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Regeneration and Hydroxyl Accessibility of Cellulose in Ultrathin Films V. Buchholz,† P. Adler,‡ M. Ba¨cker,‡ W. Ho¨lle,‡ A. Simon,‡ and G. Wegner*,† Max-Planck-Institut fu¨ r Polymerforschung, Ackermannweg 10, 55128 Mainz, Germany, and Max-Planck-Institut fu¨ r Festko¨ rperforschung, Heisenbergstrasse 1, 70569 Stuttgart, Germany Received January 28, 1997. In Final Form: April 3, 1997X
The regeneration of LB films of (trimethylsilyl)cellulose (TMSC) (1) to ultrathin films of cellulose (2) is monitored by X-ray photoelectron spectroscopy (XPS). Subsequent chemical derivatization of these regenerated cellulose films with trifluoroacetic anhydride (TFAA) (3) to give the corresponding cellulose trifluoroacetate (5) was proven and followed by XPS as a function of exposure time in order to study hydroxyl accessibility. The derivatization behavior of regenerated cellulose films is compared with literature data on bulk cellulose materials with varying degrees of crystallinity. In contrast to the latter, the functionalization kinetics of the regenerated cellulose films indicates a low hydroxyl accessibility in conjuction with a high degree of order in these films.
Introduction The silylated polysaccharide (trimethylsilyl)cellulose (TMSC) (1) contains a stiff cellulose backbone and short hydrophobic TMS side groups and hence belongs to a class of polymers referred to as hairy-rod molecules.1,2 As shown in previous studies, homogeneous Langmuir-Blodgett (LB) films of TMSC with well-defined supramolecular structures and thicknesses (9.9 Å per layer) can be prepared.3,4 Moreover, in-situ conversion of these TMSC films by exposure to gaseous, wet HCl yields regenerated ultrathin films of pure cellulose 2 possessing a spacing of 4.2 Å per layer without destruction of the film homogeneity.3-5
The polymer backbones in the LB films of TMSC and the corresponding cellulose films are oriented parallel to the dipping direction.4 Thin film architectures of cellulose obtained from TMSC LB films by the method described above provide a model system for adsorption processes, as evidenced by adsorption studies with dyes and polyelectrolytes.3,5 Following the regeneration process, chemical reactions with the free hydroxyl groups of the cellulose introduce new functionalities into the film and therefore afford the possibility of altering its properties. This approach was demonstrated through the derivatization of ultrathin cellulose films with succinic anhydride, leading to the corresponding cellulose succinate.3,5 * To whom correspondence should be addressed. † Max-Planck-Institut fu ¨ r Polymerforschung. ‡ Max-Planck-Institut fu ¨ r Festko¨rperforschung. X Abstract published in Advance ACS Abstracts, May 15, 1997. (1) Wegner, G. Ber. Bunsen-Ges. Phys. Chem. 1991, 95, 1326. (2) Wegner, G. Mol. Cryst. Liq. Cryst. 1993, 235, 1. (3) Buchholz, V.; Wegner, G.; Stemme, S.; O ¨ dberg, L. Adv. Mater. 1996, 8, 399. (4) Schaub, M.; Wenz, G.; Wegner, G.; Stein, A.; Klemm, D. Adv. Mater. 1993, 5, 919. (5) Buchholz, V. Diplomarbeit, Heinrich-Heine-Universita¨t Du¨sseldorf, 1995.
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With respect to the behavior of bulk cellulose, the questions arise if and to what extent hydrogen bonding between polymer chains is a decisive factor for the reactivity of cellulose in ultrathin films. Due to these strong hydrogen bonds, cellulose normally exhibits a microstructure containing regions that are more crystalline or more amorphous, respectively.6 The accessibility of the hydroxyl groups, and thus their reactivity, were reported to be a function of the degree of cellulose crystallinity.7 In order to determine the hydroxyl accessibility in a range of cellulosic materials, Tasker and coworkers used chemical derivatization of cellulose with trifluoroacetic anhydride (TFAA) (3) in conjunction with X-ray photoelectron spectroscopy.6 In contrast to previous techniques used for determining hydroxyl accessibility in cellulose, the derivatization with TFAA as a gas phase reagent provides a solvent-free labeling method which is far less likely to suffer from swelling effects.6 The anhydride 3 reacts exclusively with the hydroxyl groups
of the cellulose 2 to give the corresponding cellulose trifluoroacetate (5) and generates trifluoroacetic acid (4) as a volatile byproduct that is easily removed under vacuum conditions.6,8 Most noteworthy is that the reaction can be easily quantified by using X-ray photoelectron spectroscopy (XPS), since the CF3 and ester groups exhibit a large C 1s core level shift toward higher binding energy due to the high electronegativity of fluorine.9-12 (6) Tasker, S.; Badyal, J. P. S.; Backson, S. C. E.; Richards, R. W. Polymer 1994, 35, 4717. (7) Jeffries, R.; Jones, D. M.; Roberts, J. G.; Selby, K.; Simmens, S. C.; Warwicker, J. O. Cellulose Chem. Technol. 1969, 3, 255. (8) Gerenser, L. J.; Elman, J. F.; Mason, M. G.; Pochan, J. M. Polymer 1985, 26, 1162. (9) Dickie, R. A.; Hammond, J. S.; DeVries, J. E.; Holubka, J. W. Anal. Chem. 1982, 54, 2045. (10) Chilkoti, A.; Castner, D. G.; Ratner, B. D.; Briggs, D. J. Vac. Sci. Technol. 1990, A8, 2274. (11) Ameen, A. P.; Ward, R. J.; Short, R. D.; Beamson, G.; Briggs, D. Polymer 1993, 34, 1795.
