Thermotropic hquid-crystalline polymer

Rigid-rod polymers are often Hquid crystalline polymers classified as lyotropic, such as the aramids Nomex and Kevlar, or thermotropic Hquid crystalline polymers, such as Vectran. 

Park Dae Soon, Kim Seong Hun. (2003). Miscibihty study on blend of thermotropic hquid crystalline polymers and polyester, S7(ll) 1842-1851. 

Positron annihilation spectroscopy (PALS) is a technique of free volume determination in polymers that involves the injection of subatomic particles and the measurement of their decay times. This technique can be very sensitive to the degree of miscibihty and free volume behavior of polymer blends. The concept of free volume is important to understand polymer characteristics in the glassy state. For instance, PALS was used to evaluate the free volume sites of thermotropic hquid crystalline polymer blends. These blends presented smaller and fewer free volume sites than expected from a weighted average due to their intrinsic affinity. This is interesting because in contrast to thermoplastic blend results, the degree of blend miscibility alters free volume behavior as a function of blend composition [106]. In addition, the order and the dynamics in the mesophase can be accessed by nuclear magnetic resonance [107]. 

Ternary blends from a thermotropic liquid crystalline polymer, PEN, and PET were prepared by melt blending and melt spinning to fibers. The mechanical properties of ternary blend fibers could be significantly improved by annealing at 180 °C for 2 h. This is attributed to the development of more ordered crystallites and to the formation of more perfect crystalline structures [101]. The interfacial adhesion between PEN and hquid crystalline polymer phases is enhanced when the blends are processed with dibutyltindilaurate as a reactive catalyst to promote transesterification [102]. 

Side-chain homopolymers based on poly(meth)acrylic derivatives (Fig. 16.14a) have been also widely studied (Andruzzief al., 1999 Rodriguez et al, 2005, 2006). When a suitable flexible spacer mediates between the main chain and the chromophore, these side chain polymers tend to exhibit thermotropic hquid crystalline properties, which favor cooperative chromophores motion during photoorientation and lead to more stable anisotropic optical properties. The azobenzene chromophore strongly influences a material s photoresponse. As a representative example, Ikeda... 

Similady, hquid-crystal polymers exhibit considerable order in the hquid state, either in solution (lyotropic) or melt (thermotropic). When crystallized from solution or melt, they have a high degree of extended-chain crystallinity, and thus have superior mechanical properties. Kevlar (Du Pont) is an aromatic polyamide (atamid) with the repeating unit designated as (2). It is spun into... 

Figure 13.3 Typical phase diagram of polymer solutions combining isotropic solutions (Iso), crystalline phase (Cr), and the intermediate liquid crystal phase (LC). Two arrows show the dimensions for thermotropic hquid crystal and lyotropic liquid crystal, respectively. Keller [18], Reproduced with permission of lUPAC.

These thermotropic liquid-crystalhne polymers have high melting points but can be melt-processed like other thermoplastics. The macroscopic orientation of the extended-chain crystals depends on the orientation imparted by flow during processing (molding, extrusion, etc.). Because of the fibrous nature of the extended-chain crystals, these plastics behave as self-reinforced composites, with excellent mechanical properties, at least in the chain direction. This is illustrated in Table 4.3 for molded specimens of a hquid-crystalline copolyester of ethylene glycol, terephthalic acid, and / -hydroxybenzoic acid [14]. In the direction parallel to the flow, the properties listed in Table 4.3 favorably compare with ordinary crystalline thermoplastics (nylons, polyesters) reinforced with up to 30% glass fibers. 

Properties. As prepared, the polymer is not soluble in any known solvents below 200°C and has limited solubiUty in selected aromatics, halogenated aromatics, and heterocycHc Hquids above this temperature. The properties of Ryton staple fibers are in the range of most textile fibers and not in the range of the high tenacity or high modulus fibers such as the aramids. The density of the fiber is 1.37 g/cm which is about the same as polyester. However, its melting temperature of 285°C is intermediate between most common melt spun fibers (230—260°C) and Vectran thermotropic fiber (330°C). PPS fibers have a 7 of 83°C and a crystallinity of about 60%. 

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