pubs.acs.org/Langmuir © 2009 American Chemical Society
Polyvinylamine-Phenylboronic Acid Adhesion to Cellulose Hydrogel Wei Chen,† Vincent Leung,† Hubertus Kroener,‡ and Robert Pelton*,† †
Department of Chemical Engineering, McMaster University, Hamilton, Ontario L8S 4L7, Canada, and ‡ BASF Aktiengesellschaft, Ludwigshafen 67056, Germany Received January 12, 2009. Revised Manuscript Received March 5, 2009
Pairs of wet, regenerated cellulose films were laminated with polyvinylamine derivatized with phenylboronic acid (PVAm-PBA), and the forces required to delaminate the never-dried laminates were measured as functions of adhesive structure and application conditions. The greatest wet adhesion was obtained with 150 kDa PVAm, with 16% of the amines bearing phenylboronic moieties. The pH at which the PVAm-PBA was adsorbed onto the cellulose was the dominant process parameter. High adsorption pH gave high concentrations of adsorbed adhesive and the maximum adhesion. The specific role of the phenyl boronic groups was illustrated two ways: (a) replacing the B(OH)2 with OH (i.e., phenol) gave much lower adhesion, and (b) wet adhesion was greatly reduced by the presence of sorbitol, which effectively competes with cellulose for boronate-binding sites.
Introduction Tissue repair adhesives in surgery and the manufacture of paper are vastly different technologies that share a common need: instantaneous adhesion between wet, hydrophilic surfaces. In papermaking, water-swollen, wet paper webs are pulled through the papermachine at expressway speeds, causing weak webs to break and disrupting production.1 One potential solution is the use of polymeric adhesives that promote instantaneous adhesion between water-soaked cellulose fibers. Commercial polymers, called wet-strength resins, are available to strengthen wet paper and are used in applications, such as packaging and coffee filters. However, wet-strength resins do not improve strength until the wet paper web passes through the papermachine drying section, where the resins react to form various types of covalent bonds.2 Thus, conventional wet-strength polymers do not give instantaneous wet adhesion. Cationic water-soluble polymers spontaneously adsorb onto wet cellulose surfaces, owing to the presence of anionic carboxyl groups on cellulose surfaces.3 However, polymer adsorption does not guarantee strong adhesive joints. For example, we have shown that cationic polyacrylamides, poly(diallyldimethyl ammonium chloride),4 polypeptides,5 and proteins6,7 adsorb onto cellulose but do not give significant wet adhesion to cellulose. An early promising candidate was polyvinylamine (PVAm). We have shown that PVAm does strengthen cellulose-cellulose joints without a heating/curing step. However, it was necessary to nearly dry the joint to achieve a strong cellulose-cellulose adhesion when the joint was subsequently immersed in water. We proposed that, when most of the free water was removed, the *To whom correspondence should be addressed. E-mail: peltonrh@ mcmaster.ca. (1) Gurnagul, N.; Seth, R. S. Pulp Pap. Can. 1997, 98(9), 44–48. (2) Espy, H. H. Tappi J. 1995, 78(4), 90–99. (3) Wagberg, L.; Odberg, L. Nord. Pulp Pap. Res. J. 1989, 4(2), 135–140. (4) Feng, X.; Pouw, K.; Leung, V.; Pelton, R. Biomacromolecules 2007, 8(7), 2161–2166. (5) Kurosu, K.; Pelton, R. J. Pulp Pap. Sci. 2004, 30(8), 228–232. (6) Li, X.; Pelton, R. Ind. Eng. Chem. Res. 2005, 44(19), 7398–7404. (7) Su, S. X.; Pelton, R. Cellulose 2006, 13(5), 537–545. (8) DiFlavio, J. L.; Bertoia, R.; Pelton, R.; Leduc, M. The mechanism of polyvinylamine wet-strengthening. Proceedings of the 13th Fundamental Research Symposium, Cambridge, U.K., 2005; pp 1293-1316.