© 1997 American Chemical Society
Regeneration of Cellulose in Ultrathin Films
Just like bulk cellulose samples, it should also be possible to functionalize ultrathin cellulose films obtained from the regeneration of TMSC LB films with TFAA to yield the corresponding cellulose trifluoroacetate. According to the kinetics of this labeling reaction, conclusions could be drawn to compare the hydroxyl accessibility in these films to that of bulk samples.
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a
Experimental Section (Trimethylsilyl)cellulose (1) with a number average degree of polymerization, DP, of 150 and a degree of substitution, DS, of 2.73 was synthesized according to a procedure described elsewhere.13 The transfer conditions for the film-building process and the regeneration method are also given in the literature (typical regeneration time: 30 s).4,5,14 TMSC LB films of variable thickness were deposited on silicon wafers (Wacker) that were hydrophobized with ammonium fluoride etchant (Selectipur, Merck). The freshly regenerated cellulose films were dried under vacuum to ensure that the reaction products were completely removed. TFAA (3) (Fluka, >99% purity; renewed for each reaction) was used as the labeling reagent. The specially designed TFAA labeling apparatus and the corresponding reaction procedure were identical to those of Tasker and co-workers.6 The reaction apparatus and the samples were handled under argon to eliminate humidity effects. After exposure to TFAA, the samples were kept under vacuum to remove the byproduct. The reaction profile was compiled by varying the exposure time to TFAA, followed by XPS quantification of hydroxyl functionalization.6 XPS analysis was carried out with a Leybold-Heraeus X-ray photoelectron spectrometer equipped with an EA 200 hemispherical electron analyzer, operating at a pass energy of 198 and 44 eV for survey and detail spectra, respectively. For survey spectra, Mg KR (1253.6 eV) and Al KR (1486.6 eV) radiation were used as the photoexcitation source; for detail spectra, only Mg KR radiation was employed. The silicon wafers were mounted with conducting adhesive films (Leit-Tabs) onto the sample holder. On average, the vacuum in the spectrometer was better than 6 × 10-10 mbar. All spectra are referenced to the C 1s binding energy of the adventitious hydrocarbon at 284.8 eV (CxHy). The samples investigated showed charging effects between 1 and 2.9 eV. C 1s photoelectron specta were fit by a superposition of lines with mixed Gaussian and Lorentzian character (Microcal Origin, Version 4.00; Microcal Software, Inc.).
b
Results and Discussion Regeneration of a TMSC LB Film to an Ultrathin Film of Cellulose. Parts a and b of Figure 1 show XPS survey and C 1s detail spectra for a TMSC LB film (1) of 100 layers, and the corresponding spectra after regeneration to pure cellulose 2. In addition to the Auger and 1s signals for carbon and oxygen atoms that are present for both samples, the survey spectrum for TMSC (1) also contains Si 2s and Si 2p peaks corresponding to the TMS side groups. Moreover, the C 1s spectra in Figure 1b reveal the different carbon species present. In both cases there are five carbon atoms (C2 through C6) singly bonded to one oxygen atom (-C-O-), and one bridging carbon atom (C1) connected to two oxygen atoms (-O-C-O-).12 The signal for the latter is shifted to higher binding energies with respect to the former, but the signals almost completely overlap. Obviously, the C 1s spectrum for TMSC (1) shows an additional carbon signal that is assigned to the methyl carbon atoms of the TMS groups. As the degree of substitution (DS) is known to be 2.73, the ratio of backbone to TMS carbon atoms is 6:(2.73 × 3) ≈ (12) Beamson, G.; Briggs, D. High Resolution XPS of Organic Polymers. The Scienta ESCA 300 Database; John Wiley: Chichester, U.K., 1992; p 236. (13) Stein, A. Ph.D. Thesis, Friedrich-Schiller-Universita¨t Jena, 1991. (14) Schaub, M. Ph.D. Thesis, Johannes-Gutenberg-Universita¨t Mainz, 1993.
Figure 1. (a) XPS (Mg KR) survey spectra for a TMSC LB film (1) (top) and an ultrathin cellulose film (2) after regeneration (bottom). Auger lines (A) and core level peaks are indicated. Both films are multilayer systems of 100 layers. (b) C 1s core level spectra for a TMSC LB film (1) (top) and the corresponding regenerated cellulose film (2) (bottom) (100 layers each).
