Fiber crystal transition

It was pointed out in Chapter 6 that the polymorphic transitions from one crystalline form to another can be induced by the application of an external stress on an axially oriented crystalline system. Anisotropic dimensional changes usually accompany the transformation. Typical examples are the classical a-p transition of the keratins,(21) and the crystal-crystal transition in poly(l,4-trans-butadienes).(22) The dimensional changes in these cases reflect the different axial or fiber repeat distances of the two polymorphs. The dimensional change would be expected to be... 

Figure 2.50. Melting curves of quenched Nylon M5T (or as-spun Nylon M5T fiber) beyond the glass transition temperature. The crystal-to-crystal transitions are clearly visible. At 40°C/min the crystal form A->crystal form B transition does not take place (probably due to kinetic reasons), and crystal form A changes directly to crystal form C. [From Menczel et al. (1996). Reprinted with permission of Springer-Verlag.]...

Anhydrous BaCl2 exists as monoclinic or cubic crystals. The transition to cubic occurs at 925 °C. Barium chloride melts at 962°C the dihydrate, which has monoclinic crystals, loses water at 113 °C. Barium chloride, which is very hygroscopic, is sold in moisture-proof bags and steel or fiber dmms. 

The CLTE is an important consideration if dissimilar materials like one plastic to another or a plastic to metal and so forth that are to be assembled where material expansion or contraction is restricted. The CLTE is influenced by the type of plastic (liquid crystal, for example) and RP (particularly the glass fiber content and its orientation). It is especially important if the temperature range includes a thermal transition such as Tg. Normally, all this activity with dimensional changes is available from material suppliers. 

Two approaches to the attainment of the oriented states of polymer solutions and melts can be distinguished. The first one consists in the orientational crystallization of flexible-chain polymers based on the fixation by subsequent crystallization of the chains obtained as a result of melt extension. This procedure ensures the formation of a highly oriented supramolecular structure in the crystallized material. The second approach is based on the use of solutions of rigid-chain polymers in which the transition to the liquid crystalline state occurs, due to a high anisometry of the macromolecules. This state is characterized by high one-dimensional chain orientation and, as a result, by the anisotropy of the main physical properties of the material. Only slight extensions are required to obtain highly oriented films and fibers from such solutions. 

Noble metal catalysts such as Pd, Pt, and Ru and transition metals are deposited on or incorporated in the fibers by methods similar to those used in catalyst preparation. Zeolitic (e.g., TS-M, Na-X, ZSM-type) crystals have been grown on the surfaces. 

The high-throughput purification in a discovery environment and the removal of transition metals using adsorption on or crystallization in the presence of activated carbon, glass-bead sponges, polymeric fibers, or silica-bound scavengers and the preparative isolation of radiolabeled compounds are out of the scope of this contribution. 

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