Anion Specificity Effects on the Interfacial Aggregation Behavior of Poly(lauryl acrylate)-block-poly(N-isopropylacrylamide)

Abstract

Aggregation behavior of an amphiphilic diblock copolymer poly(lauryl acrylate)-block-poly(N-isopropylacrylamide) (PLA-b-PNIPAM) on neutral aqueous subphases with different salt species and salt concentrations, as well as the structures of its Langmuir−Blodgett (LB) films, were systematically studied. The presence of NaCl or Na 2 SO 4 in subphases makes PNIPAM chains shrink on the water surface and reduce their solubility underwater. On the contrary, the presence of NaNO 3 or NaSCN makes PNIPAM chains more stretched on water and increase their solubility underwater, whose stretch degree and solubility both increase with the increase of salt concentration. Solubility of PNIPAM chains in the above subphase solutions is ranked as NaSCN ≫ NaNO 3 > pure H 2 O > NaCl ≈ Na 2 SO 4 , which is almost consistent with the Hofmeister series except for the latter two close cases. All the initial LB films of PLA-b-PNIPAM exhibit tiny isolated circular micelles. Upon compression, the LB films in the case of pure H 2 O exhibit the dense mixed structures of circular micelles and wormlike aggregates. The formation of wormlike aggregates is due to connection of some adjoining cores, which is less possible in other subphase cases because of the conformation difference of PNIPAM chains.

Effects of Ionic Strength and Ion Specificity on the Interface Behavior of Poly(dimethylaminoethyl methacrylate)–Poly(lauryl methacrylate)

“…This observation is consistent with our previous results where PNIPAM blocks immerse in the water more easily due to the presence of NaSCN. 27 Furthermore, the pseudoplateau pressure in the low NaCl concentration situation is larger than that at moderate concentration, while the corresponding isotherms in the presence of NaNO 3 and NaSCN overlap. Compared with the behavior in the absence of salts, the isotherms in the presence of Na 2 SO 4 demonstrate the presence of less expanded monolayers.…”

“…26−28 The brush layer thickness of PnBA-PNIPAM on the water surface was ranked as F − > Cl − > Br − , 26 and the underwater solubility of PNIPAM blocks in poly(lauryl acrylate)-poly(N-isopropylacrylamide) followed the series NaSCN > NaNO 3 > H 2 O > NaCl ≈ Na 2 SO 4 . 27 It was reported that the average pKa value of the tertiary amine groups of PDMAEMA was ∼7.5 at 25 °C, 29 which were completely/partially protonated and unprotonated with the pH increase in/on the water solutions/surface. 25,30,31 wormlike aggregates.…”

Surface Activity and Structure of Temperature-Responsive Polymer Surfactants Based on PNIPAm at the Air/Solution Interface

“…16 You et al found that poly(lauryl acrylate)-b-poly(N-isopropylacrylamide) formed tiny isolated circular micelles at the air/solution interface and that the micelle cores coalesced with elevating temperature. 15 Amphiphilic block copolymers with different structures can form circular micelles or rod and planar aggregates at the air/ solution interface. Except for the surface structure, understanding the chain arrangement of surfactants under stimulus is also important in the rational design of polymer surfactants.…”

“…Compared with small-molecule surfactants, the structure and molecular weight of the hydrophilic or hydrophobic part of polymer surfactants can be tuned to change the surface or solution behaviors . The introduction of stimuli-responsive groups to the polymer surfactant makes the surface or interface properties change with stimuli such as temperature, light, or pH. − Stimuli-responsive surfactants are powerful tools in controlling surface properties.…”

“…You et al . found that poly­(lauryl acrylate)- b -poly­( N -isopropylacrylamide) formed tiny isolated circular micelles at the air/solution interface and that the micelle cores coalesced with elevating temperature . Amphiphilic block copolymers with different structures can form circular micelles or rod and planar aggregates at the air/solution interface.…”