Properties of LCPs

Materials with totally new property combinations may be achieved by blending two or more polymers together. Through blending of thermotropic main-chain LCPs with engineering thermoplastics, the highly ordered fibrous structure and good properties of LCPs can be transferred to the more flexible matrix polymer. LCPs are blended with thermoplastics mainly in order to reinforce the matrix polymer or to improve its dimensional stability, but LCP addition may modify several... 

Research on liquid crystalline polymers(LCP) is a fashionable subject with the goal of developing speciality polymers of superior mechanical and thermal properties. Besides these properties, other interesting properties of LCP have not been fully utilized. We are trying to use thermotropic LCP for photon-mode image recording material. 

Let us remember that the properties of LCPs are dependent on processing shear and, consequently, creep modulus varies with sample geometry. 

The physicochemical properties of LCP systems may be varied by changing the number of lipoamino acids, the length of the lipoamino acid alkyl side chains, the level of MAP system branching, and incorporation of different spacer molecules. The LCP systems are stable to a wide range of pH conditions in solution, as well as to peptidase enzymes, and do not need to be stored under refrigeration. 

Another peculiar property of LCPs is shown in Fig. 15.47, where the transient behaviour of the shear stress after start up of steady shear flow is shown for Vectra A900 at 290 °C at two shear rates. We will come back to this behaviour in Chap. 16 for lyotropic systems where this behaviour is quite common and in contradistinction to the transient behaviour of conventional polymers, as presented in Fig. 15.9. This damped oscillatory behaviour is also found for simple rheological models as the Jeffreys model (Te Nijenhuis 2005) and according to Burghardt and Fuller, it is explicable by the classic Leslie-Ericksen theory for the flow of liquid crystals, which tumble, rather than align, in shear flow. Moreover, it is extra complicated due to the interaction between the tumbling of the molecules and the evolving defect density (polynomial structure) of the LCP, which become finer, at start up, or coarser, after cessation of flow. 

It is well known that below a critical molar mass, the properties of polymers are very sensitive to changes in molar mass. Very little, however, is known about the molar mass dependence of the properties of LCPs. This is partly due to the difficulties in controlling molar mass in the reactions usually used to synthesize LCPs and partly due to the lack of good molar mass data on most LCPs, which are often soluble only in foirly harsh solvents. The chapter by Kim and Blumstein describes the preparation of series of main-chain LCPs varying in molar mass and the effect of molar mass on the thermal properties. 

Polymorphism in PPTA fibers is discussed in the chapter by Rutledge and Suter. The effect of the chemical structure on the theoretical properties of LCPs is discussed in the chapter by Dowell. 

The effects of molecular order on the gas transport mechanism in polymers are examined. Generally, orientation and crystallization of polymers improves the barrier properties of the material as a result of the increased packing efficiency of the polymer chains. Liquid crystal polymers (LCP) have a unique morphology with a high degree of molecular order. These relatively new materials have been found to exhibit excellent barrier properties. An overview of the solution and diffusion processes of small penetrants in oriented amorphous and semicrystalline polymers is followed by a closer examination of the transport properties of LCP s. 

Table 15.34. Typical properties of LCP/PPS blend (Vectra V140, Hoechst-Celanese) (40 wt% glass liber reinforced)...

In the subsequent chapters dynamic magnetic resonance experiments of thermotropic side and main chain polymers are presented. Computer simulations provide the orientational distributions and conformations of the polymer chains and the correlation times of the various motions. The results, referring to all polymer phases, are related to the exceptional material properties of LCPs. The discussion clearly demonstrates the power of dynamic magnetic resonance in characterizing such complex chemical systems. 

The relevant molecular properties of LCPs, comprising order and dynamics, are briefly discussed here. Generally, molecular order of these systons can be spedfied by contributions to three different levels [10], schematically depicted in Fig. 3. The... 

