Domasius Nwabunma

Theoretical investigation on dynamics of photopolymerization-induced phase separation and morphology development in nematic liquid crystal/polymer mixtures

A theoretical investigation of the dynamics of photopolymerization-induced phase separation (PIPS) and morphology development in a nematic liquid crystal (LC) polymer network mixture has been undertaken by incorporating photopolymerization kinetics into the coupled time-dependent Ginzburg-Landau (TDGL–Model C) equations. The simulation on the spatio-temporal evolution of the coupled LC concentration and orientation order parameters reveals that both morphological and scattering patterns for the orientation order parameter initially lag behind those of the concentration order parameter. However, the two fields evolve to the same spatial topologies with the progression of time. The PIPS dynamics is characterized only by the late stage of phase separation. We also observed a subtle change in the curvature of the growth curve associated with the onset of nematic ordering. The growth behavior and the simulated morphology consisting of LC droplets dispersed in a matrix of polymer appears the same for all compos...

Phase behavior, photopolymerization, and morphology development in mixtures of eutectic nematic liquid crystal and photocurable monomer

Abstract Phase behavior and photopolymerization-induced morphology development in mixtures of eutectic nematic liquid crystals (LC) (designated E44) and photocurative monomers (designated NOA65) have been investigated by means of optical microscopy and light scattering. The observed phase diagram of the E44/NOA65 system prior to photopolymerization is of an upper critical solution temperature (UCST) type overlapping with the nematic–isotropic transition of E44. It displayed isotropic liquid, isotropic liquid+isotropic liquid, isotropic liquid+nematic, and pure nematic coexistence regions that were verified experimentally. Using the phase diagram as a guide, photopolymerization-induced phase separation experiments were performed in the isotropic region in order to examine the morphology development in multicomponent LC (E44)-based system in comparison with that of a single component LC (K21)-based system. Of particular interest is that the emerging LC droplets turned out to be unevenly distributed in the E44/NOA65 system as opposed to the K21/NOA65 system where the droplet size was uniform. The non-uniform distribution of the droplets in the E44/NOA65 system may be attributed to compositional fractionation associated with different affinities of the constituent LC in E44 to the NOA65 networks.

Optical adhesives based on blends of methylmethacrylate-co-glycidylmethacrylate copolymer and bisphenol-A diglycidyl ether

In pursuit of nonbirefringent optical adhesives, various copolymers of methylmethacrylate and glycidylmethacrylate (MMA-co-GMA) were synthesized and subsequently blended with bisphenol-A diglycidyl ether (EPON-828) epoxy prepolymer using acetone as a common solvent. Miscibility and optical properties of these adhesive blends were investigated by means of differential scanning calorimetry (DSC), dielectric thermal analysis (DETA), and refractive index measurements. The increasing trend of the single glass transition temperatures of the copolymers as well as of their refractive indices with increasing MMA content suggests that these copolymers are probably of a random type. The adhesive blends, after curing with trimellitic anhydride (TMA), remained transparent, suggestive of a single-phase character. The complete miscibility of the adhesive blends made the refractive indices to be adjustable by simply varying the ratio of MMA-co-GMA copolymer in the blends. In addition, the positive and negative dielectric anisotropies (intrinsic birefringence) of the constituent molecules can be compensated fully in their cured states, thereby yielding significant reduction in the net birefringence in the blends of 50-80 wt % copolymer and a zero-birefringent optical adhesive at 60 wt %.

Development of Non-Birefrmgent Optical Adhesives

Abstract The birefringence compensation technique1 has been employed through polymer blending for the development of non-birefringent optical adhesives. It has been found that trimellitic anhydride(TMA)-cured polymethylmethacylate(PMMA)/Epoxy EPON-828) as a possible system resulted in phase separation due to polymerization2. To circumvent the problem of polymerization-induced phase separation, a copolymer system comprised of methylmethacrylate(MMA) and glycidylmethacrylate(GMA) was synthesiztd3. The MMA-co-GMA copolymer was subsequently blended with EPON-828, then cured thermally with TMA. The cured blend films showed single glass transition at all compositions and optical transparency suggestive of miscible character. These optical adhesive blends showed zero birefringence or zero optical retardation at compositions between 60–70-wt% copolymer. The zero Birefringence afforded by the compensation of the intrinsic birefringence of the constituent polymer chains may be attributed to the complete miscibility...