Branches in polymers

Crystallinity and Density. Crystallinity and density of HDPE resins are derivative parameters both depend primarily on the extent of short-chain branching in polymer chains and, to a lesser degree, on molecular weight. The density range for HDPE resins is between 0.960 and 0.941 g/cm. In spite of the fact that UHMWPE is a completely nonbranched ethylene homopolymer, due to its very high molecular weight, it crystallines poorly and has a density of 0.93 g/cm. ... 

Sharkey, W. H. Polymerizations Through the Carbon-Sulphur Double Bond. VoL 17, pp. 73-103. Shimidzu, T. Cooperative Actions in the Nucleophile-Containing Polymers. Vol. 23, pp. 55-102. Slichter, W. P. The Study of High Polymers by Nuclear Magnetic Resonance. VoL 1, pp. 35-74. Small, P. A. Long-Chain Branching in Polymers. VoL 18,pp. 1-64. 

Slichter, W. P. The Study of High Polymers by Nuclear Magnetic Resonance. Vol. 1, pp. 35-74. Small, P. A. Long-Chain Branching in Polymers. Vol. 18. 

Figure 9 Examples of branching in polymers LCB (long-chain branched, e.g., LDPE) and SCB (short-chain branched, e.g., LLDPE), dendrimer and hyperbranched.

Branching in polymers plays a direct role in their crystallizability, as packing configurations are directly influenced by the nature and frequency of branches along a given polymer backbone. Much has been done in the past to understand the effect of randomly placed branches on the chain, yet no study has been performed on perfectly controlled branching in polyethylene. 

Branching in polymers generally reduces the crystallization tendency for conventional polymers. Therefore, hyperbranched polymers were first believed to behave as amorphous polymers due to the highly branched backbone. Several papers have, however, shown that both liquid crystalline and crystalline hyperbranched polymers can be made from some special A B monomers or by attachment of crystallizable end groups. 

In the 1940s and 1950s, random branching in polymers and its effect on their properties was studied by Stockmayer, Flory, Zimm and many others. Their work remains a milestone on the subject to this day. Flory dedicated several chapters of his Principles of Polymer Chemistry to non-linear polymers. Especially important at that time was the view that randomly branched polymers are intermediates to polymeric networks. Further developments in randomly branched polymers came from the introduction of percolation theory. The modern aspects of this topic are elaborated here in the chapter by W. Burchard. 

Characterization of Molecular Weight Distribution and Branching in Polymers... 

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