Short chain branch

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Short chain branches are side chains of a polymer chain that are not found on every monomer unit . Furthermore, short chain branches are “short” (less than 10 structural units), which distinguishes them from long chain branches .

The definition of “short” is that these are by no means long enough to be loopy. The NMR can typically clearly distinguish side chains with a maximum of six carbon atoms from longer ones.

synthesis

Short-chain branching occurs mainly in the subgroups of polyethylene LDPE and LLDPE.

  • In the case of LDPE, 2-5 mol% of butyl side chains (short chain branches) are created by the so-called backbiting process, in which the terminal radical of a growing chain attacks the sixth from last carbon atom of the same chain and thus transfers the radical to that chain.
  • In LLDPE, the short-chain branches arise from the copolymerization of ethene with other α- olefins , usually butene , hexene and octene . By incorporating two carbons per monomer into the main chain, the short-chain branches are 2 carbon atoms shorter than the α-olefin used.

impact

Short-chain branches form a steric hindrance during crystallization , which leads to a decrease in crystallinity . This allows the crystallinity and mechanical properties of a polymer (especially an LLDPE) to be adjusted in a targeted manner. In the case of LLDPE, a comonomer content of 5 mol% octene leads to a drop in the crystallinity from approx. 60% (in the case of an ethene homopolymer) to approx. 35%. The melting point drops from 135 ° C to approx. 105 ° C. The stiffness of such a material decreases by a factor of about 10. Furthermore, such a type is almost no longer opaque , but at most slightly milky.

Comonomer contents of over 10 mol% lead i. d. Usually to amorphous LLDPEs, which are present as a melt at room temperature and can therefore no longer be used as a structural material.

Since short chain branches are not looped , the effect on the processing behavior is small. Short chain branches lead to a higher temperature dependence of the viscosity and to a slightly lower viscosity at high shear rates, since the short chain branches reduce the entanglement density.

Bibliography

  • 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 .
  • L. Almond Kernel: The crystalline state. 2nd Edition. Chap. 4, ACS, Washington DC 1993.

Individual evidence

  1. ↑ Horst Briehl: Chemistry of materials. Springer-Verlag, 2014, ISBN 978-3-658-06225-5 , p. 143 ( limited preview in Google book search).