Principles of Detection and Characterization of Branching in Synthetic and Natural Polymers by MALS

Jun 06, 2014

Branching is an important structural parameter of many synthetic and natural polymers. It can influence the mechanical and thermodynamic properties of polymers, and also affect the viscosity and rheological behaviour of polymer solutions and melts. Quantitative data about branching topology is therefore vital to understanding polymerization processes and the development of novel polymer-based materials with enhanced properties. Multi-angle light scattering (MALS) is one analytical technique that can be performed to identify branching in macromolecules. This article provides insight into the basic principles of this technique, and how it can be applied to the detection and characterization of branching.

Branching is widely recognized as relevant to synthetic polymers, but has more recently become relevant to natural polymers. For example, hyaluronic acid, an important biopolymer with numerous medical and pharmaceutical applications, was believed to have a linear structure until multi-angle light scattering (MALS) analysis proved otherwise.1

Full characterization of branching requires the coupling of a separation device to separate molecules of varying size over a period of time; and an analytical detector to determine molecular properties such as molar mass, size, or branching ratio. This coupling allows the detector to characterize each size fraction individually to obtain a complete and accurate distribution.

The most common method of separating polymers in solution is gel permeation or size-exclusion chromatography (GPC/SEC). SEC–MALS is a well-established technique for the absolute characterization of typical polymers; however, large and highly branched polymers can exhibit abnormal conformation plots in SEC.5 An alternative method is asymmetrical flow field-flow fractionation (AF4) coupled with MALS. AF4 does not require the diffusion of molecules in and out of a porous solid phase, and is therefore not subject to the "anchoring" mechanism that leads to abnormal elution behaviour. AF4–MALS is therefore ideal for the separation of large and highly branched macromolecules. MALS provides the required quantitative information about branching topology.

The Theory Behind Branching

The development of quantitative branching analysis began in 1949 when Zimm and Stockmayer2 introduced the theoretically derived "branching ratio" (g):

R2 is the mean square radius of branched and linear macromolecules having the same molar mass (M). R and M are both determined independently of MALS. A differential refractive index (dRI) detector is used for measuring concentration. The branching ratio (g) is directly related to the number of branch units in randomly branched polymers or to the number of arms in star-branched polymers.2 In general, g ≤ 1 where the equality sign stands for linear polymers. Lower values of g tend to correspond to higher degrees of branching. For example: g ≈ 0.1–0.2 indicates a highly branched structure.

Ten years after the definition of g by Zimm and Stockmayer, Zimm and Kilb3 introduced an alternative branching ratio based on intrinsic viscosity: