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Polymers are described as polydispersed compounds, that is they consist of a mixture of molecules with the same chemical composition, but with various molecular sizes. Copolymer and terpolymer systems can also vary in chemical composition. In addition they may contain monodispersed components such as polymer additives and monomers.
The type of analytical determinations in this area are in many cases unique. Broadly speaking they can be split into three areas:
(i) the characterisation of the polymer in terms of its molecular weight/ mass distribution, and where possible the production of absolute molecular weight averages;
(ii) gaining information on the chemical composition of the polymer;
(iii) identification/quantification of low molecular weight species in the polymers.
Given this, can liquid chromatography help in providing the necessary solution? Firstly, the chromatographic modes available must be considered.
9.2 Chromatographic modes used for polymer analysis
In HPLC, several modes of separation are available, adsorption, partition, ion exchange and size exclusion. The choice of separation mode depends largely on the nature of the solute to be separated. What therefore is the
nature of a polymer? What are its physical parameters that will effect the separation process:
(i) A polymer can extend from 1000 molecular mass up to ten million and above.
(ii) Most polymers have a molecular mass range covering two to three decades.
(iii) Polymers in many cases contain low molecular weight species (< 1000) such as monomers and additives.
(iv) Polymers are soluble in only a limited number of solvents, at usual HPLC operating temperatures.
(v) The diffusion coefficients of polymers in solution range between 10-6 and 10 9 cm2 s ' which is some two orders of magnitude smaller than that of small molecule substances.
(vi) The pore sizes of conventional HPLC type column packing materials are comparatively small when compared to the sizes of polymers in solution.
These properties and effects make separation by the more conventional HPLC modes of partition/adsorption difficult for polymers, in comparison to lower molecular weight species.
In partition chromatography, for example, the mass transport processes are much slower than those for low molecular weight species, resulting in poorer and often non-reproducible separations.
In adsorption systems, the physical size of the polymer will prevents its access to many of the pores within the LC stationary phase thus considerably reducing the amount of surface available for interaction. The nature of a polymerís conformation in solution can also result in complex interactions on the surface of the stationary phase, often resulting in irreversible interaction.
In addition, the very action of injecting a polymer into a HPLC system can present problems for fragile high molecular size species as the strong irregular shear forces generated by the mobile phase flowing under pressure through a tightly packed chromatographic column bed, can result in the polymer physically shearing in its passage through the system.
Thus, interactive chromatographic modes (partition/adsorption) in general are not suited for the analysis of bulk polymers. However, they can be used for the analysis of the lower molecular weight species contained in polymer systems.
For polymers, a non-reactive, gentle separation mode needs to be provided; size exclusion offers the solution. This mode of HPLC as described later is the main method of separation used in the polymer industry. Hence the majority of this chapter focusses on size exclusion HPLC and its use.
9.3 Size exclusion chromatography
Size exclusion chromatography (SEC) is a name that unites two of the more traditionally named technologies: gel filtration chromatography (GFC), used primarily with aqueous mobile phases and gel permeation chromatography (GPC), used with organic mobile phases.
Both techniques have their origins in the late 1950s, early 1960s, GFC from the work of J. Porath and P. Flodin and GPC (the more familiar name amongst polymer scientists) with the work of M.F. Vaughan and J.C. Moore. As its name suggests, SEC describes a technology where components are separated according to differences in size (hydrodynamic volume). The size in solution being a function of the sampleís molecular weight, shape and degree of solvation. It is a highly predictable technique suitable for the separation of samples where the components are sufficiently different in molecular size, and as such the technology is ideally suited to the application of polymeric species.
9.3.2 Scope of the technique
The key features of SEC are as follows:
(i) Used for the characterisation/separation of samples of molecular weight < 102 to 107.
(ii) Can be used with both non-aqueous and aqueous mobile phases.
(iii) SEC columns have limited peak capacity, thus complete resolution of complex examples is not normally achieved.