Polymer‐Dispersed Liquid Crystals Based on Polystyrene and EBBA: Analysis of Phase Diagrams and Morphologies Generated

Abstract

As an example of thermoplastic/liquid crystal blends that do not exhibit a liquid‐liquid immiscibility region in their phase diagrams, a polystyrene (PS)/N‐4‐ethoxybenzylidene‐4′‐butylaniline (EBBA) blend was analyzed. The complete phase diagram was built up using thermal transitions determined by differential scanning calorimetry (DSC) and polarized optical microscopy (POM). The boundary of the nematic + isotropic region was fitted with the Flory‐Huggins‐Maier‐Saupe model, extended to consider the polydispersity of PS. Main factors controlling the morphologies generated by thermal‐induced phase separation (TIPS) were the initial EBBA concentration in the blend and the cooling rate. Cooling at a fast rate led to small nematic domains with a narrow size distribution. Slow cooling rates led to the coexistence of very large and small dispersed domains. This was because of the large extent of coalescence of the droplets first generated associated with the continuation of the nucleation/growth process in a medium of increasing viscosity. The use of fast cooling rates might be important for the generation of a narrow size‐distribution of nematic droplets by TIPS in polymer‐dispersed liquid crystals (PDLC) used in electrooptical devices.Micrograph obtained by transmission optical microscopy without crossed polarizers, showing nematic domains dispersed in a glassy matrix arising by a fast cooling of the blend from 130 °C; composition of blend: 70 wt.‐% EBBA.magnified imageMicrograph obtained by transmission optical microscopy without crossed polarizers, showing nematic domains dispersed in a glassy matrix arising by a fast cooling of the blend from 130 °C; composition of blend: 70 wt.‐% EBBA.

Polymer dispersed liquid crystals based on poly(styrene‐b‐ethylene oxide), poly(bisphenol a carbonate) or poly(methylphenylsiloxane), and 4′‐(hexyloxy)‐4‐biphenyl‐carbonitrile: Analysis of phase diagrams and morphologies generated

“…PSEO/HOBC blends were not annealed in this study, either for DSC or for TOM1HS measurements. Moreover, the phase diagram established for PSEO/HOBC system is quite similar to the diagrams reported by Hoppe et al 28,37 for two different molecular weights PS with N-4-ethoxybenzilidene-4 0 -butylaniline (EBBA), where partial miscibility between components was confirmed by decreasing of the T g of the PS with addition of small amount of LC to the PDLC blends. The maximum solubility of EBBA in the PS was around 23 wt %, similar to the solubility of the HOBC in the PSEO reported in present work.…”

“…Consequently, controlling of the final morphology by changing the cooling rate can lead to thermoreversible materials. Hoppe et al 28,37 and Sumana et al 15,18 based on similar observation considered the PS/EBBA PDLC blends or E7 liquid crystal/polydimethyl siloxanes PDLC blends as thermoreversible materials with opto-electronic application.…”

“…It is well established that the morphology generated strongly affects the thermo-and electro-optical properties of PDLC materials. 17,[26][27][28][29][30][31] In general, the morphology is controlled by relative rates of phase separation, which depend on a number of factors such as type of LC and thermoplastic (TP) or block copolymer (BC), LC concentration, compatibility between the LC and TP or BC, and others. PDLCs have number of advantages over conventional lowmolecular-mass LC displays.…”

Phase diagrams of thermally stable, polymer‐dispersed liquid crystals: Exploring the impact of chain length and chemical structure

“…The initial and the final peaks represent the melting temperature and the clearing point of the LCs in the blends, respectively. As the LC concentration in the 8B8 blends was increased, the T g shifted from 97°C (pure PS) to 58°C (saturation temperature) at ∼15 wt% due to plasticization . Using Fig.…”

Studies on electro‐optical properties of polymer matrix/LC/ITO nanoparticles composites

“…Up to now, many efforts have focused on how to improve the electro‐optical properties of PDLC films by various polymerization methods. Generally, the PDLC films can be prepared by four methods: microencapsulation process (MP), thermally induced phase separation (TIPS), solvent‐induced phase separation (SIPS), and polymerization‐induced phase separation (PIPS) . But, the most popular method is the PIPS method, which is convenient, controllable, cheap, and solvent‐free.…”