Morphology of LCPs

The amount of a catalyst in polymerization not only determines the transition times of morphological changes but also significantly affects the morphology of final products. Figure 2.13 shows various final morphologies of LCPs synthesized from different levels of catalyst concentration. For the catalyzed systems with low catalyst concentrations of 0.002 wt% to 0.007 wt%, their morphologies are quite similar to that of an uncatalyzed system at the same reaction... 

The experimental part of this chapter consists of two separate but closely related parts. Our overall aim was to carefully study the effects of viscosity ratio (Part I) and blending conditions (Part II) on the morphology of LCP/PP (polypropylene) blends. The viscosity ratio was varied between 0.1 and 3.6 by using five different grades of polypropylene and two LCPs (see Ref. 44). 

Study of the effect of viscosity ratio on the morphology of LCP/PP blends showed the most fibrous structure to be achieved when the viscosity ratio ( Lcp/ pp) ranged from about 0.5 to 1. At even lower viscosity ratio, the fiber structure was coarser, whereas at viscosity ratios above unity, the LCP domains tended to be spherical or clusterlike. 

The chemical structures and characteristic morphology of LCPs lead to a combination of attractive features high strength, modulus and impact... 

A characteristic feature of LCP is the molecular stmcture. These polymers consist of rigid rod-like macromolecules, which align in the melt to produce liquid crystal structures. If a liquid crystal polymer melt is subjected to shear or stretching flow, as in the case of all thermoplastic processing operations, then the rigid macromolecules order themselves into fibres and fibrils, which are frozen when the melt cools. This is how the specific morphology of LCP is formed in the solid state. 

Other typical studies related to the shear flow induced morphology of LCP blends include Isayev and Modic (1987), Weiss et al. (1987), and James et al. (1987). All of them reported on fibrous structure formations at high shear rate. 

The modulus of pure LCP fibers is basic data for studying mechanical properties of LCP blends. For future studies based on the studies described in this chapter, it would help if a technique to measure the LCP fiber size in the matrix, and to determine the distribution across the thickness of the mold cavity could be developed, in order to confirm the theoretical predictions of morphology of LCP blends. One suggested method for experimental measurement of the effect of LCP fiber aspect ratio on the mechanical properties of LCP blends is to constmct a mold with several inserts. Using these inserts one can prepare ASTM D638 standard specimens with several thickness ranging from 3 mm to 100 pm or less under identical processing conditions. One can then compare the measured modulus versus thickness, with predictions. 

We cannot overemphasize that the rheological behavior and the morphology of LCPs during flow are interrelated and therefore that they are inseparable, insofar as correctly interpreting rheological measurements. What is most desirable is to conduct experiments that enable one to take simultaneous measurements of the rheological... 

The effect of viscosity ratio on the morphology of immiscible polymer blends has been studied by several researchers. Studies with blends of LCPs and thermoplastics have shown indications that for good fibrillation to be achieved the viscosity of the dispersed LCP phase should be lower than that of the matrix [22,38-44]. 

Since the processing conditions and mixing equipment have a crucial effect on the morphology of immiscible polymer blends [45], experiments were carried out in four different types of extruders to find optimal conditions for blend preparation and fibrillation. Nevertheless, the morphologies of PP-LCP blends produced by... 

In both the blends and composites, the addition of LCP reinforced the PP matrix considerably. On the basis of the fibrillar morphology throughout the specimens, even better mechanical properties were expected for the composites than for the blends. The poorer than expected reinforcement was primarily due to the lack of adhesion between fiber and matrix. 

Processing. Relatively little has been published on the processing of thermotropic LCPs. The morphology of melt-processed articles is dependent on the deformation and thermal histories. Extensional flows produce fibrillar structures with high orientation in the machine direction. Flows with complicated stress distributions and temperature gradients, such as encountered in injection molding, yield complicated morphologies. 

Melt extrusion of LCP rods and films was discussed by Chung (611 and by Ide and Chung (621. respectively. Considerably more work has been done on injection molding of LCPs, but the reults were usually less conclusive due to the complex thermomechanical histories and the resultant complex morphologies (24.63-661. The anisotropic shrinkage of molded LCP parts is discussed in the chapter by Frayer and Huspeni. 

Entries 1-6 in Table I reveal an approximate constancy of volumetric CTE through the XYDAR 300, 400, and SRT-300 series resins. This result is not surprising, since the molecular structures of these materials are similar. However, the CTE values for a molded part reflect macroscopic structure (skin/core) as well as domain or molecular level morphology. The LCP molded part must therefore be regarded as a composite structure. These considerations as well as differences in basic molecular composition may explain the lower volumetric CTE for Vectra A-950 relative to the neat XYDAR resins, at least over the measured temperature range of 0-150 C. These factors may also be responsible for the lower anisotropy of the neat XYDAR 300 series resin. 

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. 

Figure 9. Schematic drawing of the polydomain morphology of a thermotropic LCP cooled from a quiescent melt.

Figure 10. Effect of elongational flow on the morphology of thermotropic LCP s.

PP blends with a small amount of LCP are of industrial interest for two reasons (i) to improve processability, or (ii) to improve the mechanical performance. The second effect depends on the blend s morphology, i.e., on the orientation of LCP domains. The latter, depends on the concen-... 

The blend morphology was characterized by dissolving the PEBA matrix, followed by gravimetric and microscopic analysis of the LCP phase. As expected, the average fiber diameter decreased as a function of DR °. It was noted that only relatively large drops were deformed into fibers, leaving nearly 50% of LCP in the form of small dispersed nodules. The fiber content as a function of DR followed a trend parallel to that of the mechanical properties. Longitudinal and trans-... 

Two series of blends based on PEEK are available from Sumitomo, PEEK/LCP (Sumiploy EK) and PEEK/PES (Sumiploy SK). The blends are to be processed by injection molding. The critical factors affecting performance of both series of blends are crystallinity of the matrix resin (PEEK) and blend morphology. It is important to control the diameter-to-length ratio of LCP fibers or the particle size of PES dispersed in PEEK matrix to achieve the expected performance. 

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