Crystal mechanisms, polymer

The crystallization of polymer melts at high pressure (several thousand of atmospheres), just as the crystallization in flowing melts and stirred solutions, leads to the formation of ECC in which the molecules assume almost completely extended conformations. This suprising feet (the crystallization was carried out at hydrostatic compression) was first extablished experimentally by Wunderlich30 and then confirmed by other authors31 33. However, diverse opinions exist about the mechanism of ECC formation. 

In the world of molecular simulation, it would be more conventional to consider that the present model is a coarse grained model of real polymers, where the real time-scale is much longer than that of the present MD simulation time-scale. However, we did not intend to make a coarse grained model. The crystallization of polymers was shown to be rather universal. Various kinds of polymers, either fast crystallizing or slow crystallizing, were known to follow the same scheme with respect to the molecular mechanism of crystallization. So we studied this simple model expecting that the present model would also follow the same crystallization scheme and show the general molecular mechanisms of polymer crystallization. 

We note here that all the information presently available on high molecular weight polymer crystal structures is compatible with the bundle model. While very nearly all crystalline polymer polymorphs involve all-parallel chain arrangements, even the only known exception, namely y-iPP [104,105], where chains oriented at 80° to each other coexist, is characterized by bilayers of parallel chains with opposite orientation. This structure is thus easily compatible with crystallization mechanisms involving deposition of bundles of 5-10 antiparallel stems on the growing crystal surface. Also the preferred growth... 

But there are two important unresolved questions regarding the mechanism of crystallization the first question is are the primary nuclei actually formed from the earliest stage ( induction period ) of crystallization and the second is what is the role of the topological nature of polymers in the polymer crystallization mechanism ... 

On the basis of the concept described above, we propose a model for the homogeneous crystallization mechanism of one component polymers, which is schematically shown in Fig. 31. When the crystallization temperature is in the coexistence region above the binodal temperature Tb, crystal nucleation occurs directly from the melt, which is the well-known mechanism of polymer crystal nucleation. However, the rate of crystallization from the coexistence region is considered to be extremely slow, resulting in single crystals in the melt matrix. Crystallization at a greater rate always involves phase separation the quench below Tb causes phase separations. The most popular case... 

W. Brostow (Ed.). Polymer Liquid Crystals Mechanical and Thermophysical Properties. Chapman Hall, London (1996). 

Mandelkern, L. 2004. Crystallization of Polymers Kinetics and Mechanisms. Cambridge University Press, Ithaca, NY. 

Statistical mechanics was originally formulated to describe the properties of systems of identical particles such as atoms or small molecules. However, many materials of industrial and commercial importance do not fit neatly into this framework. For example, the particles in a colloidal suspension are never strictly identical to one another, but have a range of radii (and possibly surface charges, shapes, etc.). This dependence of the particle properties on one or more continuous parameters is known as polydispersity. One can regard a polydisperse fluid as a mixture of an infinite number of distinct particle species. If we label each species according to the value of its polydisperse attribute, a, the state of a polydisperse system entails specification of a density distribution p(a), rather than a finite number of density variables. It is usual to identify two distinct types of polydispersity variable and fixed. Variable polydispersity pertains to systems such as ionic micelles or oil-water emulsions, where the degree of polydispersity (as measured by the form of p(a)) can change under the influence of external factors. A more common situation is fixed polydispersity, appropriate for the description of systems such as colloidal dispersions, liquid crystals, and polymers. Here the form of p(cr) is determined by the synthesis of the fluid. 

Keywords wrinkling Thin-film Elastomeric polymer Polydimethylsiloxane Patterns Deformation Surfaces Self-assembly Polyelectrolyte multilayer films Thin-films Polymer brushes Colloidal crystallization Mechanical-properties Assembled monolayers Buckling instability Elastomeric polymer Tobacco-mosaic-virus Soft lithography Arrays... 

In man-made fibres, any stretching will irreversibly alter the crystallinity and there is no control of the lateral size of polymer crystals. Semicrystalline polymer networks typically consist of platelet type crystals whose width exceeds their thickness by several order of magnitudes because only the thickness is controlled by the chain folding [61]. In contrast to synthetic fibres, spider silk does not need any mechanical treatment by external forces the constituents self-assemble directly during the spinning-process. These examples clearly demonstrate the need for more detailed control of the mesoscopic structures for further development of man-made materials. 

A rough correlation exists between Tg and T for crystal lizable polymers, although the molecular mechanisms that underly both transitions differ. Any structural feature that enhances chain stiffness will raise Tg, since this is the temperature needed for the onset of large-scale segmental motion. Stronger intermolecular forces will also produce higher Tg s. Tliese same factors increase T, as described on page 382, in connection with Eq. (11-1). 

The polymerization of the higher aliphatic aldehydes has many similarities with formaldehyde polymerization. Notable differences are a lower ceilii temperature and the possibility of different steric configurations due to the substituted carbon atom. Especially anionic catalysts such as alkali metal alkoxides, soluble hydrides, and organo metal compounds lead to polymerizations during which crystalline isotactic polymer is produced 96). Little is known about the morphology and the detailed crystallization mechanism of the polyaldehydes. 

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