Langmuir 2003, 19, 5735-5741
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Cellulose Model SurfacessSimplified Preparation by Spin Coating and Characterization by X-ray Photoelectron Spectroscopy, Infrared Spectroscopy, and Atomic Force Microscopy Eero Kontturi,* Peter C. Thu¨ne, and J. W. (Hans) Niemantsverdriet Schuit Institute of Catalysis, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands Received January 9, 2003. In Final Form: April 17, 2003 Spin coating is introduced as a simplified method to prepare model surfaces of cellulose. Prior to spin coating, trimethylsilyl cellulose (TMSC), a nonpolar derivative of cellulose, is synthesized in order to dissolve the otherwise immiscible cellulose. After the spin coating deposition of TMSC on an untreated silicon or gold substrate, the TMSC layer is subjected to vapor phase acid hydrolysis, which regenerates it back to cellulose. X-ray photoelectron spectroscopy (XPS) and attenuated total reflectance infrared spectroscopy (ATR-IR) revealed the films’ chemical structure to be similar to pure cellulose. Spin coating parameters such as spinning speed, concentration of the coating solution, and nature of the solvent were varied to obtain diversity in morphology and thickness of the films. The optimal spin coating parameters resulted in a cellulose film with 20 nm thickness and 10% roughness, determined by atomic force microscopy (AFM). Preliminary experiments with modifying the chemistry and varying the water content of the films were also conducted.
1. Introduction In the course of the 20th century, research on the chemistry of cellulose and other carbohydrates has formed an important branch in macromolecular, organic, physical, and analytical chemistry, as well as in material science. However, ultrathin films of cellulose have received little attention in the past. Ultrathin model surfaces provide excellent means to examine the chemical and morphological changes taking place in various processes involving cellulose, for example, interactions with water (papermaking, recycling), photochemistry (yellowing of paper), thermal degradation (biomass gasification), and so forth. To our knowledge, the most established way to prepare ultrathin model surfaces from cellulose is the preparation of Langmuir-Blodgett (LB) films from trimethylsilyl cellulose (TMSC), which, once deposited on a smooth substrate, can be easily hydrolyzed back to cellulose by a simple vapor phase acid hydrolysis.1,2 The smooth LB films have been exploited further in studies with surface forces3,4 and hydroxyl accessibility.5 Recently, spin coating has also emerged as a method for cellulose model surface preparation.6,7 We feel that the model surfaces have not been used to their potential in cellulose research. The purpose of this paper is to describe a simplified procedure to prepare smooth cellulose films with a thickness on the * Corresponding author. Telephone: +31-40-2473068. Fax: +3140-2473481. E-mail address:
[email protected]. (1) Schaub, M.; Wenz, G.; Wegner, G.; Stein, A.; Klemm, D. Adv. Mater. 1993, 5, 919. (2) Buchholz, V.; Wegner, G.; Stemme, S.; O ¨ dberg, L. Adv. Mater. 1996, 8, 399. (3) Holmberg, M.; Berg, J.; Stemme, S.; O ¨ dberg, L.; Rasmusson, J.; Claesson, P. J. Colloid Interface Sci. 1997, 186, 369. (4) Holmberg, M.; Wigren, R.; Erlandsson, R.; Claesson, P. M. Colloids Surf. 1997, 129-130, 175. (5) Buchholz, V.; Adler, P.; Ba¨cker, M.; Ho¨lle, W.; Simon, A.; Wegner, G. Langmuir 1997, 13, 3206. (6) Gunnars, S.; Wågberg, L.; Cohen Stuart, M. A. Cellulose 2002, 9, 239. (7) Geffroy, C.; Labeau, M. P.; Wong, K.; Cabane, B.; Cohen Stuart, M. A. Colloids Surf. 2000, 172, 47.
