Temperature-responsive polymers for liquid-phase separations

Abstract

The needs of biotechnology for high-resolution purification and analytical technology have spurred new methods in chromatographic systems, in both terms of both columns and equipment.Stimuli-responsive polymers, which change their structure and physicochemical properties in response to external signals, comprise new materials with interesting applications in biomaterial science and technology. There are many synthetic polymers with molecular conformations sensitive to environmental factors, such as the pH [1], electric fields [2], and temperature [3]. Temperature seems to be one of the most attractive tools in such stimuli. Such polymers have been widely utilized for drug delivery systems [4], cell culture substrates [5], and bioconjugates [6]. However, there have been few reports concerning the use of these stimuli-responsive polymers in chromatographic separations.Poly N-isopropyl acrylamide (PNIPAAm) has the sharpest phase transition of the class of thermosensitive N-alkylacrylamide polymers. PNIPAAm exhibits thermally reversible soluble-insoluble changes in aqueous solution in response to temperature changes across a lower critical solution temperature (LCST) at 32°C [7]. The polymer chains show an expanded conformation in water below the LCST due to strong hydration and changes to compact forms above the LCST by dehydration. PNIPAAm-grafted surfaces show temperature-responsive hydrophilic-hydrophobic surface property alterations.By using these features, PNIPAAm and related polymers have been used to generate temperature-sensitive stationary phases for size exclusion [8], hydrophobic [9, 10, 11] (Fig. 1), ionic [12,13], and affinity chromatography [14]. These temperature-responsive stationary phases may be useful in method development as an extra tool to optimize the selectivity by adjusting the temperature rather than changing the mobile phase composition.The phase transition of PNIPAAm is due to the stability of the hydrophobic groups in the polymer chain in aqueous media. The LCST value for PNIPAAm has been regulated by the amount of hydrophobic comonomer, n-butyl methacrylate (BMA) [9]. LCSTs shift from 32°C to 20°C with increase in the mole fraction of BMA over a range of 1-5 mol% in the copolymer. With increasing BMA content, both the retention time and the affinity of the solutes were increased, even at a lower column temperature. Therefore, when separation at lower temperatures is necessary because of the stability of the proteins, use of the poly(NIPAAm-co-BMA) modified stationary phases is preferred. These hydrophobized stationary phases were also applicable for the separation of low molecular proteins, such as ribonuclease and chymotrypsinogen. Recently, the separation of basic bioactive peptides and other ionic compounds was achieved by utilizing copolymers of NIPAAm-introduced ion-exchange groups as a stationary phase [12]. PNIPAAm copolymers containing anionic acrylic acid (AAc) or cationic dimethylaminopropylacrylamide (DMAPPAm) produced alterable properties of the stationary phase with both th...

Flocculation of kaolinite clay suspensions using a temperature‐sensitive polymer

“…A detachment at a separation distance of 9 nm was observed, indicating a very compacted structure of the adsorbed polymer on the kaolinite surface, whereas the retraction profile obtained at room temperature (Figure 8a) shows a plateau of about 60 nm, that is, a long stretch of polymer molecules, indicating an extended structure. These results and interpretation are consistent with other studies 37, 38…”

“Smart” molecularly imprinted monoliths for the selective capture and easy release of proteins

“…The PNIPAAm polymer is well known for its significant changes in polarity at temperatures below and above its lower critical solution temperature (LCST) of 32°C resulting from difference in entropy at both states . Above the LCST, the PNIPAAm chains are in a collapsed dehydrated state and have a hydrophobic character attributed to the isopropyl group located on the collapsed chains.…”

“…The first paper describing use of “smart” monolith for the separation of proteins was published almost 20 years ago . Since then, several N ‐isopropylacrylamide and oligo(ethylene glycol)‐containing monolithic polymers have been described . With these materials, chromatographic separation of biomolecules can be achieved using a pure aqueous mobile phase and isocratic conditions.…”

Thermal Properties of Methyl Ester-Containing Poly(2-oxazoline)s

“…These types of polymers can be applied in temperature sensors, [18] protein chromatography [19] and various biomedical applications such as drug delivery and tissue engineering [1,20]. For these latter applications thermoresponsive polymers are needed that are biocompatible and that can be conveniently functionalized.…”