Flexible-chain polymers thermotropic

Detection of thermotropic mesophase presence in PMCS-4 has induced different opinion on poly-organocyclosiloxanes), because heretofore poly(dimethylsiloxane) had been considered one of the most flexible-chain polymers. More thorough study of the spatial structure of PMCS-4, performed by 29Si NMR spectroscopy method, has shown that HFC... 

The main part of the side group in these macromolecules consists of the alkoxy-benzoic acid moiety. This acid may form thermotropic liquid crystals. The investigation of the hydrodynamic properties of PPhEAA molecules in dilute solutions has revealed that the equilibrium rigidity of their main chains is relatively low (Table 12). Hence, since for all flexible-chain polymers, the shear optical coefficient An/Ar in PPhEAA solutions is independent of molecular weight the segmental anisotropy - tt2 and the anisotropy of the monomer unit Aa may be determined by use ofEq. (67). 

The unique molecular packing of rod-like chains in liquid crystalline polymers (LCP) closely resembles the extended chain structure of highly oriented flexible chain polymers, suggesting that these materials are good candidates for barrier applications. Thermotropic LCP s, first developed in the early 1970 s, have been the object of much interest because of their excellent mechanical properties and ease of product fabrication. Preliminary observations have shown that a commercially available wholly aromatic thermotropic copolyester has gas permeability coefficients that are lower than those of polyacrylonitrile (4.). These results raise some fundamental questions as to the nature of the mechanism for transport of small molecules through a matrix of ordered rigid rod-like chains. 

For example, if the time for the process to occur is faster than the longest relaxation time, then the fluid behaves more like an elastic solid. For the liquid crystalline systems there seems to be two relaxation times which are important. One is the time for relaxation of orientation and the other is the time for relaxation of stress. Whereas these phenomena are connected for flexible chain polymers, they seem to separate for liquid crystalline polymers. In other words, there are several stress free states for LCP. Some of the behavior observed may be partly due to the copolymer nature of thermotropic systems. However, the lyotropic systems based on the polyamide structure also exhibit similar behavior. 

This chapter deals exclusively with thermotropics (pure polymer systems). This review deals with longitudinal one-comb and network PLCs and blends of longitudinal PLCs and conventional flexible chain polymers (Figure 10.2). An excellent introduction to the subject including a presentation of definitions and classifications is given by Brostow [10]. It is important to emphasize that a given PLC at a given... 

This section deals with thermotropic PLCs. The story starts, however, with lyotropic PLCs for which the long structural relaxation times of PLCs, compared to those of conventional flexible chain polymers, were first realized from rheo-optical studies [118]. Figure 10.12 presents the fundamentals of the important early discovery a specimen first oriented by shear flow which is suddenly switched off exhibits a rapid stress relaxation within a period of seconds. The decay of birefringence is however slower and it takes a very significant period of time. 

Extended chain crystals represent the most thermodynamically stable state. However, kinetically, it is very difficult. Extended chain crystals can be grown only under extreme conditions in flexible chain polymers, such as high pressure and high temperature [9]. Nevertheless, the polymer chains of most thermotropic liquid crystalline polymers are rigid. It is unlikely for rigid, rod-like molecules to form chain-folded crystallization. Macromolecules in rigid chain polymers may take extended chain crystallization [10]. 

However, it was recently found that many flexible-chain polymers containing no mesogenic groups are capable of forming thermotropic ordered phases in thermodynamic equilibrium and intermediate in structure and properties between crystalline and amorphous without any additional orienting effect of external fields. 

The miscibility limit between thermotropic liquid crystal polymers and flexible chain polymers can be predicted by calculations corresponding to the spinodal curve at constant temperature. This miscibility is increased with the increase of the degree of disorder (y/jc,) of the liquid crystal polymer and with the decrease of the degree of polymerization. Two quantitative parameters from the Hory s lattice theory can be used to estimate the phase behavior of this kind of blend at melt processing temperature the polymer-polymer interaction parameter and the degree of disorder (y/Xj). In binary polymer blends phase separation may occur for any value of degree of disorder (y/Xj)... 

Lee HS, Jung HC, Han MS, Lee CS, Kim WN. Compatibility studies of blends of a thermotropic liquid crystalline polymer and flexible chain polymers by application of Flory s lattice theory. Polymer 2001 42 2177-84. 

Materials with totally new property combinations may be achieved by blending two or more polymers together. Through blending of thermotropic main-chain LCPs with engineering thermoplastics, the highly ordered fibrous structure and good properties of LCPs can be transferred to the more flexible matrix polymer. LCPs are blended with thermoplastics mainly in order to reinforce the matrix polymer or to improve its dimensional stability, but LCP addition may modify several... 

It is now generally accepted that folding is universal for spontaneous, free crystallisation of flexible polymer chains. It was first of all found in crystallisation from very dilute solutions, but it is beyond doubt now, that also spherulites, the normal mode of crystallisation from the melt, are aggregates of platelike crystallites with folded chains, pervaded with amorphous material. "Extended chain crystallisation" only occurs under very special conditions in the case of flexible chains for rigid polymer chains it is the natural mode ("rigid rod-crystallisation" from the melt in case of thermotropic polymers, and from solution in case of the lyotropic liquid-crystalline polymers both of them show nematic ordering in the liquid state). 

In addition to the above mentioned lyotropic cholesteric liquid crystalline polymers composed of rigid polymers, there is a diversity of thermotropic cholesteric liquid crystalline polymers which consist of a flexible chain incorporated with a mesogenic and chiral units. The thermotropic cholesteric liquid crystalline polymers are classified into two categories main chain and side chain. 

C. Noel and J. Virlet, "DSC, Miscibility and X-ray Studies of the Thermotropic Liquid Crystalline Polyesters with Aromatic Moieties and Flexible Spacers in the Main Chain", in "Liquid Crystals and Ordered Fluid, A. Griffin and A. Johnson eds., Plenvim Press, New-York, vol. 1+, 1+01 (198I+) Finkelmann, "Synthesis, Structure, and Properties of Liquid Crystalline Side Chain Polymers", in "Polymeric Liquid Crystals", A. Ciferri, W.R. Krigbaum and R.B. Meyer eds. Academic Press, New-York (I982)... 

Thermotropic PLC s evolved in academic research laboratories by incorporating known monomeric liquid crystals into polymer chains. From such studies two types of PLC s have been developed 1) side-chain polymers with variable flexibility in the main chain, and 2) semi-flexible linear polymers. In the former the monomeric mesogen appears as a pendant sidechain attached to the main chain by a flex-... 

Jackson and Kiitifuss reported a thermotropic liquid crystalline polymer which was a copolyester of poly(ethylene) terephthalate (PET) and parahydroxybenzoic acid (PHB). In this copolyester, PET has a flexible chain conformation while PHB has a stiff, rod-like conformation and the polymer is indicated to be the liquid crystal forming component. Schematically, this copolyester... 

There are two types of polymers, which form thermotropic Uquid crystals, the side-chain, Fig. 4.16a and the main-chain polymers. Fig. 4.16b. In the side-chain pol5mers the mesogenic units are attached to a backbone by more or less flexible chains. In the main-chain polymers mesogenic units are incorporated into the pol5mer backbone and separated from each other by flexible chains [9,10]. Flexible chains (spacers) are necessary to provide a certain freedom to mesogenic moieties to form an ordered state. For the side-chain polymer to be in the nematic or smectic... 

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