Development of new separation materials using stimuli-responsive polymers

Over the last decades, advances in analytical and preparative chromatography have been driven by the development of new separation materials. The aim had been to achieve improved selectivity, resolution, speed, detection sensitivity and pre-concentration/sample handling capabilities. Of these factors, the manipulation of selectivity is a considerable challenge in separation science. A highly desirable approach to manipulate selectivity is to use a stationary phase with tunable surface properties so that the separation selectivity can be adjusted without the need to physically replace the column. This project addresses the synthesis of polymer-based stimuli-responsive materials, which offer significant potential to enhance separation selectivity through better control over the molecular properties of the adsorbent surface. The novel stationary phases have been developed based on the guiding principles of 'green chemistry' for liquid chromatographic and electrochromatographic separations of low and high molecular weight analytes using immobilised stimuli-responsive polymers (SRPs). Di-substituted triazine-based macromolecules were synthesised with poly(2-dimethylaminoethyl methacrylate) (PDMAEMA) and poly(acrylic acid) (PAA) as polyelectrolyte SRPs. The diblock copolymer poly(2-dimethylaminoethyl methacrylate)-block-poly(acrylic acid) (PDMAEMA-b-PAA) was then tethered to silica particles. Batch-binding experiments were conducted to determine the static adsorption isotherms for unmodified silica, aminopropyl silica (APS) and PDMAEMA-b-PAA modified onto APS particles, termed APS-(PDMAEMA-b-PAA). A series of acidic, basic and neutral analytes were used as probes at different pH values ranging from 5.0-8.0 and at temperatures below (20 °C) and above (65 °C), the lower critical phase transition temperature of PDMAEMA-b-PAA. The results revealed the experimental conditions under which analyte binding occurred. The target analytes had very few non-specific interactions with the bare silica and APS particles, but showed binding with the polymer-modified silica particles. The APS-(PDMAEMA-b-PAA) modified particles were subsequently packed into stainless steel columns and evaluated in a high-performance liquid chromatography format. This novel SRP exhibited significant temperature, pH and/or ionic strength responses during the separation of the selected analytes, this was attributed to the analyte's hydrophilic or hydrophobic property. Subsequent studies with an extended array of analytes including proteins have documented the versatility of the novel SRP. A thermodynamic assessment of the retention of ionisable analytes on the polyelectrolyte APS-(PDMAEMA-b-PAA) stationary phase was also performed. The thermodynamic fingerprints provided important information regarding the properties of the SRP-modified stationary phase as well as the adsorption mechanisms, which were found to be multi-modal for most analytes. PDMAEMA-b-PAA was also immobilised onto the inner walls of an aminopropyl coated open tubular silica capillary. This novel polymer coated capillary contained weakly charged functional groups from PAA and PDMAEMA, enabling the generation of electroosmotic flow. The magnitude and direction of the electroosmotic flow was adjustable by changing the pH of the running buffer electrolyte. The baseline separation of a group of acidic and basic analytes with aqueous buffers of various pH values documented the zwitterion characteristic of the SRP-modified surface in the capillary electrochromatographic separation. In conclusion, novel stationary phases composed of a diblock copolymer, grafted onto silica particles or silica capillaries were designed, synthesised, characterised and used for analytical separations. These modified surfaces possessed tunable characteristics for the separation of chemical and biological compounds. Separations of analytes were performed without organic solvents, involving a temperature, ionic strength and/or pH switch under environmentally-friendly 'green' conditions.