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primary amine groups reacted with hemiacetal groups in the regenerated cellulose films to give imine and aminal linkages.8 In a research note in 2006, we reported that phenylboronic acid derivatives of polyvinylamine (PVAm-PBA; see structure in Figure 1) give instantaneous adhesion between wet cellulose films without a drying or heating step.9 This approach was inspired by the well-documented ability of phenylboronic acid groups to condense with carbohydrates to give covalent rings (see Figure 2). Details of the reactions of boric and boronic acids with carbohydrates have been extensively studied and key conclusions include: (1) Boronate-carbohydrate reactions are promoted by alkaline pH, when the boronic acid is ionized.10 However, the presence of nearby amine groups promotes bond formation at much lower pH values.11 (2) Borate binding is sensitive to the detailed structure of the carbohydrate. Some carbohydrates form relatively strong bonds (e.g., guar12 or glucose13), whereas other carbohydrates give weak (e.g., dextran14) or no borate binding (e.g., 1-o-methylglucopyranose15). In the case of cellulose, it is not entirely clear which functional groups bond to boronate. We will address this issue in a future paper, describing results from a variety of model compounds and polymers. (3) Despite the covalent appearance of the borate-carbohydrate structure in Figure 2, these bonds are weak. The binding constants typically range from 10 to 5000 L/mol, which are values more typical of hydrogen bonds than covalent ones.10 This paper documents the roles of PVAm-PBA composition, the pH during the cellulose-cellulose joint formation process, and the pH during the joint testing. In addition, we show that the presence of sorbitol, a sugar that strongly binds to phenylboronic acid, greatly decreases wet adhesion in the presence of PVAmPBA. Our adhesion measurements involve measuring the delamination force required to separate wet, regenerated cellulose (9) Chen, W.; Lu, C.; Pelton, R. Biomacromolecules 2006, 7(3), 701–702. (10) Springsteen, G.; Wang, B. H. Tetrahedron 2002, 58(26), 5291–5300. (11) Zhu, L.; Shabbir, S. H.; Gray, M.; Lynch, V. M.; Sorey, S.; Anslyn, E. V. J. Am. Chem. Soc. 2006, 128(4), 1222–1232. (12) Jasinski, R.; Redwine, D.; Rose, G. J. Polym. Sci., Part B: Polym. Phys. 1996, 34(8), 1477–1488. (13) Norrild, J. C.; Eggert, H. J. Am. Chem. Soc. 1995, 117(5), 1479–1484. (14) Shao, C. Y.; Miyazaki, Y.; Matsuoka, S.; Yoshimura, K.; Sakashita, H. Macromolecules 2000, 33(1), 19–25. (15) Nicholls, M. P.; Paul, P. K. C. Org. Biomol. Chem. 2004, 2(10), 1434–1441.
Published on Web 04/08/2009
DOI: 10.1021/la900131g
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Chen et al. Table 1. Polyvinylamine Derivativesa sample name
degree of substitution (%)
molecular weight (kDa)
PVAm-PBA-1 4 15 PVAm-PBA-4 4 150 PVAm-PBA-5 16 150 PVAm-PBA-6 29 150 PVAm-PBA-7 51 150 PVAm-PBA-8 4 34 PVAm-PBA-9 4 1500 PVAm-Ph-1 4 150 PVAm-Ph-2 17 150 PVAm-Ph-3 24 150 PVAm-Ph-4 47 150 a The corresponding structures are shown in Figure 1.
Figure 1. Polyvinylamine derivatization.
Figure 2. Reaction of phenylboronic acid with carbohydrates.
films laminated with the polymer of interest. In a companion study, atomic force microscopy was used to characterize the energetics of bond formation between two cellulose surfaces coated with PVAm-PBA.16 That work showed that the approach of two PVAm-PBA-coated surfaces was repulsive.
