Study of the Polydispersity of Grafted Poly(dimethylsiloxane) Surfaces Using Single-Molecule Atomic Force Microscopy † Sabah Al-Maawali, Jason E. Bemis, Boris B. Akhremitchev, Rojana Leecharoen, Benjamin G. Janesko, and Gilbert C. Walker* Department of Chemistry, UniVersity of Pittsburgh, Pittsburgh, PennsylVania 15260 ReceiVed: October 10, 2000; In Final Form: NoVember 29, 2000 Single-molecule atomic force microscopy (AFM) was used to study the statistical distribution of contour lengths (polydispersity) of polymer chains grafted to a surface. A poly(dimethylsiloxane) (PDMS) monolayer was grafted on a flat silicon substrate by covalently bonding Cl-terminated PDMS (M w ) 15000-20000) to an OH-silicon surface and characterized using contact angle measurements, ellipsometry, and single-molecule AFM. A model for the single-chain dynamics is presented. The statistical distributions of the polymer contour lengths were found to depend on the concentration of the PDMS polymer used in the grafting solutions. Shifts of the statistical distributions toward higher contour lengths indicated preferential adsorption of longer chains with increasing PDMS:CH 2 Cl 2 volume ratios of 0.005-0.16. The gel permeation chromatography (GPC) profile was found to correlate with the most dilute (0.005 volume ratio) AFM data. The polydispersity index (PI) calculated using AFM data was found to be 1.56 compared to 1.62 by GPC. A surface grafted with two PDMS polymer samples of average molecular weights, 3000 and 15000-20000, was found to have a bimodal distribution of contour lengths, with peaks corresponding to the two grafting samples. Introduction Man-made polymers synthesized by free radical or polycon- densation mechanisms are known to produce a wide distribution of molecular weights and hence characteristic chain lengths. 1-5 A quantity called the polydispersity index (PI) has been used as a rough guide to understand the distribution of these molecular weights: where M h w is the weight average molecular weight and M h n is the number average molecular weight. A polymer is considered to be monodisperse if PI equals 1. Different analytical methods, such as gel permeation chromatography (GPC) and combinations of light scattering and vapor pressure osmometry, are analytical tools that have been traditionally used to study the distribution of these different molecular weights (polydispersity) of polymers in solution. 1-4 On the other hand, there are few direct methods for analyzing the lengths of molecules at surfaces. 1,4 Given the importance of polymer adsorption in technologies ranging from adhesion and lubrication to biology and medicine, 1 new methods for characterizing polydispersity at surfaces are of both practical and fundamental interest. Single-molecule studies using atomic force microscopy may be able to directly characterize such surface polydispersity, and in this paper we aim to examine that potential. Apart from the usual contact adhesion observed in AFM, polymer distortions can be observed when a grafted polymer chain bridges to the AFM tip 6-12 as can be seen in Figure 1. The suggestion that this phenomenon could be used to study polymer polydispersity has been suggested by several authors, 7,8,10b,11 but has not been explored in detail. In this paper we focus on poly(dimethylsiloxane) (PDMS), mainly because of its widespread importance in industry 13 and, especially, its value for release applications due to its low surface energy. PDMS’s useful bulk properties mainly arise from its unique physical properties of flexibility, low adhesion, and low glass transition temperature. The molecular weight range studied here was, M w ) 3000, contour length of 11 nm, as well as M w ) 15000-20000, with contour lengths of ∼50-80 nm (as calculated using backbone bond lengths of 1.64 Å and bond angles Si-O-Si ) 143° and O-Si-O ) 110°). The latter one is close to the entanglement molecular weight (M w ) 18000) of PDMS. Experimental Methods Preparation of Oxidized Silicon. 14-19 The Si(100) silicon wafers (Siliconquest, CA) were cleaned by heating in a piranha solution (4H 2 SO 4 :1(30% H 2 O 2 ) (EM Science)) for 10-15 min on a hot plate at approximately 100 °C, and then rinsing in ultrapure water (Millipore). The silicon wafers were next heated in a solution of 1(30% H 2 O 2 ):1HCl:4H 2 O for 10 min on a hot plate at approximately 80 °C and then rinsed in ultrapure water. The silicon wafers were next heated again in piranha solution for 10-15 min and rinsed in ultrapure water. Preparation of Si-OH. 20,21 Hydroxyls on the silicon surface were obtained by boiling the oxidized silicon wafers for 1 h in ultrapure H 2 O (18 MΩ‚cm). The silicon wafers were then dried well with argon gas before grafting. It was observed that drying of the Si wafers with inert gas was essential before grafting of PDMS because moisture present in these experiments prevented the grafting reaction from occurring. 13,21 Grafting of PDMS of M w ) 15000-20000. Chlorine- terminated PDMS of molecular weight 20000 was used as purchased from Gelest, Inc. The solvent, anhydrous CH 2 Cl 2 , was used as purchased from Aldrich. Anhydrous pyridine used † Part of the special issue “John T. Yates, Jr. Festschrift”. * To whom correspondence should be addressed. PI ) M h w /M h n (1) 3965 J. Phys. Chem. B 2001, 105, 3965-3971 10.1021/jp0037246 CCC: $20.00 © 2001 American Chemical Society Published on Web 02/14/2001