Analyzing the chemical heterogeneity of poly(dimethylsiloxanes) and other polymers: Development and optimization of a polymer HPLC method

Polymers are used versatile and ubiquitous as raw material for plastics, fibers, elastomers, rubbers, textiles, adhesives, and packaging. In this study, the determination of these materials with polymer liquid chromatographic techniques was applied, improved, and optimized. Beginning from the analysis of low and high molar mass poly(siloxanes) or silicones, the findings were also applied to various other polymers, e.g. poly(vinylchloride) or poly(methylmethacrylate). For low molar mass poly(dimethylsiloxanes) (PDMS), a baseline resolved separation of linear and cyclic oligomers up to 30 monomer units was developed and optimized by adapting mobile and stationary phases. For high molar mass PDMS (up to 250,000 g∙mol-1), a new saw tooth gradient design was invented, enabling high-resolution measurements. In addition to and as enhancement to size exclusion chromatography, the new high-resolution polymer HPLC (HRP-HPLC) allows the assignment of retention times to the corresponding molar masses in HPLC for single resolved peaks of (complex) polymer samples. The shape of the saw tooth gradient and further significant parameters as effective step length and height are optimized by Design of Experiments (DoE). Various other polymers, e.g. poly(vinylchloride), poly(styrene), poly(methylmethacrylate), and poly(propylene glycol) were investigated with HRP-HPLC by only adapting the stationary and mobile phase. Additionally, the performance of an evaporative light scattering detector (ELSD) was improved by implementing a new parallel-path poly(tetrafluoroethylene) nebulizer instead of the originally used concentric glass nebulizer. Thus, a significant improvement of the long-term stability and sensitivity was achieved for polymer analysis. Particularly, the invention, development, and optimization of the sawtooth gradient design results to a substantial improvement in (micro) structure elucidation of especially high molar mass polymers. Moreover, (semi) preparative fraction collection of various polymers enable the off- and online coupling to other powerful analytical techniques like MALDI mass spectrometry or NMR spectroscopy