Experimental Section Materials. Commercial polyvinylamine (PVAm) with viscosity molecular weights of 15, 34, 150, and 1500 kDa was obtained from BASF, Ludwigshafen. The PVAm samples were purified and derivatized to give pendant phenylboronic acid groups (PVAm-PBA) or pendant phenol groups (PVAm-Ph) using carbodiimide-activated condensation with carboxyls. The structures are shown in Figure 1, and the properties are summarized in Table 1. Details about the preparation, purification, and phase behavior of the PVAm derivatives were recently published.17 Regenerated cellulose dialysis tubing (Spectra/Por 4 product 132682, 12 kDa MWCO, Spectrum Laboratories, Inc.) was cut to give top (2 6 cm) and bottom (3 6 cm) membranes. The long axis of the membrane strips corresponded to the lengthwise dimension of the original cellulose tubing. Only the interior surface of the tubing was used to form adhesive joints. The cellulose membranes were boiled for 2 h to remove any preservatives (glycerin). The cellulose films were then rinsed thoroughly and stored in water with a small amount of methanol (to prevent spoilage) at 4 °C. The cleaned membranes contained approximately 50% (w/w) water and had an average roughness value of 5 nm dry and 50 nm wet.18 Wet Laminate Fabrication and Testing. Cellulose/PVAmxx/cellulose laminates were prepared using two different methods for treating the cellulose membranes with the polyvinylamine derivatives: direct application and adsorption application.5,8 (16) Notley, S.; Chen, W.; Pelton, R. Langmuir 2009, in press. (17) Chen, W.; Pelton, R.; Leung, V. Macromolecules 2009, 42(4), 1300–1305. (18) DiFlavio, J. L.; Pelton, R.; Leduc, M.; Champ, S.; Essig, M.; Frechen, T. Cellulose 2007, 14(3), 257–268.
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(a) Direct Adhesive Application. In the direct application method, the bottom cellulose membrane was placed on a polished stainless-steel disk (TAPPI standard drying plate), and excess surface water was removed by dabbing with a lintfree tissue. A 40 12.7 mm long piece of Teflon tape (G.F. Thompson Co. Ltd., TWB480P) was placed across one end of the bottom membrane. The purpose of the Teflon tape, which was removed before peel testing, was to give two cellulose film tails for attachment to Instron clamps and to give a uniform crack in the laminate. In the next step, a 15 μL drop of aqueous PVAm derivative was applied using a 20 μL micropipet (Gilson) onto the base membrane near the Teflon tape. The top membrane was progressively placed over the bottom membrane starting at the end of the Teflon tape. If performed carefully, the 15 μL polymer solution droplet uniformly spread between the top and bottom membrane with negligible loss of polymer solution. The laminate was placed between two TAPPI standard blotters and lightly pressed (49 N) for 5 min. The wet laminate was sealed in a Ziplock sandwich bag, to prevent moisture loss, and was further pressed with a force of 88 kN for 3 min at room temperature. The delamination forces were measured immediately after pressing. A 90° peeling apparatus was used to measure the peel delamination forces using an extensional rate of 20 mm/min. Our apparatus and data analysis were described in detail previously.5,8 (b) Adsorption Application. The pH and ionic strength of PVAm-PBA or PVAm-Ph solutions, typically 0.1 g/L, were adjusted to give the desired “adsorption pH”. Pairs of wet membranes were immersed in the adsorption solutions for 10 min and then immersed in a polymer-free salt rinse solution at the same pH as the adsorption solution. Finally, the polymercoated membranes were soaked in a “final solution”, whose pH could differ from the “adsorption solution”. The laminates were assembled on polished stainless-steel disks, and the wet laminates were carefully transferred between two blotters and pressed at 88 kN for 3 min at room temperature. The wet laminates were immediately tested using the procedure described above. Adhesion versus Water Content. Experiments were conducted to determine the influence of laminate moisture content on adhesion. Membranes were treated with polymers using the adsorption method. The wet laminates were assembled in the usual way and then placed on top of dry, lint-free tissues. The laminates and tissues were then sealed in a Ziplock sandwich bag that was then pressed with a force of 88 kN for 3 min at room temperature. The water contents of the laminates were measured gravimetrically. The water contents in the laminates after pressing varied inversely with the number of tissues placed in the sandwich bag. Measuring PVAm-PBA Adsorption onto Cellulose. 125I was grafted onto PVAm-PBA by using the iodogen method.19 (19) Korde, A.; Venkatesh, M.; Sarma, H. D.; Pillai, M. R. A. J. Radioanal. Nucl. Chem. 2000, 246(1), 173–178.
