(650i) Main-Chain Liquid Crystalline Networks Synthesized Using Click Chemistry

Liquid crystalline (LC) networks (LCNs) are unique materials with stimuli-responsive properties (e.g. shape memory, actuation, and optical properties) that are attributed to mesogen ordering within the matrix. The unique features of LCNs have been proposed for use in biomedical and biomimetic applications, including as artificial muscles, biomimetic iris-like apertures, and scaffolds for cell culture. The challenge with many existing networks is that they employ side-chain liquid crystals attached as pendant groups to the polymer chain, and this reduces the coupling between LC ordering and polymer conformation. To facilitate transport in hydrated applications, this work focused on developing main-chain LC hydrogels, where the backbone of the polymer would maintain its liquid crystallinity even in a swollen, hydrated state. Previous work by others in the field reported on poly(ethylene glycol) (PEG)-based main-chain LCNs using thiol-click chemistry, where it was found that PEG crystallization disrupted mesogen ordering in networks. A challenge with this synthetic route is that thiol-click chemistry requires the use of a photoinitiator or elevated temperatures, both of which are not ideal for biological applications.

This work focused on the synthesis and characterization of main-chain LCNs using alkyne-azide Click Chemistry, which led to high yields using mild reaction conditions. Two different mesogens (5yH and 5yMe) and a non-LC monomer (5yTe) were coupled with different spacers, poly(ethylene glycol) and poly(propylene glycol), and a crosslinker to prepare the networks. Varying the length and chemical composition of the polyether spacer was found to tailor LCN phase behavior and mechanical properties. Of particular importance is the role of chemical composition and length of the polyether spacer, which was found to control hydrophilicity, crystallization, and LC phase formation. The cooperation of LC and polyether spaces gives these LCNs unique microstructure and properties in the dry and hydrated states and enables future use in biological applications.