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Polyvinylamine-graft-TEMPO Adsorbs onto, Oxidizes, and Covalently Bonds to Wet Cellulose Robert Pelton,*,† Pengchao Ren,† Jieyi Liu,† and Darijo Mijolovic‡ † ‡
Department of Chemical Engineering, McMaster University, Hamilton, Canada BASF Aktiengesellschaft, Ludwigshafen 67056, Germany ABSTRACT: Described is a new, greener approach to increasing adhesion between wet cellulose surfaces. Polyvinylamine (PVAm) with grafted TEMPO spontaneously adsorbs onto cellulose and oxidizes the C6 hydroxyl to aldehyde groups that react to form covalent bonds with primary amines on PVAm. Grafted TEMPO offers two important advantages over solutions of low-molecular-weight water-soluble TEMPO derivatives. First, the oxidation of porous cellulose wood fibers is restricted to the exterior surfaces accessible to high-molecularweight PVAm. Thus, fibers are not weakened by excessive oxidation of the interior fiber wall surfaces. The second advantage of tethered TEMPO is that the total dose of TEMPO required to oxidize dilute fiber suspensions is much less than that required by water-soluble TEMPO derivatives. PVAmTEMPO is stable under oxidizing conditions. The oxidation activity of the immobilized TEMPO was demonstrated by the conversion of methylglyoxal to pyruvic acid.
’ INTRODUCTION Cellulose, the most abundant organic chemical on earth, is a critical component of any strategy to move to renewable materials and fuels. One of the challenges for some applications is that cellulose is very hydrophilic; it adsorbs water and when wet it is difficult to form strong adhesive joints between cellulose surfaces. Our interests center on developing new, greener approaches to induce adhesion between wet cellulose surfaces. Much of the early literature describes improving fiber-fiber bond adhesion in wet paper. For example, Luner et al. showed in 1967 that the introduction of aldehydes (by strong, nonspecific oxidants) increased the wet strength of paper.1 Whether for an old technology like paper or for the latest cellulosic composites,2,3 the adhesion of wet cellulose to itself or another matrix, such as hydrophobic plastic or hydrophilic biological tissue, is important. We showed in 2004 that polyvinylamine (PVAm) is a good adhesive for wet cellulose films if the carbohydrate surface is activated by the introduction of aldehyde groups.4 In subsequent work, we proposed that the primary amines on PVAm react with the aldehydes to give imine (Schiff base) and aminal covalent linkages; see Figure 1.5,6 Controlling cellulose oxidation was critical; nonoxidized cellulose or cellulose fully oxidized5,7 to carboxyls did not adhere strongly to PVAm. We employed 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) radicalmediated oxidation to specifically convert the C6 hydroxyl groups on cellulose to the corresponding aldehyde. Kitaoka r 2011 American Chemical Society
and Isogai were the first to show that TEMPO oxidation improved paper wet strength, and they argued that aldehyde groups formed cross-links in the fiber-fiber joints.7,8 At about the same time, Jaschinski et al. disclosed in a patent the use of TEMPO to improve the strength of paper products particularly when wet.9 TEMPO is not very water-soluble, and this patent also claims 4-hydroxy-TEMPO and 4-acetamido-TEMPO, which are more water-soluble. TEMPO-catalyzed oxidations have been described since the mid sixties.10 Perhaps the most common implementation of TEMPO oxidation was described by Anelli, who used NaOCl/ NaBr to regenerate TEMPO.11 TEMPO oxidation is attractive because it is highly selective toward primary versus secondary alcohols.12 The first report of TEMPO-oxidized carbohydrate was by Davis and Flitsch,13 and the first report of cellulose oxidation was by Chang and Robyt.14 A series of papers from Isogai’s group disclosed many details regarding TEMPO oxidation of cellulose.7,8,15-31 Virtually all of the above publications describe the organic chemistry of TEMPO mediated oxidation; the details are not repeated here. Key features of the TEMPO system include: the nitroxyl radical must be in molecular contact with the target hydroxyl; the oxidation products include aldehyde and carboxyl groups at a ratio depending upon conditions; and, for a TEMPO Received: October 24, 2010 Published: March 10, 2011 942
