Dispersion of Polystyrene Inside Polystyrene-b- poly(N-isopropylacrylamide) Micelles in Water XIAODONG YE, JINGYI FEI, JUAN GUAN, XUECHANG ZHOU, GUANGZHAO ZHANG Hefei National Laboratory for Physical Sciences at the Microscale, Department of Chemical Physics, University of Science and Technology of China, Hefei, Anhui 230026, China Received 25 September 2009; revised 28 December 2009; accepted 29 December 2009 DOI: 10.1002/polb.21948 Published online in Wiley InterScience (www.interscience.wiley.com). ABSTRACT: The addition of mixture of polystyrene-b-poly(N-isopro- pylacrylamide) (PS-b-PNIPAM) and polystyrene homopolymer (h- PS) in tetrahydrofuran dropwise into water leads to nanoparticles with a PS core and a thermally sensitive PNIPAM shell. The effects of the ratio of the homopolymer to copolymer and temperature on the formation and stabilization of the dispersion were investigated by using a combination of static and dynamic laser light scattering. PNIPAM shell continuously collapses as temperature increases in the range 20–40  C. Such formed particles are stable even at tem- peratures much higher than lower critical solution temperature (LCST  32  C) of PNIPAM. Our results reveal that the area occu- pied per hydrophilic PNIPAM chain on the hydrophobic PS core remains nearly a constant regardless of the amount of h-PS in the polymer mixture. This clearly indicates that the surface area occu- pied per hydrophilic group is a critical parameter for stabilizing par- ticles dispersed in water. V C 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 749–755, 2010 KEYWORDS: diblock copolymers; dispersions; light scattering; micelles; poly(ethylene oxide); poly(N-isopropylacrylamide); polystyrene; stabilization; stimuli-sensitive polymers INTRODUCTION As an old issue, colloidal stabilization relates to phase behavior, structure, and properties of particulate suspensions. It is important in pharmacy, ceramics, paints, pigments, and other industries today. Because of the large attractive van der Waals’ interactions, colloids usually irre- versibly aggregate. Generally, to stabilize colloidal disper- sions, charged groups or short polymer chains are intro- duced to colloid surface, so that the interactions between the attractive van der Waals’ forces can be balanced by Coulom- bic or other steric-repulsive interactions. 1,2 Recently, a so- called nanoparticle haloing strategy was introduced where colloids were stabilized by highly charged small nanopar- ticles. 3 Such nanoparticles induce effective repulsion. Consid- ering that the small nanoparticles, charged groups, polymer chains, and surfactants are stabilizers of colloids, quantita- tively understanding the role of a stabilizer is important for controlling the colloidal stabilization. On the basis of the assumption that the hydrophobic tail of the stabilizer is on the surface of the particles, which is an idealized micelle structure, Antonietti et al. 4,5 proposed a model, where the particle radius can be calculated with the weight ratio of emulsifier to polymer. Using a more simple model, Wu pre- dicted that for a given dispersion, the particle surface area occupied per stabilizer is a constant. 6 Such a model has been tested valid in several polymeric colloid systems. 7,8 Besides small nanoparticles, charged groups, polymer chains, and surfactants, block copolymers can also be used as stabil- izers. When a copolymer is dissolved in a solvent which is a nonsolvent for one block and a good solvent for the other, the copolymer may form micelles in solution. The micelles have the ability to solubilize homopolymer in solution and form stable dispersions, which have been studied both theoretically 9–11 and experimentally. 12–16 Theoretically, Viduna et al. studied the conformation of solubilized homo- polymer chains and core-forming blocks by Monte Carlo Sim- ulation. 9 Izzo and Marques found that the maximum amount of solubilized homopolymer in 1 g of copolymer decreases with the polymerization index of the homopolymer. 10 Experi- mentally, Tuzar et al. investigated the solubilization of two polybutadiene homopolymers and a triblock copolymer poly- styrene-b-polybutadiene-b-polystyrene micelles with a high content of butadiene in the polybutadiene cores of polysty- rene-b-polybutadiene-b-polystyrene micelles by light scatter- ing. 12 They found that the maximum amount of solubilized polybutadiene based on the weight of the copolymer was 60 wt %. Using laser light scattering, Quintana et al. studied the solubilization of polyisobutylene by micelles of polystyrene- b-poly(ethylene/propylene) block copolymer in several selec- tive solvents and found that the maximum of solubilized polyisobutylene decreases with the polymerization index of the polyisobutylene homopolymer. 13,14 Besides the systems in organic solvents, aqueous systems have also been studied. Eisenberg et al. studied the formation of crew-cut aggregates from blends of polystyrene-b-poly(acrylic acid) diblock co- polymer and polystyrene homopolymer in aqueous solution Correspondence to: X. Ye (E-mail: xdye@ustc.edu.cn) Journal of Polymer Science: Part B: Polymer Physics, Vol. 48, 749–755 (2010) V C 2010 Wiley Periodicals, Inc. DISPERSION OF POLYSTYRENE INSIDE PS-b-PNIPAM, YE ET AL. 749