Crystallites extended chain

Table 19.2 gives a survey of the morphology of polymer crystallisation. The survey is self-explanatory it demonstrates an almost continuous transition from the pure folded chain to the pure extended-chain crystallite. 

Extended, or fully extended, chain crystallites contain straight chains at least 200 nm long and have been obtained for only a few polymers, such as polytetrafluoroethylene (PTFE), polyethylene and polychlorotrifluoro-ethylene, using special crystallisation techniques. Extended-chain polytetrafluoroethylene can be obtained by slow crystallisation from the melt the other two are obtained by crystallisation from the melt under elevated pressure. Solution crystallisation has so far not been shown to give rise to extended-chain crystals. 

Mendelkem [42] noted that there are three different interfacial free energies that are characteristic of crystallites. One, is for the equilibrium extended chain crystallite, a second one a c represents the mature, but nOTi-eqiulibrium crystallite, and the third one is Uen is the interfacial free energy involved in forming a nucleus. These quantities caimot be identified with one another. Because only portions of the polymeric chains participate in the formations of crystallites, the section or sections of the chains of x length that participate in crystallite formation can be designated as (e and the sections of the chains that remain in disorder and amorphous, as x — Ce -... 

Tsuji H, Ikarashi K. In vitro hydrolysis of poly(L-lactide) crystalline residues as extended-chain crystallites. Part I Long-term hydrolysis in phosphate-buffered solution at 37 degrees C. Biomaterials 2004 25 5449—55. 

The rearrangement of the polymer chains of the as-polymerized sample takes place by annealing at temperatures below the melting point. The annealing effects on the long spacing of 100 A and the melting behavior of poly(tetroxocane) confirm the coexistence of lamellar crystallites which are situated between the fibrillar bundles of the extended chain crystallite. 

Oriented Crystallization.—Hoffman " has developed a theory for the growth of fibrous crystals with extended-chain morphology i.e., the core fibril or shish which develops on flow-induced crystallization. An embryonic fibril connected by bundle nuclei is produced. End surfaces resulting from the repulsion of amorphous chains in the regions between the nuclei build up commutatively as the nuclei mature. Volume strain in each nuclei limit the diameter of the core fibril to 15—50 nm. The model leads to a set of extended chains crystallites of stable diameters interrupted by short and highly strained amorphous regions. 

FIGURE 21.7 Schematic representation of structures of crystallized PLA material before and after hydrolytic degradation, or of the formation of crystalline residues (or extended-chain crystallites) [62, 268]. 

Also, Tsuji et al. [68] prepared block copolymers by ringopening polymerization of DLLA coinitiated with PLLA crystalline residues (or extended-chain crystallites) prepared by NHD of crystallized PLLA films. Extended-chain crystallites can act as a macrocoinitiator because they have many hydroxyl groups as terminal groups on their surface. In spite of the activity as the coinitiator, the chains inside the extended-chain crystallites are inert and expected to be protected from intersegmental transesterification between block chains synthesized in the first step (L-lactide chains) and the second step (DL-lactide chains) due to the rigidity of the crystalline lattice. 

The foregoing analysis of stress-induced crystallization has deliberately been limited to equilibrium concepts. By definition, therefore, only extended chain crystallites are being considered. Even with this restriction, theory and experiment are not in as complete harmony with one another as would be desired. Several... 

Tsuji, H., Ikarashi, K., Fukuda, N., 2004. Poly(L-lactide) Xll, Formation, growth, and morphology of crystalline residues as extended-chain crystallites through hydrolysis of poly(L-lactide) films in phosphate-buffered solution. Polym. Degrad. Stabil. 84, 515-523. 

Still higher degrees of chain orientation are found in the extended chain crystallites, row structures, and highly lamellar hard-elastic or springy forms of crystallizable polymers which recently have become of considerable interest. Follow-... 

Extended-chain crystallites have been studied by several authors (cf. 2 IC), most recently by Gogolewski and Pennings et al. [201-203]. 

The overall crystallization kinetics of the high molecular weight n-alkanes yields superposable, classical isotherms of the Avrami type. An example is given in Fig. 9.73, where ln(l - X(t)) is plotted against In t for Ci92H3g6.(278) The studied temperature interval of 118-124 °C encompasses the time scale of about 0.25-200 min for the detection of crystallinity. Low frequency Raman acoustical mode (LAM) studies indicate that extended chain crystallites are formed at all temperatures in... 

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