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Figure 2. Interaction of PVAm-TEMPO with cellulose. The relevant chemistry is magnified in the right-hand structures. and used as received. The concentration of sodium hypochlorite was determined as 1.98 mol/L by iodometric titration. Regenerated cellulose dialysis membrane (purchased from Spectra/Por 3, product no.: 132724, 3500 Da MWCO, Spectrum Laboratories) was boiled in water to remove plasticizer and was cut into two sizes: top membranes (2 cm 6 cm) and bottom membranes (3 cm 6 cm). All solutions were prepared with Milli-Q water. PVAm-TEMPO Preparation and Characterization. PVAmTEMPO was prepared by EDC/sulfo-NHS-mediated conjugation of 4-carboxy-TEMPO to PVAm; see Figure 3. In a typical experiment, 120 mg freeze-dried PVAm and 170 mg 4-carboxy-TEMPO were first dissolved in 60 mL of Milli-Q water, followed by 3 g EDC and 100 mg sulfo-NHS. The reaction mixture was stirred for 2.5 h at room temperature, and the solution pH was maintained at 6 by 1.0 N HCl addition. The product was dialyzed against water for 2 weeks and then freeze-dried and stored at 4 °C for further use. The TEMPO content of the product was determined by conductometric titration using a PC titrate (Man-Tech Associates). Typically 7.2 mg lyophilized PVAm-TEMPO was dissolved in water, and the initial pH was adjusted to 3. The solution was titrated with 0.1 N NaOH, and both pH and conductivity were recorded until pH 11. The primary amine content was clearly evident from the flat portion of the conductivity curve. The TEMPO content was calculated as the change in titratable amine when PVAm was converted to PVAm-TEMPO. Example titration curves are given in Ren’s thesis.32 PVAm-TEMPO Stability. TEMPO-mediated oxidations are generally conducted in the presence of an oxidant at alkaline pH. We were concerned that PVAm-TEMPO may degrade under oxidation conditions. A stability test of PVAm-TEMPO was conducted as follows: three bottom cellulose membranes and two top membranes (0.548 g) were soaked in 100 mL of water containing 20 mg PVAm-TEMPO and 15 mg sodium bromide. We then added 75 μL of sodium hypochlorite (1.98 mol/L) to the solution to start the reaction. The pH of the reaction mixture was maintained at 10.3 by 0.1 N NaOH addition. Samples of the polymer solution were isolated after 4 and 8 h oxidation at room temperature and purified by exhaustive dialysis against distilled water. FTIR of isolated samples confirmed that the PVAm-TEMPO was stable; see the Results section. Methylglyoxal Oxidation. Methylglyoxal was oxidized in homogeneous solution to confirm that PVAm-TEMPO was catalytically
Figure 1. Coupling of PVAm with TEMPO oxidized cellulose to form imine and aminal linkages.
moiety to participate in multiple oxidations, the radical must be regenerated by a secondary oxidant. The practical utility of TEMPO oxidization for increased cellulose adhesion is limited by two difficulties. First, with porous cellulose substrates such as membranes or wood pulp fibers, TEMPO catalyzes the oxidation of the interior pore surfaces, causing a significant decrease in mechanical properties. Second, in dilute systems such as wood pulp fiber processing, large volumes of TEMPO solutions would have an environmental and financial impact. In this Article, we report a completely new approach to exploit the adhesion advantages of TEMPO oxidation while circumventing the above problems. Herein we show that by grafting a TEMPO derivative to polyvinylamine (PVAm-TEMPO), the resulting polymer will: adsorb onto cellulose, induce oxidation underneath the adsorbed polymer, and give strong wet adhesion when the amines react with newly formed aldehydes; see Figure 2. We believe that this is the first example of a synthetic bioadhesive (or adhesive of any kind) in which a surface oxidation (activation) catalyst is covalently attached to the adhesive chain.
