Polymer stabilized cholesteric liquid crystals (PSCLCs) provide many advantages over other electro-optical materials. The unique helical structure of the cholesteric liquid crystal (CLC) creates a natural gradient for light interacting across each CLC domain layer. Not only does the CLC helical structure greatly increase the bandwidth tuning and broadening range, it also allows CLCs to act as a polarizer, notch filter, reflector, and optical rotator all in one material. However, while many novel PSCLC materials have been created, little is understood about how complex initial system interactions affect final electro-optical (e-o) properties.1,2
In this work, the principal variables affecting PSCLC blue shift electro-optical behavior have been determined through structural analysis and measurement of electro-optical properties. Typical PSCLC materials must meet both formulation and photopolymerization processing requirements to display blue shift e-o properties. Threshold photoinitiator concentrations (0.5-1.5 wt%) and morpholine containing group concentrations (0.25-1.0 wt%) were both shown to be primary factors, along with sufficient UV exposure time (10-30 min) and light intensity (500 mW/cm2, 365 nm), for PSCLC blue shift bandwidth tuning/broadening to occur. Morpholine was initially identified as a component of photoinitators Irgacure 369 and 907 and was proven to increase PSCLC ion density altering LC-polymer network interactions with several proposed theories included later in this work. The use of an appropriate morpholine containing LC monomer to directly incorporate morpholine into the LC-polymer network was shown to greatly improve PSCLC sample stability. Through the results of this research we successfully induced blue shift e-o behavior in a previous red shift only PSCLC using only 30% of the UV exposure that a model PSCLC blue shift sample required while extending the blue shift broadening range over threefold (from 75 nm to 250 nm). The fundamental understanding and design of PSCLC systems described herein serves as a starting point for engineering PSCLC materials with specific and desirable electro-optical properties.