Insights into Entropy Change of Β2 Relaxation in Retraction of Uniaxially Stretched Polyimide Films

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 (T-g) stretchability and retraction after stretching are systematically investigated. Based on X-ray diffraction and dynamic mechanical analysis, it is revealed that both the sub-T-g stretchability and retraction correlates with beta(2) 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 alpha 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.