Solution-crystallized polyethylene

Fig. 3.15. Growth rates of (110) and (200) sectors of polyethylene single crystals grown from solution in tetradecanol (courtesy of S. Organ)...

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. 5 Initial fold length L against undercooling AT = To - T for both melt- and solution-crystallized polyethylene, from different solvents [16]. Dashed line gives previous calculations (Model A [9], old model in Fig. 4), solid line shows the results from the new model in Fig. 4 after re-adjusting the energy of fusion per - CH2 - unit from E= 1.07 to E- 1.42 kcal/mol...

When polyethylene is crystallized from its solutions in p-xylene at suitable temperatures (around 70°C), single crystals of polyethylene are formed as... 

In addition to these studies on the branched polyethylene, fractions of linear polyethylene prepared by large-scale gel-permeation chromatography are being characterized for certification in the near future. These should be useful for gel-permeation chromatography calibration. We also expect them to be particularly valuable in dilute solution, crystallization, and rheological studies. 

The recent studies of the phase structure of linear polyethylene by refined NMR analyses are reviewed. The phase structure of the polymer in various crystalline forms, including bulk-crystals, solution-crystals and drawn fibers, is discussed in terms of different modes of molecular mobilities in a wide range of temperature. 

In this article we have reviewed our recent work with NMR analysis on various kinds of linear polyethylene samples. It has become evident that the refined NMR analysis gives us much important information on the phase structure of samples in terms of molecular mobility, and establishes that there is no unified phase structure for polymer samples. The phase structure of samples varies over a very wide range, depending strongly on the sort of samples involved as well as on the mode of crystallization or the history of those samples. We should emphasize that there are significant differences in phase structure among the bulk-crystals, the solution-crystals, and the fiber samples, particularly in the conformation of molecular chains in the noncrystalline content. We should not confuse these phase structures with each other. The phase structures are evidently different, sample by sample, as their macroscopic properties also differ one from another. 

Fig. 15 Lateral habit of solution- and melt-grown polyethylene single crystals. The drawing are based on electron micrographs of Organ and Keller [64]—grown from solution—and of Toda [107]—grown from the melt...

To help the reader unfamiliar with polyethylene single crystal habits, Fig. 10 summarizes the most typical types [32] (a) the rhombic lozenge, normally observed in PE solution crystallized at lower temperatures (b) the... 

This shows that by means of synchrotron radiation, for the first time the primary lamellar thickening on melt crystallization in polyethylene could be observed. This enabled also the determination of the true crystal thickness as a function of supercooling AT. Most satisfyingly it was found to be closely identical to that obtained in solution crystallization were no isothermal thickening takes place. In conclusion, the hitherto existing gap between melt and solution crystallization has been removed and the corresponding f versus AT curves brought in coincidence. 

A band of moderate intensity has been observed at 1080 cm-1 in the Raman spectrum of once-folded solution-crystallized n-alkane C168H338.72 This spectral region is associated with C—C stretching vibrations of methylene sequences containing gauche bonds. Extended-chain shorter alkanes (/v-CixHox, n-C72H146) did not show this band. The 1080 cm-1 band is also observed in semicrystalline polyethylene.73 Low-temperature spectra were not reported. 

Figure 2. Exhaustive oxidation of molded (O) and solution-crystallized linear polyethylene and poly(4-methylpent-l-ene)...

SAXS measurements of the lamellae thickness of solution crystallized polyethylene (PE) as a function of supercooling AT = Tm — Tc, where Tm and Tc are the melting and crystallization temperatures respectively, are in good agreement with the theory. It predicts, that the thickness of the lamellae, lc, is determined by the size of the critical nucleus, which is given by [30]... 

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