Mechanical deformation behavior in highly anisotropic elastomers made from ferroelectric liquid crystalline polymers

Cross-linked ferroelectric liquid crystalline polymers are studied by atomic force microscopy. Polysiloxane copolymers have been synthesized with mesogenic and photo-cross-linkable side groups, the latter connected either directly to the backbone via a short spacer or as terminal groups on a part of the mesogens. Although the polymers are otherwise identical, the restrictions imposed on the network formation process by the anisotropy of the smectic mesophase are different for the two positions of the cross-linkable group: In the first case ("intralayer cross-linking"), a predominantly two-dimensional network is formed in the backbone layers separating the smectic layers; in the second case ("interlayer cross-linking"), a primarily three-dimensional network is established which is dependent on the mesophase of cross-linking. These elastomers are prepared as thin freely suspended films in homeotropic orientation. The topography consists of plateaus separated by steps of characteristic height, corresponding to the surfaces and edges of smectic layers. If a film of "intralayer cross-linked" elastomer (network formation in the microphase-separated backbone layers) is elongated, the layers can slide on one another, showing occasional tears but no surface roughening (roughness 0.5 nm at 30% elongation). In an "interlayer cross-linked" film (network formation via the mesogens), the three-dimensional network introduces forces perpendicular to the direction of the mechanical deformation, leading to a characteristic depression pattern on the surface which indicates a distortion of the smectic order.