Polymer single crystals chain folds

While the lamellar structures present in spherulites are similar to those present in polymer single crystals, the folding of chains in spherulites is less organized. Further, the structures that exist between these lamellar structures are generally occupied by amorphous structures including atactic chain segments, low molecular weight chains, and impurities. 

With the first successful growth of a polymer single crystal in the 1950s it was found that the polymer chains are folded back and forth many times inside the crystal [161]. 

The. folded-chain lamella theory arose in the last 1950s when polymer single crystals in the form of thin platelets termed lamella, measuring about 10,000 A x 100 A, were grown from polymer solutions. Contrary to previous expectations, X-ray diffraction patterns showed the polymer chain axes to be parallel to the smaller dimension of the platelet. Since polymer molecules are much longer than 100 A, the polymer molecules are presumed to fold back and forth on themselves in an accordionlike manner in the process of crystallization. Chain... 

Figure 3. Schematic representation of a polymer single crystal, illustrating the principle of chain folding (13)...

As a model of the nucleus in polymer crystallisation one often takes a rectangular prism. A breakthrough in this respect was the discovery and exploration of polymer single crystals (Schlesinger (1953) and Keller (1957)) which are indeed small prisms, platelets of polymeric chains, folded back and forth in a direction perpendicular to the basal plane (see Fig. 19.1)... 

Fig. 14.—Polymer Single Crystal from Esparto-grass Xylan. (Screw dislocations and lamellar texture are typical of these crystals, which contain folded polymer-chains. The insert shows the schematic diagram of the electron diffractogram that corresponds to the dry crystal form (see Section V,l, p. 460), and confirms that the molecular axis is normal to the lamellar plane).

The above results have obvious implications for the biosynthesis of cellulose mlcrofIbrlls. The parallel chain structure of cellulose I rules out any kind of regularly folded chain structure, and reveals the mlcrofibrils to be extended chain polymer single crystals, which leads to optimum tensile properties. Work by Brown and co-workers (22) on the mechanism of biosynthesis points to synthesis of arrays of cellulose chains from banks of enzyme complexes on the cell wall. These complexes produce a bundle of chains with the same sense, which crystallize almost immediately afterwards to form cellulose I mlcroflbrlls there is no opportunity to rearrange to form a more stable anti-parallel cellulose II structure. Electron microscopy by Hleta et al. (23) confirms the parallel sense of cellulose chains within the individual mlcroflbrlls stains reactive at the reducing end of the cellulose molecule stain only one end of the mlcroflbrll. 

Figure 8.6. Three models of the folded chain surfaces in polymer single crystals.

A first and quite comprehensive discussion of chain-folded crystals was given by Geil PH (1963) Polymer Single Crystals. Interscience, New York. 

In this work, I consider several aspects of the equiUbrium state of polymer single crystals using the model proposed by Muthukumar which will be extended to multi-chain crystals. In particular I will show that extended chain crystals are the equilibrium form for many chain crystals if sufficiently many chains are accessible and I will give a simple argument for their thermodynamic stability with reference to folded chain crystals. Furthermore, the role of finite flexibiUty of chains is discussed as well as the tilt of stems in extended chain crystals. 

The above insight, gained soon after the discovery of polymer single crystals, and the subsequent discovery of chain folding (1-3), has mapped out the route taken by much of the research into polymer crystallization over the subsequent decades. It soon became clear that the thickness of polymer lamellae was controlled by the supercooling at which they were crystallized and defined by the kinetics of crystallization. The crystal thickness, or alternatively the thickness of each new crystalline layer in a growing crystal, is the one that grows the fastest (4,5) rather than the one that is at equilibrium (6). There is now a wealth of information available on the crystallization of many polymers, as well as several theories that aim to predict the crystallization rates, crystal shapes, and lamellar thickness. 

While the lamellar structures in the spherulites are the analogue of the single crystals, the folding of the chains is much more irregular, as will be developed further in Section 6.6.2.3. In between the lamellar structures lies amorphous material. This portion is rich in components such as atactic polymers, low-molecular-weight material, or impurities of various kinds. 

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