Thermotropic, definition

A Hquid crystal compound in more cases than not takes on more than one type of mesomorphic stmcture as the conditions of temperature or solvent are changed. In thermotropic Hquid crystals, transitions between various phases occur at definite temperatures and are usually accompanied by a latent heat. 

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]. 

Gray has provided an excellent book on the liquid crystalline state as known in 196222. It includes many detailed definitions. Although he notes the wide occurrence of liquid crystals in biological tissue, he limited his discussion of such crystals to the introduction. Materials in the liquid crystalline state can be divided into two major groups, those that are thermotropic and lyotropic. 

Further theoretical studies by Floryconcerned the clarification of phase transitions for the systems with a definite distribution of macromolecules over the length and also the analysis of equilibrium for a model system composed of macromolecules in which rigid blocks are separated by flexible units. An example of such systems are copolyesters exhibiting the thermotropic transition into the liquid crystalline state in the absence of a solvent... 

Liquid crystals (LCs) are molecules that have the ability to self-assemble into organized mesophases with properties intermediate between those of crystalline solids and isotropic liquids [1,2]. In LC phases, the molecules are dynamic and collectively behave as a viscous liquid but retain on average a degree of organization reminiscent of an ordered, crystalline solid. Consequently, they can be considered ordered fluids, as a more accurate definition. LCs can be subdivided into two general classes—thermotropic LCs and lyotropic LCs—depending on the environmental and molecular factors that govern how they form ordered fluid phases. 

Thermotropic liquid crystal phases are formed by rodlike or disclike molecules. However, in the following we consider orientational ordering of rodlike molecules for definiteness, although the same parameters can be used for discotics. In a liquid crystal phase, the anisotropic molecules tend to point along the same direction. This is known as the director, which is a unit vector denoted n. 

Copolyesters of poly(ethylene terephthalate) and 4-acetoxybenzoic acid (PET / oxybenzoate) were synthesized by Jackson et al. through high temperature melt transesterification. However intricate details pertaining to the polyesterification kinetics have remained unexamined. In this system, insertion of 4-oxybenzoate moieties, with stiff rod like conformations, into flexible PET chains fosters the development of thermotropic character within a definite range of copolyester composition. 

Liquid crystals is one of the names by which certain mesomorphic phases or mesophases are known, and as the name implies refers to materials which show a high degree of structural order, yet are liquid. The topic has been reviewed recently. " Those most used in NMR work have been the thermotropic variety which have a definite meso-phase range between the melting point and a second transition producing the isotropic liquid. Both pure substances (e.g. p-azoxyanisole) and mixtures (e.g. butyl-/7-(p-ethoxybenzoyl)phenyl carbonate and p-(/7-ethoxyphenylazo)phenyl heptanoate) which provide a mesophase at room temperature, have been used. 

This implies a fast, and complete, reorientation of the side groups, as in the molten state. Nevertheless, local anisotropy of motion is retained up to the much higher isotropization temperature (Ti). Great heat absorption is involved in the Tc transition, while a minor DSC signal is detected at the melting. Such a dramatic transformation of a large mass of the solid at Tc temperature supports the definition of a crystalline-to-mesophase thermotropic transition. 

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... 

Table I of Baek et al. [46] also indicates that negative may have been seen in thermotropes, but they conclude that these observations are not definitive nor unambiguous. There is a report of negative in blends of thermotropic polyester and PET [52]. We hope to return to the issue of negative in thermotropes in a future paper, possibly attempting to correlate with observations of negative die swell, but at this point we state merely that the case for negative in thermotropes is tenuous.

A cholesteric phase represents a special case of a nematic phase. The planes of adjacent molecules are also parallel, but the longitudinal axis of adjacent planes are turned about a definite angle. The result is a screw structure of the system. Cholesteric thermotropic liquid crystalline phthalocyanines are also known. ( + )-2,3,9,10,16,17,23,24-octakis[4-(dodecyloxy)-2-oxapentyl]phthalo-cyanine (preparation see Schemes 13 and 14) shows a texture typical for cholesteric phases [155], At 160°C, this compound gives a fluid isotropic phase. Upon cooling an anisotropic phase appears at 153 °C. The texture similar to platelets (blue phase) changes at 66 °C to typical cholesteric fan-shape structure and remains fan-shaped down to room temperature. Except for the discotic... 

Thus, all types of liquid crystals are thermodynamically stable over a definite temperature range. In depending on the conditions of LC formation, liquid crystals are divided into two groups. An LC phase formed by heating a solid or cooling an isotropic liquid is usually called thermotropic liquid crystals. ... 

Broadline nmr, relating melt order with polymer composition and temperature, also provided evidence supporting the thermotropic nature of these materials. The physical properties of molded parts of certain compositions showed a definite mechanical anisotropy. Strength and modulus values in the machine direction were considerably higher than those values transverse to the machine direction, that is, perpendicular to the direction of flow. Again, facile and high molecular orientation was noted for these polymers in the flow direction and this became more pronounced as part thickness was reduced. Further, the melt viscosities of the X7G copolyesters, at about equivalent molecular weights, were... 

The monofunctional P-U complex (supermolecule 19, Table 3) has been shown to form disklike dimers that can self-assemble into columns displaying thermotropic behavior. Each disk has a thickness/diameter ratio of 0.1 [155], The formation of columnar mesophases by similar discotic supermolecules is described in the previous section when cases in which liquid crystallinity and growth are hierarchically related, coupled, or totally uncoupled were considered. Lack of data on the equilibrium constants, or DP, prevents a definite assessment of the assembling mechanism of discotics based on supermolecule 19. 

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