THERMALLY STABLE, DISPERSIBLE CELLULOSE NANOCRYSTALS

Abstract

The present application relates to cellulose nanocrystals and other anionic carbohydrates and methods of preparation thereof. Specifically, in certain embodiments, the cellulose nanocrystals are modified using ion exchange technology to yield thermally stable or task-specific, dispersible cellulose nanocrystals.

Claims

1. (canceled)

2. (canceled)

3. (canceled)

4. (canceled)

5. A thermally stable, dispersible cellulose nanocrystal composition comprising: a cellulose nanocrystal having the formula: ##STR00002## where each A is independently selected from O.sub.2.sup.− or an esterified oxyacid anion, where each R is independently selected from H.sup.+, Na.sup.+, Im.sup.+, HdMe.sub.2Im.sup.+, MePh.sub.3P.sup.+, Me.sub.3PhP.sup.+, Me.sub.3Im.sup.+, NH.sub.4.sup.+ or other exchangeable cation, with the proviso that at least one A-R is not A-H or A-Na, and where Y is in the range of 10-30 monomer units.

6. The composition of claim 5, wherein A=SO.sub.4.sup.2−.

7. The composition of claim 5, wherein A=PO.sub.4.sup.3−, PO.sub.3.sup.3−, or O.sub.2.sup.−.

8. The composition of claim 5, wherein the exchangeable cation comprises imidazolium, phosphonium, pyridinium, pyrollidinium, piperdinium, morpholinium, sulfonium, unsymmetrical quaternary ammonium, basic dye cations, silver cations, amino acids, or protonated amines.

9. The composition of claim 5, further comprising a polymer resin.

10. The composition of claim 9, wherein the polymer resin is selected from epoxy thermosets, phenolic thermosets, polystyrene (PS), poly(acrylonitrile-co-butadiene-co-styrene) (ABS), high impact polystyrene (HIPS), poly(styrene-co-butadiene-co-styrene) (SBS), polyethylene (PE), polypropylene (PP), polylactic acid (PLA), polycaprolactone (PCL), polyamides (Nylons), polyacrylates including poly(methylmethacrylate) and poly(ethylmethacrylate), and sodium polyacrylate, cellulose acetate (CA), cellulose acetate acrylate (CAB), polyacrylonitrile (PAN), poly(styrene-co-acrylontrile) (SAN), polybutadiene, poly(ethylene-co-vinyl acetate) (EVA), polyvinyl acetate (PVAc), poly(ethylene terephthalate) (PET), polycarbonate (PC), thermoplastic polyurethane (PU), and other extrudable polymers.

11. (canceled)

12. (canceled)

13. (canceled)

14. (canceled)

15. (canceled)

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee.

[0017] FIG. 1 shows dynamic water vapor sorption of freeze-dried, exchanged CNCs in accordance with certain embodiments of the invention.

[0018] FIG. 2 shows dispersive component of surface energy for Na-CNC and MePh.sub.3P—CNC at 0% relative humidity using iGC.

[0019] FIG. 3 shows acid-base components of surface energy for Na-CNC and MePh.sub.3P—CNC at 0% relative humidity using iGC.

[0020] FIGS. 4A-B show microscopic images (20×) of DGEBA/JA230 composites containing (a) 0.5% Na-CNC and (b) 0.5% MePh.sub.3P—CNC.

[0021] FIG. 5 shows heat release rate measured by microcombustion calorimeter analysis of exchanged CNCs according to one embodiment.

[0022] FIG. 6 shows dispersion of exchanged CNCs according to one embodiment; left side—PS+0.5% Na-CNC, mixed 195° C., 200 rpm, 3 min, Pressed 160° C., 5 min; right side—PS+0.5% MePh.sub.3P—CNC, mixed 195° C., 200 rpm, 3 min, Pressed 160° C., 5 min.

[0023] FIGS. 7A-D show laser excitation (λ=405 nm) confocal images of (A) PS, (B) PS+1% Na-CNC, (C) PS+0.5% (HdMe.sub.2Im)Na.sub.0.9—CNC, and (D) PS+1% MePh.sub.3P—CNC.

[0024] FIGS. 8A-C show microscopic images (20×) of polystyrene composites containing (A) 3% Na-CNC freeze dried from water, (B) 3% Na-CNC freeze dried from 9% t-BuOH (aq), and (C) 3% MePh.sub.3P—CNC freeze dried from water.

[0025] FIG. 9 shows transmission spectra of 3% CNC-PS composites, showing dispersion and reduction in aggregation of exchanged CNCs according to one embodiment.

[0026] FIG. 10 shows absorbance and fluorescence (λ=520 nm) of Na-CNC and MePh.sub.3P—CNC exchanged with cation mixture containing 1/100 Rhodamine 6G.