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The unbound 125I was removed by dialysis against water for 3 days. A total of 0.5 mL of 0.5 g/L labeled PVAm-PBA was mixed with 125 mL of 0.1 g/L PVAm-PBA solution. The “adsorption application” method, described above, was used to apply labeled PVAm-PBA to the wet cellulose; however, no pressing and peeling procedures were performed. Instead, the treated cellulose membranes with labeled polymers were placed in measurement vials, and the radioactivity was measured with a WizardTM 300 1480 Automatic Gamma Counter (Perkin-Elmer Life Sciences). The radioactivity of 500 μL aliquot of the above PVAm-PBA solution was counted and used as a control to quantify the adsorption amounts of PVAm-PBAs on the cellulose membranes.
Results Polyvinylamine homopolymers were derivatized to give pendant phenylboronic acid or pendant phenol groups (see Figure 1). The polymer names and compositions are summarized in Table 1, where the degree of substitution is defined as the percentage of the PVAm nitrogen atoms with a substituent. The phenolic derivatives PVAm-Ph served as control polymers containing aromatic functionality without the boronic acid moiety. The solution properties of some of the polymers in Table 1 were reported previously.17 For pH values less than 10, the polymers were positively charged. The 150 kDa PVAm-PBA polymers tended to phase separate to form colloidal or macroscopic precipitates when the pH approached the isoelectric points. For example, Figure 3 shows the phase boundaries for two polymers used herein. The pH range over which the polymers phase-separated was sensitive to the degree of PBA substitution. PVAm-PBA-5 with 16% substitution phase-separated from pH 7.5 to 9.5,
Figure 3. Phase boundaries of PVAm-PBA as functions of pH and polymer concentration for two polymers. All measurements were made in 5 mM NaCl at 25 °C. The solutions were considered single phase if the transmittance at 600 nm was higher than 99%. The colloidally dispersed regions corresponded to transmittance values between 90 and 99%, whereas more turbid solutions contained macroscopic precipitates, which settled quickly.17 Langmuir 2009, 25(12), 6863–6868
whereas PVAm-PBA-7 with 51% substitution was phase-separated over most of the pH range.17 Although we are ultimately interested in wet adhesion between cellulose surfaces in fiber networks during papermaking, in this work, we employed model experiments in which two wet, regenerated cellulose films were laminated with a layer of polymer serving as the adhesive. Two methods were used to treat the cellulose films with polymer before lamination: direct application and adsorption. With direct application, a known amount of polymer solution is spread between the membranes before lamination. In contrast, in the adsorption method, the cellulose films are soaked in polymer solution and then rinsed before lamination. In both cases, the pH and ionic strength are carefully controlled. The advantage of the direction application method is that the adhesive concentration in the joint is controlled and known. In addition, non-adsorbing polymers and very thick adhesive layers can be employed. The advantage of the adsorption method is that it mimics the papermaking processes, where cellulose fibers are saturated with adsorbed polymer before fiber-fiber bonds are formed. Results from the two polymer application methods are compared in Figure 4. The top figure shows the average delamination force for PVAm-PBA-xx and PVAm-Ph-xx as functions of the degree of substitution. The pH for polymer application and subsequent delamination was 10.5, and the polymer content of the joint was 30 mg/m2, which corresponds to 10 or more polymer layers. Supporting the results from our preliminary work,9 the phenylboronic acid derivatives gave much stronger adhesion than
Figure 4. Influence of the degree of substitution on adhesion. The parent PVAm had a molecular weight of 150 kDa. For the direct application experiments, the applied PVAm-PBA and PVAmPh were 0.1 wt %, with 0.03 M NaCl aqueous solution, at 25 °C. For the adsorption application, the applied PVAm-PBA and PVAm-Ph solutions were 0.01 wt %, with 0.03 M NaCl aqueous solution, at 25 °C. The error bars are the standard derivations of the mean based on four measurements. DOI: 10.1021/la900131g
