Temperature-Induced Intermicellization of “Hairy” and “Crew-Cut” Micelles in an Aqueous Solution of a Thermoresponsive Copolymer

Abstract

Temperature-induced intermicellar structures in aqueous solutions of the thermoresponsive methoxypoly(ethylene glycol)-block-poly(N-isopropylacrylamide) (MPEGn-b-NIPAAM71) copolymer that exhibit a lower critical solution temperature were studied by means of turbidimetry, dynamic light scattering (DLS), shear viscosity, and rheo small-angle light scattering (rheo-SALS) methods. The length of the hydrophilic chains (MPEG) of the copolymer varies from n=0 to n=114. It is shown that this change has a major impact on the temperature-induced association behavior of the polymer in solution. The turbidity results at quiescent conditions revealed a transition peak in the turbidity curve at intermediate temperatures, and this peak as well as the cloud point is shifted toward higher temperatures with increasing length of the hydrophilic chains of the copolymer. The DLS measurements disclosed a fast and a slow relaxation mode, which both are diffusive. From the fast and slow relaxation times the sizes of unimers/micelles and intermicellar clusters, respectively, can be determined. The temperature-induced aggregation is less pronounced in solutions of copolymers with long hydrophilic chains, and the intermicellar structures exhibit an interesting transition at intermediate temperatures. In the shear viscosity measurements large association complexes are formed at high temperatures and at low shear flow for the polymers with short hydrophilic chains, whereas at high shear rates breakup of interaggregate chains was observed. For the copolymer with the highest number of hydrophilic chains (n=114), a novel transition peak was found in the viscosity data. The rheo-SALS results divulged shear-induced structural changes of the association complexes at elevated temperatures. For copolymers with short hydrophilic chains, shear-induced disruption of association complexes was found at higher temperatures, whereas for hairy micelles augmented shear flow promoted the growth of complexes.

Effects of addition of anionic and cationic surfactants to poly(N-isopropylacrylamide) microgels with and without acrylic acid groups

“…This indicates that the particles with PAA are less compressed than the particles without PAA, an effect that is attributed to the electrostatic repulsions of the PAA groups [5]. In agreement with a previous study [36], copolymerization of NIPAAM with a hydrophilic monomer shifts the transition temperature of the resultant copolymer in aqueous solution to a higher temperature. That paper demonstrates a clear effect on the LCST of longer hydrophilic chains.…”

Thermoresponsive Poly(2-oxazoline) Block Copolymers Exhibiting Two Cloud Points: Complex Multistep Assembly Behavior

“…22,52,53 In addition, a temperature increase will render the polymers more hydrophobic over a wide range of temperatures both before and after the cloud point temperature is reached. 30,48,49,52 Keeping these general considerations in mind, we will now examine the size changes in our poly(2-oxazoline) systems.…”

Characterization of temperature induced changes in liposomes coated with poly( N -isopropylacrylamide- co -methacrylic acid)

“…For both methods, each sample of 0.5 ml was allowed to equilibrate for 5 min before the experiment started. A more detailed description of the Rheo-SALS set-up is described by Zhu et al [36]. The total scattered intensity is integrated over the whole region captured by the camera, and the wave vector (q) dependency of the scattered intensity was calculated from the pictures using a specially designed program.…”