Thermodynamics of Flexible and Rigid Rod Polymer Blends

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 concentration. 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 [3-8], wormlike chain model [9-11], 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[. 

Encydopedia of Polymer Blends.Volume 1 Fundamentals. Edited by Avraam I. Isayev 

Copyright 2010 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim ISBN 978-3-527-31929-9 

In this section, we introduce a basis of the Onsager model for a nematic phase, which will help us to understand subsequent sections. 

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In the excellent review by Schartel and Wendorff [30] on molecular composites, after analyzing the thermodynamic requirements for the preparation of molecular reinforced composites and stressing that blends of rigid rod-hke polymers with flexible-chain polymers are thermodynamically nonmiscible, they describe possible routes for obtaining homogeneous mixtures. Further, they draw attention to the fact that none of these approaches could avoid the dephasing completely. Therefore, better results in this respect can be obtained by a combination of various approaches. 

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