’ EXPERIMENTAL SECTION Materials. PVAm was the commercial polymer Lupamin 5095 (BASF, Ludwigshafen) with a number-average molecular weight of 45 000 Da, and the degree of hydrolysis specified by BASF was >90%. PVAm was purified by exhaustive dialysis against water and followed by lyophilization. N-Hydroxysulfosuccinimide sodium salt (sulfo-NHS), TEMPO, 4-carboxy-TEMPO, tris(hydroxymethyl) aminomethane (Tris), 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC), methylglyoxal solution (∼40% in water), pyruvic acid (98%), sodium bromide, and sodium hypochlorite were purchased from Sigma-Aldrich 943
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procedure was developed to simulate fiber-fiber adhesion in wet paper and was performed as follows. The bottom cellulose membrane was placed on a polished stainless-steel disk (TAPPI standard drying plate), and excess surface water was removed by gently blotting with a lint-free tissue. A 40 12.7 mm long piece of Teflon tape (G.F.Thompson, 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 pipet (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 pressed (88 N) for 10 min in a Carver press. The pressed membranes were dried at 23 °C and 50% humidity for 24 h. The laminates were soaked in a mixture of 3 mmol/L NaCl and 10 mmol/L Tris-HCl buffer at pH 7.5 for 30 min. The laminates were then placed between two pieces of blotting paper, and the excess water was removed by two passes with a 2.4 kg stainless steel roller. The delamination forces were measured immediately with a 90° peeling apparatus using an extensional rate of 20 mm/min. Our apparatus and data analysis were previously described in detail.4,5
Figure 3. Preparation of PVAm-TEMPO. active. The oxidation of methylglyoxal to pyruvic acid was performed with a membrane reactor in which the PVAm-TEMPO was confined in a dialysis bag and whereas the reactants and products could pass freely through the membrane. The reactor was based on a Spectra/Por 137002 Tube-A-Lyzer, which consists of a cellulose ester dialysis tube (1 cm diameter, 15 cm long) with a molecular weight cutoff of 0.1 to 0.5 KD. The dialysis tube was sealed at one end and was suspended inside a plastic tube with a capacity of ∼55 mL. The plastic tube had inlet and outlet spigots through which ∼200 mL of solution was circulated past the dialysis tube using a peristaltic pump. The external solution consisted of 225 mL of water to which was added 2.5 mL of methylglyoxal solution (40% weight concentration), 200 mg of NaBr, and 8.42 mL(1.98 mol/L) of sodium hypochlorite. The pH of the external solution was maintained at 10 with 1 N NaOH. The external solution was circulated through the membrane chamber at a rate of 60 mL/min. The methylglyoxal concentration was followed by UV absorbance at 321 nm; calibration curves and further details are given in Ren’s thesis.32 Cellulose Oxidation with PVAm-TEMPO. In a typical experiment, 15 pairs of washed cellulose strips were immersed in 225 mL of solution of 67 mg/L solution of PVAm-TEMPO and 3 mM NaCl. The suspension was stirred for 10 min to give a saturated monolayer of PVAm-TEMPO adsorbed on the cellulose strips. Room-temperature oxidation was initiated by adding NaBr (82.5 mg/L) and NaOCl (26.3 mmol/L). The pH was maintained at 10.3 with 0.1N NaCl. The oxidation was quenched after 2 h with 10 mL of ethanol, and the cellulose strips were separated and rinsed three times with water. Cellulose Laminate Fabrication and Delamination. Wet adhesion between two cellulose films was measured by a delamination procedure developed in our laboratory and previously described.5 The