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the phenolic derivatives or the starting PVAm. Maximum adhesion was obtained with PVAm-PBA-5, in which 16% of the amine groups supported a phenyl boronate moiety. The corresponding experiments in which films were treated by the adsorption method are shown in the lower set of curves in Figure 4. The major trends are the same: PVAm-PBA was superior to PVAm-Ph, and the optimum degree of substitution was 16%. The adsorption method gave almost twice the adhesion, although, as will be shown below, the adsorption method puts far less polymer in the joint. The laminate preparation procedure included a room-temperature pressing step, which squeezed the water out of the laminates. The influence of laminate water content on laminate strength is shown in Figure 5. With adsorbed PVAm-PBA-5, the adhesion was zero when the laminate water content was 60 wt %. However, adhesion increased geometrically as more and more water was squeezed out during the lamination process. In contrast, adhesion was very low over the whole range of water contents for laminates prepared without polymer or with the phenolic PVAm derivative, PVAm-Ph-2, The water content for our standard laminate preparations, which generated the results in the other graphs, was 44.3 ( 1.5 wt %. Figure 6 shows laminate wet strength as a function of the pH (labeled “adsorption pH”) at which PVAm-PBA-5 was adsorbed onto the cellulose. In this series of experiments, both the laminate formation and the delamination steps were conducted at a “final” pH of 5. In a parallel set of experiments, membranes were treated
Figure 7. Wet adhesion as a function of final pH. PVAm-PBA-5 applied was 0.01 wt %, with 0.03 M NaCl aqueous solution, at 25 °C. Plot A is the wet adhesion curves at different adsorption pH as functions of the final pH. The error bars are the standard derivations of the mean based on four measurements. Plot B is the contour plot of the delamination force against adsorption and final pH.
Figure 5. Influence of water content on wet delamination force. The polymers were applied using the adsorption application method using 0.1 wt % solutions in 0.03 M NaCl. The adsorption pH was 7.5, and the final pH was 9.5.
Figure 8. Effect of the molecular weight of PVAm-PBA on the adhesion strength. The membranes were treated with 0.5 wt % PVAm-PBA dissolved in 0.03 M NaCl at 25 °C. The error bars are the standard derivations of the mean based on four measurements.
Figure 6. Delamination peeling strength and maximum adsorbed amount of PVAm-PBA-5 onto cellulose membranes as functions of adsorption pH. The applied PVAm-PBA-5 was 0.01 wt %, with 0.03 M NaCl aqueous solution, at 25 °C. The error bars are the standard derivations of the mean based on four measurements. The inset shows adhesion as a function of the corresponding coverage of adsorbed PVAm-PBA-5. 6866 DOI: 10.1021/la900131g
with isotope-labeled PVAm-PBA, rinsed, and soaked at pH 5.9. The polymer content in the films was measured, and the results are also shown in Figure 6. A comparison of Figure 3 to Figure 6 reveals that, at pH 9.5, where the adsorbed amount was maximum, the polymer was adsorbed as a colloidal dispersion and not a true solution. The adsorption results mirror the adhesion data. In the inset, the delamination force is plotted against the corresponding estimate of the polymer content in the joints. At pH 9-10, the polymer solution was present as a colloidal dispersion of Langmuir 2009, 25(12), 6863–6868
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Article Table 2. Influence of the Salt Concentration on Wet Adhesiona
NaCl concentration (mol/L) 0 0.01 0.3 0.6 delamination force (N/m) 26.96 ( 3.58 27.85 ( 1.21 30.8 ( 1.35 31.49 ( 2.64 a The membranes were treated with 0.1 wt % PVAm-PBA-5 dissolved in 0.03 M NaCl at 25 °C to give a coverage of 30 mg/m2. The errors limits are the standard derivations of the mean based on four measurements.
