Microstructure of LCPs

Characterization of the optical textures of LCPs is used in the identification of the specific phases present and in understanding the structure and its relation to the solid state properties. Dynamic hot stage microscopy experiments with videotape recording provide images of the texture associated with phase changes, as a function of temperature and time. The majority of the 

Optical studies of uniaxially aligned TLCP fibers, films, and ribbons observed in the orthogonal position in polarized light exhibit a salt and pepper texture and incomplete extinction. Close examination shows a fine domain texture with individual domains, about 0.5 /tm in diameter, regions of local order (Fig. 5.131). There is a slight color variation between domains. 

An in situ rheo-optical and dynamic x-ray scattering study has given insight on formation of microstructures [646] showing the banded texture develops after cessation of shear. An example of the banded structures is shown in 

Banding has been observed in both lyotropic and thermotropic polymers examined by optical and electron imaging techniques [613,628,637-644] banding is a result of extensional or shear fiow. Incomplete extinction has been observed for some of the aramids where bands, normal to the fiber axis, are observed in polarized light [638] (Fig. 5.132). It is known that the aramids exhibit axial banding having periodicities of about 500 nm, observed by dark field TEM [645]. Simmens and Hearle [638] proposed that the optical observations and the pleated sheet model of Dobb et al. [645] are compatible and that the optical bands are the bends or folds between the pleats, which might well exhibit the local density differences observed by DF TEM. 

Thermotropic LCPs are melt processed by injection molding and extrusion to form highly oriented rods, strands, and molded articles. It is well known [595, 647, 648] that extrudates have high molecular orientation that develops as a result of the effect of the flow field on the easily oriented extended chain molecules. Molded articles composed of thermotropic LCPs have properties that are better than short fiber reinforced composites and thus have been termed self-reinforcing [649,650]. Highly anisotropic physical properties are explained by the highly anisotropic structures layers, normal to the flow direction bands, parallel to the flow and a skin-core structure. The skin-core 

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The unifying feature of all fibers spun from LCPs is the very high axial molecular orientation, which leads to extreme anisotropy of microstructure and mechanical properties. In the transverse direction, the strength is only about 20% of the axial strength and the modulus is typically less than 10% of the axial value. The microstructure of LCP fibers reflects the very... 

One mitigating feature of LCP polymer microstructures is that they often are quenchable, due to the relatively slow kinetics of ciystallization. Thus one can examine microstructures at resolutions that are inaccessible while the specimen is enclosed in a heating stage. (Objectives with the highest numerical apertures have a short working distance, and all objectives should be protected from above-ambient temperatures.) However, the gain in resolving power may not be sufficient to offset the decrease in microstructural scale. 

The distinction is easy if the microstructure is sufficiently coarse to yield interference figures for the individual domains. However, sufficiently large domains are uncommon in the case of LCPs. Magnetic or electric fields are sometimes used to coarsen the micro structure (15.161. though this is only reliable if one can be certain that the field does not induce artificial biaxiality. 

Fig. 5.7 Core and skin morphology formation of LCPs during processing, (a) Elongated microstructure. (b) Ellipsoidal microstructuFe...

Blends. There has been considerable research in recent years on polymer blends that contain an LCP. This subject was recently reviewed by Dutta et al. (67). The addition of an LCP to another thermoplastic melt effectively lowers the melt viscosity and improves processability. In addition, if the flow field contains an extensional stress component, the LCP dispersed phase is extended into a fibrous morphology and oriented in the flow direction. This microstructure can be retained in the solidified blend to provide self-reinforcement. 

SEM micrographs and visual appearances of the fracture surfaces revealed the presences of a hierarchical organization and microstructures in the macrolayers. Starting torn the macroscopic level, five macrolayers were observed two outer skins, two mid layers with a core in between. Due to differences in color, the macrolayers were readily visible to the naked eye. This skin-core morphology is a characteristic of many injection-molded LCPs (7-81. 

The microstructure and polymer-solvent interactions of lyotropic cellulosic mesophases can be derived from rheological studies. The lyotropic LCP solution is a complicated system and a wide range of unusual rheological phenomena have been observed. 

Detailed analysis of microstructure development within the extruder showed that, in the process of microstructure formation of extmdates, the deformation, coalescence, breakup, and relaxation of the dispersed phase were all involved. The process of deformation ofLCP domains in the shear flow before the extmder die was controlled by the viscosity. The shear flow before the die could result in the deformation and fibrillation ofLCP droplets, if the viscosity ratio (0.01 or smaller) favored the fibrillation. The coalescence and further deformation of the LCP domains in the die entrance lead to the increase in volume and aspect ratio of the fibrils [20]. 

Microstructural Evolution During Shear Startup of Thermotropic Main-Chain LCPs... 

The double architectures shown in Table 1.1 correspond to LCPs more commonly referred to as combined , indicating that their structures combine the features of main chain and side chain LCPs. Thus mesogenic entities are located both in the side chains and in the polymer backbone, and mesophase microstructures tend to contain mesogens from each of these environments. Ordering that involves main chain components implies a tendency away from the random-coil conformation associated with flexible, non-mesogenic backbones this is possibly why double LCP materials are often crystalline and tend to exhibit at least one smectic mesophase (sometimes several) in their thermal profiles. 

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