Chemical Cross-Linking, Surface Compressional Modulus, and Viscosity of n-Octadecyltrimethoxy Silane Monolayers Stephen R. Carino, † Randolph S. Duran,* ,† Ronald H. Baney, ‡ Laurie A. Gower, ‡ Liu He, ‡ and Piyush K. Sheth ‡ Butler Polymer Laboratory, Department of Chemistry Department of Materials Science and Engineering UniVersity of Florida, GainesVille, Florida 32611-7200 ReceiVed August 25, 2000 Monomolecular-thin films prepared at an aqueous interface continue to provide an attractive means to form well-defined molecular assemblies for surface modification and ultrathin-film applications. For these purposes, high-molecular weight, poly- meric Langmuir-Blodgett films 1 have the potential of being more robust materials compared to their low-molecular weight ana- logues. Of course, the same entanglement of polymer backbones which gives rise to the transient network formation and attractive mechanical properties in bulk polymeric materials is less effective in a quasi-two-dimensional LB layer. Several groups have considered cross-linking monolayers as a means of enhancing properties of these materials. 2a-f We recently demonstrated that an extended network formed from polymerization of bolaform alkylaniline monolayers results in a sufficiently stable and flexible material in which individual self-supporting monolayers could be drawn from the water surface as thin films and fibers of macroscopic size. 3 Can the chemical transformations in these types of reactions be more quantitatively correlated to the resulting physical properties? In this work, we present some insight on the network formation of amphiphilic n-octadecyltrimethoxysilane (OTMS, CH 3 (CH 2 ) 17 - Si(OCH 3 ) 3 ) at air/water interfaces by measuring changes in compressive mechanical properties and surface rheology of the reacting system in real time. Such measurements should be broadly applicable to reactive amphiphiles. In bulk solutions, alkoxysilanes are well-known to undergo acid- or base-catalyzed hydrolysis and to form siliceous materials. 4 Polysilsesquioxanes, RSiO 3/2 , offer intriguing models to examine this; as in this work, the R group can be strongly hydrophobic. Such materials form oligomeric cage, polymeric ladder, and three- dimensional network structures depending upon reaction condi- tions and the nature of the R group. 5 Langmuir monolayers of OTMS at the air/water interface have been reported 6a,b to form a condensation product. While the molecular weight and architecture of the product have not been reported, some have suggested that OTMS and similar materials should form linear 6a,7 rather than network polymers due to the 2D restrictions in the environment around the silane headgroup. Experimental. All monolayer and polymerization studies were conducted on KSV LB5000 equipment at 25 °C using trough and barriers made of PTFE. The surface pressure (Π) measurements were obtained using the Wilhelmy plate technique. High-purity water (resistivity g 18 MΩ-cm) from a Milli-Q (Millipore) filter system was used, and the pH was adjusted with HCl. n- Octadecyltrimethoxy silane (>95%) obtained from Gelest Inc. was used as received. Isotherm studies (not shown) reproduced previously published work. 6 Surface Compressional Modulus and Reaction. The surface compressional modulus of an insoluble monolayer is a measure of the film stiffness and generally should increase as molecular weight increases. The modulus was obtained by applying brief mechanical stimuli in situ during the course of the hydrolysis and condensation reactions. This was accomplished by introducing periodic compression-expansion cycles as shown schematically in Figure 1a. Each compression-expansion cycle gave rise to two isotherm curves; Figure 1b is a plot of the resulting surface pressure/area/reaction time curves describing the compression cycles. Each curve is an instantaneous record of mechanical properties at a defined point in the chemical reaction. The surface compressional modulus is defined as K s ≡ -dΠ/d ln A where Π is the surface pressure and A is the molecular area in the film. 8 In the work, K s was calculated directly from the slope of the condensed region of the corresponding isotherm, at Π ≈ 20 mN/ m. Between compression-expansion cycles (∼80% of total reaction time) the monolayer was maintained at isobaric conditions of Π ) 8 mN/m. Control experiments at isobaric conditions determined that the mechanical stimulus did not measurably change the reaction kinetics. As shown in Figure 1c, K s for the reacting monolayer increases about 10-fold with polymerization time in a nonlinear manner. At the start of the reaction, K s has a value of 30 mN/m, increasing to plateau at about 70 mN/m from 20 to 60 min. The modulus then increases sharply and eventually becomes more constant at a value of about 280 mN/m. The sharp jump in the modulus near the end of the reaction suggests that a threshold degree of reaction is required to attain a critical concentration where entanglements * To whom correspondence should be addressed. † Butler Polymer Laboratory, Department of Chemistry. ‡ Department of Materials Science and Engineering. (1) Miyashita, T. Prog. Polym. Sci. 1993, 18(2), 263. (2) (a) Miyano, K.; Veyssie ´, M. Phys. ReV. Lett. 1984, 52(15), 1318. (b) Heger, R.; Goedel, W. A. Supramol. Sci. 1997, 4, 301. (c) Ko ¨lchens, S.; Lamparski, H.; O’Brien, D. F. Macromolecules 1993, 26, 398. (d) Sisson, T. M.; Lamparski, H. G.; Ko ¨lchens, S.; Elayadi, A.; O’Brien, D. F. Macromol- ecules 1996, 29, 8321. (e) Zhang, L.; Hendel, R. A.; Cozzi, P. G.; Regen, S. L. J. Am. Chem. Soc. 1999, 121, 1(7), 1621. (f) Schoberl, U.; Magnera, T. F.; Harrison, R. M.; Fleischer, F.; Pflug, J. L.; Schwab, P. F. H.; Meng, X. S.; Lipiak, D.; Noll, B. C.; Allured, V. S.; Rudalevige, T.; Lee, S.; Michl, J. J. Am. Chem. Soc. 1997, 119, 9(17), 3907. (3) Kloeppner, L. J.; Duran, R. S. J. Am. Chem. Soc. 1999, 121, 1(35), 8108. (4) Parikh, A. N.;, Schivley, M. A.; Koo, E.; Seshadri, K.; Aurentz, D.; Mueller, K.; Allara, D. L. J. Am. Chem. Soc. 1997, 119, 9(13), 3135. (5) Baney, R. H.; Itoh, M.; Sakakibara, A.; Suzuki, T. Chem. ReV. 1995, 95, 1409. (6) (a) Vidon, S.; Leblanc, R. M. J. Phys. Chem. B 1998, 102, 1279. (b) Britt, D. W.; Hlady, V. J. Phys. Chem. B 1999, 103, 2749. (7) Sjo ¨blom, J.; Stakkestad, G.; Ebeltoft, H.; Friberg, S. E.; Claesson, P.; Langmuir 1995, 11, 2652. (8) Gaines, G. L., Jr. Insoluble Monolayers at Liquid-Gas Interfaces; Interscience Publishers: New York, 1966; pp 24-25. Figure 1. (a) Example of two cycles of the oscillatory compressive stimuli applied to the reacting monolayer; the bottom curve represents the measured surface pressure response. (b) Pressure-area isotherms from which the plot in (c) was calculated. Obtained between 0 and 25 mN/m at 25 °C. (c) Surface compressional modulus of OTMS during reaction on a pH 3.5 subphase and at Π ) 8 mN/m. 2103 J. Am. Chem. Soc. 2001, 123, 2103-2104 10.1021/ja0055514 CCC: $20.00 © 2001 American Chemical Society Published on Web 02/06/2001