Polyethylene long chain branches

The more recently developed so-called linear low-density polyethylenes are virtually free of long chain branches but do contain short side chains as a result of copolymerising ethylene with a smaller amount of a higher alkene such as oct-1-ene. Such branching interferes with the ability of the polymer to crystallise as with the older low-density polymers and like them have low densities. The word linear in this case is used to imply the absence of long chain branches. 

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). 

Copolymerization e.g., of 1-butene or 1-hexene with ethylene, gives short-chain branching-, e.g., the branches contain three or five carbon atoms. The random location of the side-chains lowers the crystallinity and density. Long-chain branching refers to branches that are similar in length to the polymer backbone and this type occurs in polyethylene manufactured using the... 

The method outlined above for characterizing branched polymers will hereafter be referred to as the molecular weight and branching distribution (MWBD) method. In the following sections, its application to the long chain branching in polyvinyl acetate and high pressure low density polyethylene will be demonstrated. 

There are numerous variations on the basic linear structure of polymers. Returning to our example of polyethylene, we find short chain branches and long chain branches, as shown in Figs. 1.2 and 1.3, respectively. The number and type of these branches strongly influences the way that the molecules pack in the solid state, and hence affect the physical properties. Long... 

Figure 1.3 Polyethylene molecule with a long chain branch...

We use carbon-13 NMR spectrometry to identify the monomer units present in copolymers, their absolute concentrations, the probability that two or more monomer units occur in proximity, and long chain branching concentrations. For instance, in the case of polyethylene, we can not only distinguish and quantify ethyl, butyl, and hexyl branches, but we can also determine whether branches are present on carbon backbone atoms separated by up to four bonds. We can compare the observed adjacency of branches to a theoretical value calculated for random comonomer incorporation. By this method, we can determine whether comonomers are incorporated at random, as blocks, or in some intermediate fashion. 

Figure 5.13 Example of backbiting reaction to form a long chain branch during the high pressure polymerization of polyethylene...

The most common copolymer of this type is ethylene-vinyl acetate, which we normally refer to as EVA. This variety of polyethylene is illustrated in Fig. 18.2 e), in which the ester branches are indicated by the symbol VA This family of copolymers is commercially available containing vinyl acetate concentrations of up to approximately 25 mole %. In addition to the randomly distributed ester branches, these resins also contain the short and long chain branches that are characteristic of low density polyethylene. 

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. 

How do chain transfer events generate long chain branches in polyethylene ... 

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