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42825-G7
Polymer Dispersed Liquid Crystal Confined in Nanoscopic Templates
Polymer dispersed liquid crystals (PDLCs) are an important new class of materials for potential applications in the areas of light shutters, flat panel displays, microlens, etc. The performance of PDLC strongly depends on the final morphology of LC domains in polymer matrix. The size, shape, and distribution of LC domains are generally not only dictated by thermodynamic phase equilibria but also strongly depend on phase separation kinetics and anisotropic ordering of LC as most polymer systems hardly reach a thermodynamic equilibrium state. Thus, the fundamental understanding of the phase equilibrium and phase separation kinetics of mixtures of polymer/LC is of crucial importance for optimizing the performance of PDLC materials. Here, we report the formation of ordered structures in a polymer dispersed liquid crystals (PDLC) film by allowing polymer and LC to phase separate on a chemically patterned substrate (i.e., concentric Au rings patterned ITO substrate). The patterns on the substrate were successfully transferred to the PDLC film, resulting in alternating LC rich phase and polymer rich phase as confirmed by polarized optical microscope and Raman spectroscopy measurements. A unique surface-induced phase separation process was observed for the first time. The time evolutions of the size, number, and total area of LC domains were quantified. This simple approach offers a new means to organize micrometer-sized LC domains into well-ordered structures in polymer matrix of PDLC. Polystyrene (PS; Mw = 5,100 g/mol and PDI = 1.07) and a thermotropic liquid crystal (LC), 4-n-pentyl-4'-cyanobiphenyl (5CB) were selected as model systems. The concentric Au rings patterned ITO substrate (i.e., chemically patterned) was fabricated using the methods introduced previously by us. The width of Au rings, wAu and the center-to-center distance between Au rings, lamda_Au are ~10 um and ~20 um, respectively. Surface-induced phase separation kinetics of PDLC on chemically patterned substrate. A drop of PS/5CB (50:50) toluene solution was cast on the Au rings patterned ITO substrate. As PS and 5CB are incompatible, demixing between PS and 5CB took place during the solvent evaporation. Subsequently, the time evolution of PDLC morphology was monitored in-situ polarized OM. At t = t0 + 191 min, the size of randomly dispersed 5CB domains increased and the number of domains decreased. As time progressed, 5CB domains were seen to preferentially segregate to the Au rings and grow into bigger droplets (t = t0 + 501 min and t = t0 + 791 min). The phase separation completed at t = t0 + 1492 min. As a result, a thin microstructured PDLC film on chemically patterned substrate was formed, that is, 5CB-rich phase on Au rings alternating periodically with PS-rich phase on ITO rings. Evolution of LC domains and chemical composition of PDLC on chemically patterned substrate. The time evolution of the average size (in terms of LC domain area), number, and total area of 5CB droplets on Au rings were quantified. The initial diameter of the droplets was much less than half of the Au ring width, w (w = ~10 um) (t = t0 = 0 min and t = 191 min, respectively). As time progressed, the droplets on Au rings condensed together and increased successively in size, forming domains of 5-10 um in diameter (t = 501 min and t = 791 min, respectively). The final size of the domains was determined by the Au ring width (t = 1492 min). The number of droplets on the Au rings decreased with time, while the total area covered by the 5CB droplets on the Au rings increased with time. The resulting PDLC film comprised of alternating stripes of 5CB rich phase and PS rich phase. To confirm that, the chemical identification of PDLC on the patterned substrate was performed using spatially-resolved Raman spectroscopy. The Raman line scanning was conducted in the direction perpendicular to the Au rings by focusing on the CN stretch vibration spectrum (2228 cm-1) in the frequency region from 2210 cm-1 to 2250 cm-1. In summary, we have demonstrated that chemically-patterned-surface-induced phase separation of PDLC could afford a unique means to control the distribution of LC and organize micrometer-sized LC in a well-ordered fashion in polymer matrix over a large area with no need for photo irradiation. Due to strong chemical affinity between LC and Au, the LC droplets wet, grow, and coalescence preferentially along the Au rings. The patterns on the substrate were transferred to the PDLC film as a direct consequence of the incompatibility of PS and 5CB, the preferential interaction between 5CB and Au, and the comparable pattern size to the length of the domain morphology on the homogeneous substrate.