’ RESULTS PVAm-TEMPO was prepared by the EDC-catalyzed condensation of 4-carboxy-TEMPO to give the structure shown in Figure 3. The average number of TEMPO groups per amine was determined by conductometric titration, which measures the content of unsubstituted amines. The PVAm-TEMPO used for this work had a TEMPO content of 0.29 TEMPO moieties per amine group; the influence of the extent of TEMPO substitution will be addressed in future work. Because we employed NaOCl/NaBr as oxidants, we were concerned that the PVAm-TEMPO would degrade under oxidizing conditions. PVAm-TEMPO was exposed to a solution of NaOCl þ NaBr at pH 10.3, and changes in the polymer structure were probed with FTIR. After 8 h at room temperature, there was no significant change in the PVAm-TEMPO spectra; see Figure 4. The ability of PVAm-TEMPO to catalyze alcohol oxidation was confirmed by the oxidation of methylglyoxal to pyruvic acid in a membrane reactor. Two hours of reaction at pH 10 and room temperature gave 78% conversion, demonstrating the catalytic efficacy of PVAm-TEMPO. Wet adhesion experiments demonstrated the ability of PVAmTEMPO to oxidize and react with cellulose. Our standard experiment consists of: adsorbing a saturated monolayer PVAm-TEMPO onto regenerated cellulose strips; oxidization by activation with NaOCl þ NaBr at pH 10.3; room-temperature laminating pairs of strips using additional PVAm as an adhesive; drying at room temperature and 50% relative humidity; rewetting with a buffer solution; and finally, using 90° peeling to determine the delamination force. Our previous work has shown that without PVAm or the oxidation step, the adhesion is essentially zero.5 Table 1 summarizes the results of three key adhesion experiments. Experiment number 3 shows that oxidizing cellulose with PVAm-TEMPO gave strong wet adhesions, whereas the two control experiments gave very low adhesion. The first control 944
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(experiment 1) was performed without PVAm-TEMPO or NaBr and NaOCl. The second control (experiment 2) was performed with everything but the PVAm-TEMPO catalyst. Note that for all three experiments the cellulose strips were coated with PVAm adhesive before the lamination. Figure 5 shows the effect of oxidation time on wet adhesion. Two sets of results are shown - one, with PVAm-TEMPO and the other with soluble TEMPO reproduced from DiFlavio et al.5 In both cases, about 10 min of oxidation time was required to observe increased adhesion. The soluble TEMPO increased adhesion at shorter times; a detailed kinetic comparison of PVAm-TEMPO with soluble TEMPO is ongoing. At 80 min, the soluble TEMPO degraded the cellulose substrate so much that substrate failure occurred instead of delamination, whereas the PVAm-TEMPO did not degrade the cellulose membrane to the same extent. This difference arises because the PVAm-TEMPO is restricted to the film exterior surface, whereas TEMPO can diffuse into the porous membrane. All experimental steps for the results in Table 1 were conducted at room temperature. A series of experiments was conducted in which the temperature of the cellulose film lamination step was varied. Two sets of data are summarized in
Figure 6: one set where the cellulose was oxidized with PVAmTEMPO before lamination and one without the oxidation step. Without oxidation, the wet adhesion was extremely low, irrespective of the lamination temperature. With PVAm-TEMPO oxidation, the adhesion was high across the temperature range, with the intermediate temperatures giving the highest adhesion. Sodium bromide is commonly used to increase the rate of TEMPO oxidations.33,34 A series of experiments was conducted to determine the influence of NaBr concentration on wet adhesion, and the results are summarized in Figure 7. With the exception of the 45 mg/L data set, the adhesion was rather insensitive to the presence of NaBr. Perhaps adhesion drops with too much sodium bromide because the aldehydes are converted to carboxyls before the amines have a chance to react.
