Direct View of Structural Regimes of End-Grafted Polymer Monolayers: A Scanning Force Microscopy Study V. Koutsos, E. W. van der Vegte, and G. Hadziioannou* Department of Polymer Chemistry, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands Received November 18, 1997; Revised Manuscript Received July 17, 1998 ABSTRACT: Scanning force microscopy images of thiol-end-functionalized polystyrene monolayers, end- grafted to gold substrates from good solvent, showed two structural regimes in bad-solvent conditions. At low and moderate grafting densities the experimental results agree with the scaling arguments and can be situated correctly to the corresponding diagram of states for single end-grafted chains and pinned micelles, respectively. A third structural regime, of semicontinuous dimples, has been observed when adsorption was performed from poor-solvent conditions. Introduction End-grafted polymer monolayers play an important role in colloidal stabilization, 1,2 adhesion, 3 and biocom- patibility of artificial organs in medicine. 4 In good- solvent conditions the physical picture is rather simple. In connection with grafting density, two limiting re- gimes can be distinguished: 5 at low grafting densities, each chain is isolated, occupying roughly a half-sphere with a radius comparable to the radius of gyration (“mushroom” regime); at high grafting densities, poly- mers stretch away from the interface to avoid overlap- ping, forming a uniform polymer “brush”. In bad-solvent conditions (dry state) the structural forms have a wider variety. When the grafting density is low enough, the polymer chains collapse separately. 6 If the polymer chains overlap with each other, they collapse in groups due to the polymer-polymer attrac- tive interactions and the constraints of the grafted tethers, forming clusters. This microphase segregation has been detected by several scanning force microscopy studies 6,7 in agreement with theoretical and numerical predictions. 8 For even higher grafting densities the situation becomes trivial: a homogeneous layer can be formed. 7b Recently, much work has been focused on the struc- tural states of the end-grafted monolayers in bad-solvent conditions. In a previous study of ours 6 we used the spontaneous chemisorption of thiol-terminated polysty- rene (PS-SH) on gold surfaces from toluene solutions to form monolayers of end-grafted PS. We described the single chains-cluster transition and showed that the cluster dimensions agree with scaling predictions for pinned micelles. 8g,h In this paper (a) we present some more data on the single chain regime and demonstrate that our previous and recent data fit well in a diagram of states deduced by scaling arguments and (b) we show that, for high grafting densities and before a homogeneous layer is formed, there is an intermediate regime of semicontinu- ous dimples. Experimental Part Gold Substrate Preparation. Mica was cleaved in air and immediately placed in the vacuum chamber of a diffusion pump thermal evaporator (Edwards Auto 306). The mica sheets were heated to 400 °C at a pressure below 1 × 10 -4 mbar. Gold was evaporated from resistively heated tungsten boats at a pressure below 5 × 10 -6 mbar. The thickness of the gold layer was monitored with a quartz crystal oscillator and eventually amounted to 35 nm (deposition rate 3 nm/min). The gold substrates were left at 400 °C for 2 h and then cooled to room temperature in a vacuum. The vacuum chamber was filled with prepurified nitrogen, and the gold substrates were removed and stored under nitrogen until further use. This way of gold film preparation produces atomically flat gold islands separated by deep channels. 9,10 Sample Preparation. Thiol-terminated polystyrene (PS- SH) was prepared by anionic polymerization. 11,12 For the first topic of this study the samples were prepared by exposing the gold substrates to a 0.1 mg/mL toluene (good solvent for PS) solution of PS 1400-SH (molecular weight 144 000, polydisper- sity 1.2) for relatively short incubation time (so that low grafting density is attained 6 ). In addition, a group of data from our paper, ref 6, was used. The gold substrates were immersed in 2 mg/mL solutions of PS x-SH (five different molecular weights) for 48 h (Table 1). For one molecular weight (PS1400- SH), we also employed a 0.1 mg/mL solution and 24 h of incubation time. Using long incubation times, we obtained higher surface coverages. 6 For all samples, the substrates were removed from the adsorption solution and immediately rinsed exhaustively with fresh toluene. The samples were dried under a stream of argon and were placed in a vacuum at 50 °C for 1 h and stored in nitrogen. For the second topic (higher grafting densities-dimples formation) we used a 0.1 mg/mL cyclohexane (poor solvent for PS at room temperature) solution of PS 1400-SH. The gold substrates were kept in the solution for 1 h. Immediately after their removal from the cyclohexane solution and the rinsing with fresh toluene, they were dipped in pure toluene for about 1 h. This step is important since we want to exclude the possibility of physisorbed polymer chains on the substrate. * To whom correspondence should be addressed. E-mail Hadzii@ chem.rug.nl. Table 1. Molecular Characteristics of the PSx-SH r Polymers and Grafting Densities Estimated from SFM Images polymer Mw Mw/Mn σ (no. of chains/μm 2 ) PS400-SH 41 500 1.09 22 839 PS500-SH 51 500 1.1 19 929 PS800-SH 85 000 1.2 14 688 PS1400-SH 144 000 1.2 12 672 PS2500-SH 258 000 1.1 4 268 PS1400-SH (lower coverage) 144 000 1.2 3 800 R The subscript refers to the approximate degree of polymeri- zation. 1233 Macromolecules 1999, 32, 1233-1236 10.1021/ma971702k CCC: $18.00 © 1999 American Chemical Society Published on Web 02/02/1999