Langmuir 2007, 23, 10801-10806
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Bi-phobic Cellulose Fibers Derivatives via Surface Trifluoropropanoylation Ana G. Cunha,† Carmen S. R. Freire,*,† Armando J. D. Silvestre,† Carlos Pascoal Neto,† Alessandro Gandini,† Elina Orblin,‡ and Pedro Fardim‡ CICECO and Department of Chemistry, UniVersity of AVeiro, Campus de Santiago, 3810-193 AVeiro, Portugal, and Laboratory of Fibre and Cellulose Technology, Åbo Akademi UniVersity, Porthansgatan 3, FI-20500, Turku/Åbo, Finland ReceiVed June 11, 2007. In Final Form: July 30, 2007 The surface modification of cellulose fibers with 3,3,3-trifluoropropanoyl chloride (TFP) was studied in a toluene suspension. The characterization of the modified fibers was performed by elemental analysis, Fourier transform infrared (FTIR), 13C-solid-state NMR, X-ray diffraction, thermogravimetry, and surface analysis (XPS, ToF-SIMS, and contact angles measurements). The degree of substitution (DS) of the ensuing trifluoropropanoylated fibers ranged from less than 0.006 to 0.30, and in all instances the fibers’ surface acquired a high hydrophobicity and lipophobicity resulting from a drastic reduction in its energy. The hydrolytic stability of these cellulose derivatives was also evaluated and shown to be permanent in time in the presence of neutral water, still appreciable in basic aqueous solution at pH 9, but, as expected quite poor at pH 12.
Introduction In the past few decades, mankind has been confronted with the inevitability of preserving the environment for its own survival. Vital issues, such as the handling of massive amounts of often harmful rejects, the progressive degradation of the environment, and the dwindling of fossil resources, require urgent attention. Accordingly, the specific realm of polymer science is witnessing a rapidly growing quest for novel materials, mostly based on renewable resources, capable of replacing petroleumbased macromolecules. Among these, polysaccharide derivatives are playing a fundamental role, with particular emphasis on cellulose, which is the most important and widespread biopolymer on earth. Because of its abundance, biodegradability, and remarkable properties, cellulose has always been intensively exploited as a source of materials, e.g., paper, cotton, and cellulose acetate, etc. The recent surge of activities related to its chemical modification stems from the growing interest in the development of a variety of value-added, environment friendly, biocompatible, and functional materials.1,2 Surface hydrophobicity has become a hot research topic which is stimulating a rich variety of studies and approaches.3-5 However, relatively little has been reported on the modification of the surface of natural polymers like cellulose. The most effective treatments append fluorinated moieties through chemical or physical means, which induce both hydrophobic and lipophobic * Corresponding author. Telephone. +351 234 401405. Fax +351 234 370084. E-mail:
[email protected]. † University of Aveiro. ‡ Åbo Akademi University. (1) Klemm, D.; Heublein, B.; Fink, H. P.; Bohn, A. Cellulose: Fascinating biopolymer and sustainable raw material. Angew. Chem., Int. Ed. 2005, 44 (22), 3358-3393. (2) Edgar, K. J.; Buchanan, C. M.; Debenham, J. S.; Rundquist, P. A.; Seiler, B. D.; Shelton, M. C.; Tindall, D. Advances in cellulose ester performance and application. Prog. Polym. Sci. 2001, 26 (9), 1605-1688. (3) Ma, M. L.; Hill, R. M. Superhydrophobic surfaces. Curr. Opin. Colloid Interface Sci. 2006, 11 (4), 193-202. (4) Gao, L. C.; McCarthy, T. J. A perfectly hydrophobic surface (θA/θR ) 180°/180°). J. Am. Chem. Soc. 2006, 128, 9052-9053. (5) Wier, K. A.; McCarthy, T. J. Condensation on ultrahydrophobic surfaces and its effect on droplet mobility: Ultrahydrophobic surfaces are not always water repellant. Langmuir 2006, 22, 2433-2436.
