The modeling method and simulation study on the biaxially anisotropic polymer films

Biaxially anisotropic polymer (BAP) films are very common materials, usually prepared by stretching linear polymers along two orthogonal directions in sequence. Most molecular chains of BAPs are oriented in the two stretching directions with a crossed arrangement, leading to unique properties of BAP films. However, the theoretical and simulation studies on BAPs and BAP-based devices are few until now. Here, we developed entity modeling methods to successfully simulate actual BAPs: through constructing the model of crossed rebars embedded in matrix, which respectively imitate crossed molecular chains and inter-chain van der Waals interactions, the mechanical properties of the BAP films/strips can be reproduced. Different from uniaxially anisotropic polymers, the longitudinal Young’ moduli of BAP strips have two maxima in 0°/90° directions, and their coefficients of thermal expansion (CTEs) also have two maxima in ±45° directions. By introducing asymmetry, the BAP film model becomes from symmetric to moderately asymmetric and then to highly asymmetric: the ratio of two modulus maxima (in 0°/90°) increases (from 1 to 1.13, then to 1.22), and the directions of maximum CTEs also change (from ±45° to ±35°, then to ±27°), making the simulated BAP films closer to reality. Moreover, we systematically studied the helical morphing behaviors of the BAP-based bimorph actuator by finite element simulations. The chirality of this strip actuator is reversed four times, while its cutting angle changes within 180°. Besides, as the asymmetry of BAP film increases, the angle of chirality reversion of the BAP-based actuators changes from ±45° to ±60°, then to ±70°.