Dynamic Contact Angle Studies of Self-Assembled Thin Films from Fluorinated Alkyltrichlorosilanes 1 Mark J. Pellerite,* ,† Erika J. Wood, † and Vivian W. Jones ‡ 3M Company, 3M Center 201-1W-28, St. Paul, Minnesota 55144 ReceiVed: October 16, 2001; In Final Form: February 10, 2002 Dynamic contact angle studies on self-assembled thin films from C 7 F 15 CH 2 OCH 2 CH 2 CH 2 SiCl 3 reveal a range of interesting behavior. Solution-based processing conditions have been identified that allow preparation of essentially monolayer films on quartz exhibiting water adv/rec contact angles of 119/104 ( 2° and extremely low contact angle hysteresis (hexadecane adv/rec ) 74/70°, heptane adv/rec ) 56/55°) with hydrocarbon liquids. This compound provides an example of a fluorinated trichlorosilane that is able to deliver low- hysteresis films by deposition at room temperature. Adsorption of silane oligomers, formed by hydrolysis and condensation reactions in solution, was also found to occur, slower than but competitive with monolayer formation. This process became more significant as dip coating times increased. Ellipsometric data on silicon wafers confirmed that film thicknesses increased with dip time, while AFM imaging showed that the oligomeric material was deposited in the form of particulates. The effects of this process on water dynamic contact angles are discussed. We also compare contact angles on these films with those on films prepared from C n F 2n+1 CH 2 CH 2 SiCl 3 (n ) 6, 8, 10) and draw some conclusions regarding structure-property effects in these systems. Finally, we propose a mechanism that can account qualitatively for the bulk of the results observed here. Its central feature is reaction of the fluorinated alkyltrichlorosilane with surface-adsorbed water to yield a self-assembled monolayer consisting of silanetriol molecules hydrogen-bonded to the substrate. Effects of high humidity aging on dynamic contact angles of these films suggest that they are at most only lightly cross-linked when prepared under conditions utilized here. Introduction Alkylsilanes RSiX 3 (R ) alkyl, X ) Cl, alkoxy) on siliceous surfaces and other substrates represent one of the most extensively investigated classes of self-assembling monolayer systems. Since the pioneering work of Sagiv and co-workers, 2 virtually every tool available to the surface chemist has been brought to bear in the study of these systems. 3-35 The picture that has emerged 2,10,25 regarding the structure of these mono- layers revolves around a two-dimensional cross-linked alkyl- siloxane network, formed by hydrolysis of the trichlorosilane and condensation of the resulting silanol groups, partially bound to the substrate through additional condensation with surface silanols (Figure 1), although the extent and distribution of siloxane linkages is currently subject to debate. This view has recently been challenged, however, 36 so despite the voluminous literature in this area, research is extremely active and basic mechanistic questions remain to be answered. Fluorinated alkylsilanes have been somewhat less well- studied 48 than their hydrocarbon counterparts, even though such materials are finding application as low-surface-energy films for optical substrates 49 and lubricating treatments for micro- electromechanical systems (MEMS) devices. 37 For instance, dynamic contact angles in these films are not well characterized, and most of the work reported to date has involved silanes of the general structure 1; little systematic data exist on structure- property relationships and the effect on film properties of the linking group structure between the perfluoroalkyl segment and the silyl headgroup. 22,34,38,45-47 This study presents results from dynamic contact angle measurements on self-assembled films derived from fluorinated trichlorosilane 2 on quartz. Contact angle data before and after aging in dry and humid environments, along with ellipsometry and AFM data on coated silicon wafers, are discussed within the framework of a mechanistic hypothesis for the formation and structure of these self-assembled thin films. Also presented are some limited dynamic contact angle data on films from 1 obtained on quartz under the same conditions, to probe the significance of structure-property interactions in these films. Where possible, we attempt to place the data from this study, and the picture that emerges therefrom, in the context of results from previous work 22,37 on mechanism and structure in fluori- nated alkyltrichlorosilane self-assembly. We prefer the general term “thin films” because, as we show, any description of self- assembled films from 2 strictly in terms of monolayers is not only overly confining, but at times incorrect. Experimental Section Materials. All solvents and reagents were standard com- mercial grade and used as received. Methyl perfluorobutyl ether * To whom correspondence should be directed. E-mail: mjpellerite1@ mmm.com. Fax: 651-737-5335. † Advanced Materials Technology Center. ‡ Corporate Analytical Technology Center. C n F 2n+1 CH 2 CH 2 SiCl 3 1; n ) 6, 8, 10 C 7 F 15 CH 2 OCH 2 CH 2 CH 2 SiCl 3 2 4746 J. Phys. Chem. B 2002, 106, 4746-4754 10.1021/jp013820m CCC: $22.00 © 2002 American Chemical Society Published on Web 04/09/2002