Folded-Chain Single Crystals

Fig. 11-4. Possible conformations of poly inerchains al the surfaces of chain-folded single crystals, (a) Adjacent reentry model with smooth, regular chain folds, (b) adjacent reentry model with rough fold surface, and (c) random reentry (switchboard) model.

Chain folding — Single crystals from 0.03-3% Parallelogram-shaped (65, 66)... 

Welland, E. L., Stejny, J., Halter, A. and Keller, A. (1989) Selective degradation of chain folded single crystals of poly() -hydroxybutyrate), Polym. Common, 30, 302-4. 

E.L. Welland, J. Stejny, A. Halter and A. Keller, "Selective degradation of chain folded single crystals of poly(p-hydroxybutyrate)", Polym. Commun., 30, 302-304, 1989. 

A miscible blend of amorphous and crystalline polymers usually means a single phase in the melt and a neat crystalline phase with a mixed amorphous region in the sohd. Because of chain folding during crystallization, the crystal lamellae are formed. Their radical growth usually lead to the formation of spheniUtes [Nadkami and Jog, 1991]. 

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. 

Fig. 24a. Melting point df folded-chain polyethylene single crystals of apjffoxi-matdy 130 A fold length as a function of luting rate. The bars represent tl limits of a large number of data collected at a particular heating rate. [Re 104)2...

The melting point of folded chain polyethylene single crystals as shown in curve 4 has been studied over a wide heating rate range by Wunderlich 104). 

Crystallization from the melt also results in chain-folded lamellar crystals and for numerous polymers single lamellae have been studied by SFM approaches. Since typical lamellar thicknesses, depending on the polymer and the crystallization temperature, are in a range of 10 nm to several tens of nanometers, the finite size of the SFM probe tip leads to convoluted data, if lamellae are viewed edge-on (33). Among the coimtless examples are isotactic PS (157,158), PEO (159,160), PP (161-163), PE (164), polycarbonate (PC) (165), etc. 

Anisotropic friction was observed for all samples discussed in this paper. The anisotropy is directly correlated with the directionality of the polymer molecules at the surface of the specimens. The directionality was either confirmed experimentally by AFM (for uniaxially oriented polymers and the transcrystallized PEO) or reported in the literature in numerous diffraction studies for extended-chain polymer single crystals. The existence of regularly packed folds at the surface of lamellar polymer crystals is still a matter of discussions. 

The several models involving chain folding and dimensions have two elements in common (1) regular or irregular folded sequences connected by tie molecules, and (2) some random coil characteristics, i.e., dimensions perfectly folded single crystal has... 

Amines can also swell the polymer, lea ding to very rapid reactions. Pyridine, for example, would be a fairly good solvent for a VDC copolymer if it did not attack the polymer chemically. However, when pyridine is part of a solvent mixture that does not dissolve the polymer, pyridine does not penetrate into the polymer phase (108). Studies of single crystals indicate that pyridine removes hydrogen chloride only from the surface. Kinetic studies and product characterizations suggest that the reaction of two units in each chain-fold can easily take place further reaction is greatiy retarded either by the inabiUty of pyridine to diffuse into the crystal or by steric factors. 

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 single crystal of a polymer is a lamellar structure with a thin plateletlike form, and the chain runs perpendicular to the lamella. The crystal is thinner than the polymer chain length. The chain folds back and forth on the top and bottom surfaces. Since the fold costs extra energy, this folded chain crystal (FCC) should be metastable with respect to the thermodynamically more stable extended chain crystal (ECC) without folds. 

Extrapolation of the molecular structure of an a-maltohexaose duplex com-plexed with triiodide in single crystals leads to a left-handed, 8-fold, antiparallel double-helix for amylose.90 The pitch of this idealized helix is 18.6 A, so h is only 2.33 A. Although this model is no contender to the fiber data, in terms of biosynthesis, it is doubtful that the native amylose helix favors antiparallel chains. 

One of the most remarkable features of polymer crystallization is that such chain molecules can form lamellar crystals that contain heavily folded polymer chains. In experiments, the structural analysis of these lamellar crystals became possible when polyethylene single crystals were first prepared from a solution [100-102]. It was found that the orientation of the polymer chains... 

Fig. 9 Snapshot of a single crystal of lattice polymers viewed from the chain direction. The bonds are drawn as solid cylinders. The viewing angle is large for better observation of folds. The chain length is 512 units and the thickness of the crystallite is about 12 units. The dissolved chains are not shown for clarity [57]...

Then we extended the 2D-model to a 3D one [21]. We considered crystallization of a single polymer chain C500 from a vapor phase onto a solid substrate, taking into account detailed interactions between the chain and the substrate. Though the polymer molecule in a vacuum was collapsed, like in a very poor solvent, under the influence of bare van der Waals interactions between atoms, the molecule was found to show quick adsorption and crystallization into a rather neat chain folded lamella. 

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