3:4, which is in qualitative agreement with the intensity ratio of the signals (Figure 1b, top). However, it is also noted that the spectrum of 2 reveals an additional C 1s signal at a binding energy that is higher than that for the TMS signal, but lower than that for the -C-O- signal. This band is attributed to adventitious carbon contamination (CxHy) arising from the spectrometer and was used for calibrating the binding energy scale. Especially noteworthy is that the spectra confirm the regeneration process to be complete after exposure to gaseous HCl for 30 s and that all byproducts were completely removed. This is clearly demonstrated by the disappearance of the silicon signals in the survey spectra and the TMS signal in the C 1s region after regeneration (see Figure 1a,b, bottom). These results support previous investigations using polarized transmission infrared spectroscopy, X-ray reflectometry, and surface plasmon resonance (SPR).3,5 Experiments with a varying number
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Figure 3. Rate of hydroxyl functionalization by TFAA in ultrathin cellulose films (100 layers) as determined by XPS (open squares). For comparison, bulk data from Tasker and co-workers for Avicell (solid circles) and bacterial cellulose (solid diamonds) are included.6
carbon atoms can be assigned to the carbon atoms of the trifluoroacetate group (>CdO) and the trifluoromethyl group (-CF3), respectively.12 The percentage of labeled hydroxyl groups can be determined from the peak areas using the following equation:6 % labeling =
Figure 2. (a) XPS (Mg KR) survey spectra for a 91% TFAA labeled cellulose film (5) (top) and for a pure cellulose film (2) (bottom). Auger lines (A) and core level peaks are indicated. Both films are multilayer systems of 100 layers. (b) C 1s core level spectra for multilayer systems of 100 cellulose layers exposed to TFAA for different times to yield cellulose trifluoroacetate (5): 30 min exposure gives 12% (top), and 2600 min exposure leads to 91% hydroxyl functionalization. The solid lines represent fits of the experimental data (dashed line). See text for further details.
of cellulose layers reveal that for these multilayer systems the Si signals due to ionization of the underlying silicon wafer start to vanish once the number of cellulose layers exceeds 50, corresponding to a total thickness of 210 Å. Derivatization of cellulose with TFAA. XPS spectra of pure and labeled cellulose films of 100 layers are shown in Figure 2a,b. In the former figure, a survey spectrum of a pure cellulose film 2 is compared with one that has been exposed to TFAA for 2600 min (5). In the spectrum of the labeled cellulose film, F 1s and F Auger signals are observed arising from the trifluoroacetate groups. Figure 2b shows two C 1s spectra for different reaction times with TFAA. The two additional C 1s signals shifted to higher binding energies with respect to the backbone
area (–CF3) + area (>C O) area (–C O–) + area (–O C O–)
x 100
Thus, the two spectra in Figure 2b represent labeling rates of 12% and 91%, employing exposure times of 30 and 2600 min, respectively. In XPS, a shift in binding energy reflects changes in the chemical bonding of the various atoms. Here, the binding energy of three backbone carbon atoms increases with increasing hydroxyl functionalization. For example, referring to the samples of Figure 2b, the binding energy of the signal due to the -C-O- backbone carbon atoms shifts from 286.7 eV for the 12% labeled sample to 287.4 eV for the 91% functionalized sample. This increase is due to the strong electronegative effect of the trifluoroacetate groups that causes the signal for those -C-Obackbone carbon atoms that are bonded to these groups to move to higher binding energies (-I effect). The chemically different -C-O- carbon atoms are, however, not resolved in the spectra. The observation of the overall -C-O- shift confirms that the cellulose is really functionalized with trifluoroacetate groups. The esterification reaction could also be proven using polarized transmission infrared spectroscopy that reveals additional signals for the trifluoroacetate groups.5 In Figure 3, the extent of hydroxyl derivatization (% labeling) is plotted against the exposure time. For comparison, this figure also contains two examples for the derivatization behavior of bulk cellulose samples (Avicell [Fluka] and bacterial cellulose [BPS Separations Ltd.]).6 The highest functionalization achieved for ultrathin cellulose films was around 91% of the maximum theoretical value using 2600 min exposure time. Compared to bulk cellulose samples, the degree of hydroxyl functionalization is higher, because the latter never exceed a limiting level of approximately 80%.6 Moreover, bulk cellulose samples are functionalized much quicker than the regenerated cellulose in ultrathin films. For example, the longest time for maximum derivatization of a bulk cellulose sample is only 1500 min, representing Avicell cellulose with a crystallinity ratio of 0.85, as determined by X-ray diffraction (see Figure 3 for comparison).6 As put forward by Tasker and co-workers, the hydroxyl accessibility is dependent upon the strength of hydrogen bonding.6 It was concluded that the ease with which a hydroxyl center in a cellulosic material can be derivatized
Regeneration of Cellulose in Ultrathin Films
(e.g., with TFAA) decreases with increasing degree of cellulose crystallinity.6 Thus, the less crystalline or less close packed the sample, the easier the derivatization, and the shorter the period required to reach the maximum degree of derivatization. In the case of cellulose ultrathin films, it is concluded from the functionalization behavior described above that the degree of order in the film must be higher than in the bulk cellulose samples investigated
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previously,6 because the latter are much more readily functionalized. Acknowledgment. The authors wish to express their thanks to T. Jaworek and Dr. A. Esker for helpful advice and valuable discussions. LA970085Q
