Synthesis and properties of liquid crystalline urethane methacrylates for dental composite applications

Article abstract of DOI:10.1002/pola.24693

Three novel liquid crystalline methacrylates have been synthesized and characterized to be tested as comonomers in light-curing dental resin-based composites. The selected formulations consist of an alkylammonium or cholesteryl urethane methacrylate and 2,2-bis[4-(2-hydroxy-3-methacryloyloxypropyl)phenyl] propane (BisGMA) or a BisGMA derivate modified with urethane methacrylate groups, further diluted with triethyleneglycol dimethacrylate (TEGDMA) and reinforced with 70% filler (zirconium silicate nanopowder, silanized filler). This study addresses the relationships between the LC monomer structure, photopolymerization rates (by differential scanning photo calorimetry), and specific properties of the dental resin composites (volumetric shrinkage, water sorption, water solubility, and hydrophobicity). The investigation of LC properties by differential scanning calorimetry and polarizing microscopy indicated that the LC mesophase is stable to room temperature (cationic monomers) or at 40 °C (cholesteryl methacrylate). It was found that the polymerization rate for LC urethane methacrylates used in combination with BisGMA/TEGDMA (0.122-0.136 s^-1) is higher than that of the mesogenic monomers alone (0.085-0.107 s^-1). The structures of the urethane monomers and, consequently, the viscosity of the comonomer mixture influence both the rate and the degree of conversion (44.8-67.5 %) of the photopolymerization process. Polymerization shrinkage measured by pycnometry showed lower values for LC monomers (3.25-3.43 vol %) comparatively with the monomer mixture (5.19-6.65 vol %). Preliminarily, the effect of ammonium groups from two resin composites incorporating alkylammonium structures (4.5 wt %) was tested on Streptococcus mutans, and distinct zone of inhibition was observed.