relaxation in Retraction of Uniaxially Stretched Polyimide Films

Abstract

Stretching has been extensively applied for decades in the production of polyimide (PI) materials with improved mechanical, electrical, and thermal properties. Meanwhile, a detailed understanding of the molecular motions during stretching is still limited. Herein, the thermomechanical behaviors of uniaxially stretched PI films prepared from 3,3′,4,4′‐biphenyl tetracarboxylic dianhydride (BPDA) and 2,2′‐bis (trifluoromethyl) benzidine (TFDB) are studied, and the possible molecular motions responsible for the sub‐glass transition temperature (Tg) stretchability and retraction after stretching are systematically investigated. Based on X‐ray diffraction and dynamic mechanical analysis, it is revealed that both the sub‐Tg stretchability and retraction correlates with β2 relaxation, which originates from the highly cooperative molecular motions of conformational rotations involving segmented BPDA units. A thermodynamic model is established and applied for the analysis of the retraction behaviors of stretched BPDA‐TFDB PI films. It is found that the increase of entropy under stretching plays a significant role in most retraction cases. However, as the stretching temperature increases, the α relaxation‐correlated large‐scale molecular motions emerge. Such motions partially counterbalance the conformational rotations of BPDA segments, leading to an enhanced contribution of enthalpy decrease during the retraction of BPDA‐TFDB PI films.

Preparation and Properties of Low-Dielectric Polyimide Films Containing Tert-Butyl

“…In this context, low-dielectric materials have attracted much interest. Particularly, polyimide (PI) films are considered as one of the most promising candidates for the next generation of dielectrics due to their excellent thermal [6][7][8], mechanical [9,10], and dielectric properties [11][12][13][14][15], and have been applied in insulation layers, buffer coatings, and passivation layers in the microelectronics industry [16][17][18][19]. However, it is a fact that the dielectric constant of most PI films is not low enough (approximately 3.2-4.0).…”