High-pressure crystallized polyethylene

An unusual application of size elution chromatography is the determination of lamellar thicknesses in high pressure crystallized polyethylene [4]. Nitric acid etching was used to remove all but the crystalline molecular stems, which were... 

Fig. 13. Mass fractions of three phases of bulk polyethylene versus weight-average molecular weight, obtained by broad-line JH NMR analyses. Solid lines indicate high-pressure crystallized samples, while broken lines show samples which were isothermally crystallized under atmospheric pressure...

Figure 13. Cell dimensions plotted against the lattice distortion parameter of polyethylene (40) ((O) melt crystallized, A (9) melt crystallized, B (Is) single crystal ( ) cast film high pressure crystallized, C)...

FIG. 19.1 Morphological models of some polymeric crystalline structures. (A) Model of a single crystal structure with macromolecules within the crystal (Keller, 1957). (B) Model of part of a spherulite (Van Antwerpen, 1971) A, Amorphous regions C, Crystalline regions lamellae of folded chains. (C). Model of high pressure crystallised polyethylene (Ward, 1985). (E) Model of a shish kebab structure (Pennings et al., 1970). (E) Model of paracrystalline structure of extended chains (aramid fibre). (El) lengthwise section (Northolt, 1984). (E2) cross section (Dobb, 1985). 

DILATOMETRIC STUDY OF THE MELTING POINT OF THREE HIGH-PRESSURE CRYSTALLIZED SAMPLES OF MARLEX POLYETHYLENE. 

Extended-chain crystals of polyethylene are produced by high-pressure crystallization at elevated temperatures, typically at 0.5 MPa and 245 °C. These micrometre-thick crystals display a distinctly different melting behaviour from that of the thin folded-chain single crystals grown from solution. The recorded... 

Ferguson RC, Hoehn HH. Effect of molecular-weight on high-pressure crystallization of linear polyethylene. 2. Physical and chemical characterizations of crystallinity and morphology. Polym Eng Sci 1978 18(6) 466-71. 

Yasuniwa M, Nakafuku C. High-pressure crystallization of ultra-high molecular-weight polyethylene. Po/ym /1987 19(7) 805-13. 

Turell, M.E., Bellare, A. Effect of crystallization temperature on the morphology of high pressure crystallized ultra-high molecular weight polyethylene. Abstra Paper Am Chem Soc 2002 Aug 18 224 U508-U508. 

As with ceramics and metals, polymer crystals can have multiple crystal forms. Polyethylene has a metastable monoclinic form and a orthohexagonal high pressure form. A list of some of the more common polymers and their corresponding crystal strnctnres is given in Table 1.24. Finally, X-ray diffraction can be used to determine the amorphous to crystalline ratio in semicrystalUne polymers in much the same way that Eq. (1.61) can be used. Figure 1.66 shows a schematic illustration of the X-ray diffraction patterns for semicrystalline and amorphous polyethylene. The estimation of crystalline content is based upon a ratio of the peak areas in the two samples. 

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