Thermotropic polymers structure

Perhaps the most widely utilized (and studied) lyotropic LCP is poly j -phenylene terephthalamide (PPTA), more commonly known as Kevlar (see Figure 1.70). Kevlar belongs to the class of aramids that are well known for their LCP properties. Because these polymers are crystalline in solution, they are often spun into filaments, from which the solvent is subsequently removed in order to retain the aligned polymer structure. The result is a highly oriented, strong filament that can be used for a wide variety of structural applications. Most thermotropic LCPs are polyesters or copolymers that can be melted and molded into strong, durable objects. 

The development is reviewed of liquid-crystalline polymers whose mesophase formation derives from the nature of the chemical units in the main chain. The emphasis lies primarily on highly aromatic condensation polymers and their applications. The general properties of nematic phases formed by such polymers are surveyed and some chemical structures capable of producing nematic phases are classified in relation to their ability to form lyotropic and thermotropic systems. The synthesis, properties, physical structure and applications of two of the most important lyotropic systems and of a range of potentially important thermotropic polymers are discussed with particular reference to the production and use of fibres, films and anisotropic mouldings. 

The design and synthesis of new liquid crystalline polymeric materials endowed with intrinsc chirality deserve attention, as chirality can offer probes of the supermolecular structure and a tool for modulating specific responses of the polymers (1). The chemical transformation of preformed thermotropic polymers can add novel opportunities for the realization of various molecular architectures conventionally unfeasible and best suited for mesophase modification. 

A series of thermotropic poly(amide-imide)s were synthesised [38] from diamines produced by alkylation of silylated 4-nitrophenol with oc,co-dibromoal-kanes, followed by hydrogenation of the resulting a,co-bis(nitrophenoxy)al-kanes. The diamines were reacted with trimellitic anhydride chloride in boiling m-cresol to produce polymer structure 21. 

To date, reports of investigations on the gas transport properties of main chain liquid crystalline polymers appear to have been limited to the work conducted in our laboratory. Chiou and Paul (4.) have briefly described the transport parameters of an extruded film of an LCP having a similar structure to the commercial product Vectra. This copolyester belongs to the family of napthylene thermotropic polymers (NTP s) commercialized by Hoechst-Celanese Corp. whose synthesis and properties have been described previously (iLS.). Transient permeation experiments were conducted with a series of gases. The effective... 

Butzbach GD, Wendorff JH, Zimmermann HJ (1986) Structure and structure formation of a main chain thermotropic polymer. Polymer 27 1337... 

Hence, the analysis of conformational properties of an extensive class of comb-like polymers with mesogenic side groups showed that the appearance of thermotropic mesomorphic structures in the melts of these polymers is determined by the spontaneous formation of liquid crystalline fragments on a molecular level as a result of the interaction between the side groups. 

Figure 14.3 Chemical structure of the monomer units of some commercial thermotropic polymer liquid crystals.

Donald et al. [2] reported banded structures formed by several thermotropic polymers oriented by shear at temperatures above their softening points. Similar structures were also noted in fibers drawn from polymers with rigid backbones above the softening points. Viney et al. [3] point out that the banded structures observed in shear are due to the variation in the direction of the long molecular axis with respect to the direction of shear. Evidence obtained by both optical microscopy and electron diffraction measurements supports this view. Donald and Windle [4] studied the banded structure by electron microscopy and commented that The near sinusoidal variation in the direction of the principal axis of the refractive index ellipsoid is indeed reflecting the variations in the molecular orientation. Their transmission electron microscopy indicates that the transition from... 

Although the banded structures have, as stated, been observed in sheared solutions and in films prepared from these solutions as well as in sheared melts of thermotropic polymers, they have not, to our knowledge, been reported for electric field-aligned films, nor have the internal laminae been observed previously. 

The results above show that the Frank moduli are determined mainly by the structure of mesogenic units which are similar for conventional nematics and thermotropic polymers (the situation changes considerably for the lyotropic solutions of long rod-like polymeric molecules, see the next section). On the other hand, the dynamics of reorientation are strongly influenced by the backbone. Field response and relaxation times depend dramatically on the molecular mass of a polymer though, in the first approximation, obey the same equations (4.30, 4.31). Figure 4.42 shows field-response times as a function of temperature for a comb-like acryl polymer H... 

Thus, the factors critical in the molecular design of a commercially viable thermotropic polymer for fiber/resin applications include (1) processability — ideally the polymer should process in the range of 250-350°C (2) melt anisotropy — a careful balance of molecular symmetry is required (3) end-use properties -- considerable structure-property experimentation needed to optimize and, of course, (4) minimum monomer cost. 

It must be emphasized that neither the lyotropic nor the thermotropic polymers are ordinarily used in the liquid crystalline state. The LC state is highly convenient for processing, yielding highly ordered structures with low... 

Liquid crystalline polymers [326,329-336] have found application as high modulus fibers and films with unique properties due to the formation of ordered lyotropic solutions or thermotropic melts which transform easily into highly oriented, extended chain structures in the soUd state. Thermotropic polymers are melt process-able and thick extrudates and molded parts are formed with high strength characteristics as in fiber reinforced thermoplastics. 

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