Comparatively Speaking: Linear vs. Branched vs. Dendrimer Polymer Structures

Apr 6, 2010 | Contact Author | By: Anthony J. O'Lenick, Jr., Siltech LLC
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Title: Comparatively Speaking: Linear vs. Branched vs. Dendrimer Polymer Structures
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(more on chemistry)

Industry expert Tony O'Lenick explains the difference between a linear, branched and dendrimer polymer structure.

The simplest polymer architecture is a linear chain: a single backbone with no branches. A branched polymer molecule is composed of a main chain with one or more substituent side chains or branches. Special types of branched polymers include dendrimers.

The branching of polymer chains affects their ability to slide past one another by altering intermolecular forces, in turn affecting bulk physical polymer properties. Long chain branches may increase polymer strength, toughness and the glass transition temperature due to an increase in the number of entanglements per chain. The effect of such long-chain branches on the size of the polymer in solution is characterized by the branching index. Random length and atactic short chains, on the other hand, may reduce polymer strength due to disruption of organization and may likewise reduce the crystallinity of the polymer.

Dendrimers are a special case of maromolecules wherein every monomer unit is branched. This class of highly specialized polymers is grown by a series of tandem reactions, providing perfect branching. This tends to reduce intermolecular chain entanglement and crystallization. Alternatively, there is a class of dendritic polymers that is not perfectly branched but that shares similar properties to dendrimers due to a high degree of branching. The major advantage of a perfectly branched dendrimer is the highly functionalized surface and “open” core. This technology is commonly used to encapsulate actives and for this reason, is highly interesting to the cosmetic chemist.

The architecture of the polymer is often physically determined by the functionality of the monomers from which it is formed. This property of a monomer is defined as the number of reaction sites at which chemical covalent bonds may form. The basic functionality required for forming even a linear chain is two bonding sites. Higher functionality yields branched or even cross-linked or networked polymer chains.

A cross-link suggests a branch point from which four or more distinct chains emanate. A polymer molecule with a high degree of cross-linking is referred to as a polymer network. Sufficiently high cross-link concentrations may lead to the formation of an infinite network, also known as a gel, in which networks of chains are of unlimited extent—essentially all chains have linked into one molecule.