Branching in low density polyethylene

The presence of long chain branches in low density polyethylene (LDPE) accounts for the difference in properties e.g. higher melt strength, greater toughness for the same average molecular weight) between LDPE and linear low density polyethylene (LLDPE, made by coordination polymerization). 

Isolated butyl branches in low-density polyethylene are formed by an intrachain radical rearrangement that is followed by repeated addition of ethylene without further rearrangement. Here, stereochemical selectivity during the formation of CH2R—CH2-CHR—CH2-branches in the free radical initiated polymerization of monosubstituted vinyl monomers is Investigated. The configuration partition functions are denoted by Zm and Zn respectively. They can be written as 2 Um Up Iv1 v2 v3]T and Zr = U U2 Up Ur Up [v3 v2 v3lT. Numerical values... 

Chain Branching by Hydrogen Abstraction Low-density polyethylene is soft and flimsy because it has a highly branched, amorphous structure. (High-density polyethylene, discussed in Section 26-4, is much stronger because of the orderly structure of unbranched linear polymer chains.) Chain branching in low-density polyethylene results from abstraction of a hydrogen atom in the middle of a chain by the free radical... 

Beer, F. Capaccio, G. Rose, L.J. High molecular weight tail and long-chain branching in low-density polyethylenes. J. Appl. Polym. Sci. 2001, 80, 2815-2822. 

Nishoika and co-workers [29] determined the degree of chain branching in low-density polyethylene using proton Fourier transform NMR at 100 MHz and Fourier transform NMR at 25 MHz with concentrated solutions at approximately 100 °C. Methyl concentrations obtained agreed well with those obtained by IR based on the absorbance at 1378 cm" (7.25 pm). 

Bugada and Rudin [30] combined exclusion chromatography with C-NMR to determine long-chain branching in low-density polyethylene. 

Pyrograms were also prepared for a range of polyethylene containing different amounts of short chain branching (see Table 10.10 for details of samples). Previous work by high-energy electron irradiation and mass spectrometry has shown that the short branches in low-density polyethylenes are mainly ethyl and n-butyl groups, but other short branches have also been supposed to be present. 

Romanini, D., Savadori, A., and Gianotti, G., Long chain branching in low density polyethylene. 2. Rheological behaviour of the polymers, Pofymer, 21,1092-1101 (1980). 

Shirayama, K. Okada, T. and Kita, S., Distribution of short-chain branching in low-density polyethylene,/. Polym. Set, Part A-2,3(3), 907-916 (1965). 

Ethylene ionomers consist of copolymers of ethylene and an organic add, such as methacrylic acid, the acid moieties of which have been neutralized to form a metal salt. The metal salts from neighboring chains tend to form clusters, such as the one shown schematically in Fig. 18.3. The net result is the overall structure shown in Fig. 18.2 g), in which the ionic clusters form weak crosslinks between adjacent chains. Ionomers also contain short and long chain branches, which are similar to those found in low density polyethylene. 

The crystallinity of a polymer such as polyethylene typically increases as the molecular weight and the structural regularity increase but decreases as the extent of irregular branching in the polymer molecule increases. Thus because of its regular structure, hdpe, like linear paraffins, readily forms crystals. In contrast, branched or low-density polyethylene (ldpe) is less crystalline because of its more irregular structure. 

We will deal in this review article with monodisperse, model-branched polymers in order to describe the basic relaxation modes of branched polymers. The concepts described below are the source of current attempts to describe the viscoelastic properties of complex tree-like structures which are close to those fo md in low density polyethylene, for example. One may foimd interesting approaches of that problem in recent papers presented by Mac Leish et al [10]. 

Chain branching in low density versions of polyethylene is common. Extent and length of branching stem primarily from the mechanism of polymerization and incorporation of comonomers. Branching is classified as long chain branching (LCB) or short chain branching (SCB). By convention, SCB implies branches of 6 or fewer carbon atoms. LDPE contains extensive LCB and branches can contain hundreds of carbon atoms. Branches on branches are also common in LDPE. 

The formation of the backbone radical site depends on the nature of the polyolefin. The presence of tertiary carbon atoms as in polypropylene and branch points in low-density polyethylene, LDPE, would be expected to provide the preferred site for hydrogen-atom abstraction. However, it has been noted (Russell, 2002) that the difference between the reactivity of tertiary and secondary carbon atoms decreases at elevated temperatures. 

It has been noted (Guan et al, 1999) that polyethylene is not just branched (as in linear low-density polyethylene, where branching is controlled by the copolymerization of 1-hexene, or in low-density polyethylene, where it is uncontrolled due to back-biting) but may, under certain circumstances, also be hyperbranched. The mechanism for hyperbranching is to create the branch point by controlled isomerization of the active site by an appropriate choice of both co-ordination catalyst and pressure. 

Pure polyethylene should not absorb ultraviolet radiation of wavelength above 200 nm since pure paraffins are transparent in that region of the spectrum. However, it is well established [ 20] that even carefully purified polyethylene differs from a simple high molecular weight straight chain paraffin in being to some extent unsaturated. The total unsaturation has been estimated to be about 0.25% C=C by weight [21]. Olefinic unsaturation of different types has been detected by infrared spectroscopy [21, 22] it seems to be mainly of the vinyl type in linear polyethylene, while most unsaturation is of the vinylidene type in branched polyethylene [22]. Attention has also been drawn to the fact that a structure seems to be present in low density polyethylene which leads to a triene on ultraviolet irradiation [23]. 

When chain transfer occurs to a finished polymer, branches are formed. This phenomenon is found in low-density polyethylene, which exhibits various kinds of branches extending from the main chain. 

This kind of sequence defect occurs in the statistical copolymers, where the species of monomers can crystallize. On the backbone of polyethylene chains, the short branches can be regarded as the non-crystallizable comonomers. In high-density polyethylene (HOPE), the branching probability is about 3 branches/1,000 backbone carbon atoms, and its crystallinity can reach levels as high as 90 % while in low-density polyethylene (LDPE), the branching probability is about 30 branches/ 1,000 backbone carbon atoms, and its crystallinity reaches only 50 %. The most common industry product is actually linear low-density polyethylene (LLDPE), and its branching probability is determined by the copolymerization process of CH2 = CH2 and CH2 = CHR (R means side alkane groups for short branches). 

A nonlinear polymer in which the molecules consist of linear main chain to which there are randomly attached secondary chain branches, viz., low density polyethylene. A fraction of repeat units in a polymer that statistically contain one branch is defined as the branching density X = ab/n, where a is the branching coefficient (dependent on functionality of the branch point), b is the number of branch points, and n is the number of repeat units. 

Branches of comparable length as the main polymer chain as in low-density polyethylene, polyvinylchloride, etc. 

Fig. 17. In an ethyl-branched linear low-density polyethylene (7-5 QHs/ 1000 C atoms) crystallized alongside the sample of Fig. 16, there are no very thick lamellae. This is a consequence of exclusion of ethyl branches from the crystal lattice, in contrast to methyl branches which are included. Replica of an...

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