Vat photopolymerization of semicrystalline polymers: Effects of crosslinking on crystallinity

Abstract

In vat photopolymerization (VP), a crosslinked polymer network can enhance the dimensional stability and modulus of printed parts, but semicrystalline materials have not been extensively studied in VP. This work explores a semicrystalline polymer system suitable for VP, consisting of the monomers 1,6-hexanedithiol (HDT) and diallyl terephthalate (DAT), which was crosslinked using triallyloxy-1,3,5-triazine (TAT). While crosslinking improves certain mechanical properties, it can also restrict the formation of crystalline domains, thereby affecting material performance. Additionally, crosslinking can slow crystallization kinetics by influencing chain mobility. Therefore, to gain insight into the crystallization and recrystallization behavior of the HDT-DAT polymer, networks crosslinked with 0, 2, 5, 10, and 20 mol % of TAT in bulk form and printed using VP were compared. Differential scanning calorimetry (DSC) was employed to assess the crystalline and recrystallization behaviors, and X-ray diffraction was used to analyze samples with 20 mol % of crosslinker. It was found that at crosslinker concentrations of 5 mol % or higher, longer recrystallization times (over 20 h) are required to reestablish ordered crystalline regions in bulk polymerized films. In contrast, 3D printed specimens with similar compositions exhibited significantly shorter recrystallization times during annealing at room temperature, which can be attributed to their lower degree of crosslinking as evidenced by the lower gel fraction in the 3D printed samples.