’ DISCUSSION We propose that the mechanism shown in Figure 2 explains the enhanced wet adhesion with PVAm-TEMPO. In the first step, PVAm-TEMPO adsorbs onto the cellulose surface. PVAm is ∼50% ionized at neutral pH, and most cellulose surfaces are negatively charged, and thus the adsorption is driven by electrostatics. In the second step, the TEMPO moieties catalyze the oxidation. Finally, in the third step, primary amines react with the freshly generated aldehyde groups. This is supported by the following evidence. Adsorption, the first step, is not controversial; scores of papers exist describing the electrostatically driven adsorption of cationic polymers onto cellulose,35-37 including PVAm.38-41
Figure 5. Influence of oxidation time on cellulose-PVAm-cellulose wet adhesion comparing soluble TEMPO5 with PVAm-TEMPO oxidation catalysts. For the soluble TEMPO experiments the PVAm molecular weight was 150 kDa. For the PVAm-TEMPO experiments, the PVAm and PVAm-TEMPO molecular weights were 45 kDa, and the degree of TEMPO substitution was 25%. For all experiments, the dried samples were soaked for 30 min in pH 6.5, 5 mM NaCl and then delaminated at a peel angle of 90° and a peel rate of 20 mm/min.
Figure 4. FTIR showing that PVAm-TEMPO is stable under oxidizing conditions at room temperature. The peak at 1040 cm-1 is due to grafted TEMPO.
Table 1. Wet Adhesion of Treated Cellulose Filmsa experiment
NaOCl (mmol/g cellulose)
NaOCl (mmol/L)
coated PVAm (mg/m2)
delamination force (N/m)
0
6.25
1.9
26.3
6.25
2.9
6.25
25.2
NaBr (mg/L)
PVAm-TEMPO (mg/L)
1
0
0
0
2
82.5
0
1.8
3
82.5
66.7
1.8
26.3
a
For experiments 2 and 3, the oxidation lasted for 2 h at pH 10.3, with a starting concentration of each ingredient shown below. For experiment 1, the samples were not oxidized. All films were laminated with 6.25 mg/m2 PVAm for the delamination test. 945
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Figure 7. Influence of NaBr concentration on wet adhesion. The error bars denote the range of triplicate measurements.
Figure 6. Influence of lamination temperature on wet adhesion.
Step 2 oxidation: There are many publications describing oxidations with immobilized TEMPO.12 Furthermore, we have shown that PVAm-TEMPO will catalyze the oxidation of methylglyoxal to pyruvic acid.32 Although we have presented no spectroscopic or other direct evidence that PVAm-TEMPO is catalyzing cellulose oxidation, the indirect evidence is substantial. The results in Table 1 and Figure 5 show that without PVAmTEMPO, the adhesion is low. In addition, the wet delamination forces with PVAm-TEMPO are similar to values we reported for cellulose oxidized with TEMPO in solution.5 Step 3: Amine coupling to newly formed aldehyde groups. Sato and Isogai argued that amine-containing polymers formed imines with aldehydes on TEMPO-oxidized cellulose giving enhanced adhesion.26 At the same time, we provided substantial indirect evidence that the primary amines on PVAm form imine and aminal linkages (Figure 1) with TEMPO-oxidized cellulose;5,6 the same arguments hold for PVAm-TEMPO and are not repeated here. By tethering TEMPO to PVAm, TEMPO is concentrated at the cellulose surface. For example, if we assume that the saturation adsorbed density of PVAm on cellulose is 1 mg/m2 and that the thickness of the adsorbed monolayer is 5 nm, the concentration of TEMPO moieties in the adsorbed PVAm-TEMPO layer is 0.51 mol/L. This order-of-magnitude analysis suggests that the effective TEMPO concentrations when using PVAm-TEMPO are >1000 times higher than Saito’s recipe, assuming