Photopolymerization of Semi-Crystalline Polymers: Fundamental Characterization and Opportunities for Additive Manufacturing

With the growing importance of additive manufacturing (AM), there is a need for new materials development to match the property demands. While extrusion-based AM has been applied to many material classes, the property scope of resins amenable to light-based approaches to AM is limited. One challenge in this area is the development of formulations that polymerize rapidly at low intensities and exhibit targeted, high performance mechanical and physical properties upon curing. Most conventional photopolymerization-based AM resins rely on chemically crosslinked polymer networks to improve cure speed, but these resins tend to be brittle and lack the ability to be thermally reprocessed. To address these challenges, our group has recently developed a new class of semicrystalline thiol-ene polymers that polymerize rapidly in a spatiotemporally controlled manner to form linear polymers that can be melted at reasonable, super-ambient temperatures.

In this work, the goals were to investigate the relationships between polymer structure and properties such as molecular weight, melting temperature, and kinetics of polymerization and crystallization. By implementing off-stoichiometric polymerizations, the molecular weight and thermomechanical behavior of the polymer was controlled. The ability to tune these mechanical properties and the kinetics of polymerization and crystallization is useful in 3D printing materials designed for applications such as cast molding jewelry, dental implants, or other customized molds.

The focus of this work was on the step growth polymerization of the thiol-ene “click” reaction of diallyl terephthalate (DAT) and hexanedithiol (HDT) initiated by TPO. The molecular weight of the polymer after polymerization was studied by GPC. Polymerization kinetics were measured with FTIR, and crystallization analysis was done by rheology, differential scanning calorimetry (DSC), and principal component analysis (PCA). It was found that changing the stoichiometric ratio of the materials system allows the molecular weight and rates of crystallization to be judiciously controlled. Also, increasing the temperature at which the reaction takes place was appropriate for controlling the crystallization rate. By adjusting these variables, the processes of polymerization and crystallization - which normally are coupled - were temporally separated as necessary for AM. Furthermore, it was found that adding Cr nanoparticles to the material system enabled induction heating with a simple radiofrequency field to be used to melt the linear polymer uniformly and rapidly. Overall, using photopolymerization to form this new class of semicrystalline polymers holds great potential in AM and is further enabled by utilizing off stoichiometric mixtures along with 3D printing and induction heating.