Factors Affecting the Phase Separation of Liquid Crystals from Acrylate-based Polymer Matrices

The work presented in this dissertation covers the study of phase separation of small-molecule liquid crystals (LC's) from acrylate-based polymer matrices. These materials are used in the construction of polymer dispersed liquid crystals, or PDLC's, an emerging technology with applications ranging from privacy windows to photonic materials.

The first part of this study involves the determination of the effect of increasing polymer molecular weight on the solubility of the LC 4' -octyl-4-biphenyl carbonitrile, or 8CB, in poly(methyl methacrylate), or PMMA. Optical microscopy is used to determine the equilibrium phase diagrams for blends of 8CB and PMMA with weight-average polymer molecular weights ranging from 23,000-600,000 g/mol, and differential scanning calorimetry, DSC, is used to determine the fraction of 8CB that remains trapped in the polymer matrix, or the solubility limit of 8CB. Phase diagrams show what appears to be an upper limit to the effect of polymer molecular weight. The effect of polymer molecular weight on the phase behavior is quantified by extraction of the Flory-Huggins interaction parameter from fits of the microscopy data to the Flory-Huggins theory for polymer solutions. The solubility limit data also show a limit to the effect of polymer molecular weight, and when compared to solubility limit data from previous studies that use different polymer matrices, the data supports the independence of the solubility limit from polymer composition.

The second part of this work changes the emphasis of the study from the effect of polymer molecular weight to fluorination of the polymer matrix. Monomers of 2,2,2,- trifluoro ethyl methacrylate, TFEMA, and methyl methacrylate , MMA, are polymerized by atom transfer radical polymerization, or ATRP, to form copolymers with 8, 19, 25, 44, and 70% TFEMA content. The copolymers are blended with 8CB, and phase diagrams of the blends are determined by optical microscopy. As the TFEMA content of the copolymer increases, a corresponding increase in the region of immiscibility of 8CB is observed. In order to quantify the effect of increasing TFEMA content, the Flory-Huggins interaction parameters for each blend are determined from the fitting procedure used in the previous section.

The final part of this thesis employs time-resolved light scattering to study the phase separation kinetics of the LC blend E7 from a polymer matrix formed by polymerization-induced phase separation, or PIPS. The light scattering experiments start with syrups that consist of two different E7 fractions, 40 and 50% by mass. The syrups are cured by a green diode laser under four different cure beam intensities. The scattering profiles for the lowest cure beam intensity exhibit behavior that supports phase separation by a spinodal decomposition mechanism.