that soluble TEMPO does not accumulate on the cellulose surface.7 The work herein is the first publication describing PVAmTEMPO and its interaction with cellulose. There are many opportunities to optimize the system. The analysis in the previous paragraph suggests that far lower degrees of TEMPO substitution than those used here should be effective. For example, if each PVAm chain requires only two covalent linkages to cellulose for strong adhesion, then perhaps as few as two TEMPO units per chain would suffice, whereas herein we employed one TEMPO for about every three amines on PVAm. The oxidization conditions and kinetics also need to be optimized. Two hours of oxidation at pH 10 in the presence of NaOCl is not practical in many applications; shorter oxidation times and neutral pH would be preferable. Furthermore, because each TEMPO needs only oxidize one hydroxyl, it may not be necessary to use NaOCl or another co-oxidant to regenerate the TEMPO radicals. These issues are currently under investigation
From a technological perspective, polymer-immobilized TEMPO offers a number of advantages over conventional water-soluble TEMPO recipes including: 1 Much less TEMPO is required. With low-molecular-weight TEMPO and its derivatives, much of the catalyst will be in solution, whereas it is possible to have most of the added PVAm-TEMPO adsorbed on fibers surfaces. In other words, PVAm-TEMPO, like all high-molecular-weight cationic polymers, will display high affinity binding, whereas low-molecular-weight TEMPO species will have a lower binding constant and thus partition into the aqueous phase. 2 The TEMPO grafted to high-molecular-weight PVAm is restricted to the exterior surfaces of porous fiber walls, preventing fiber weakening by oxidation of the cell wall interior surfaces. 3 The proximity of amine groups to the TEMPO moieties in the adsorbed PVAm-TEMPO layer allows amines to react with freshly formed aldehydes before they are further oxidized to acid groups. By contrast, when using lowmolecular-weight TEMPO, the oxidation step is conducted first; then, the cellulose is exposed to PVAm or other polymers, giving the aldehydes opportunity to oxidize further or to form hemiacetals within the fiber wall. 4 The environmental impact of immobilized TEMPO adsorbed on surfaces will be less than that of water-soluble TEMPO derivatives because higher quantities of watersoluble TEMPO derivatives must be added, leaving significant quantities remaining in the aqueous phase. In contrast, most of the PVAm-immobilized TEMPO will be sequestered by adsorption onto surfaces.
’ CONCLUSIONS In conclusion, we describe PVAm-TEMPO, a novel polymeric adhesive that promotes cellulose-to-cellulose wet adhesion. By grafting the TEMPO oxidation catalyst onto the PVAm adhesive chains, many of the negative economic and environmental impacts of water-soluble TEMPO oxidation are minimized because the cationic polymer fixes the TEMPO moieties onto cellulose surfaces. Furthermore, the high-molecular-weight PVAm carrier restricts the TEMPO catalyst to exterior surfaces of the porous cellulose films, reducing cohesive film failure at longer oxidation times. Similar cellulose-PVAm-cellulose wet adhesion strengths were observed with TEMPO compared with 946
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PVAm-TEMPO oxidized cellulose, suggesting similar adhesion mechanisms. On the basis of the literature, the increased wet adhesion with oxidation likely involves covalent bond formation between primary amine groups and aldehydes on oxidized cellulose.
