Phase diagram approach to fabricating electro-active flexible films: Highly conductive, stretchable polymeric solid electrolytes and cholesteric liquid crystal flexible displays.

摘要

The ultimate goal of this work is to fabricate self-standing polymer lithium electrolytes and flexible reflective liquid crystal displays by first understanding the equilibrium phase behavior of their constituent mixtures. The isotropic phase facilitates processing and allows better control of the final morphology. It is anticipated that ionic conductivity in polymer lithium electrolytes is favored with isotropic morphology which means that initial amorphous structure should be preserved in the final self-standing membranes. On the other hand, phase separation induced by polymerization is a necessary condition to generate cholesteric liquid crystal dispersions.;To understand the effect of morphology on the ionic conductivity, a ternary phase diagram of polyethylene oxide (PEO), succinonitrile (SCN) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) was established. Ionic conductivity was found to improve in the isotropic region containing high concentrations of SCN. Later, polyethylene glycol diacrylate (PEGDA) having a lower molecular weight of 700 g/mol was used in lieu of PEO and a room temperature ternary phase diagram of PEGDA/SCN/LiTFSI was constructed. Isotropic membranes with ionic conductivities between 10-5 S/cm to 10-3 S/cm at room temperature were achieved. Furthermore, membranes fabricated with PEGDA having a molecular weight of 6000 g/mol, SCN and LiTFSI have a higher ionic conductivity of 2.9x10-3 S/cm at room temperature and increase to 10-2 S/cm at 60 C. This material also exhibits stronger tensile strength and modulus that can be further improved with the addition of trimethylolpropane triacrylate (TMPTA) crosslinker.;The fabrication of polymer dispersed cholesteric liquid crystals (CLC) was carried out by first studying the phase behavior of EMA/TMPTA/CLC mixtures. Ternary phase diagram of EMA, TMPTA and CLC was constructed in order to identify the isotropic region necessary to select an appropriate precursor mixture. Reflectivity and electro-optical properties of final displays were characterized showing that the CLC domains are switchable even under deformation. Finally, the mechanical peel strength of the flexible display was improved by adding small concentrations of glycidyl methacrylate (GMA) as a dual curing agent. GMA can be integrated to the polyacrylate network through its acrylate group. A post thermal curing process is employed to activate the ring opening polymerization of epoxy groups of GMA with a thermal crosslinker such as trimellitic anhydride (TMA). During the peel tests, the regular displays containing no GMA fail at the interface between the polymer network and the substrate surface. On the other hand, materials containing GMA exhibit a fractured seismic periodic striation pattern that propagates through the crosslinked network. This stick-slip fracture behavior is a typical signature of strong adhesives, implying that GMA significantly improves peel strength up to 85 % relative to those of the cured materials containing no GMA. Supported by National Science Foundation: Award Numbers NSF-STTR 1010240 (with KDI) and NSF-DMR 1161070.