Branching in high-density polyethylene

In a relatively short period of time the Lab Connections Transform system found its way into a large number of laboratories. Applications of the technique have been discussed in various fields. Willis and Wheeler demonstrated the determination of the vinyl acetate distribution in ethylene-vinyl acetate copolymers, the analysis of branching in high-density polyethylene, and the analysis of the chemical composition of a jet oil lubricant [143].Provder et al. [144] showed... 

The concentrations of structures produced in irradiated polyethylene are on the order of 1 per 10,000 carbon atoms for absorbed doses of approximately 2.0 Mrad. Although the approach of examining polyethylenes irradiated with absorbed doses less than the gel dose placed a premium on sensitivity, we were able to detect the first direct radiation induced long chain branches in high density polyethylene (4). 

Spevacek J. Short chain branching in high density polyethylene 13C n.m.r. study. Polymer 1978 19 1149-1152. 

The polyethylene produced by radical polymerization is referred to as low-density polyethylene (LDPE) or high-pressure polyethylene to distinguish it from the polyethylene synthesized using coordination catalysts (Sec. 8-1 lb). The latter polyethylene is referred to as high-density polyethylene (HDPE) or low-pressure polyethylene. Low-density polyethylene is more highly branched (both short and long branches) than high-density polyethylene and is therefore lower in crystallinity (40-60% vs. 70-90%) and density (0.91-0.93 g cm 3 vs. 0.94-0.96 g cm-3). 

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... 

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). 

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. 

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. 

It is very diffieult to eliminate internal haze in high density polyethylene resins because the wavelengths of light span a range that overlaps broadly with that of the spherulitie diameters. The short-chain branching in linear low density polyethylene reduces the level of crystallinity and the size of the spherulites and consequently reduces the level of haze relative to that of unbranched resins. Fur-... 

Branching occurs to some extent and can be controlled. Minimum branching results in a high-density polyethylene because of its closely packed molecular chains. More branching gives a less compact solid known as low-density polyethylene. 

Branching can to some extent reduce the ability to crystallise. The frequent, but irregular, presence of side groups will interfere with the ability to pack. Branched polyethylenes, such as are made by high-pressure processes, are less crystalline and of lower density than less branched structures prepared using metal oxide catalysts. In extreme cases crystallisation could be almost completely inhibited. (Crystallisation in high-pressure polyethylenes is restricted more by the frequent short branches rather than by the occasional long branch.)... 

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