DETAILED DESCRIPTION

[0027] The following detailed description will illustrate the general principles of the invention, embodiments of which are additionally provided in the accompanying examples.

[0028] The disclosed preparations and methods involve the preparation of cationic surfactant exchanged cellulose nanocrystals, including preparation using an ion exchange column. The disclosed methods involve the direct exchange of CNC cations with cationic surfactants, which simultaneously alters both the surface energy and thermal stabilities of the CNCs, and thus improves the quality of melt-blended polymer nanocomposites and other products incorporating the CNCs produced according to the disclosed methods. Results have shown an ability to modify the surface energy of the cellulose and great advantages to this technique as compared to other techniques used to alter the surface energy of the CNCs. The CNCs prepared according to the disclosed methods are suitable for dispersion even when dried.

[0029] The disclosed method can be carried out using ion exchange methodologies and apparatuses known in the art. In one embodiment, a strong acid ion exchange resin is washed in an ammonium hydroxide solution to form an ammonium ion form of the resin. The resin is then stirred in a 4-6 wt % Na-CNC solution. The solution is filtered through a screen to remove the resin, and fresh ammonium ion form resin is added to the solution. This “batch exchange” is repeated, for example four times, to produce nearly 100% NH.sub.4—CNC. The process is applicable for any cationic surfactant solution to obtain surfactant exchanged CNCs.

[0030] In another embodiment, a columnar ion exchange process is used to prepare a number of modified CNCs for miscibility in polymers. CNCs are initially prepared using acid hydrolysis techniques known in the art. An ion exchange column is loaded with resin beads with a cationic surfactant for ion exchange. A solution containing the CNCs is then flowed through the column, resulting in the direct exchange of the cationic surfactant for at least some of the cationic sites on the CNCs (i.e., to replace the hydrogen ions of one or more of the acid groups of the esterified CNCs). The disclosed technique is suitable for the modification of cellulose nanocrystals formed during H.sub.2SO.sub.4, H.sub.3PO.sub.4, and any other oxyacid hydrolysis or during TEMPO processing, all of which produce negative-charged moieties on the cellulose surface through side reactions at the C-6 hydroxyl position.

[0031] Suitable cationic surfactants according to the invention include ammonium, imidazolium and phosphonium cations, covering a range of surface energies. Imidazolium and phosphonium-based surfactants have been shown to have higher thermal stabilities than ammonium based surfactants. CNCs modified with cations with longer alkyl chains generally exhibit better thermal stability than CNCs modified with small, more hydrophilic cations. However, resin selectivity complicates attachment of longer chain surfactants. In embodiments where high thermal stability is not required, any of a variety of other cationic surfactants known in the art may be used.

[0032] The exchange of cations does not need to be complete. Using partially loaded resin, flow through a column will produce CNCs that have a mixture of cations. The amount of cations transferred will depend on the selectivity of the cation towards the resin, the cellulose, and water. For instance, partially loading an acid form ion exchange resin with hexadecyl-dimethylimidazolium cations (HdMe.sub.2Im.sup.+) and flowing a solution of sodium form cellulose (Na-CNC) through a column filled with this resin produces cellulose with both protons and HdMe.sub.2Im.sup.+ cation with a higher HdMe.sub.2Im.sup.+:H.sup.+ molar ratio than was originally on the resin.

[0033] A mixture of cations can be exchanged onto the CNCs. Loading the ion exchange column with a mixture of cations will produce CNCs with a mixture of cations. The ratio of ions will be dependent on the relative affinity they have for both the resin and the cellulose. For instance, loading sodium form resin with (100:1) molar ratio of methyl(triphenyl)phosphonium bromide and rhodamine 6G and flowing a solution of Na-CNC through a column filled with this resin produces fluorescent nanocrystals with a surface energy similar to that of nanocrystals completely exchanged with methyl(triphenyl)phosphonium ions.

[0034] The ion exchange process can also be used to attach cellulose nanocrystals to an immobile cationic surface. For example, the surface of glass fibers can be chemically modified with reactive groups, such as amines or epoxides. Flow of acid form cellulose nanocrystals (H-CNC) over glass fibers with an aminated surface can produce “fuzzy” glass fibers decorated with cellulose nanocrystals. The hydrophobicity or surface energy of these fibers can be altered by using mixed cation cellulose nanocrystals, such as HdMe.sub.2Im.sup.+/H.sup.+-CNC produced using partially loaded resins. The process can be extended to other aminated surfaces, such as chitosan; aminated gold; aminated carbon; aminated aramid; or lysine-, histidine-, and arginine-rich polypeptides using methods known in the art.

[0035] The flow of cellulose solutions through the ion exchange column also purifies the CNCs, producing a product that is whiter (a more desirable) and that has a higher cation exchange capacity as compared to the state of the art CNC production methods. The higher cation exchange capacity may be due to the removal of non-ionic cellulose or non-ionic impurities. For CNCs in the disclosed process, only cations are exchanged, and salt impurities are not retained by the resin, so washing by filtration, dialysis, or solvent extraction will not be necessary.

