Polyethylene supermolecular structure

Figure 15 Morphological map of linear polyethylene fractions. Plot of molecular weight against crystallization temperature. The types of supermolecular structures are represented by symbols. Patterns a, b and c represent spherulitic structures with deteriorating order from a to c. Patterns g and d represent rods or sheet-like structures whose breadth is comparable to their length g or display a different aspect ratio d. Pattern h represents randomly oriented lamellae. Neither h nor g patterns have azimuthal dependence of the scattering. Reproduced with permission from Ref. [223]. Copyright 1981 American Chemical Society. (See Ref. [223] for full details.) Note the pattern a is actually located as o in the figure this was an error on the original.

The linewidth-temperature relation of the polyethylene oxide samples are given in Fig. 5. Despite the large differences in molecular weight, these samples have about the same linewidth, 300-350 Hz, in the crystalline state at 25°C. They all also possess a spherulitic type of morphology. The influence on the linewidth of the different types of supermolecular structures... 

As has been emphasized previously (IJ), the level of crystallinity is not the major determinant of the linewidth in the semicrystalline state. Rather the supermolecular structure or morphology is a major factor in governing the magnitude of the linewidth. Structural factors and crystallization conditions under which low density (branched) polyethylene forms... 

Comparing the WAXS data determined in bulk to the ones eharacter-izing the surface layer of the systems studied, one can find that their relation does depend on supermolecular structure of components. Sequenced elastomer matrix always produces significantly lower than in bulk degree of crystallinity, no matter the structure of plastomer, whereas the same is followed by amorphous elastomer matrix only when branched polyethylene (LDPE2) of lower crystallinity is added. Amorphous EPDM matrix facilitates crystallization of low molecular weight polyethylene of higher crystallinity (LDPEl) on the surface. 

Mandelkern recently drew a morphological map for polyethylene (55). He showed that the supermolecular structures become less ordered as the molecular weight is increased or the temperature of crystallization is decreased. 

Fig. 4.48. A plot of a- and p-transition temperatures, at frequency 3.5 Hz, for a variety of linear and branched polyethylenes representing the complete range of supermolecular structures. From [242].

Low-magnification transmission electron microscopy of chlorosulphonated polyethylene samples also provides information about the supermolecular structure. 

In this review the crystal structure and the super-molecular structure of the most used polyolefins is discussed. In particular the latest papers on the morphology of polyethylene, isotactic and syndiotactic polypropylene, isotactic poly(l-butene), and finally isotactic poly(4-methylpentene-l) are summarized and integrated with the fundamental work on the topic. After a short general introduction, the first part of the chapter is dedicated to the analysis of the order at the molecular level (the crystal structure), and the second part deals with the supermolecular structures. 

Mandelkern and coworkers [194] studied the morphology of linear and branched polyethylenes crystallized under controlled nonisothermal conditions. They proved that various morphological forms could develop by varying molecular mass, concentration of branch groups, and quenching temperature. A review of the supermolecular structures of polyethylene developed in nonisothermal crystallization conditions and the related morphological maps has been presented in a previous chapter of this handbook by Silvestre et al. [4]. 

The relation between n and the supermolecular structure that is formed is of interest. The low molecular weight polyethylene fractions that have n values of 4 form a uiuque type of superstructure. They can he represented by either rods or a rod-like assembly of the lamellar crystallites.(5) For molecular weights 7800 and 11 500, crystallized at high temperatures, 129 °C and 130°C, n is also equal to 4 and similar superstructures are observed. In this molecular weight range the high crystallization temperatures are borderline between the different type superstructures that are formed by hnear polyethylene.(5) As the crystallization temperature... 

UHMWPE comes from a family of polymers with a deceptively simple chemical composition, consisting of only hydrogen and carbon. However, the simplicity inherent in its chemical composition belies a more complex hierarchy of organizational structures at the molecular and supermolecular length scales. At a molecular level, the carbon backbone of polyethylene can twist, rotate, and fold into ordered crystalline regions. At a supermolecular level, the UHMWPE consists of powder (also known as resin or flake) that must be consolidated at elevated temperatures and pressures to form a bulk material. Further layers of complexity are introduced by chemical changes that arise in UHMWPE due to radiation sterilization and processing. 

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