Copolymer characterization and 2D Chromatography
(Theoretical background )
Gel Filtration, Gel Permeation, Size Exclusion Chromatography
In addition to the molar mass distribution copolymers have a chemical composition distribution (comonomer distribution), therefore they are complex polymers with more than one molecular heterogeneity and multiple distributions. Other distributions may additionally be present, e.g functional distribution, architecture distribution, structural distribution.
For the characterization of the different types of molecular heterogeneity it is necessary to use a wide range of analytical techniques. Preferably, these techniques should be selective towards a specific type of heterogeneity, e.g. size (molecular weight - GPC/SEC) and composition (Polymer-HPLC, LAC). The combination of two or more selective analytical techniques is assumed to yield multidimensional information on the molecular heterogeneity.
For a detailed review see also P. Kilz, H. Pasch, Encyclopedia of Analytical Chemistry, R.A. Meyers (Ed.)
Complex (co)polymers have multiple distributions. For comprehensive analysis 1-dimensional characterization is not sufficient.
Homopolymers vs. copolymers in GPC/SEC
GPC/SEC separates according to hydrodynamic volume, independent on the chemical composition of the investigated macromolecules.
|Click picture for close up view|
For Copolymers it has to be taken into account that the hydrodynamic volume, Vh, depends on the degree of polymerization, Pn, as well as on the composition x. Therefore two random or sequential copolymers which differ in Pn (proportional to molar mass M) and in composition can elute at the same elution volume. If only GPC/SEC is used, it is not possible to distinguish between these copolymers. Even with advanced detection (light scattering, viscometry, and/or Triple detection) a detailed analysis is not possible. For these polymers efficient separation techniques like Polymer-HPLC/LAC and 2D Chromatography are needed.
For block copolymers synthesized using controlled or living polymerization the GPC/SEC approach with 2 concentration detectors is also very useful. However, the increased peak efficiency of 2D Chromatography can also add additional information for block copolymers and residual homopolymer.
Advantages of 2D Chromatography
2D Chromatography allows to separate selective towards a specific type of heterogeneity, e.g. composition and molar mass. The peak capacity in an isocratic 1-dimensional separation is very limited. Coelution can be a serious problem. The corresponding peak capacity in an n-dimensional separation is considerably higher due to the fact that each dimension contributes to the total peak capacity.