Polymer analysis absolute molecular weight of synthetic polymer analysis

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Polymer analysis absolute molecular weight of synthetic polymer analysis

Polymers

One of the key advantages of MALDI MS for synthetic polymer analysis is that the absolute molecular weights of oligomers can be determined as opposed to obtaining relative molecular weights by chromatographic techniques.

MALDI polymer analysis does not require polymer standards to assign molecular weights to oligomers, and the technique permits accurate determination of molecular weights from narrowly distributed polymers (polydispersity <1.2).

Using submilligram amounts of sample material, the actual analysis can be accomplished in a few minutes. Therefore, the speed and information obtained by MALDI are significantly greater than with other conventional molecular weight determination techniques. In addition, MALDI can determine molecular weight independent of polymer structure.

For example, rigid-rod polymers such as tetrahydropyrene are a challenge to analyze by conventional polymer analysis methods such as gel permeation chromatography (GPC) because these polymers have a linear stick like geometry.

Most synthetic polymers have structures with flexible backbones that form random coil conformations in solution.

These polymers are readily analyzed by GPC methods, which are based on correlating the hydrodynamic volume of the coiled polymer chains with molecular weight.

However, the hydrodynamic volume of rigid-rod polymers does not correlate with polymer molecular weight; hence, molecular weights determined by GPC analysis of these structures often deviate strongly from the true values. Thus MALDI MS can be used to analyze materials that are difficult to characterize by GPC methods, because of either solvent or column incompatibilities.

In addition, GPC does not perform well in low-molecular-weight regions (masses less than a few kilodaltons), whereas MALDI works very effectively in this mass range.

Polymer characteristics that can readily be determined by MALDI MS include molecular weight averages (including the number [Mn] and weight [Mw] averaged molecular weights), polydispersity, mass of repeat units, and end group mass structure. Molecular weight and polydispersity data can be used to verify synthetic pathways, study degradation mechanisms, look for additives and impurities, compare product formulations, and provide QC data on batch-to-batch compositional variations.

End-group and chemical structure data are critical to understanding structure-property relationships of polymer formulations.

MALDI MS provides a direct method of determining end-group mass and composition as well as the mass and composition of oligomer repeat units. The successful characterization of synthetic polymers by MALDI MS is strongly dependent on the chemical structure of the polymer.

It is important to point out that not all synthetic polymers are readily analyzed by MALDI MS, and polymers with different chemical structures may require different sample preparation for a successful analysis. From a practical standpoint, analyzable polymers can be classified into four groups : water-soluble polymers, such as poly(acrylic acid) and poly(ethylene glycol) (PEG); polar organic-soluble polymers, such as acrylics and poly(methyl methacrylate) (PMMA); non polar organic-soluble polymers, such as polystyrene (PS), polyvinyl chloride,and polyethylene; and low-solubility polymers, such as cured polyimide. This last class includes polymers soluble in solvents that are not compatible with matrix materials, and these are the most difficult to analyze by MALDI because a homogeneous mixture of matrix and analyte is not easily prepared.