Structure-property relationship in phase-separated (meth)acrylate photopolymers

Abstract

(Meth)acrylate monomers and oligomers dominate the photopolymer-based additive manufacturing (AM) market because their rapid polymerization under ambient conditions generates complex and precise structures that, in addition to high strength, offer high resistance to chemicals and heat. The major drawbacks of such structures, however, include high shrinkage stress and low fracture toughness, which originate from their highly crosslinked networks in irregular architectures [1, 2]. These challenges in photocurable systems cannot be widely addressed by common physical heterogenization approaches, such as employing fillers, mainly due to the possible negative effects on curing efficiency and monomer conversion [3]. This work focuses on the chemical heterogenization of (meth)acrylates using photopolymerization-induced phase separation (PIPS) and investigates its potential effect on the mechanical behavior of the resulting photopolymers. Different comonomer ratios of a diluted Urethane Diacrylate (UDA; Miramar 5216) oligomer and Bisphenol-A-Ethoxylate di-Methacrylate (Bis-EMA; SR348C) monomer were in different comonomer ratios, and the presence of a TPO photoinitiator polymerized where the PIPS was evidenced by obvious haziness during and after the polymerization of initially transparent photosensitive mixtures. Atomic Force Microscopy (AFM) data showed that with the addition of UDA content, the underlaid morphology transformed from a continuous hard phase with dispersed soft droplet-like domains to a co-continuous hard/soft morphology. Moreover, by increasing the UDA content, Dynamic Mechanical Analysis (DMA) showed an increase in Full Width at Half Maximum (FWHM), i.e., a measure of glass transition in such heterogeneous systems, of approximately 10 degrees. Furthermore, small quantities of trimethylolpropane tri(meth)acrylates, namely TMPTA (Laromer, BASF) as a compatible crosslinker for the acrylate-rich phase (UDA) and TMPTMA (Sigma-Aldrich) as the compatible crosslinkers for the methacrylate-rich phase (BisEMA), were used to alter the gel point as well as the magnitude of PIPS. A bespoke setup was then developed to monitor these effects by accurately measuring the PIPS onset, while real-time photorheometry was employed to determine the gel point. The obtained kinetical values and the corresponding AFM data were then analyzed with the tensile test results and discussed from the viewpoint of structure manipulation's effect on mechanical behavior.