[0036] Since the number of anionic sites on the CNCs is limited, the exchange of cations according to the disclosed method can alter surface energies without completely covering the surface of the crystals with surfactant. Accordingly, in some embodiments, the inventive CNCs maintain an ability to form hydrogen bonded 3-D networks in composites.

[0037] CNCs according to the present disclosure may have the following structure:

##STR00001##

where each A is independently selected from O.sub.2.sup.− or an esterified oxyacid anion, where each R is independently selected from H.sup.+, Na.sup.+, HdMe.sub.2Im.sup.+, MePh.sub.3P.sup.+, Me.sub.3PhP.sup.+, Me.sub.3Im.sup.+, NH.sub.4.sup.+ or other exchangeable cation, with the proviso that at least one A-R is not A-H or A-Na, and where Y is typically in the range of 10-30 monomer units. In one embodiment, A=SO.sub.4.sup.2−. In other embodiments, A=PO.sub.4.sup.3−, A=PO.sub.3.sup.3−, or A=O.sub.2.sup.−.

[0038] The ion exchange process may be used to prepare fully or partially exchanged CNCs that have high thermal stabilities and a broad range of surface energies, as required for a specific melt-blended polymer composition or other products. We have found that surfactant cation exchanged CNCs have the ability to improve the organo-clay thermal stability enough to enable exfoliated clay in melt-blended polyethylene terephthalate and polystyrene, de-bundle and disperse multi-walled carbon nanotubes in melt-blended polystyrene nanocomposites, and improve the thermal stability of fluorinated synthetic mica by 100° C. The disclosed process produces similar improvements for CNCs. The modified CNCs may be melt-blended with several polymers commonly used in packaging applications, for example with polystyrene (PS), poly(acrylonitrile-co-butadiene-co-styrene) (ABS), high density polyethylene (LDPE), and polylactic acid (PLA).

[0039] Supportive Data

[0040] Using the disclosed technique, a wide variety of modified CNCs were produced. Ag.sup.+ exchanged CNCs were prepared and dispersed in polyvinyl alcohol. ICP analysis confirmed the cations were exchanged. The x-ray diffraction patterns of Ag-CNC and a 4% Ag-CNC in PVOH had no discernable peaks associated with Ag(0), indicating well dispersed single ions and the absence of silver nanoparticles. Phosphonium and imidazolium surfactants were exchanged and used in a variety of polymer composites. The exchange efficiencies can be deduced from the analysis of cations after exchange. Concentrations can be measured using ion selective electrodes (e.g. Na.sup.+ or H.sup.+), UV spectroscopy (e.g. HdMe.sub.2Im.sup.+ or MePh.sub.3P.sup.+), fluorescence spectroscopy (e.g. rhodamine 6G), or ICP (e.g. Na, S, C, P, Ag). As shown in Table 1, below, the more hydrophobic cations do not fully exchange the sites on cellulose.

TABLE-US-00001 TABLE 1 Exchange efficiency of cellulose nanocrystals [Na.sup.+] [H.sup.+] [IL] CNC mM mM mM meqv/L g CNC/100 g meqv/g Na-CNC 5.5 0.0 0.0 5.5 2.51 0.219 H-CNC 0.2 4.4 0.0 4.6 1.88 0.245 Me.sub.3Im-CNC 0.6 0.0 3.7 4.3 1.85 0.232 (post EtOH) 2.0 0.0 3.2 5.2 2.18 0.239 HxMe.sub.2Im-CNC 4.3 0.0 2.1 6.4 2.66 0.241 HdMe.sub.2Im-CNC 0.0 1.4 3.2 4.6 Not tested (Na.sup.+ column) 4.4 0.0 0.5 4.9 Not tested Me.sub.3Ph-CNC 0.5 0.0 4.6 5.1 2.30 0.222

[0041] This is at least in part due to the inability to fully exchange the cation resin. Since the solubility of the hexadecylimidazolium salt is largely driven by the solubility of the anion (Cl.sup.− salt is readily soluble, but BF.sub.4.sup.− and PF.sub.6.sup.− salts are practically insoluble in water), the inability to solvate the polyanionic resin leads to a very low exchange of the cation. The exchange on the cellulose is significantly higher than the loading on the resin, indicating a higher affinity for cellulose over the solid resin. The drop in exchange efficiency after an ethanol wash is because the resin had to be re-suspended to rehydrate the beads and remove all air bubbles. As a result, the column no longer contained any regions where there was only ionic liquid cation. This indicates that the cellulose selectivity of hydrophobic cations is less than or equal to Na.sup.+ cations. The concentration of cellulose decreases after flow through the column, which indicates that there is some affinity between the cellulose and the resin. However, the average number of exchange sites after flow through the column increases, which might indicate the removal of low exchange site impurities, such as residual lignin.