order of 10-50 nm and to explore the properties of these surfaces. The seminal difficulty of creating ultrathin films as model surfaces of cellulose has been its reluctance to dissolve into conventional organic and inorganic solvents. A number of solvents is available for cellulose, such as lithium chloride in dimethylacid amide,8 N-methylmorpholine-N-oxide (NMMO) monohydrate,9 or the cadmium complex with ethylenediamine (Cadoxen),10 but while they are suitable for analytical and synthetic purposes, they are very difficult to work with from the point of view of model surface preparation, although recently NMMO has been exploited with model surfaces.6 This is also the reason easily dissolving cellulose derivatives have been subjects for model surfaces with a wider interest than that for cellulose itself.11-14 The elegance of the aforementioned LB technique1,2 lies in the easy transformation of the TMSC derivative to cellulose after the TMSC has already been coated on the substrate (Scheme 1). The LB technique, however, requires special equipment, like the Langmuir trough, and rather demanding conditions, for example, a computer controlled thermostat. Therefore, we set out to establish a new, simplified method based on spin coating to prepare smooth, ultrathin films of cellulose. Spin coating is a process in which a polymer (8) Dawsey, T. R.; McCormick, C. L. JMS-Rev. Marcomol. Chem. Phys. 1990, C30 (3 and 4), 405. (9) Rosenau, T.; Potthast, A.; Sixta, H.; Kosma, P. Prog. Polym. Sci. 2001, 26, 1763. (10) Nehls, I.; Wagenknecht, W.; Philipp, B. Cellul. Chem. Technol. 1995, 29, 243. (11) Maaroufi, A.; Mao, L.; Ritcey, A. M. Macromol. Symp. 1994, 87, 25. (12) Feigin, L.; Klechkovskaya, V.; Stepina, N.; Tolstikhina, A.; Khripunov, A.; Baklagina, Y.; Volkov, A.; Antolini, R. Colloids Surf. 2002, 198-200, 13. (13) Kimura, S.; Kitagawa, M.; Kusano, H.; Kobayashi, H. Polym. Adv. Technol. 2000, 11, 723. (14) Plagge, A.; Stratmann, M.; Kowalik, T.; Adler, H. Macromol. Symp. 1999, 145, 103.
10.1021/la0340394 CCC: $25.00 © 2003 American Chemical Society Published on Web 06/06/2003
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Langmuir, Vol. 19, No. 14, 2003
Scheme 1. Conversion of Cellulose to Trimethylcellulose (TMSC) with Hexamethyldisilazane (HMDS), and the Subsequent Hydrolysis Back to Cellulose with HCl
or colloid containing liquid film is spread by centrifugal forces onto a rotating substrate and the liquid is evaporated with the help of high-speed spinning, leaving a smooth, uniform film with thickness that can be modified by altering parameters such as original solution concentration, spinning speed, and choice of solvent.15-17 In addition, spin coaters are widely abundant in laboratories of polymer and surface science throughout the world. Spin coating is not a new technique concerning cellulose model surfaces. The earliest attempt was to spin coat cellulose straight from trifluoroacetic acid,18 which unfortunately has an obvious disadvantage of chemically modifying the cellulose.19 Geffroy et al. exploited spin coating of TMSC and subsequent acid hydrolysis to cellulose,7 but regrettably the characterization of these films was minimal and the method was left unexplored. Furthermore, NMMO has been used as a direct solvent to spin coat cellulose on smooth silica surfaces.6 This method, however, lacks the reproducibility to an extent and the characterization of the chemical structure of the films is still meager. Both recent methods6,7 also involve a use of anchoring polymer between the substrate, and especially spin coating from NMMO6 is a lengthy procedure including pretreatment of the silica substrate and a prolonged washing stage after the deposition. With the disadavantages of the aforementioned methods in mind, we wanted to establish a fast, simplified, reproducible method to prepare ultrathin model surfaces of cellulose, which is comprised of four stages: (i) Synthesis of TMSC from cellulose, performed in a homogeneous environment. (ii) Spin coating of the dissolved TMSC on a smooth substrate. The concentration of the TMSC solution, choice of solvent, and spin coating speed are the parameters that determine the morphology and thickness of the layer. (iii) Hydrolysis of the coated TMSC layer back to cellulose in a vapor phase acid hydrolysis. The hydrolysis happens very fast (
