Influence of variations in liquid-crystalline content upon the self-assembly behavior of siloxane-based block copolymers

Abstract

A series of well-defined smectic side chain liquid-crystalline (LC) block copolymers with a low glass transition (T_g) siloxane block has been synthesized viaanionic polymerization; these systems consist of a glassy polystyrene block and a unique low glass transition temperature LC block based on poly(vinylmethylsiloxane) to which six different LCs have been synthesized and attached. The synthesis techniques used provide systematic control over covalent LC side chain content, allowing for a range of morphologies to be obtained from a single block copolymer backbone during a one-step LC attachment reaction. Variations in the LC structure and content significantly affect the morphology of the LC mesophase, allowing the smectic-to-isotropic transition temperature to be tuned from room temperature up to 150 °C. There are two key driving forces in the self-assembly behavior of these materials that are significantly affected by the LC content. The first is the segmental interaction parameter (χ) between the blocks, which is a function of the amount of LC attached to the siloxane block. The attachment percent of the LCs to the siloxane block determines the packing density, which affects the stability of the LC mesophase and its interactions with the inter-material dividing surface. The self-assembled morphologies are characterized as a function of LC content and the mechanisms for the observed behavior are detailed. Additional insights into the interactions between the LC and block copolymer mesophases are gained by investigating the morphologies in response to mechanical deformation. The elastic modulus of this system can be tailored over several orders of magnitude by controlling the LC content, and the thermo-mechanical behavior is also highly dependent. The ability to precisely control the degree of LC functionalization enables the custom design and tailoring of material properties for specific applications such as electro-mechanical, damping, and mechano-optical devices.