Orientational order of LCPs is conveniently described in terms of two differoit order parameters, characterizing the average orientation of the mesogenic units within a molecular domain (microorder) and the macroscopic alignment of the domains (see Fig. 3) [10,35]. The measurement of these parameters is important for relating macroscopic physical properties of LCPs to their molecular structure. Various methods have been used to measure order parameters in these systems, including the use of X-ray diffraction [128-130], birefringence [103, 104], and linear dichroism [104,131], but these methods can not essentially separate the two types of orientational order. 

Special interest in these polymers is stipulated by intensive progress in computer technology. In the last 5-10 years, a great list of publications concerning structure and properties of LCP was presented. However, there is little information on thermoresistance and specific features of ECP degradative behaviour. Authors have studied degradative transformations... 

From the standpoint of polymer applications, two properties of LCPs are of major interest—the effect of order on polymer melt viscosity, and the ability of the polymer to retain its ordered configuration when cooled down to the solid state. 

Among the first polymers observed to exhibit the aforesaid properties of LCPs were copolyesters (I) prepared from terephthalic acid, ethylene glycol, and p-hydroxybenzoic acid... 

Vectra LCPs consist of parallel, closely packed, fibrous chains in injection molded parts which give the polymer a wood-like appearance at fracture surfaces and also give it seU-reinforcing properties that are at least as good as those of conventional fiher-reinforced thermoplastics. ICI supphes LCPs (self-reinforcing polymers) under the trade name Victrex SRP. Excellent physical properties, low comhnstihility and smoke generation, resistance to chemicals and solvents, and stahihty towards radiation are some of the important properties of LCPs. 

The influence of chain packing (Le. free volume) on solubility, diffusivity and permeability in liquid crystalline polymers can be studied by comparing properties of LCPs in the disordered, isotropic state with those in the ordered, liquid crystalline state. HIQ-40 is a random, glassy, thermotropic, nematogenic terpolymer synthesized from 40 mole percent p-hydroxybenzoic acid and 30 mole percent each of isophthalic acid and hydroquinone. The chemical structures of the constituent monomers for fflQ-40are ... 

Not only do LCP exhibit superior performance under high temperatures, but are of the few plastic materials which perform well under cryogenic temperatures. For most plastics, there is a certain temperature between -50 C and -100°C where the physical properties reach a ductile-brittle transition point. Even under cryogenic temperatures as low as that of liquid nitrogen, the mechanical properties of LCP remain unaffected. 

In recent years, nanotechnology and nanomaterials have been widely used in designing advanced functional materials based on photoresponsive LCPs. Although several reviews have concentrated on the photodeformable effect of LCPs and their applications in soft actuators [22], to date the influence of nanostructures and nanomaterials on the photodeformable properties of LCPs has not been summarized. In this chapter, we mainly focus on the utilization of special nanostructures and amazing physicochemical properties of nanomaterials to manipulate the photomechanical behaviors of LCPs. Fmthermore, their potential applications as light-driven devices and other future prospects are proposed. 

On a microscopic scale just as for low molecular weight liquid crystals, nematic and cholesteric mesophases have been identified. Whether the smectic structure actually exists for polymeric systems is still open to question. However, it seems to be the macroscopic structure which accounts for the rheological properties of LCP. 

The electrical properties of LCPs and polybutylene terephthalate (PBT) resins are comparable although LCPs offer at least a few advantages over PBT in electric applications, i.e., low mold shrinkage, fast cycling time, ease of molding thin parts, low moisture regain, and excellent chemical and mechanical properties. 

What are the effects of reaction conditions on the formation of liquid crystallinity and the end-use properties of LCPs ... 

As we know, the dynamic properties of LCP are quite different from that of LC, but the static properties of LCP are similar to that of LC [1]. In the... 

La Mantia et al. [19] also noticed the importance of the spinnability, for mechanical properties of LCP blend fibers, although they did not emphasize... 

TABLE 8.3. Effect of Mold Thickness on the Mechanical Property of LCP and Thermoplastic PCT. 

Figure 11.4 shows the type of orientation in two films, one a uniaxial film extruded through a slot die and the other with biaxial orientation [8-11]. Table 11.1 shows the tensile properties of LCP film made by conventional processing for 0° orientation. We note that MD tensile properties are about 10 times higher than TD properties. 

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