pubs.acs.org/Macromolecules Published on Web 10/20/2009 r 2009 American Chemical Society 9096 Macromolecules 2009, 42, 9096–9102 DOI: 10.1021/ma901567d Forces of Interaction between Poly(2-vinylpyridine) Brushes As Measured by Optical Tweezers Mahdy M. Elmahdy, †,§ Alla Synytska, ‡ Astrid Drechsler, ‡ Christof Gutsche, † Petra Uhlmann, ‡ Manfred Stamm, ‡ and Friedrich Kremer* ,† † Institute of Experimental Physics I, Leipzig University, Linn estrasse 5, 04103 Leipzig, Germany, ‡ Leibniz Institute of Polymer Research Dresden, Hohe Str. 6, 01069 Dresden, Germany, and § Department of Physics, Mansoura University, Mansoura 35516, Egypt Received July 17, 2009; Revised Manuscript Received October 6, 2009 ABSTRACT: Forces of interaction within single pairs of poly(2-vinylpyridine) (P2VP) grafted colloids have been measured by optical tweezers (OT) with an extraordinary resolution of (0.5 pN. Parameters to be varied are the concentration and type of salt (KCl, CaCl 2 , and LaCl 3 ) of the surrounding medium as well as its pH. The observed force-distance relation is quantitatively described by the Jusufi model [Colloid Polym. Sci. 2004, 282, 910-917] for spherical polyelectrolyte brushes which takes into account the entropic effect of the counterions and enables one to estimate the ionic concentration inside the brush. The transition from an osmotic to the salted brush regime is analyzed in detail. For the scaling of the brush height a power law is found having an exponent of 0.24 ( 0.01, which ranges between the values expected for spherical and planar brushes. At pH 4 a strong transition from a brush to a pancake conformation takes place. Introduction Polyelectrolytes are macromolecules whose repeating units bear an electrolyte group. They are used in many applications: for instance, for rheology control, as wet and dry strength additives, as flocculating or dispersing agents, as core-shell particles and hollow capsules for controlled drug delivery. 1-4 If linear polyelectrolyte chains are grafted densely to planar sur- faces, then the result is a planar polyelectrolyte brush while grafting linear polyelectrolyte chains densely to colloidal core latex particles leads to spherical polyelectrolyte brushes in which the curvature of the cores becomes a new decisive length para- meter. 5,6 The conformations of the polymer brushes are affected by a number of factors: the polymer architecture, solvent affinity, grafting density, pH, and the ionic strength of the surrounding medium. 7 Figure 1 shows the schematic structure of a spherical polyelec- trolyte that will form the subject of this article. The core is SiO 2 , which is inert and totally impenetrable for the dispersion medium water, and the polyelectrolyte chains are P2VP which are weak cationic polyelectrolyte and pH-responsive polymers. The ratio of the contour length (128 nm) of the P2VP brushes and the radius of the SiO 2 colloids (775 nm) is ∼1/6, a value too high to consider the surfaces as planar but not large enough to treat them as spherical. The positive charge on the chain in a broad range of pH permits the interaction with negatively charged counterparts. In the present study the counterions are the Cl - ions whereas K þ , Ca 2þ , and La 3þ are the co-ions. If two spherical polyelectrolyte brushes approach each other, the respective surface layers over- lap. Hence, the polymer chains of the two particles start to interact. This leads to steric and electrostatic interactions. The steric interaction is controlled by the geometry and graft- ing density of the polyelectrolyte chains while the electrostatic interaction is controlled by the charge of the chain and the ionic strength of the medium. The translational entropy of the coun- terions as well as the elastic entropy of the chain controls the space distribution of the counterions and the conformation of the chain. For better understanding of the interactions between positively charged P2VP-grafted colloids, it is of great interest to know the interaction forces. Force measurements were carried out using surface force apparatus, 8-11 AFM colloidal probe technique, 12,13 and optical tweezers. 14-20 Some previous surface force apparatus (SFA) studies of polyelectrolyte brushes have used brushes formed from diblock copolymers. 21-24 In particular, the force measurements between diblock copolymers of poly(tert-butylstyrene) and poly(styrene- sulfonate) (PtBS-PSS) adsorbed on both a hydrophilic surface Figure 1. Sketch of the spherical polyelectrolyte brushes investigated. Linear chains of poly(2-vinylpyridine) (P2VP) are densely grafted onto the surface of a colloidal SiO 2 particle by the “grafting to” technique. The radius R c of the core particle is 1.55 ( 0.04 μm. The thickness L of the P2VP brush on the surface of the particles is determined by the optical tweezers measurements. *Corresponding author: Tel þ49 341 9732551; fax þ49 341 9732599; e-mail friedrich.kremer@physik.uni-leipzig.de.