Intriguing three-component four-layer stacking nanostructures formed by the self-assembly of main-chain liquid-crystalline polyurethane elastomers, as analyzed by two-dimensional small-angle X-ray scattering

Abstract

A peculiar two-dimensional small-angle X-ray scattering (2D-SAXS) pattern, exhibiting an X-shaped feature, was observed for a main-chain liquid-crystalline (LC) polymer in the uniaxially stretched state. The main-chain LC polymer examined in this study exhibits shape memory depending on temperature, which can be applied as a novel thermoresponsive soft actuator with a highly accurate and robust performance. The one-dimensional SAXS profile indicates a strange feature that is the unusually low intensity of the first-order reflection peak compared with the intensity of the second-order peak (inversion of the intensity). These peaks are ascribed to the periodic stacking of LC and other component layers. To explain this peculiar SAXS profile, model calculations were conducted by the Fourier transform of a plausible electron density profile. As a result, it was found that physical constraints in the main-chain LC polymer due to the forced formation of the LC phase by mesogenic units caused the extraordinarily sparse (low mass density) layer of the normal alkane moiety in a three-component four-layer stacking nanostructure. Other effects of physical constraints in the main-chain LC polymer on thermal properties were examined by differential scanning calorimetry (DSC) analysis. The DSC results reveal that the melting temperature (T_m) of the LC phase and the degree of liquid crystallinity are lowered in the main-chain LC polymer compared with the precursory polyol biphenyl sample. The reflection peaks are broadened in the main-chain LC polymer, suggesting an extraordinary reduction in the grains of the stacking layers, to which the tremendous lowering of T_m of the LC phase is also ascribed. Thus, many effects due to physical constraints in the main-chain LC polymer were found. To further reveal the details of the nanostructure, the 2D-SAXS patterns exhibiting the X-shaped feature were measured during uniaxial elongation. Then, it was found that the crossing angle between the two representative directions in which the reflection peaks appeared increased and leveled off at the limiting value of 64°, which implies an inclination angle of 32° for the mesogenic units in the LC layer.