Thermotropic PLCs

Isotropic polymeric systems as well as particulate systems might also show time-dependent moduli after cessation of flow. As long as the shear does not induce structure growth, the moduli always increase with time after flow. An increase of the moduli upon cessation of flow has also been reported for thermotropic PLCs (18) as well as for lyotropic solutions of hydroxy propyl cellulose in water (19) and in acetic add (20). The possibility of changing in either direction seems to be characteristic for mesomorphic materials. A fundamental theory for describing complex moduli does not exist for such materials. The present results, combined with the information about optical relaxation mentioned above, could be explained on the basis of reorientation of domains or defects. The different domains orient differently, even randomly, at rest whereas flow causes an overall orientation. Depending on the molecular interaction the flow could then cause an increase or decrease in moduli as recently suggested by Larson (21). 

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-... 

The investigations of thermotropic PLCs commenced almost two decades after studies of mesophases employing rodlike polymers(21) and virus particles(22) in solution were initiated. Like their MLC analogues (aqueous solutions of amphiphilic molecules) the polymeric lyotropic liquid crystals which form in solutions of rigid, rodlike polymers (helical biopolymers, and more recently, semiflexible aromatic amides) constitute a distinct class of liquid crystals. Unlike the MLC analogues, however, the lyotropic PLCs are not necessarily stabilized by specific interactions between the polymer chain... 

Although instances of lyotropic PLCs predate studies of thermotropic PLCs, as they involved solutions of comparatively esoteric species — virus particles and helical polypeptides — studies of these liquid crystals were isolated to a few laboratories. Nevertheless, observations on these lyotropic PLCs did stimulate the first convincing theoretical rationalizations of spontaneously ordered fluid phases (see below). Much of the early experimental work was devoted to characterizing the texture of polypeptide solutions. (23) The chiral polypeptides (helical rods) generate a cholesteric structure in the solution the cholesteric pitch is strongly dependent on polymer concentration, dielectric properties of the solvent, and polymer molecular weight. Variable pitch (<1 - 100 pm) may be stabilized and locked into the solid state by (for example) evaporating the solvent in the presence of a nonvolatile plasticizer.(24)... 

In the following, attention is focused only on some results obtained for solutions of thermotropic PLCs, and particularly on those of side group polymer liquid crystals (SGPLCs) since main chain polymer liquid crystals (MCPLCs) are normally difficult to dissolve and at least are insoluble in non-protonated solvents. The inclusion of lyotropic systems would be beyond the scope of this chapter and also would give no contribution to solving the question of how the properties of solutions of PLCs differ from those of non-LC polymers with similar molecular design. 

The most widely used and researched rigid thermotropic PLCs have been the Vectra series by Hoechst Celanese (USA). Although section... 

Engberg and coworkers [14,98] investigated the blends of thermotropic PLCs with fully amorphous polymers polyethersulfone (PES), polycarbonate (PC) and aromatic poly (ester carbonate) (APEC) [98] and semicrystalline polymers like polybutylene terephthalate (PBT) [14]. aL was calculated according to the Takayanaga model [101] as follows ... 

NMR spectroscopy to characterize thermotropic PLCs was first reported by McFarlane et al. [21] and Calundann and Jaffe [22], although the first actual measurements of the nematic order parameter (i.e. the domain order parameter of the nematic polymer and the fraction of nematic phase (in the biphasic region) was first made by Blumstein and CO workers [23-25] and Samulski [26] using and D-NMR. The nematic order parameter was calculated on the basis of the dipolar and quadrupolar splitting of the NMR spectrum when cooling from the isotropic phase. 

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. 

We consider now the size requirement for samples in immersion measurements. The ultrasonic pulse typically used consists of only a few periods. At the high frequency of 10 MHz, measurements can be made on samples as thin as 0.7 mm without the problem of pulse overlap arising from multiple reflections in the sample. The ultrasonic attenuation in typical thermotropic PLC at room temperature is not very high, so the upper limit in thickness is about 6 mm. The use of a lower frequency would raise both the upper and lower limits. 

Because of these reasons, it would be desirable to find an approach in which the reinforcing elements are not present before processing but are formed during the extrusion or injection molding process. To produce these in situ composites, a thermotropic PLC is first blended with a thermoplastic in the melt. During the subsequent extrusion or injection molding, the dispersed PLC phase is deformed into the fibrillar... 

Consider now using thermotropic PLCs instead of HCs. The problems associated with HCs are eliminated from the start. The thermophysical properties are also better than those of the HCs the service temperature range is large and the isobaiic expansivily small sometimes even negative expansivily is observed. Thus, we find that mechanical and thermophysical properties of PLCs are better than those of HCs and much better than those of EPs. 

A very important feature of PLCs has been already mentimied in Section 41.1.4 thermotropic PLCs are often processable with conventional processing equipment for thermoplastics— in fact, given the orientation of LC sequences, more easily than thermoplastics. It is this ease of orientatiOTi which is the reason for one more name for PLCs, already mentioned above, namely self-reinforcing plastics. 

Viscoelasticity is perhaps the most ubiquitous characteristic of high molecular weight polymers at temperatures above Tg. Here we consider the implications of coupling rubber elasticity and mesomorphism via synthesizing covalent networks from conventional elastomers (siloxanes, isoprenes, etc.) and typical MLC mesogenic cores such networks are thermotropic PLCs. At low levels of crosslink densities in the PLC network, there is no appreciable change in the transition temperatures (Tg, Td, etc.) from those of the uncross-linked PLC (and its ancestral MLC) [68]. Below Td, rather modest mechanical deformations (extension ratio A < 1.5) may convert an initial random and disclination-ridden texture into a... 

The dependence of C NMR chemical shifts on conformations has been used by Uryu and Kato to study the solid-state structure of the following thermotropic PLC ... 

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