Thermally induced phase separation dispersions

Liquid crystal and polymer dispersions are fabricated using thermally-induced phase separation (TIPS), solvent-induced phase separation (SIPS), or Polymerization-induced phase separation (PIPSX/I)- For TIPS, a homogeneous mixture of a low-molecular weight liquid crystal and thermoplastic polymer is cooled below the critical phase separation temperature to induce phase separation into liquid crystal rich and polymer rich domains. The morphological properties (domain size, number of domains per unit volume, and the composition of the domains) depend primarily on the choice of liquid crystal and thermoplastic polymer, the initial weight fraction of liquid crystal in die initial mixture, and the rate of cooling. 

Brinkman WF, Cladis PE (1982) Defects in liquid crystals. Phys Today 35(5) 48-54 Chandrasekhar S (1977) Liquid crystals. Cambridge University Press, Cambridge Chen YD, Fuh AYG, Cheng KT (2012) Particular thermally induced phase separation of liquid crystal and poly(N-vinyl carbazole) films and its application. Opt Express 20 16777-16785 Cho JD, Lee SS, Park SC, Kim YB, Hong JW (2013) Optimization of LC droplet size and electro-optical properties of acrylate-based polymer-dispersed liquid crystal by controlling photocure rate. J Appl Polym Sci 130 3098-3105... 

Experimental work on molecular composites has focused on attempting to kinetically delay the phase separation with a desirable morphology before the thermodynamics leads to complete immiscibility. Most of this work was initially performed as part of a program sponsored by the United States Air Force. For example, poly (p-phenylene terephthalamide) (PPTA) and poly (p-phenylene benozbisthiazole) (PBT) have been successfully dispersed in a Nylon 66 matrix.i 22 J y gf 23 reported that there are interactions present in a PPTA and Nylon 6 system and that phase separation can be thermally induced in molecular composites based on these two polymers. Thermally induced phase separation has also been observed in the PBT/ Nylon 6 system, when the melting temperature of the Nylon component is reached. Finally, Moore and Mathias reported a unique method for the preparation of molecular composites using an in situ polymerization process in which the anion of the PPTA was used as the initiator for the anionic polymerization of acrylamide in the formation of a Nylon 3 matrix. 

Binary mixtures of a flexible polymer and a rigid rod-like molecule (nematogen or liquid crystal) play an important role in electro-optical devices, such as light shutters and displays. Since the miscibility or phase separation controls the performance of the materials, the phase behavior and phase separation kinetics have been of fundamental and practical interests. Liquid crystalline domains dispersed in a polymer matrix are called polymer dispersed-liquid crystals (PDLCs), or polymer-stabilized liquid crystals (PSLCs), where the polymer forces the liquid crystals to phase separate into droplets surrounding by the polymer matrix [2]. Practically, there are many ways to create PD LCs by mixing polymers and liquid crystals the emulsion method [37] and phase separation method [38], including polymerization-, thermally-, and solvent-induced phase separations. The reader is referred to text books [1, 2] for details of PDLC and a review [39] for the rheological and mechanical properties. 

Polymer-dispersed liquid crystals can be prepared in several different ways including encapsulation and phase separation. The latter process has become the primary method of manufacture having basically three modes, viz., polymerization induced, thermally induced and solvent induced. Each method produces PDLCs having different properties and characteristics. The size and morphology of the droplets, the types of polymer and liquid crystal used, and cooling and heating rates in production are the main parameters deciding the properties of PDLC. 

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