properties to the ensuing low-energy surfaces. Additionally, the presence of these moieties tends to impart a high thermal stability, reduce the chemical and biological fragility, and enhance interactions with various gases. The surface modification of cellulose fibers with fluorinated reagents represents therefore a promising strategy for the development of innovative functional biopolymeric materials for numerous potential applications. To date, only a few studies dealing with this topic have been published, namely, trifluoroacetylation in both homogeneous and heterogeneous systems,6-8 homogeneous modification with tri- and difluoroethoxy acetic acid,9,10 surface modification with perfluorinated oligo(ethylene oxide),11 plasma treatment with carbon tetrafluoride12 and fluorotrimethylsilane,13 surface graft-copolymerization with glycidyl methacrylate followed by coupling with pentadecafluorooctanoyl chloride,14 and surface modification with pentafluorobenzoyl chloride.15 The present paper describes the heterogeneous surface modification of cellulose fibers by the esterification of some of its OH groups with trifluoropropanoyl chloride (TFP), viz., (6) Cunha, A. G.; Freire, C. S. R.; Silvestre, A. J. D.; Neto, C. P.; Gandini, A. Reversible hydrophobization and lipophobization of cellulose fibers via trifluoroacetylation. J. Colloid Interface Sci. 2006, 301 (1), 333-336. (7) Liebert, T.; Schnabelrauch, M.; Klemm, D.; Erler, U. Readily hydrolyzable cellulose esters as intermediates for the regioselective derivatization of cellulose. 2. Soluble, highly substituted cellulose trifluoroacetates. Cellulose 1994, 1 (4), 249-258. (8) Yuan, H. H.; Nishiyama, Y.; Kuga, S. Surface esterification of cellulose by vapor-phase treatment with trifluoroacetic anhydride. Cellulose 2005, 12 (5), 543-549. (9) Glasser, W. G.; Becker, U.; Todd, J. G. Novel cellulose derivatives. Part VI. Preparation and thermal analysis of two novel cellulose esters with fluorinecontaining substituents. Carbohydr. Polym. 2000, 42 (4), 393-400. (10) Sealey, J. E.; Frazier, C. E.; Samaranayake, G.; Glasser, W. G. Novel cellulose derivatives. V. Synthesis and thermal properties of esters with trifluoroethoxy acetic acid. J. Polym. Sci., Part B: Polym. Phys. 2000, 38 (3), 486-494. (11) Fabbri, P.; Champon, G.; Castellano, M.; Belgacem, M. N.; Gandini, A. Reactions of cellulose and wood superficial hydroxy groups with organometallic compounds. Polym. Int. 2004, 53 (1), 7-11. (12) Sahin, H. T.; Manolache, S.; Young, R. A.; Denes, F. Surface fluorination of paper in CF4-RF plasma environments. Cellulose 2002, 9 (2), 171-181. (13) Navarro, F.; Davalos, F.; Denes, F.; Cruz, L. E.; Young, R. A.; Ramos, J. Highly hydrophobic sisal chemithermomechanical pulp (CTMP) paper by fluorotrimethylsilane plasma treatment. Cellulose 2003, 10 (4), 411-424.
