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Effects of Sodium Dodecyl Sulfate on Structures of Poly(N-isopropylacrylamide) at Particle Surface Peng Wei Zhu J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/jp510350w • Publication Date (Web): 10 Dec 2014 Downloaded from http://pubs.acs.org on December 16, 2014
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The Journal of Physical Chemistry
Effects of Sodium Dodecyl Sulfate on Structures of Poly(N-isopropylacrylamide) at Particle Surface
Peng Wei Zhu* Department of Materials Engineering, Monash University, Clayton, VIC 3800, Australia
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The Journal of Physical Chemistry
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ABSTRACT: The effect of sodium dodecyl sulfate (SDS) on the poly(N-isopropylacrylamide) (PNIPAM) tethered to nanoparticles was experimentally investigated using dynamic light scattering below the lower critical solution temperature. A mean-field analytical model was used to calculate the parameters of interfacial PNIPAM-SDS complexes. Particularly, the magnitude of SDS adsorption energy obtained decreases with decreasing the excluded volume parameter, implying that the partially collapsed PNIPAM brush virtually favours the adsorption of SDS onto the PNIPAM chains. A self-consistent field theory (SCFT) model was used to get a detailed quantitative description of monomer density distribution. By lowering the solvent quality, a number of phenomena related to the non-continuity of monomer density are revealed. These phenomena are either referred to as the vertical phase separation or as its precursor, which can be delayed and eventually eliminated as the SDS coverage is increased. The distribution of free chain ends was calculated using the SCFT model. Increasing the SDS coverage gives rise to a broader and more asymmetric distribution of free chain ends, accompanied with a considerable expansion of the dead zone (below which the free chain ends do not exist), but lowering the solvent quality has opposite influences. The relative thickness of the dead zone can be scaled to a master curve, regardless of SDS coverage.
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The Journal of Physical Chemistry
1. INTRODUCTION Aqueous solutions of polymers and surfactants have been studied over the past several decades.1-5 These mixtures are tremendous importance in a wide variety of applications such as detergents, paints, cosmetics, pharmaceuticals, foods, drug and pesticide formulations, and oil recovery fluids.13
They also form the basis for understanding more complex systems such as supramolecular
organisation, self-assembly, and molecular complexation.6,
7
In terms of polymer-surfactant
interactions, the mixtures can be classified into two broad categories. The first system contains surfactants, mainly anionic ones, and neutral polymers. The second system contains ionic polymers or polyelectrolytes and oppositely charged ionic surfactants. Polymer-surfactant interactions result in the ionization of neutral polymers or the neutralisation of polyelectrolytes. Studies of polymer-surfactant mixtures have been extended to systems with constrained environments.8-13 Of particular relevance to the present work are polymer brushes which have distinctly different conformations from chemically identical polymers.14-18 Figure 1 schematically shows neutral polymers tethered to the particle surface with a radius R. In Fig. 1, d is a distance between grafting sites (the grafting density σ=1/d2) and r is a distance between grafting site and blob periphery. When d>2r, tethered chains are so-called mushrooms and the brush height H of mushrooms is independent of grafting density σ. When d∼2r, tethered chains start to overlap. When d
