Specific ion modulated thermoresponse of poly(N-isopropylacrylamide) brushes

Abstract

The influence of specific anions on the equilibrium thermoresponse of poly(N-isopropylacrylamide) (pNIPAM) brushes has been studied using in situ ellipsometry, quartz crystal microbalance with dissipation (QCM-D) and static contact angle measurements between 20 and 45 °C in the presence of up to 250 mM acetate and thiocyanate anions in water. The thickness and changes in dissipation exhibited a broad swelling transition spanning approximately 15 °C from collapsed (high temperatures) to swollen conformation (low temperatures) while the brush surface wettability changed over approximately 2 °C. In the presence of the kosmotropic acetate anions, the measured lower critical solution temperature (LCST) by the three techniques was very similar and decreased linearly as a function of ionic strength. Conversely, increasing the concentration of the chaotropic thiocyanate anions raised the LCST of the pNIPAM brushes with variation in the measured LCST between the three techniques increasing with ionic strength. The thickness of the pNIPAM brush was seen to progressively increase with increasing thiocyanate concentration at all temperatures. It is proposed that specific ion binding of the chaotropic thiocyanate anion with pNIPAM amide moieties increases the electrostatic intra- and intermolecular repulsion within and between pNIPAM chains. This allows the brush to begin to swell at higher temperatures and to an overall greater extent.

Mixed micelles of oppositely charged poly(N-isopropylacrylamide) diblock copolymers

“…The corresponded well with the temperature of the heat‐induced dehydration and collapse of PNIPAAM brushes at planar surfaces. In such a brush, the outermost polymer segments are solvated until about 32 °C, while the densely packed segments within the brush undergo a dehydration and collapse to a rigid conformation over a broad temperature range, which can span up to 20 °C . In contrast, of the PNIPAAM 30 homopolymer studied here was higher in comparison (38.6 °C), so that LCST is elevated in this case.…”

Molecular description of the coil-to-globule transition of Poly(N-isopropylacrylamide) in water/ethanol mixture at low alcohol concentration

“…As a result, in the last years several experimental and numerical studies have been carried out to investigate PNIPAM chains and microgels solution behavior [1,10]. The LCST/VPTT control can be achieved by variations of chemical [11] and stereochemical [12,13,14] composition of the polymer and by variations of the aqueous suspension medium, concerning ionic strength [15,16,17] and cosolute [18,19] or cosolvent addition, the latter being one of the simplest and less expensive ways to decrease the transition temperature [20,21,22]. In particular, the presence of ethanol as water cosolvent has a drastic impact on PNIPAM solution behavior, since this polymer, soluble in ethanol at all temperatures and in water at T ≤ 305 K, is actually insoluble in ethanol/water mixtures having an ethanol mass fraction approximately from 20% to 60%, i.e.…”

Optical characterization of salt-induced swelling behavior ofpoly(N-isopropylacrylamide) terpolymer hydrogel layers

“…2 ) on the equilibrium thermoresponse of PNIPAAm brushes was studied using in situ ellipsometry, quartz crystal QCM-D, static contact angle measurements, neutron reflectometry (NR), and AFM [31,32]. It was found that the shift in transition temperature was in line with the Hofmeister series where transition temperature was shifted to lower temperature by kosmotropic acetate ions and higher temperature by chaotropic thiocyanate ions [31,32].…”

“…2 ) on the equilibrium thermoresponse of PNIPAAm brushes was studied using in situ ellipsometry, quartz crystal QCM-D, static contact angle measurements, neutron reflectometry (NR), and AFM [31,32]. It was found that the shift in transition temperature was in line with the Hofmeister series where transition temperature was shifted to lower temperature by kosmotropic acetate ions and higher temperature by chaotropic thiocyanate ions [31,32]. Koenig et al studied the effect of NaCl salt on the thermoresponsive behavior of PNIPAAm brushes using QCM and ellipsometry and found that transition temperature was shifted to lower temperatures by the addition of NaCl salt due to the interaction of the ions with the amide groups and the hindrance of hydrogen-bonding formation [33].…”

“…Besides temperature and pH, the salt-induced swelling behavior of PNIPAAm brushes and thin gel films was studied by several research groups [17,22,[28][29][30][31][32][33]. Ishida et al studied the NA 2 SO 4 salt-induced changes in grafted PNIPAAm layers using quartz crystal microbalance (QCM-D) with dissipation and AFM and used high salt concentrations in the range of 0.1 to 1 M [29].…”