Effects of Surfactants on Thermally Collapsed Poly(N-isopropylacrylamide) Macromolecules

Abstract

Poly(N-isopropylacrylamide) macromolecules (PNIPAM) dissolve in water at temperatures below 33 °C; at higher temperatures, due to hydrophobic interactions, they precipitate out of solution. Addition of a small quantity of charged surfactant resolubilizes the precipitated polymer. We have used small angle neutron scattering to determine the structures of aqueous solutions of PNIPAM and sodium dodecyl sulfate (SDS). At temperatures above the precipitation temperature of the pure polymer solution, two different structures have been found, depending on the ratio of surfactant to polymer. At high ratios, SDS micelles bind to individual macromolecules and pull them into solution; the distance between consecutive micelles in a “necklace” is found to remain constant at around 60 Å. At low ratios, the macromolecules collapse to form small colloidal particles rather than forming necklaces with micelles spaced further apart. The collapse transition is abrupt in the sense that no intermediate states between the stretched necklaces and the collapsed colloidal particles have been found. A critical number of micelles per unit length of polymer is required for complete solubilization.

Erratum to: Solution behavior of aqueous mixtures of low and high molecular weight hydrophobic amphiphiles

“…It is possible then that first AOT, L121 and pyrene molecules associate in some kind of structures that are more "open", so that the local pyrene concentration is not much different to that of the bulk solution but still the probe "feels" a different environment. This kind of association could be similar to the well-known pearl-necklace model for surfactant/polymer interactions [38]. The second kind of association will be closer to typical aggregates in which local pyrene concentration is different from that of the bulk.We followed the criteria of Alexandridis et al [35], and took the second break as the concentration corresponding to the formation of mixed micellar-like aggregates (cac*).…”

Influence of topology of LCST-based graft copolymers on responsive assembling in aqueous media

“…Concurrently the thermothickening properties progressively decrease and we could expect to totally suppress the thermoassociation behavior at very high SDS concentrations, when all PNIPA chains will be saturated by surfactant micelles forming water-soluble polyelectrolyte necklaces [47]. Such behavior is expected at high temperature for weight ratio SDS/PNIPA (S/P) higher than 0.4 [38,47] …”

“…In the case of SDS, it was proposed that above a critical aggregation concentration (~ 0.8 mM), that is well below the critical micelle concentration of SDS (8.6 mM), surfactant micelles bound to PNIPA segments via hydrophobic interactions, forming polyelectrolyte necklaces that progressively hinder the formation of PNIPA aggregates at high temperatures [38,[47][48]. Indeed, as shown in Figure 12, the addition of SDS into PD-N and PN-D solutions has the opposite effect of salt with an increase of the association temperature and a decrease of the transition enthalpy.…”

Polymeric Nanoparticles Stabilized by Surfactants: Controlled Phase Separation Approach

“…Three types of nanoparticles were tested: Earlier Cabane and Lee in their pioneer work have investigated similar the PNIPAM-SDS system by SANS (Lee & Cabane, 1997 Our work is thus a research on a similar system with different conditions. Fig.…”