Systematic Modulation and Structure-Property Relationships in Photopolymerizable Thermoplastics

Thermoplastics encompass the majority of commercial plastics but are limited to manufacturing techniques that require heat and/or solvent to enable material reprocessing and reshaping. Photopolymerization offers a readily accessible in situ alternative to conventional processing techniques but has typically been restricted to cross-linked networks, thus removing the potential for recycling/reprocessing of the materials. In this work, the paradigm of photopolymerizable thermoplastics consisting of semicrystalline thiol-ene polymers with repeat units structures mimicking those of poly(ethylene terephthalate) is explored. These materials are rapidly photopolymerized (<10 s) under mild irradiation conditions by using commercially available, unpurified dithiol and diene monomers to give polymeric materials with distinctive mechanical properties. The complex relationships between monomer structure, crystallization, and material properties (optical, thermal, and mechanical) were investigated by modifying the dithiol alkyl chain length (xDT, x = 2-10). Though subtle, these monomeric structural perturbations significantly impacted both the rate (∼0.3-19 MPa min-1) and extent of crystallization as well as the corresponding material properties (e.g., Young's modulus and elongation to break ranging from 80-280 MPa and 500-840%, respectively). To confirm the expansive applicability of this facile photopolymerization method, substitutions to the diene functionalities that participate in crystallization were explored to understand the relationship between the monomer structure, the extent of crystallinity, and the mechanical performance. The photopolymers reported herein provide a basic framework to expand access to photocurable step-growth linear polymers with well-controlled polymerization-induced crystallization for a variety of applications, including for the rapid manufacture of prototypical or sacrificial components in 3D printing.