Novel Liquid Crystal Monomers for Stereolithography: Reaction Rates and Photopolymerization Conversion

Liquid crystal (LC) monomers are a novel type of resin for stereolithography that result in polymers having unique physical and mechanical properties. These monomers consist of rigid central cores connected to acrylate functional groups by short aliphatic chains. Because of the rigid-rod structure of the monomer the cross-linked polymer networks formed have high glass transition temperatures (Tg). The high TglS result in particularly high upper-use temperatures for stereolithography parts. This paper reports on photopolymerization reaction rates and monomer conversion for two acrylate monomers measured using reflectance real-time infra-red spectroscopy (RRTIR). The RRTIR method measures the disappearance of reactive acrylate groups in the monomer as a function oftime while the monomers are being exposed to UV light. For the two new resins, UV irradiation using an argon ion laser gives rapid photopolymerization with acrylate conversion as high as 95 %. Conversion and polymerization rates in these monomers are dependent upon photo-initiator selection and concentration. In addition, the results indicate that conversion increases with increased laser intensity and elevated temperatures.