Printable Semicrystalline Polymers Exhibiting High Dimensional Extensibility and Recovery

Objectives: A semicrystalline thermoplastic thiol-ene with stoichiometric amounts of 1,6-hexanedithiol (HDT) and diallyl terephthalate (DAT) exhibiting rapid photopolymerization, high molecular weights and excellent ductility/toughness was recently reported. Here, a crosslinker, 2,4,6-triallyloxy-1,3,5-triazine (TAT), was added to the HDT-DAT formulation to improve the mechanical properties obtained for 3D printed structures.
Methods: Real-time photopolymerization and crystallization rates were obtained for bulk thiol-ene formulations composed of 1 wt% BAPO and 1:1 HDT:DAT, 1:0.95:0.05 HDT:DAT:TAT, 1:0.9:0.1 HDT:DAT:TAT using FTIR, photo-rheology and DSC. Bulk-cured and 3D printed dogbone specimens were uniaxially strained at 5 mm/min until failure or halted at significant levels of sub-failure strain. Thermal conditioning of the printed structures was performed above each formulation’s melt transition (T_m), and recovery of glassy deformation was observed at 85-90°C.
Results: Near-quantitative thiol and alkene conversions were observed for each bulk thiol-ene formulation (0, 5, and 10 mol% TAT) upon irradiation with a 405 nm LED at 1 mW/cm². Degrees of crystallinity for the 5 and 10 mol% TAT were 86% and 64% of that obtained without crosslinker, respectively, and the rates of crystallinity observed via a secondary modulus development with photo-rheology were reduced with increasing crosslinking densities. Excellent mechanical properties, specifically elongation at break (ε_max) and toughness (T), were observed for printed samples with 5 mol% (ε_max=532±5%, T=59±5 MJ/m³) and 10 mol% TAT (ε_max=787±7%, T=95±1 MJ/m³) thermally conditioned at 150 and 200°C, respectively. Printed dogbones strained to 500% demonstrated full shape recovery within seconds upon heating at 85-90°C.
Conclusions: The impressive ductility and toughness of the bulk photocured linear thiol-ene polymer was not replicated with 3D printing; however, heating crosslinked printed materials above T_m provided substantially enhanced interlayer adhesion such that both bulk and layered printed samples displayed tough, mechanically robust properties that notably include an ability to recover the original shape of a dramatically deformed printed structure upon heating.