Long chain branch

A long-chain branch is a side chain of a polymer molecule that occurs only at a few points in the molecule, i.e. not at every repeat unit, such as the methyl side group of polypropylene , but typically in concentrations of 0.1 to 100 long-chain branches per polymer molecule. Long chain branches are side chains that are called long, that is, the length of which is greater than a certain minimum length, which has a different value depending on the definition and which is usually not quantified in more detail.

They thus differ from the short chain branch , which has a precisely defined length and usually occurs in higher concentrations. Due to their low concentration, long-chain branches have only a very small influence on the properties of the solid , including the crystallinity , but they play an essential role in processing, as they have a decisive influence on the rheological behavior .

The definitions for the minimum length of a side chain to be a long chain branch are

• 6 carbon atoms in measurements with NMR , as this cannot distinguish between side chains with 6 or more carbon atoms,
• The entanglement molar mass in measurements by means of rheology above the side chains (i.e. long chain branches) have a significant influence on the material behavior in the melt, which is expressed in particular by the more pronounced structural viscosity and the strain hardening of long-chain branched materials, both of which are advantageous for thermoplastic processing.

Examples of long chain branched polymers

• Low density polyethylene (LDPE)
• metallocene-catalyzed polyethylene (LCB-mLLDPE, some types)
• Hollow body blow types of polycarbonate (some types)
• high-melt-strength polypropylene (HMS-PP)

literature

• J. Dealy, RG Larson: Structure and Rheology of Molten Polymers - From Structure to Flow Behavior and Back Again . Hanser, Munich 2006, ISBN 3-446-21771-1 .