10.1021/la7017192 CCC: $37.00 © 2007 American Chemical Society Published on Web 09/14/2007
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cell-OH + CF3-CH2COCl f cell-O-CO-CH2-CF3 + HCl The interest of this novel system stems from the fact that the ester moieties it generates are stable to neutral hydrolysis, in stark contrast with the very high reactivity of the counterparts bearing perfluorinated carbon atoms at the ester junction.16 Indeed, we showed previously that trifluoroacetyl cellulose is extremely prone to hydrolysis when placed in contact with neutral liquid water and even with significant concentrations of moisture.6 Likewise, the perfluorinated papers reported by Nystrom et al.14 are susceptible to the same hydrolytic sensitivity, despite their reported initial superhydrophobicity, as confirmed by our observations that these groups were completely removed within a day by a water/tetrahydrofuran solution.17 The insertion of a methylene spacer between the trifluoromethyl group and the ester function was deemed a good way to preserve the hydrophobic role of the appended moiety and at the same time to minimize its sensitivity to hydrolysis in mild aqueous media. Experimental Section Materials. The cellulose fibers (PC) used in this study were in the form of Eucalyptus globulus ECF (DEDED) industrial bleached kraft pulp and pure cellulose Schleicher & Schuell Microscience filter paper, the latter being used to provide flat surfaces for the measurements of contact angles. Both samples were vacuum-dried at 60 °C. 3,3,3-Trifluoropropionic acid was supplied by Apollo Scientific Ltd. and used as received. Thionyl chloride was supplied by Fluka. Toluene was dried over sodium wire. Pyridine was purified and dried by distillation over sodium hydroxide. Fiber Modification. 3,3,3-Trifluoropropanoyl chloride was prepared by the reaction of the 3,3,3-trifluoropropionic acid with thionyl chloride. A 1 equiv amount of 3,3,3-trifluoropropionic acid was placed in a 25 mL round-bottom flask, and then 1.1 equiv of thionyl chloride was added dropwise under magnetic stirring. The mixture was then refluxed at 100 °C for 3 h. The condenser was connected to a washing bottle, filled with a concentrated aqueous sodium hydroxide solution, through a glass tube packed with activated silica gel. At the end of the reaction, the excess of thionyl chloride was removed by vacuum evaporation at room temperature. The ensuing TFP was used directly to esterify the cellulose fibers in a heterogeneous medium. Thus, dry toluene, pyridine (1 equiv), and finally cellulose (1 equiv of OH functions) were added to it in a nitrogen atmosphere. Reactions were conducted under magnetic stirring at different temperatures (room temperature, 50, 65, 80, 90, and 100 °C) and times (2 and 15 h). The esterified fibers were filtered and sequentially washed with dichloromethane, acetone, ethanol, and again with acetone and dichloromethane before being submitted to a Soxhlet extraction with dichloromethane for 12 h and dried at 60 °C for 24 h. Hydrolitic Stability Evaluation. Aliquots (∼50 mg) of the modified fibers (treated for 2 h at 80 °C) were placed in an Erlenmeyer flask containing 20 mL of water, and the ensuing suspension was stirred for different times (1-96 h). These experiments were also carried out without stirring (1-8 days) and under alkaline conditions, at pH 9 and pH 12 (2-96 h) at room temperature, and at pH 9 (2-96 (14) Nystrom, D.; Lindqvist, J.; Ostmark, E.; Hult, A. Malmstrom, E. Superhydrophobic bio-fibre surfaces via tailored grafting architecture. Chem. Commun. (Cambridge) 2006, 3594-3596. (15) Cunha, A. G.; Freire, C. S. R.; Silvestre, A. J. D.; Neto, C. P.; Gandini, A.; Orblin, E.; Fardim, P. Highly hydrophobic biopolymers prepared by the surface pentafluorobenzoylation of cellulose substrates. Biomacromolecules 2007, 8, 1347-1352. (16) Winter, G.; Scott, J. M. W. Studies in solvolysis. I. Neutral hydrolysis of some alkyl trifluoroacetates in water and deuterium oxide. Can. J. Chem. 1968, 46 (18), 2887-2894. (17) Cunha, A. G.; Freire, C. S. R.; Silvestre, A. J. D.; Neto, C. P.; Gandini, A. Unpublished work.
