Chapter 20
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Synthesis and Chain Rigidity of MesogenJacketed Liquid-Crystalline Polymers Based on Poly(2,5-disubstituted-styrene) Qi-Feng Zhou, Xin-Hua Wan, Dong Zhang, and Xin-De Feng Institute of Polymer Science, College of Chemistry, Peking University, Beijing 100871, China
The evolution of the concept "Mesogen-Jacketed Liquid Crystal Polymer (MJLCP)" is described. When mesogenic units attached through no or only very short spacer to the main chain with a side-on attachment, there should form a "jacket" of mesogenic units around each chain backbone. The polymer chains are forced to take extended conformations and show properties of rigid or semi-rigid macromolecules. MJLCPs thus may be termed also "Rigid SC-LCPs" because they are not only liquid crystal polymers of the side chain type when viewed chemically, but also of significant rigidity when viewed physically as are many of the main chain type liquid crystal polymers. The synthesis and properties (with emphasis on chain rigidity) of representative MJLCPs are discussed. This concept would have bridged the gap present in between the conventional side -chain LCPs and the main-chain LCPs.
It has been practice to classify Hquid crystal polymers into two basic types, namely the side chain type and the main chain type. A side chain liquid crystal polymer (SC-LCP) has mesogenic units as side groups, while a main chain liquid crystal polymer (MC-LCP) has mesogenic units as main chain segments. Because of this kind of molecular constitution and the inherentrigidityof rod-like mesogenic units, MC-LCPs are usually rigid or semi-rigid. The liquid crystalline aromatic polyesters and polyamides are examples of mis category. For SC-LCPs, on the other hand, flexible spacers are used to decouple the movements of the main chain and of the mesogenic side groups in order to realize the liquid crystallinity (1% such that the mesogenic side groups, however rigid, will have minor influence on theflexibilityof the polymer backbones. The reverse is true so that die statistical movements and conformational changes of the main chain backbones will have only minor effects on the mesomorphic organizations of the mesogenic units. As a result, SC-LCPs are mostly flexible polymers and are thus expected to show rapid response to foreign electro-magneticfields(2).Onthe other hand
0097-6156/96/0632-0344$15.00/0 © 1996 American Chemical Society In Liquid-Crystalline Polymer Systems; Isayev, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.
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20. ZHOU ET AL.
Synthesis of Mesogen-Jacketed LCPs
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MC-LCPs are applicable as high strength and high modulus materials with high heat deflection temperatures (3). Our efforts described in mis article have been to contribute a third class of liquid crystal polymers mat have chemical structures similar to conventional SC-LCPs, but have chain properties, chain rigidity in particular, similar to that of MC-LCPs. They are "Mesogen-Jacketed Liquid Crystal Polymers" or "MJLCPs" for short. Chainrigiditymay be evaluated by a variety of methods such as viscometry and light scattering. These methods will result in values for Maric-Houwink-Sakurada equation exponent ct and for persistence length q of polymers. The parameter