(15) Isogai, A.; Kato, Y. Preparation of polyglucuronic acid from cellulose by tempo-mediated oxidation. Cellulose 1998, 5, 153–164. (16) Kato, Y.; Matsuo, R.; Isogai, A. Oxidation process of watersoluble starch in tempo-mediated system. Carbohydr. Polym. 2003, 51, 69–75. (17) Shibata, I.; Isogai, A. Depolymerization of cellouronic acid during tempo-mediated oxidation. Cellulose 2003, 10, 151–158. (18) Saito, T.; Isogai, A. Tempo-mediated oxidation of native cellulose. The effect of oxidation conditions on chemical and crystal structures of the water-insoluble fractions. Biomacromolecules 2004, 5, 1983–1989. (19) Kato, Y.; Kaminaga, J.; Matsuo, R.; Isogai, A. Oxygen permeability and biodegradability of polyuronic acids prepared from polysaccharides by tempo-mediated oxidation. J. Polym. Environ. 2005, 13, 261–266. (20) Saito, T.; Isogai, A. Ion-exchange behavior of carboxylate groups in fibrous cellulose oxidized by the tempo-mediated system. Carbohydr. Polym. 2005, 61, 183–190. (21) Saito, T.; Shibata, I.; Isogai, A.; Suguri, N.; Sumikawa, N. Distribution of carboxylate groups introduced into cotton linters by the tempo-mediated oxidation. Carbohydr. Polym. 2005, 61, 414–419. (22) Saito, T.; Yanagisawa, M.; Isogai, A. Tempo-mediated oxidation of native cellulose: SEC-MALLS analysis of water-soluble and -insoluble fractions in the oxidized products. Cellulose 2005, 12, 305–315. (23) Saito, T.; Nishiyama, Y.; Putaux, J. L.; Vignon, M.; Isogai, A. Homogeneous suspensions of individualized microfibrils from tempocatalyzed oxidation of native cellulose. Biomacromolecules 2006, 7, 1687–1691. (24) Saito, T.; Okita, Y.; Nge, T. T.; Sugiyama, J.; Isogai, A. Tempomediated oxidation of native cellulose: microscopic analysis of fibrous fractions in the oxidized products. Carbohydr. Polym. 2006, 65, 435–440. (25) Shibata, I.; Yanagisawa, M.; Saito, T.; Isogai, A. SEC-MALS analysis of cellouronic acid prepared from regenerated cellulose by tempo-mediated oxidation. Cellulose 2006, 13, 73–80. (26) Saito, T.; Isogai, A. Wet strength improvement of tempooxidized cellulose sheets prepared with cationic polymers. Ind. Eng. Chem. Res. 2007, 46, 773–780. (27) Saito, T.; Kimura, S.; Nishiyama, Y.; Isogai, A. Cellulose nanofibers prepared by tempo-mediated oxidation of native cellulose. Biomacromolecules 2007, 8, 2485–2491. (28) Fan, Y. M.; Saito, T.; Isogai, A. Tempo-mediated oxidation of beta-chitin to prepare individual nanofibrils. Carbohydr. Polym. 2009, 77, 832–838. (29) Hirota, M.; Tamura, N.; Saito, T.; Isogai, A. Surface carboxylation of porous regenerated cellulose beads by 4-acetamide-tempo/ naclo/naclo2 system. Cellulose 2009, 16, 841–851. (30) Hirota, M.; Tamura, N.; Saito, T.; Isogai, A. Oxidation of regenerated cellulose with naclo2 catalyzed by tempo and naclo under acid-neutral conditions. Carbohydr. Polym. 2009, 78, 330–335. (31) Saito, T.; Hirota, M.; Tamura, N.; Kimura, S.; Fukuzumi, H.; Heux, L.; Isogai, A. Individualization of nano-sized plant cellulose fibrils by direct surface carboxylation using tempo catalyst under neutral conditions. Biomacromolecules 2009, 10, 1992–1996. (32) Ren, P. R. the Use of Polyvinylamine-Supported Tempo Oxidation in Wet-Strengthening of Paper. Masters Thesis, McMaster University, Hamilton, Ontario, 2010. (33) De Nooy, A. E. J.; Besemer, A. C.; Vanbekkum, H. Highly selective nitroxyl radical-mediated oxidation of primary alcohol groups in water-soluble glucans. Carbohydr. Res. 1995, 269, 89–98. (34) Jiang, B.; Drouet, E.; Milas, M.; Rinaudo, M. Study on tempomediated selective oxidation of hyaluronan and the effects of salt on the reaction kinetics. Carbohydr. Res. 2000, 327, 455–461. (35) Pelton, R. H. Electrolyte effects in the adsorption and desorption of a cationic polyacrylamide on cellulose fibers. J. Colloid Interface Sci. 1986, 111, 475–485. (36) Tanaka, H.; Odberg, L.; Wagberg, L.; Lindstrom, T. Adsorption of cationic polyacrylamides onto monodisperse polystyrene lattices and
