Surface Interaction Forces of Well-Defined, High-Density Polymer Brushes Studied by Atomic Force Microscopy. 1. Effect of Chain Length Shinpei Yamamoto, Muhammad Ejaz, Yoshinobu Tsujii, Mutsuo Matsumoto, and Takeshi Fukuda* Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan Received October 18, 1999; Revised Manuscript Received April 24, 2000 ABSTRACT: We made direct force measurements by atomic force microscopy (AFM) at surfaces of polymer brushes comprised of low-polydispersity poly(methyl methacrylate) (PMMA) chains densely end-grafted on a silicon substrate by living radical polymerization. These brushes are characterized by an exceptionally high, nearly constant graft density (approximately 0.4 chains/nm 2 ) and a wide range of molecular weights of the graft chains. This graft density is at least 10 times larger than those of the previously studied polymer brushes prepared by the adsorption of block copolymers. Force measurements were made in toluene with a silica particle-attached cantilever. The hysteresis behavior of a piezo actuator used in the AFM was corrected by simultaneously measuring its piezo current. The true distance D between the substrate surface and the silica probe, which usually is difficult to define in AFM experiments, was successfully determined by AFM imaging across the boundary of a scratched and an unscratched region on the sample surface. In this way, we could obtain quantitative force-distance profiles. The repulsive force was observed to rapidly increase with decreasing separation. The equilibrium thickness L e of the brushes, i.e., the critical distance at which a repulsive force was detectable, was found to be proportional to the weight-average chain contour length Lc,w, giving Le/Lc,w ) 0.6. This indicates formation of a homogeneous polymer layer with highly stretched graft chains. Unlike the previously reported results for lower density polymer brushes, the force-distance profiles for different graft chain lengths were not scaled by the reduced distance D/L e. Longer brushes were more resistant to compression: for example, the longest studied brush was compressible only to D/Le ) 0.8, which was about 3 times as large as the scaled dry thickness Ld/Le. Introduction Polymers end-grafted on a solid surface play an important role in many areas of science and technology, e.g., colloid stabilization, adhesion, lubrication, tribol- ogy, and rheology. 1-5 The conformation of those poly- mers in a good solvent can dramatically change with graft density; 6-8 at low graft densities, a “mushroom” structure will be formed with the coil dimension similar to that of ungrafted chains. With increasing graft density, graft chains will be obliged to stretch away from the surface, forming a so-called “polymer brush”. Poly- mer brushes have been extensively studied: for ex- ample, the brush height and the segment density profile in a good solvent were studied by neutron reflectom- etry, 9-11 and the forces at polymer brush surfaces were directly measured by a surface force apparatus (SFA) 12-14 and an atomic force microscope (AFM). 15-18 Most of the polymer brushes experimentally studied so far were prepared by end-functionalized polymers or block copolymers with terminal group or one block selectively adsorbed on the surface. These systems, however, had a rather low graft density, typically 0.001-0.05 chains/nm 2 . These graft densities corre- spond to the “moderate density” regime, in which graft chains overlap each other, but the volume fraction of polymer in the layer is still so low that the interaction free energy is dominated by binary interactions and the elastic free energy is approximately Gaussian. Theoreti- cal analyses of polymer brushes with higher graft densities which take account of higher-order interac- tions predict that the repulsive force steeply increases with increasing graft density. 19,20 By the adsorption procedure with preformed polymers, however, it is difficult to obtain such high graft densities. 21 An alter- native method is the graft polymerization starting with initiating sites fixed on a surface, but it usually results in a poor control of chain length and its distribution. 23-25 Recently, living polymerization techniques were suc- cessfully applied to the surface-initiated graft polym- erization to prepare a dense polymer brush with these parameters controlled. 26-31 Some research groups 27-31 have explored the applicability of living radical polym- erization (LRP), which has been attracting much atten- tion as a new route to well-defined polymers with low polydispersities. 32 Its clear advantage over other po- lymerization techniques may be the applicability to a large variety of monomers without involved procedures including perfect removal of water and other impurities and protection and deprotection of reactive groups. We were the first to succeed in preparing a low-polydisper- sity polymer brush with an exceptionally high graft density on a silicon substrate, 27 in which we made a combined use of two independent techniques: one was the Langmuir-Blodgett (LB) technique 33 to provide a well-organized set of initiating sites on the surface, and the other was the atom transfer radical polymerization (ATRP) technique, 34 a variant of LRP, to achieve a controlled graft polymerization. In this work, we have made an AFM study on the structure and interaction forces of poly(methyl meth- acrylate) (PMMA) brushes prepared by the surface- initiated ATRP technique. The graft density of the studied PMMA brushes was estimated to be as high as 0.4 chains/nm 2 , which is one of the highest ever reported * To whom correspondence should be addressed: e-mail fukuda@scl.kyoto-u.ac.jp. 5602 Macromolecules 2000, 33, 5602-5607 10.1021/ma991733a CCC: $19.00 © 2000 American Chemical Society Published on Web 07/06/2000