Peng Wei Zhu

Volume phase transitions of poly(N-isopropylacrylamide) latex particles in mixed water-N,N-dimethylformamide solutions

Abstract Volume phase transitions of poly(N-isopropylacrylamide) (PNIPAM) latex particles in mixed water-N,N-dimethylformamide (DMF) solutions were studied at different temperatures and crosslink densities using dynamic light scattering. A collapse-swelling behaviour was observed on the addition of N,N-dimethylformamide to an aqueous dispersion medium. The degree of swelling was found to be dependent on the temperature and crosslink density. The transition between the collapsed state and the swollen state was continuous. The cononsolvency of water and DMF for PNIPAM latex particles has been interpreted in terms of the changes in the interactions between their components.

Light scattering studies of poly(N-isopropylacrylamide) microgel particles in mixed water-acetic acid solvents

Properties of poly(N-isopropylacrylamide) (PNIPAM) microgel particles were examined in mixtures of water and acetic acid at 20°C using dynamic light scattering in order to investigate the cononsolvency phenomenon. It was found that the cononsolvency exhibited by the PNIPAM-water-acetic acid ternary system results in five regions in response to changes in solvent composition. Upon the addition of acetic acid to the aqueous dispersion medium, the regions appeared successively: volume change from a swollen to a collapsed state, unstable microgel aggregates (in water rich range), precipitation, unstable microgel aggregates (in acetic acid rich range), and swelling. The volume change from swollen to collapsed states starts before the phase-separation composition is achieved. Both volume phase transition and swelling are continuous but undergo in a sharp way. In a very narrow range of solvent composition, the microgel aggregates formed are unstable, while the immiscible phase was found to span a wide range. The degree of swelling is dependent on the crosslinker concentration in the solutions of very higher acetic acid content. The cononsolvency of the mixed water-acetic acid solutions for PNIPAM microgel particles is not only involved in changes of the interactions between their components but also in the partial neutralisation of surface charges of the PNIPAM microgel particles.

The longer time collapse kinetics of interfacial poly(N-isopropylacrylamide) in water

The kinetics of the later stages of the temperature induced collapse transition of swollen poly(N-isopropylacrylamide) (PNIPAM) chains at interfaces in the presence of the surfactant sodium dodecyl sulfate were studied using dynamic light scattering (DLS). The longer time collapse processes were interpreted in terms of the “globule growth” model of Kuznetsov, Timoshenko, and Dawson (KTD) [J. Chem. Phys. 103, 4807 (1995)]. The relaxation times of the collapse transition were determined from the longer time kinetics and were found to decrease with increasing quench temperature above the θ temperature.

Interfacial coil-to-globule transitions: the effects of molecular weight

Abstract The coil-to-globule transitions of fractionated poly( N -isopropylacrylamide) chains attached to electrosterically stabilized polystyrene latex particles were studied for molecular weights ranging from 3 × 10 5 to 2 × 10 6 . The shapes of the transitions proved to be a sensitive function of the polymer molecular weight: the component of the transition that occurred under better than θ-solvency conditions increased significantly (from 30 to 80%) as the molecular weight was decreased by an order of magnitude; concomitantly, the component that occurred under worse than θ-solvency conditions was substantially decreased. Studies carried out with 13 C NMR supported the hypothesis that these two types of transitions were phenomenologically different. The transitions in worse than θ-solvents can be attibuted to the strong attractive binary interactions that characterize worse than θ-solvents, whereas the transitions in better than θ-solvents arose from the weaker attractive many-body interactions, i.e. n -clustering. Simple scaling law considerations imply that the component of the coil-to-globule transitions arising from n -clusters should decrease with increasing molecular weight of the interfacial polymer, in conformity with the experimental data.

The effects of different electrolytes on the fractal aggregation of polystyrene latexes coated by polymers

Abstract The effects of different electrolytes on the aggregation kinetics and fractal dimensions of aggregates of polystyrene latex particles coated by poly( N -isopropylacrylamide) (PNIPAM) and poly(ethylene oxide) (PEO) have been studied using dynamic light scattering. Whilst PNIPAM is soluble in water at low temperatures, on increasing the temperature and/or adding electrolyte it adopts a globular conformation and becomes hydrophobic, in comparison to the more hydrophilic PEO. For the PNIPAM-coated latexes, the aggregation induced by various sodium salts was found to decrease in the following order: NaCl > NaBr > NaNO 3 > NaI > NaSCN. This sequence is similar to that found previously for latexes coated by PEO. The concentrations of electrolyte required to reach the same fractal dimension at longer times corresponded to the reverse order. The longer time fractal dimensions were found to increase with increasing electrolyte concentration so that the aggregate structures tended to become more densely packed. All the results can be correlated with the sequential order of the viscosity coefficients B η of the electrolytes studied. For the PEO-coated latex, the fractal dimension was found to be insensitive to electrolyte concentration. With respect to temperature, the fractal dimension changed only slightly between 40 and 47.5°C, but dropped dramatically up to 50°C. The observations have been interpreted in terms of the nature of the interparticle interactions.

The dynamics of aqueous crosslinked poly(N-isopropyl-acrylamide) microgel particles under good solvency conditions

From a dynamic light scattering study of the concentration and temperature dependencies of weakly crosslinked poly(N-isopropylacrylamide) (PNIPAM) microgel particles in aqueous solutions it was found that the decay of the time correlation function is markedly slowed down with increasing microgel concentration and with decreasing temperature. The microgel particles exhibit an exponential decay in very dilute solutions or at higher temperatures; this changes to a stretched exponential at higher concentrations or at lower temperatures. The observed slowing down occurrs over a relatively narrow concentration range, which has not been observed in some corresponding systems consisting of hard spheres. The time correlation function data were analyzed with the aid of a coupling model. The relaxation time that was calculated from this model increases with increasing concentration by a power law (τ c 2.6 ) in higher concentration or with decreasing temperature, the result that is physically in accord with the change in the value of the stretched exponent. A wave vector dependence of the relaxation time was observed. The results have been interpreted as arising from the formation of neighboring cages of the microgel particles. Over the concentration range studied, the apparent activation energy of the relaxation processes operating in the cage structures was found initially to increase with increasing microgel concentration and then to approach a plateau value.

Viscosity study of oligomer-induced coil-globule transition of a polystyrene

Abstract The oligomer-induced coil-globule transition in a single polystyrene chain in benzene is reported. The observed transition is in good agreement with the prediction of theory.