From liquid crystalline building blocks to semicrystalline photopolymers

Abstract

Hot Lithography is a 3D printing technique that enables additive manufacturing of photocurable polymers at high temperature. This technology greatly expands the spectrum of monomers to include those that are too unreactive, viscous or simply solid at room temperature.In this work, multiple liquid crystalline (LC) monomers were synthesized to be polymerized using the photoinitiated thiol-ene click reaction. An extensive literature search was conducted to gain information about the LC phase behavior of certain structures and derive promising LC motifs for dithiol or diene monomer and addition-fragmentation chain transfer (AFCT) agent design. While polymer networks derived from AFCT agents failed to cause crystallinity in polymers, they still exhibited efficient chain transferring capabilities during polymerization.The synthesis of liquid crystalline thiol monomers proved unfeasible due to oligomerization of the functional chain ends. Synthesis of the chosen liquid crystalline terminal alkene monomer structures was successful and they were combined with a variety of comonomers to obtain photoreactive formulations, most of which exhibited LC phases in narrow temperature ranges. A heated polymerization chamber was built to polymerize precisely within the LC temperature windows.The resulting polymer networks exhibited high degrees of crystallinity, which resulted in high mechanical strength and toughness. Additionally, multiple materials were found to have shape memory properties with excellent shape imprintability and recovery. One combination of monomers exhibited fully controllable polymer crystallinity based on the curing parameters. Moderate curing temperatures within the LC temperature window resulted in opaque, hard, crystalline polymers while high temperatures above the LC temperature window led to transparent, soft, amorphous polymers.IVThis was successfully utilized to print multi-material parts via Hot Lithography. The highly tunable mechanical and optical properties were proven to be variable pixel to pixel, within each printed layer.The research performed during this thesis opens effective new ways of introducing crystallinity into rather densely crosslinked polymer networks to significantly enhance mechanical and functional properties, which are complimented by the ability to 3D print them.