Multiplicity of shape selection in functionally graded liquid crystalline polymers

Abstract

The synergy of through-thickness gradation in the orientation of the molecular director and the extent of polymerization is shown to offer a framework for controlling shape selection in integral polymer films. Native curvatures are realized under ambient conditions in splayed liquid crystalline polymers that are photopolymerized on anchoring surfaces, while being exposed to the atmosphere. Residual multiaxial polymerization strains drive the spontaneous assembly of a range of geometries (tape springs, helical coils and arches) in strips that are excised from the as-prepared films. Gradients in the director orientation and the cross linking through the thickness enable a temperature dependent structural evolution. Following a moderate temperature rise (<70 °C), the samples are found to intensify their native shapes. However, further temperature rise leads to a relaxation of the strains followed by an eversion of the geometry (>100 °C), which generates curvature orthogonal to that in the native state. This multiplicity in shape selection, which spontaneously emerges without requiring any mechanical training offers a useful framework for actuation and morphing. In a prototypical demonstration, a suitably excised sample is shown to spontaneously jump when placed on a hot-plate as a result of the eversion. Also, when confined in ring-like geometries, hinge-like structures are generated due to the interplay of the imposed bending strains with that existing in the native state. The evolution of the curvatures as a function of temperature offers control over the active hinge/fold and expands the multiplicity of shapes that can be realized.