Cunha et al. h) at 50 °C. The progress of the hydrolysis was followed by Fourier transfom infrared (FTIR) spectroscopy and contact angle measurements. Modified Fibers Characterization. The degree of substitution (DS) of the modified fibers, i.e., the number of trifluoropropanoate groups per cellulose monosaccharide unit, was calculated from their fluorine content (obtained in duplicate at the CNRS “Service Central d’Analyze”, Vernaison, France) using the following equation: DS )
162(%F) 5700 - 110(%F)
where 162 ) Mw(anhydroglucose unit), 5700 ) [3Mw(F)] × 100, and 110 ) Mw(CF3CH2CO) - 1. The FTIR spectra were recorded with a Bru¨cker IFS FTIR spectrometer equipped with a single horizontal Golden Gate ATR cell. For the X-ray diffraction (XRD) measurements, the fibers were pressed into small pellets using a laboratory press and analyzed with a Phillips X’pert MPD diffractometer using Cu KR radiation. The thermogravimetric assays were carried out with a Shimadzu TGA 50 analyzer equipped with a platinum cell. Samples were heated at a constant rate of 10 °C/min from around 22 °C up to 800 °C, under a nitrogen flow of 20 mL/min. The thermal decomposition temperature was taken at the onset of significant (g0.5%) weight loss from the heated sample, after the moisture loss. Contact angles with water and diiodomethane were measured with a “Surface Energy Evaluation System” commercialized by Brno University (Czech Republic). Each θ value was the average of 5-10 determinations. These values were used to calculate the dispersive and polar contributions to the surface energy of the fibers, using the Owens-Wendt’s approach.18 X-ray photoelectron spectra of pulp hand sheet surfaces were obtained with a Physical Electronics PHI Quantum 2000 ESCA instrument equipped with a monochromatic Al KR X-ray source and operated at 25 W with a combination of an electron flood gun and ion bombarding for charge compensation. The takeoff angle was 45° in relation to the sample surface. The analyzed area was 100 × 100 µm. At least 3 different spots were analyzed on each sample. A Gaussian curve fitting program was used to treat the C1s signal and the following binding energies, relative to the C-C position at 285 eV, were employed for the relevant moieties: 1.7 ( 0.2 eV for C-O, 3.1 ( 0.3 eV for O-C-O or CdO, 4.6 ( 0.3 eV for OdC-O, and 8.4 ( 0.3 eV for C-F. Secondary ion mass spectra were recorded using a Physical Eletronics ToF-SIMS TRIFT II spectrometer. A primary ion beam of 69Ga+ liquid metal ion source (LMIS), with a 15 kV applied voltage, a 600 pA aperture current, and a bunched pulse width of 20 ns, was used in both positive and negative modes. A raster size of 200 × 200 µm was scanned, and at least three different spots were analyzed. The surface distribution of the trifluoropropanoyl moieties was obtained with the best spatial resolution using the ion gun operating at 25 kV, 600 pA aperture current, and an unbunched pulse width of 20 ns. The spectra were acquired for 6 min with a fluency of ∼1012 ions/cm2, ensuring static conditions. Charge compensation was achieved with an electron flood gun pulsed out of phase with respect to the ion gun. 13C solid-state cross-polarized magic-angle spinning nuclear magnetic resonance (13C CP-MAS NMR) spectra were recorded on a Bruker Avance 400 spectrometer. The samples were packed into a zirconia rotor sealed with Kel-F caps and spun at 7 kHz. The acquisition parameters were as follows: 4 µs 90° pulse width, 2 ms contact time, and 4 s dead time delay.
Results and Discussion Cellulose fibers were modified with 3,3,3-trifluoropropanoyl chloride at several reaction times and temperatures with the intent (18) Owens, D. K.; Wendt, R. C. Estimation of surface free energy of polymers. J. Appl. Polym. Sci. 1969, 13 (8), 1741-1747.
Bi-phobic Cellulose Fiber DeriVatiVes with TFP
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Figure 1. FTIR spectra of plant cellulose fibers before and after trifluoropropanoylation at different reaction temperatures and times.
of studying the effect of these reaction parameters on the extent of esterification and hence on the properties of the modified fibers. The occurrence of the reactions, as well as their progress, was assessed by FTIR spectroscopy, through the monitoring of a new carbonyl ester band at 1760 cm-1 (Figure 1). Moreover, the emergence of new absorptions in the range of 1000-1500 cm-1, characteristic of C-F stretching modes,19 corroborated the presence of fluorine-containing moieties. These results showed that while the reaction time had only a slight effect on the extent of esterification, the temperature played a key role, particularly above 65 °C (Figure 1). For the characterization studies, six samples were selected, namely, the cellulose fibers and the filter papers treated for 2 h at room temperature (RT), 65 and 100 °C, which gave DS values of
