Polymer crystalline order

Polymers of different tacticity have quite different properties, especially in the solid state. One of the requirements for polymer crystallinity is a high degree of microstructural regularity to enable the chains to pack in an orderly manner. Thus atactic polypropylene is a soft, tacky substance, whereas both isotactic and syndiotactic polypropylenes are highly crystalline. 

Papkov, S. P. Liquid Crystalline Order in Solutions of Rigid-Chain Polymers. Vol. 59, pp. 75— 102. 

Besides crystalline order and structure, the chain conformation and segment orientation of polymer molecules in the vicinity of the surface are also expected to be modified due to the specific interaction and boundary condition at the surface between polymers and air (Fig. 1 a). According to detailed computer simulations [127, 128], the chain conformation at the free polymer surface is disturbed over a distance corresponding approximately to the radius of gyration of one chain. The chain segments in the outermost layers are expected to be oriented parallel to the surface and chain ends will be enriched at the surface. Experiments on the chain conformation in this region are not available, but might be feasible with evanescent wave techniques described previously. Surface structure on a micrometer scale is observed with IR-ATR techniques [129],... 

Blumstein, A., Liquid Crystalline Order in Polymers , Academic Press, N. Y., 1978... 

Schneider NS, Desper CR,Beres JJ (1978) In Blumstein A (ed) Liquid crystalline order in polymers. Academic, New York, chap 9, p 299... 

The connection between polymer chemistry and ceramic science is found in the ways in which linear macromolecules can be converted into giant ultrastructure systems, in which the whole solid material comprises one giant molecule. This transformation can be accomplished in two ways—first by the formation of covalent, ionic, or coordinate crosslinks between polymer chains, and second, by the introduction of crystalline order. In the second approach, strong van der Waals forces within the crystalline domains confer rigidity and strength not unlike that found when covalent crosslinks are present. 

A difference between microcrystallite-based ultrastructure and covalently-crosslinked systems is that microcrystallites melt at specific temperatures, allowing the polymer to be fabricated by heating at modest temperatures. Subsequent cooling of the system below the crystallization temperature allows the physical property advantages of the solid state to become manifest. Liquid crystallinity is also possible if some order is retained in the molten state. Crystalline order not only adds mechanical strength, it also provides opportunities for the appearance of other properties that depend on solid state order—such as electronic conductivity. 

Pre-crystalline order in PET has been investigated by a number of different groups and in the present volume the issue is reviewed under different perspectives in two other contributions [25,26]. Nodular structures measuring ca. 7.5 nm and ca 15 nm apart in PET quenched from the melt close to Tg were first described by Geil [27,28]. Such structures are essentially amorphous, albeit characterized by some degree of orientational order and by a significantly higher density as compared to the fully isotropic polymer. They are indeed qualitatively compatible with the bundle model we propose [29]. Apparently... 

As discussed in Section 1, general requirements for the crystallizability of polymers are the regularity of the chemical constitution and of the configuration of long sequences of monomeric units. In these conditions the conformation of the chains is also regular (equivalence principle). However, some three-dimensional long-range (crystalline) order may be maintained even when disorder is present in the conformation of the polymer chains. 

The four-parameter model is very simple and often a reasonable first-order model for polymer crystalline solids and polymeric fluids near the transition temperature. The model requires two spring constants, a viscosity for the fluid component and a viscosity for the solid structured component. The time-dependent creep strain is the summation of the three time-dependent elements (the Voigt element acts as a single time-dependent element) ... 

Blumstein, A., ed.. Liquid Crystalline Order in Polymers, Academic Press, New York, 1978. Cifferi, A., Kriegbaum, W.R., and Meyer, R.B., eds.. Polymer Liquid Crystals, Aeademie Press, New York, 1982. 

The most relevant property of stereoregular polymers is their ability to crystallize. This fact became evident through the work of Natta and his school, as the result of the simultaneous development of new synthetic methods and of extensive stractural investigations. Previously, the presence of crystalline order had been ascertained only in a few natural polymers (cellulose, natural rubber, bal-ata, etc.) and in synthetic polymers devoid of stereogenic centers (polyethylene, polytetrafluoroethylene, polyamids, polyesters, etc.). After the pioneering work of Meyer and Mark (70), important theoretical and experimental contributions to the study of crystalline polymers were made by Bunn (159-161), who predicted the most probable chain conformation of linear polymers and determined the crystalline structure of several macromolecular compounds. 

Clarity is typical for light passing through a homogeneous material, such as a crystalline-ordered polymer or completely amorphous polymer. Interference occurs when the light beam... 

When the glass transition temperature of the polymer sample is reached in the DSC experiment, the plot will show an incline. It is obvious that the heat capacity increases at T, and therefore DSC can monitor the of a polymer. Usually the middle of the incline is taken to be the Tg. Above Tg, the polymer chains are much more mobile and thus might move into a more ordered arrangement they may assume crystalline or liquid-crystalline order. When polymers self-or-ganize in that way, they give off heat which can be seen as an exothermal peak in... 

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