Figure 9. Influence of sorbitol on PVAm-PBA adhesion to cellulose.
phase-separated polymer,17 giving high polymer contents in the joint and high wet adhesion strengths. To further clarify the interplay between the adsorption pH and the final pH at which the delamination tests were conducted, a series of experiments was conducted. The top of Figure 7 shows data lines at constant adsorption pH, whereas the bottom curve shows the same data converted to lines of constant wet adhesion mapped against the adsorption and final pH values. This contour plot was generated with Origin software (version 8.0). The adsorption pH was the most important variable; high wet strengths corresponded to adsorption pH values above 8. In contrast, wet adhesion strength was rather insensitive to the final pH. This is surprising because, as discussed in the Introduction, boronic acid chemistry is pH-sensitive. The influence of PVAm-PBA molecular weight is shown in Figure 8. The degree of substitution was low, 4%, and the application and testing pH values were 9.5. As commonly observed with adhesives, adhesion increased with the molecular weight. The influence of electrolyte concentration was evaluated, and the results are summarized in Table 2. The salt concentration had very little influence on adhesion strength, suggesting that nonspecific electrostatic interactions were not dominating. A body of evidence is growing to support the proposal that high wet adhesion with PVAm-PBA is in part due to boronate ester formation with cellulose. To test this hypothesis further, cellulose films with adsorbed PVAm-PBA-5 were soaked in sorbitol before lamination. Figure 9 shows that the presence of sorbitol greatly decreased adhesion with PVAm-PBA-5, suggesting that reactive phenylboronic acid sites were sequestered by ester formation with sorbitol. The binding constant for phenylboronic acid with sorbitol is 2000 L/mol,20 which is 40 times higher than the binding constant to cellobiose.21
Discussion The PVAm-PBA polymers are the first water-soluble polymers that we have found to strengthen never-dried cellulose/ PVAm-PBA/cellulose laminates. From an engineering perspec(20) Shiino, D.; Murata, Y.; Kataoka, K.; Koyama, Y.; Yokoyama, M.; Okano, T.; Sakurai, Y. Biomaterials 1994, 15(2), 121–128. (21) Bishop, M.; Shahid, N.; Yang, J.; Barron, A. R. Dalton Trans. 2004, No.17, 2621–2634.
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tive, this work has identified some key properties of the adhesive and application conditions: specifically, PVAm-PBA-5 adsorbed at high pH gave the strongest laminates. Figure 3 shows that the PVAm-PBA copolymers with 51% PBA substitution formed phase-separated suspensions over most of the pH range. In contrast, the copolymer with 16% PBA displayed a narrow phase separation range around pH 9. Thus, the poorer performance of the high DS copolymers may be linked to their poor water solubility. From a scientific perspective, perhaps the most surprising finding was that adhesion strength was rather insensitive to the pH and ionic strength of the laminates before testing (i.e., the final pH). This behavior is illustrated by the horizontal nature of the higher iso-adhesion lines in Figure 7. This is surprising because we anticipated that adhesion would be low for laminates soaked at low pH because we did not expect boronate ester formation with cellulose under these conditions. However, there are many reports in the literature supporting the idea that boronate esters can form under acidic conditions in an environment high in amine groups.11 For papermaking applications, the requirement for high pH during the adsorption step is not practical in most cases. However, the results in Figure 6 suggest the major contribution of high pH adsorption is to maximize the amount of adsorbed PVAm-PBA in the joint. Furthermore, as described in the Results, the high adsorbed amounts at pH 9.5 reflect the fact that the PVAm-PBA was present as a colloidal dispersion during the adsorption step at this pH. Other strategies for achieving high polymer concentrations in the joints, such as using microgels,22-24 will be described in a future paper. A common question in any adhesion experiment involves the locus of failure; for