Thermodynamics of Flexible and Rigid Rod Polymer Blends

Abstract

Binary mixtures of a flexible polymer and a low molecular weight liquid crystalline molecule, or a rigid rod-like molecule, are of interest because of their important technological applications in high modulus fibers, nonlinear optics, and electro optical devices. These blends are basic materials for recent new technologies of liquid crystal displays [1,2]. The performances of these systems are closely related to phase separations and conformations of polymer chains dissolved in a liquid crystalline phase. One of the main problems is to examine the location of various phases such as isotropic, nematic, and smectic phases, depending on temperature and concentra tion. To understand the thermodynamics and thermal instability of these blends, it is important to consider the co-occurrences between liquid crystalline ordering and phase separations.Generally, polymer chains exhibit a variety of flexibility, from flexible to rigid rodlike polymers. Polymer blends containing a liquid crystalline ordering have received considerable attention. The volume of literature on the theories and experiments of liquid crystallinity in polymer blends is now very extensive. There are many theoretical models for liquid crystalline polymers, such as the Flory lattice model [3][4][5][6][7][8], wormlike chain model [9][10][11], and Onsager virial theory [12] for rigid rods. An example of a recent review of liquid crystalline polymers is the text book of Donald and Windle [13].In this chapter we introduce the thermodynamics, or phase behavior, of binary mixtures of a flexible polymer and a rigid rod-like molecule. Here the flexible polymer means that liquid crystalline ordering does not occur in the constituent pure polymer. We first describe the fundamentals of a nematic phase, based on the Onsager theory for a rigid rod, where the entropy-induced nematic ordering is discussed. This will be the basis of the following sections. In Section 2.3, we focus on the phase separations of mixtures of a flexible polymer and a rigid rod-like molecule, or a low molecular weight liquid crystalline molecule, in a thermal equilibrium state. We introduce recent theoretical models to describe phase behavior, combining the Flory-Huggins Edited by Avraam I. Isayev

Thermoreversible Gel-Dispersed Liquid Crystals

“…When τ < τ N I , the nematic phase is stable and when τ > τ N I , the isotropic phase is stable. The orientational order parameter S jumps from 0 to 0.44 at τ = τ N I [11]. For the pure LCs (φ = 0), the NIT temperature is given by τ…”

“…The third term in Equation (1) is the free energy for nematic ordering of LCs. Using the Maier-Saupe model for nematic ordering [10,11,26], it is given by…”

“…The coupled equations above (Equations ( 28) and ( 30)) correspond to the common tangent method, namely the free energy has a common tangent at φ and φ [11].…”

“…[1,2]. In these systems, nematic LC microdroplets are dispersed in a polymer matrix by using the phase separation between isotropic polymer-rich and nematic LC-rich phases [3][4][5][6][7][8][9][10][11]. Typical examples of LC/polymer mixtures are mixtures of (p-ethoxybenzylidene)-p-n-butylaniline (EBBA) and polystyrene (PS) [3] and mixtures of 4-cyano-4 -n-heptylbiphenyl (7CB) and PS [7], etc.…”

“…We introduce a simple model to describe the phase behaviours of binary mixtures of a thermoreversible gel and an LC. Based on the molecular-field theories for nematic ordering of PDLCs [8][9][10][11] and for thermoreversible gelation [21][22][23], we construct the free energy of binary mixtures of reactive molecules and LCs. We calculate phase diagrams on the temperature-concentration plane and predict novel phase behaviours, such as phase separations between nematic sol and isotropic gel phases, caused by three different phase transitions: sol-gel transition, nematic-isotropic phase transition (NIT), and phase separation.…”