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT We thank the Natural Sciences and Engineering Research Council of Canada for funding this work through a cooperative research grant with BASF Canada. The authors also thank the Canada Foundation for Innovation for support of this work. R.P. holds the Canada Research Chair in Interfacial Technologies. ’ REFERENCES (1) Luner, P.; Eriksson, E.; Vemuri, K. P.; Leopold, B. The effect of chemical modification on the mechanical properties of paper. 1. Oxidation and reduction of rayon fibers. Tappi 1967, 50, 37–39. (2) Eichhorn, S.; Dufresne, A.; Aranguren, M.; Marcovich, N.; Capadona, J.; Rowan, S.; Weder, C.; Thielemans, W.; Roman, M.; Renneckar, S.; Gindl, W.; Veigel, S.; Keckes, J.; Yano, H.; Abe, K.; Nogi, M.; Nakagaito, A.; Mangalam, A.; Simonsen, J.; Benight, A.; Bismarck, A.; Berglund, L.; Peijs, T. Review: Current international research into cellulose nanofibres and nanocomposites. J. Mater. Sci. 2010, 45, 1–33. (3) Gardner, D. J.; Oporto, G. S.; Mills, R.; Samir, M. Adhesion and surface issues in cellulose and nanocellulose. J. Adhes. Sci. Technol. 2008, 22, 545–567. (4) Kurosu, K.; Pelton, R. Simple lysine-containing polypeptide and polyvinylamine adhesives for wet cellulose. J. Pulp Paper Sci. 2004, 30, 228–232. (5) Diflavio, J. L.; Bertoia, R.; Pelton, R.; Leduc, M. The Mechanism of Polyvinylamine Wet-Strengthening; Pulp & Paper Fundamental Research Society: Cambridge, U.K., 2005; Vol. 1, pp 1293-1316. (6) Diflavio, J. L.; Pelton, R.; Leduc, M.; Champ, S.; Essig, M.; Frechen, T. The role of mild tempo-nabr-naclo oxidation on the wet adhesion of regenerated cellulose membranes with polyvinylamine. Cellulose 2007, 14, 257–268. (7) Saito, T.; Isogai, A. Introduction of aldehyde groups on surfaces of native cellulose fibers by tempo-mediated oxidation. Colloids Surf., A 2006, 289, 219–225. (8) Kitaoka, T.; Isogai, A.; Onabe, F. Chemical modification of pulp fibers by tempo-mediated oxidation. Nord. Pulp Pap. Res. J. 1999, 14, 279–284. (9) Jaschinski, T.; Gunnars, S.; Besemer, A. C.; Bragd, P. Oxidized polymeric carbohydrates and products made thereof. U.S. Patent 6,987,181 B2, 2006. . G.; Neiman, M. B. Some reactions (10) Golubev, V. A.; Rozantsev, E of free iminoxyl radicals with the participation of the unpaired electron. Russ. Chem. Bull. 1965, 14, 1898–1904. (11) Anelli, P. L.; Biffi, C.; Montanari, F.; Quici, S. Fast and selective oxidation of primary alcohols to aldehydes or to carboxylic-acids and of secondary alcohols to ketones mediated by oxoammonium salts under 2-phase conditions. J. Org. Chem. 1987, 52, 2559–2562. (12) Bragd, P. L.; Van Bekkum, H.; Besemer, A. C. Tempo-mediated oxidation of polysaccharides: survey of methods and applications. Top. Catal. 2004, 27, 49–66. (13) Davis, N.; Flitsch, S. Selective oxidation of monosaccharide derivatives to uronic acids. Tetrahedron Lett. 1993, 34, 1181–1184. (14) Chang, P. S.; Robyt, J. F. Oxidation of primary alcohol groups of naturally occurring polysaccharides with 2,2,6,6-tetramethyl-1-piperidine oxoammonium ion. J. Carbohydr. Chem. 1996, 15, 819–830. 947
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dx.doi.org/10.1021/bm200101b |Biomacromolecules 2011, 12, 942–948
