Polyethylene molecular characteristics

A further striking example can be given of the relation between molecular characteristics and phases occurring in the solution. In non-ionic surfactants, the length of the polyethylene hydrophilic segment can be varied in an almost continuous manner. Phase diagrams then show steady evolution in the extent of the various phases. 

Modulated DSC has been nsed to evaluate correlations between polymer chain molecular characteristics and crystallization behavior in polyethylene (156). It has also been used in the study of thermal treatments of PET (157) and the kinetics of the Tg (158). 

Polyethylene oxides and amyloses (Mr > 4000) readily complex with polyphenols but quantitative studies have been severely limited by the availability of water-soluble polysaccharides with clearly defined molecular characteristics. Semi-quantitative studies show that the association of polyphenols with polysaccharides is - in contrast to that with proteins - broadly independent of pH. Molecular size and flexibility are likewise critical factors but, significantly, where the polysaccharide can sequester the hydrophobic aryl residues of the polyphenol - holes in a crystal lattice (cellulose) or hydrophobic cavities (amylose and polysaccharide gels) - then complexation is substantially enhanced. Open, flexible, filamentous polysaccharides, such as the l-a-6-dextrans conversely bind phenolic substrates very poorly. It is interesting to note that model polysaccharide holes - in the form of the a- and 3- cyclodextrins - can sequester the aryl residues of certain polyphenols in the core of the molecule. In doing... 

These values determine the type of end-use application that this particular sample may be utilized for in a commercial application. The Melt Index (MI) value is a relative measure of the polyethylene molecular weight, while the Flow Index (FI) value is a relative measure of the processability of a sample and the Melt Flow Ratio (MFR) value is a relative measure of the shear-thinning behavior of the polyethylene sample which correlates with the MWD of the sample. A relative increase in the shearthinning characteristics denotes an increase in the MWD of the polymer. 

These values of G and Xi constitute the relaxation spectrum of a polymer, and are useful in correlating the viscoelastic behavior of polymers with their molecular characteristics, such as long-chain branching in polyethylene (Figure 13.33) [79]. 

Kozlov, G. V, Gazaev, M. A., Bloshenko, V. A., Varyukhin, V. N., Slobodina, V. G. (1995). Intercommunication of Molecular Characteristics and Molecular Draw Ratio for Oriented Polyethylene and Compositions on its Basis. Ukrainskii Fizicheskii Zhumal, 40(8), 883-886. 

Kakiage M, Yamanobe T, Komoto T, Murakami S, Uehara H. Effeets of molecular characteristics and processing conditions on melt-drawing behavior of ultrahigh molecular weight polyethylene. J Polym Sci B 2006 44 2455. 

Semicrystalline polymers are those that consist of two or more sohd phases, in at least one of which molecular chain segments are organized into a regular three-dimensional array, and in one or more other phases chains are disordered. The nonciystalline phases form a continuous matrix in which the crystalline regions are embedded. Most polyolefins are semicrystalline their specific morphology is governed by molecular characteristics and preparation conditions. Polyethylene is no exception to this mle it is all but impossible to prepare a solid specimen of polyethylene that is not semicrystalline. All commercial polyethylene products are semicrystalline. The physical properties exhibited by polyethylene products are governed by the relative proportions of the crystalline and noncrystalline phases and their size, shape, orientation, connectivity, etc. with respect to one another. 

The conditions under which polyethylene crystallizes influence the mechanisms by which the process takes place. Therefore, controlling the rate of crystallization regulates the properties of the product within limits imposed by its molecular character. For example, two polyethylene samples with very different molecular characteristics may be made to behave similarly in the solid-state by appropriate control of their crystallization rates. Thus, rapidly quenched high density polyethylene with a molecular weight of 500,000 and a broad molecular weight distribution exhibits tensile characteristics similar to those of a linear low density polyethylene with a molecular weight of 100,000 and a narrow molecular weight distribution that is slowly cooled from the melt [74]. 

The molecular characteristics of a polyethylene resin control its melt rheological properties. These characteristics include the distribution of molecular lengths and the number and type of branches (if any). Except in the case of polar copolymers, there is very little interaction between adjacent polyethylene chains in the melt. The combination of limited chain interaction and a flexible backbone of carbon-carbon bonds results in polymer melts that are highly mobile on a local scale. 

In this chapter no attempt is made to list the mechanical properties of all the polyethylene resins available. It is more important to understand the basic relationships that govern such properties. The nature of a specimen s response to applied stress can be correlated with its morphological and molecular characteristics it is these relationships that are emphasized. The mechanical properties of a specimen are controlled by its processing history within the limits imposed by its molecular characteristics. The nature of the molecular mechanisms involved in the physical deformation of polyethylene is discussed in Chapter 8. 

The mechanical propterties of polyethylene may be divided into two broad categories (1) low strain properties sueh as yield stress and initial modulus and (2) high strain properties, typified by ultimate tensile strength and draw ratio at break. To a first approximation, the low strain properties are controlled by a sample s morphological features, and the high strain properties by its molecular characteristics. 

The molecular characteristics of the polyethylene resin from which an article is fabricated play a role secondary to the roughness of the counterface with which it come in contact [53]. 

The melt index (Ml)—also known as the melt flow index (MFl)—of a polyethylene resin refers to the rate at whieh it extrudes from a capillary die under a standard set of conditions. The method by which it is determined is described in Chapter 6. The melt index of a polyethylene resin depends on its molecular characteristics, primarily average molecular weight, molecular weight distribution, and branching characteristics—short chain versus long chain, concentration, and distribution. The melt index reflects the average dimensions of the molecules in a resin and their entanglements with one another. From a commercial point... 

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