Metastable folded chain

There is no unanimity in regard to the exact mechanism of ECC formation under high pressure. Wunderlich et al. [11-18] suggested that when a flexible polymer molecule crystallizes from the melt under high pressure, it does not grow in the form of a stable extended chain, rather it deposits as a metastable folded chain. 

According to Rule No. 1, polyethylene chains tend to form the most stable aU-trans conformation in the crystal. In practice, however, such an extending of chain conformation could not easily be realized. The crystallization process often chooses the metastable folded-chain conformatirHi to form the lamellar crystals. 

The decrease of T with increasing rate which diminishes at fast rates is typical for reorganization. The limit at about 50° C/min corresponds to the zero entropy p-oduction path for this polymer in which the metastable folded chain conformations are directly transformed into the supercooled melt. 

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. 

The development of a pressure-temperature phase diagram (141) for polyethylene showed that orthorhombic (folded chain), o, and hexagonal (extended chain), /i, crystal domains were placed in such a way that on cooling from the melt above about 4 kbar, first the hexagonal crystal structure was encountered and then the orthorhombic. The surprising conclusion was that at room temperature and one atmosphere, the hexagonal structure was metastable. 

Design of Folded-Chain Lamellae, Folded-chain lamellar crystals are a dominant morphological feature of natural and synthetic polymers of repetitive chemical structure. In synthetic polymers, the folded-chain structure is dictated by the kinetics, rather than the thermodynamics, of the crystallization process, and is metastable. In our early experiments... 

L.H. Tung (Dow Chemical Company, Midland, Michigan) Thermodynamically, the folded-chain crystal is metastable. In that case, why does one normally get folded-chain crystallization ... 

The bundle model of polymer crystallization will be discussed first. This mean-field approach describes the metastable configurational equilibrium of the undercooled polymer chain in solution or in the melt, and we will summarize and update previous work of ours [7-9]. The concept of the bundle, i.e. an aggregate of a few parallel polymer segments or stems connected by folds (see Fig. 1 see also [10]) and stabilized by attractive crystal-like interac-... 

The lamellar thickening proceeds through many metastable states, each metastable state corresponding to a particular number of folds per chain, as illustrated in Fig. 8. In the original simulations of [22], Kg was monitored. Rg is actually very close to the lamellar thickness due to the asymmetric shape of the lamella. The number of folds indicated in Fig. 8 were identified by inspection of the coordinates of the united atoms. This quantization of the number of folds has been observed in experiments [50], as already mentioned. The process by which a state with p folds changes into a state with p - 1 folds is highly cooperative. The precursor lives in a quiescent state for a substantial time and suddenly it converts into the next state. By a succession of such processes, crystals thicken. If the simulation is run for a reasonably long time, the lamella settles down to the equilibrium number of folds per chain. 

The lamellar thickening proceeds through many metastable states, each metastable state corresponding to a particular number of folds per chain, as illustrated in Figure 1.25. This quantization of number of folds has been observed in experiments [25], as already mentioned. The process by which a state with p folds changes into a state with p + I folds is highly cooperative. The... 

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