DOI: 10.1002/adma.200602376 Molecular Pressure Sensors** By Hui Xu, Frank C. Sun, David G. Shirvanyants, Michael Rubinstein, Denis Shabratov, Kathryn L. Beers, Krzysztof Matyjaszewski, and Sergei S. Sheiko* Flow properties of molecularly thin films are at the founda- tion of many practical applications such as lithography, micro- fluidics, coatings, and lubrication. [1–10] Further advances in these fields depend on understanding the mechanisms that control the kinetics of flow. [11–14] However, one of the prob- lems in flowing monolayers is the independent characteriza- tion of the driving and frictional forces that are intimately coupled through the molecular interactions between the fluid and the substrate. In this regard, the visualization of compres- sible macromolecules during flow provides an exceptional op- portunity to study these forces. [15] Here we report on the mon- itoring of brush-like macromolecules as they change their shape in response to variations in the film pressure during flow. After appropriate calibration, these molecular sensors can be used to gauge both the pressure gradient and the friction coefficient at the substrate. We anticipate the utiliza- tion of such miniature sensors for probing flow properties on nanometer length scales. The design of the pressure-responsive macromolecules is based on brush-like polymer architectures comprised of a flexible backbone surrounded by a dense shell of side chains (Fig. 1a). The characteristic property of these macromolecules is their ability to change shape upon lateral compression on a substrate. [16–18] If the film pressure increases, the number of side chains adsorbed to the surface decreases allowing the backbone to coil. This causes the macromolecules to become more compact and occupy less area on the substrate. There- fore, the area per molecule can be used as a pressure sensitive parameter to gauge the variations of film pressure within flowing monolayers. Molecular brushes (Fig. 1a) with the same degree of poly- merization of a poly(2-hydroxyethyl methacrylate) backbone (n = 570 ± 50) and different degrees of polymerization of poly(n-butyl acrylate) (pBA) side chains (n = 35 ± 5 and n = 51 ± 5) were synthesized by atom transfer radical polymer- ization. [19,20] At room temperature, the materials are fluid melts that spontaneously spread when placed on higher sur- face energy substrates such as mica and graphite. [21] Small drops of molecular brushes (volume ∼ 1 nl, radius ∼ 100 lm) were deposited on a substrate inside an environmental cham- ber under controlled temperature (T = 25 °C) and relative humidity (RH = 30–99 %). Like many other fluids, the drops first spread by generating a molecularly thin precursor film moving ahead of the macroscopic drop (Fig. 1b). [22] Using an atomic force microscope (Multimode, Nanoscope 3A, Veeco Metrology Group), we monitored the spreading process over a broad range of length scales ranging from the motion of the film front all the way down to the movements of the individu- al molecules within the film. [15,23] Figure 1c shows the time dependence of the film length L = R – R 0 observed on mica at a high relative humidity (RH = 99 %), where R 0 = 63 lm is the initial drop radius and R is the total radius of the film at time t (Table 1). The length follows the well-known law, observed for different types of simple and complex fluids, (R – R 0 ) 2  Dt, where D is the spreading rate having the dimension of a diffusion coeffi- COMMUNICATION 2930 © 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Mater. 2007, 19, 2930–2934 – [*] Prof. S. S. Sheiko, Dr. H. Xu, [+] F. C. Sun, D. G. Shirvanyants, Prof. M. Rubinstein Department of Chemistry, University of North Carolina at Chapel Hill CB# 3290, Chapel Hill, NC 27599 (USA) E-mail: sergei@email.unc.edu Dr. D. Shabratov Estonian Nanotechnology Competence Center Riia 142, Tartu 51014 (Estonia) Dr. K. L. Beers, [++] Prof. K. Matyjaszewski Department of Chemistry, Carnegie Mellon University 4400 Fifth Avenue, Pittsburgh, PA 15213 (USA) [+] Current address: Caliper Life Sciences, 605 Fairchild Drive, Moun- tain View, CA 94043, USA. [++] Current address: NIST Combinatorial Methods Center, Polymers Division, National Institute of Standards and Technology, Gaithers- burg, MD 20899, USA. [**] This research is supported by grants from the National Science Foundation (CBET 0609087 and DMR 0606086) and the Petroleum Research Foundation (PRF-46204-AC7). Supporting Information is available online from Wiley InterScience or from the authors. Table 1. Time dependence of the film length at different relative humidity (RH). # [a] N [b] RH (%) [c] t (min) [d] R 0 (lm) [e] R(lm) [f ] 1 35 99 10 63 638 2 3 4 5 35 35 50 50 97 95 99 30 10 10 10 900 40 52 38 150 126 59 359 162 [a] experiments #1–3: spreading of pBA brushes with n = 35 ± 5 (M n = 2.7 × 10 6 g mol –1 ) on mica substrate at different relative humidities; experiment #4: spreading of pBA brushes with longer side chains (n = 51 ± 5, M n = 3.8 × 10 6 g mol –1 ) on mica substrate at RH = 99 %; exper- iment #5: spreading of pBA brushes with n = 50 ± 5 on highly oriented pyrolytic graphite at RH = 30 %. [b] degree of polymerization of the side chains of brush molecules. [c] relative humidity within the chamber. [d] time allowed for spreading process [e] the initial radius of the drop does not change significantly during the spreading process. [f ] radius of the precursor film at time t, R(t)= R 0 + L(t).