Design of Advanced Materials by Exploiting Hierarchically-Structured Liquid Crystalline Block Copolymers

Date of Completion

January 2011

Keywords

Chemistry, Polymer

Degree

Ph.D.

Abstract

The self-assembly of block copolymers (BCPs) is a promising "bottom-up" approach for the preparation of advanced materials of nanoscaled sizes and precisely controlled periodical structures. Among various BCPs, side-chain liquid crystalline block copolymers (SCLCBCPs) are of special interests because they are able to form sophisticated hierarchical structures where BCPs microphase-segregate at around 10–100 nm length scale with LC ordering confined in microsegregated domains. These complex hierarchical structures allow the tuning of the morphologies and functionalities at different length scales and the decoupling of the components providing different properties; furthermore, the LC phase may endow stimuli-responsive behaviors to the resulting materials. Therefore, SCLCBCPs can serve as promising candidates for the design and preparation of novel advanced smart materials. ^ In this work, we first developed a reversible addition-fragmentation chain transfer (RAFT) polymerization protocol through the preparation of a model block copolymers poly(cholesteryl methacrylate-b-2-hydroxyethyl methacrylate). This versatile synthetic method can be used to prepare well-defined SCLCBCPs comprising one block bearing side-chain cholesteryl mesogens and the second block with different functionalities. With this RAFT polymerization method, PEO-based LC triblock copolymers bearing cholesteryl side-chain mesogens have been prepared. By varying the LC content in these LC triblock copolymers, unique hierarchical structures including "LC lamellae in PEO lamellae" and "PEO cylinders in LC matrix" are obtained, which dictate the PEO crystallization behaviors in the polymers. With this approach, nanostructured amorphous PEO domains have been obtained at room temperature, which can be potentially used as scaffolds to fabrication solid polymer electrolytes for lithium batteries. ^ With the same RAFT polymerization method, we have also prepared amphiphilic LC pentablock copolymers, which contains PEO, polymethacrylate bearing side-chain cholesteryl mesogens (PC5MA) and poly(acrylic acid) (PAA) in the sequence of PAA-PC5MA-PEO-PC5MA-PAA. These pentablock copolymers, when dispersed in an aqueous solution of magnetic nanoparticles (MNPs), self-assemble to form hierarchically-structured multilayered micelles encapsulating MNPs. Freeze-drying the MNP micellar solutions results in polymer/MNP nanocomposites, which can swell in water to give free-standing magnetic hydrogels (ferrogels) with thermotropic LC domains and MNPs functioning as dual physical crosslinkers. These hierarchically-structured LC ferrogels with good mechanical properties and being responsive to magnetic field can be used for fabricating soft actuators or artificial muscles.^

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