A Two-Stage Polymerization Strategy for Preparing Polymer-Network Liquid Crystals with Oxygen-Sensing Property

Polymer-network liquid crystal (PNLC) possesses both advantages of low-molecular-weight liquid crystal (LMWLC) and liquid crystal (LC) polymer. Herein, a two-stage polymerization strategy for the formation of unique PNLC with oxygen-sensing properties is reported. The reaction mixture consists of 6% diacrylate RM257, 93.5% LMWLC 4-cyano-4′-pentylbiphenyl (5CB), and 0.5% photoinitiator dimethoxy-2-phenylacetophenone (DMPA). In the first stage, the mixture is exposed to UV light for 2 min to form a primary polymer network, which is highly uniform with a planar orientation. However, some free radicals are trapped inside the LC due to the short UV exposure time. Subsequently, the trapped free radicals are released by heating the PNLC sample into an isotropic state. Under this condition, the free radicals can move freely and react with surrounding monomers to form a secondary polymer network, which is highly disordered and scatters light strongly. Because oxygen can deactivate free radicals trapped inside the PNLC, the phenomenon to detect oxygen and monitor the diffusion of oxygen through the PNLC is exploited. The PNLC-based oxygen sensor is potentially useful for the detection of oxygen and monitoring the exposure time to oxygen.