Fibrillar crystallites

Crystallizable polymers tend to form randomly oriented crystallites which are oriented when the polymer is stretched or cold drawn at temperatures below the Tm. Crystallization under pressure may result in a fibrillar structure or extended chain structure. 

Numerous studies of the structure and properties of drawn crystalline polymers have led to the microfibrillar model of fibrous morphology177 179 180. According to Peterlin 179) and Prevorsek et al. 180), the long and thin microfibrils are the basic elements of the fibrous structure. The microfibrils consist of alternating folded chain crystallites and amorphous regions. The axial connection between the crystallites is accomplished by intrafibrillar tie-molecules inside each microfibril and by inter-fibrillar tie-molecules between adjacent microfibrils. 

As already illustrated by a previous paper (5 ) on the structure of high modulus fibers, electron microscopy can be very successful when applied to these beam-resistant materials, and so constitutes an essential complement to x-ray studies. In the present work, electron diffraction coupled with BF and DF imaging has allowed detection of the best ordered zones within PBT fibers which illustrates the structure possibly obtainable by fiber processing refinement. The well ordered structures observed thus far compare rather well, with the exception of their fibrillar texture, to the structure of PPT high-modulus fibers. The two dimensional character of the crystallites is likely due to the freedom of axial translation of the molecules. Future work should determine if this feature is a direct consequence of the chemical structure of the PBT molecule or is simply the result of non-optimized processing conditions. 

Although the typical high tensile strength carbon fibers (obtained at heat treatment <1600 °C) are certainly considerably less ordered, and less densely packed, their structure may be assumed to be somewhere in-between the fibrillar structure with some pseudocrystalline lamellar order within the fibrils, as suggested by Warner for stabilized fiber, and the graphitic crystallites suggested by Crawford . The voids determine the relatively low density of the carbon fibers (for PAN based fibers... 

Recent studies in the fine structures of radiation polymerized PTOX show that they are disordered crystals in which maln-crystal-lites and sub-crystallites are arranged in series (7)- The fine structures of PTEOX are even more disordered and complex than those of PTOX, and are believed to possess an oriented lamellar morphology. However, when the post-polymerization is carried out at temperatures above 90 C, the sub-crystallites disappear. WAXS studies indicate that the (009) and (0018) reflections of PTEOX have an asymmetrical profile, suggesting the existence of two different lattice spacings along the fiber axis. Odajima, et al. (8) suggested that there may be two kinds of crystallites present, namely those with the extended fibrillar and the folded lamellar morphologies. 

Drawn fibrillar morphology (developed when a spherulitic polymer is stretched below its melting point and the original lamellar crystallites are fragmented and rearranged into an oriented fibrous structure) (29)... 

The model suggests the occurrence of fibrillar crystallites, which can grow both parallel and perpendicular to the chain axes. Each chain... 

Shortly afterwards (in 1957) Fischer showed by electron microscopy that the crystallites in melt-grown spherulites of polyethylene and nylon were most likely to be lamellar rather than fibrillar, as would be expected from the fringed-micelle model. It is now accepted that chain-folded lamellar crystallites play an important part in the structure of most ordinary... 

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