Development of temperature-responsive ion-exchange resins

‘Smart’ or stimuli-responsive polymers represent new classes of materials that are currently under development. These novel polymeric materials undergo conformational rearrangement in response to small changes in their environment, such as temperature, pH, UV irradiation, ionic strength or electric field. These environmental changes alter the structure of stimuli-responsive polymers and increase or decrease their overall hydrophobicity, resulting in reversible collapse, dehydration or hydrophobic layer formation. With further research into their synthesis, behaviour and application, these novel materials have great potential to become the ‘next generation’ of separation media for cost effective and environmentally-friendly extraction and purification of high value bio-molecules from agri-food and other raw materials. A temperature-responsive ion-exchange resin has been prepared by grafting poly(N-isopropylacrylamide-co-tert-butylacrylamide-co-acrylic acid; ItBA) onto cross-linked agarose. A carboxymethylated ion-exchanger (CM) of similar charge density was also prepared. Maximum adsorption capacities (Bmax) for lactoferrin at 20ºC and 50ºC were determined for both resins by batch equilibrium adsorption procedures. Dynamic adsorption and desorption characteristics of CM and ItBA with lactoferrin were established, as well as the ability of ItBA to selectively adsorb and desorb lactoferrin in the presence of other proteins. With the CM-agarose resin, there was no significant difference between the Bmax values obtained at 20ºC and 50ºC. However, for the agarose based ItBA resin, the Bmax value at 50ºC was almost 3 times higher than the Bmax value at 20ºC. Dynamically, lactoferrin adsorbed to the ItBA packed column at 50ºC, with a significant proportion of the adsorbed lactoferrin desorbed by reducing the temperature to 20ºC. In addition, anionic proteins did not adsorb to the ItBA packed column, and did not interfere with the dynamic adsorption/desorption behaviour of lactoferrin. These results indicate that this new temperature responsive agarose-based ItBA resin has potential for the fractionation of whey proteins, with good selectivity for cationic proteins. The synthesis of new, improved temperature responsive resins resulted in resins which had greater differential binding at low and high temperatures compared to ItBA. The first strategy was the preparation of a resin with higher polymer content (ItBA-H) by increasing the loading of the polymerisation initiator. The resin (ItBA-H) had higher protein adsorption capacities at both 20ºC and 50ºC but the differential binding between high and low temperature was reduced. Free polymers were prepared to investigate the effect of polymer composition on the lower critical solution temperature (LCST). Introduction of the more hydrophobic phenyl group significantly decreased the LCST of the polymer. Several resins were then synthesised with different monomer compositions and using different ratios of these monomers. Among this array of resins synthesised, poly(N-isopropylacrylamide-co-phenylacrylamide-co-acrylic acid) (IPhA) was the best performer. Compared to ItBA, the IPhA resin showed a much higher differential binding between the low and high temperatures. Similar to ItBA, IPhA also showed a great potential in dynamic studies. Most of the protein adsorbed onto the resin at 40ºC could be eluted by changing the temperature to 4ºC. Under static conditions, the resin also showed selective adsorption for cationic proteins. The attributes of these new temperature-responsive resins (ItBA and IPhA) demonstrate their potential for use in protein separations where salt usage, costs, and the environmental impact of the ion-exchange separation process represent key considerations.