Structure-property relations in LCPs

In the last decade, major technological developments have occurred in the production of polymer fibers with high mechanical strength and stiffness. In concert with these efforts, studies have been directed toward a better understanding of the relationship among chemical composition, physical structure and mechanical properties. One goal is to develop predictive structure-property models to develop marketable technologies. The discussion that follows includes examples of the types of nucroscopy 

One general concept that has received much attention is the notion that the microfibril is the fundamental building block in polymers made from flexible linear molecules. A decade ago Sawyer and George [504] proposed a basic microfibrillar building block for both natural and synthetic materials. The microfibrils have been Imown to exist in the natural materials since the 1950s [505, 506] when they were imaged in the earliest TEMs. This minimum stable sized structure appears to be the building block of 

A fibrillar texture has been shown to exist for many LCPs including the aromatic copolyesters, the aramids and the rigid rod polymers. A conunon manifestation of the highly oriented 

Martin used HREM to image the deformation and disorder in extended chain polymer fibers, such as PBZT and PBO [509] showing that deformation occurs by strain localization into kink bands. The ultrastructure of the fibers as a function of process conditions showed structural 

Polarized light microscopy has shown micrometer-sized domains in the thermotropic LCPs aligned along the fiber axis. The meander of the domains is consistent with their polarization colors (Fig. 5.87, color section). The domains 

Microscopy techniques are used for the development of predictive structure-property-process models to develop marketable technologies, such as for TLCPs [613]. Models are used to fully describe the macro and microstructure of materials such as fibers, moldings, and extrudates. As with most polymers, process history and temperature affect these structures and the resulting properties. The discussion that follows includes examples of the types of microscopy techniques that can be helpful for 

A common feature of highly oriented fibers is poor compressive properties, demonstrated by kink bands that can be seen in the OM and the SEM. Kink bands have been studied [677,679-682] and a mechanism for their formation, consistent with tensile loss, was proposed. Martin [693, 705, 706] addressed this issue by use of HREM, imaging the deformation and disorder in extended chain polymer fibers [705], showing that deformation occurs by strain localization into kink bands. High resolution EM images revealed the crystalhte size, shape, orientation, and internal perfection. The nature of the disorder within a kink band was imaged, modeled, and compared with diffraction data. Kinks were observed [706] by TEM investigation of PBZT 

Vectran LCP fiber (Fig. 5.153A) and Kevlar (Fig. 5.153B) have a periodic texture of about 50 nm observed across a group of microfibrils, arranged normal to the microfibril axis this texture appears similar in size and spacing to the nonperiodic layer (NPL) crystals observed by Donald and Windle [707]. 

Microfibrils are also shown in Fig. 5.153, and measurements of the thickness of a few of the smallest microfibrils were ca. 3-5 nm wide and about 1 nm thick with a tape-like shape. Thus, the organization of the LCPs appears to be flat or tape-like microfibrils arranged within fibrils. 

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