thick adhesive layers, it is often possible to distinguish cohesive failure (i.e., failure within the adhesive layer) versus adhesive failure (i.e., failure between the adhesive film and the substrate surface). With experiments involving the direct application of adhesive (Figures 4 and 8 and Table 2), there should exist a thin, distinct adhesive layer. The fact that these samples displayed significant wet adhesion suggests that PVAmPBA films had significant cohesive strength, perhaps because of ionic bonding between the ammonium and borate groups. For our experiments involving the adsorption application, the cellulose/PVAm-PBA/cellulose joint contains the equivalent of two saturated adsorbed layers; thus, there is no distinct, continuous adhesive layer for cohesive failure. A number of scientific issues remain to be resolved. Our crude macroscopic delamination measurements give no information about the energetics of joint formation. In addition, the literature is not clear as to whether or not boronate esters form with cellulose. In unpublished work, we have shown that PVAmPBA condenses with cellobiose, the sugar dimer repeat unit of cellulose. On the other hand, PVAm-PBA does not interact with hydroxyethyl cellulose. Thus, we propose that the phenylboronic acid groups condense only on the oxidizing end of cellulose chains. In the case of the regenerated cellulose used as a substrate in this work, the degree of polymerization is 250-500,25 which is low, ensuring many end groups for boronate binding. (22) Miao, C. W.; Pelton, R.; Chen, X. N.; Leduc, M. Appita J. 2007, 60(6), 465–468. (23) Miao, C.; Leduc, M.; Pelton, R. J. Pulp Pap. Sci. 2008, 34(1), 69–75. (24) Miao, C.; Chen, X.; Pelton, R. Ind. Eng. Chem. Res. 2007, 46(20), 6486–6493. (25) Klemm, D.; Heublein, B.; Fink, H. P.; Bohn, A. Angew. Chem., Int. Ed. 2005, 44(22), 3358–3393.
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Finally, there has been a resurgence in research on wood and other cellulosic materials as a source for fuel, chemicals, implantable biomaterials,26 and nanocomposites.27 One could imagine applications involving implantable biomaterials or nanocomposites where having a polymer giving instantaneous wet adhesion between cellulose and other tissues could be important. The combination of primary amine and phenylboronic groups seems particularly potent in terms of wet adhesion.
Conclusions (1) The phenylboronic acid groups are responsible for the instantaneous wet adhesion in cellulose/PVAm-PBA/cellulose laminates. Most of the advantages conferred by the phenylboronic acid groups were lost in the presence of sorbitol, which complexes with boronate. (2) The largest wet adhesion measurements occurred when cellulose films were exposed to PVAmPBA solutions, whose pH was 9.5. Under these conditions, the (26) Czaja, W. K.; Young, D. J.; Kawecki, M.; Brown, R. M. Biomacromolecules 2007, 8(1), 1–12. (27) Dufresne, A. Can. J. Chem. 2008, 86(6), 484–494.
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polymer was present for colloidally dispersed particles that deposited onto the cellulose to give the greatest coverage of polymeric adhesive. (3) Wet adhesion was surprisingly insensitive to the pH under which the laminates were tested. We propose that the high local concentration of ammonium ions on the PVAm backbone extend the ionization pH range of phenylboronic acid below pH 7, enabling borate-carbohydrate bonding under acid conditions. (4) Strong adhesion was observed with thick PVAmPBA films, suggesting good cohesion and adhesion. We propose that adhesion reflects the boronate ester formation with cellulose, whereas ionic bonding between ammonium and borate groups enhanced PVAm-PBA cohesion. (5) The highest delamination force was measured with 150 kDa PVAm, having 16% of the amine groups substituted with phenylboronic acid. Shorter chains or higher degrees of substitution gave weaker laminates. Acknowledgment. The authors acknowledge the Natural Science and Engineering Research Council of Canada and BASF Canada for financial support and Dr. Marc Leduc, BASF, for useful discussions. R.P. holds the Canada Research Chair in Interfacial Technologies.
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