Nematics melting processes

It was first reported in the early 1970s that these melt processible polymers could best be described as thermotropic systems which usually display an nematic texture in the melt phase [5]. Subsequently, a number of additional phases have been reported ranging from discotic structures to highly ordered smectic E G systems with three dimensional order. In the last several years an IUPAC sponsored study on nomenclature on thermotropic LPCs has been underway. A more complete set of definitions will be available shortly as a result of Recommendation No. 199 IUPAC [6]. 

On the other hand, as opposed to the randomizing reactions which occur in the nematic melt if one anneals these copolyesters near their crystal nematic transition a completely different process appears to be operative. Thus several workers [11, 14], have reported that heating the HBA/HNA system near its melting point results in a dramatic increase in Tcn by approximately 50 °C. As... 

Liquid crystalline aromatic polyamides were the first chemical class to be commercialised. The best known example is Kevlar fibre which is spun from liquid crystalline solution to obtain the benefit of the high orientation present in the nematic phase. Subsequently, melt-processable main-chain polyesters were developed and brought to the market (Amoco with Xydar, Hoechst-Celanese with Vectra). 

Not only does the melting process depend on the thermal history of the sample, the isotropization of the polymeric nematic phase is also history-dependent. Feijoo and coworkers (1988 1990) are apparently the first to have studied in detail the effect on the isotropization of the thermal treatment. According to these researchers the establishment of thermodynamic equilibrium in the nematic phase was not as quick as has been commonly believed, for both the nematic and the isotropic phases are fluid states. They found that the heat and temperature of isotropization of the polymeric nematic phase were remarkably affected by the preceding... 

In the following, a description of the melting processes of liquid crystals will be given, and then the general structures of the nematic, smectic A and smectic C achiral and chiral phases that are involved in TGB phenomena will be discussed. When mesophases are formed by molecules (such as 1) that have asymmetric or dissymmetric structures, a reduction in the environmental space symmetry occurs, which in some cases can induce the creation of helical... 

Presence of small amounts of isotropic material within the mesophase can have important practical implications in processing of polydisperse samples. We have observed, for example, that upon fiber extrusion from the nematic melt, optimum mechanical strength is not developed until extrusion temperatures are below the l/N transition temperature of the shortest components. Similarly, measurement of order parameter shows that presence of a minor isotropic component deters optimum overall alignment until a temperature some 25-30 C below the transition to a "homogeneous nematic phase, as observed by microscopy. ... 

Crystallization of an oriented mesophase> The crystallization process of oriented nematic melts can lead in certain cases to highly oriented crystalline fibers while, in other cases, the crystallite orientation is totally or partly disrupted by the process of crystalliza-tion% Thus, DDA-8, MAA-8 and its copolymers MAA-DDA-8 lead to strongly oriented crystalline fibers on crystallization from nematic melts oriented in a magnetic field of 12 Tesla On the other hand, DDA-9 and its copolymers are much less oriented under the same circumstances. 

Thermotropic polycarbonates have been prepared from mixtures of 4,4-dihydroxybiphenyl and various diphenols (12). Nematic melts were found for copolycarbonates prepared from methylhydroquinone, chlorohydroquinone, 4,4-dihydroxydiphenyl ether, and 4,4-dihydroxybenzophenone. Slightly crystalline polycarbonates have been prepared from mixtures of hydroquinone and BPA (14) [Tg = 154°C, Tm = 313°C, AiT = 11.0 J/g (2.63 cal/g)l, and ahighly crystalline, high heat polycarbonate has been prepared from methylhydroquinone (21) [Tg = 155°C, Tm = 289°C, AH = 31.0 J/g (7.41 cal/g)]. While the former (hydroquinone-BPA) copolymer has only been prepared via cyclic oligomers, the methylhydroquinone polycarbonate can be prepared via a melt process. 

Thermotropic aromatic copolyester fibers are produced by melt spinning since the combination of an aromatic backbone and flexible segments results in an LCP which can be melt processed. Copolyesters, such as p-hydroxybenzoic add (PHBA) modified poly(ethylene terephthalate) (PET), 60/40 PHBA/PET (X7G) polymers, have been produced by the Tennessee Eastman Company [425-427]. Other melt processable nematic thermotropic LCPs (TLCP), based on combinations of 2,6-naphthalene dicarboxylic add (NDA), 2,6-dihydroxynaphthalene (DHN) and 6-hydroxy-2-naphthoic acid (HNA), and referred to as Vectra LCP resins, have been conunercialized (trademark by Hoechst Celanese Corporation [406]). 

Melt-processable liquid-crystalline polymers should be nematic over an extensive temperature range. The melting point should be depressed with respect to the isotropization temperature. It is also... 

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