[0042] Despite the relatively low number of exchange sites (1 out of every 20 glucose units), the exchanged CNCs were significantly less hygroscopic and had lower surface energies than the Na-CNCs. The water sorption profiles for several exchanged CNCs are shown in FIG. 1. As expected, the more hydrophobic cations resulted in lower levels of water adsorbed. In this series of exchanges, MePh.sub.3P—CNC adsorbed 30% less water at 70% relative humidity. The hysteresis (additional water retention) upon desorption was also lower for the CNCs exchanged with the more hydrophobic cations. Exchanged CNCs also had lower surface energies. Using inverse gas chromatography, it was observed that both the dispersive component and the acid base character of the surface energy are lowered. The comparative surface energies of Na-CNC and MePh.sub.3P—CNC are shown in FIG. 2 and FIG. 3. As expected, the surface energies at 50% relative humidity are also lowered, as there is less water adsorbed to the cellulose surface.

[0043] Bisphenol A based epoxies are hydrophobic with aromatic character. In addition to lower surface energies, MePh.sub.3P-CNCs have aromatic character, which improves the adhesion in these epoxies. Na-CNC and MePh.sub.3P—CNC were shear mixed with bisphenol A diglycyl ether (DGEBA) and cured with a standard polyethylene glycol diamine. As shown in FIG. 4, the exchanged CNCs exhibited much better dispersion and formed smaller aggregates than the Na-CNCs. This level of dispersion is typically achieved with cellulose only when using a surfactant or chemical grafting technique.

[0044] Replacing the sodium ion or proton in sulfated CNCs with imidazolium and quaternary phosphonium cations not only alters the surface energies, but also increases the thermal stability of the crystals. As a result, CNC polymer nanocomposites can be prepared for a large range of polymers using a melt-blending technique. We found that 1-hexadecyl-2,3-dimethylimidazolium cation (HdMe.sub.2Im.sup.+) and methyl(triphenyl)phosphosphonium cation (MePh.sub.3P.sup.+) exchanged CNCs have thermal stabilities 50° C. greater than the initial CNCs used (i.e., Na-CNC and H-CNC), as shown by the microcombustion calorimeter analysis in FIG. 5. As shown in FIG. 6, MePh.sub.3P.sup.+ exchanged CNCs show superb dispersion and no degradation as compared to Na-CNCs, which form large, partially degraded aggregates when melt blended with polystyrene. Both polystyrene and the cellulose nanocrystals show some fluorescence. The exchange did not appear to significantly reduce the fluorescence of the CNCs, though intensities and quantum yields have not been determined. The fluorescence allowed for dispersion characterization by laser scanning confocal fluorescence microscopy (LSCFM). The images provided in FIG. 7 show that the ionic liquid cation exchange leads to much smaller particle sizes and good dispersion throughout the matrix. The smallest particle sizes were obtained using MePh.sub.3P—CNC, leading to transparent composites at both 0.5% and 1.0% loadings. Freeze dried Na-CNCs have recently become commercially available. These crystals are typically freeze dried with a small amount of t-butanol (t-BuOH). This reduces the ice crystal size and improves redispersibility into polar solvents, such as water and DMF. In addition, it adds a small amount of t-BuOH to the crystal structure, which can aid in dispersion in more hydrophobic media, such as polymers. As observed in FIG. 8a and FIG. 8b, CNC agglomerates are indeed reduced, from about 100 μm diameter particles to about 50 μm diameter particles. In addition, there are more rod like structures. Using the disclosed process, separation and dispersion is further improved (cf FIG. 8c), reducing the largest aggregates to about 10 μm×20 μm. Almost all the particles are now rod-like, and the average aspect ratio is much closer to that of the original CNCs. The smaller sized aggregates result in higher transmission through the polymer, as shown in FIG. 9.

[0045] CNCs modified with arylphosphonium and alkylimidazolium cations are more thermally stable than CNCs modified with alkylammonium cations. As shown in FIG. 5 and Table 2, the use of long alkyl chain surfactants does indeed improve the thermal stability of the cellulose nanocrystals. Use of smaller, more hydrophilic cations do not show the same thermal stability improvement. The microcombustion calorimetry data also shows that the long alkyl chains add some fuel to the material, increasing the total heat released. The flammability of these nanocrystals are still lower than pure cellulose crystals, likely due to the presence of sulfate groups, which have been shown to increase char formation and reduce both the peak heat release rate (HRC) and the total amount of material consumed (THR).

TABLE-US-00002 TABLE 2 Microcombustion calorimetry data for exchanged cellulose nanocrystals THR HRC T.sub.peak % CNC (kJ/g) (J/gK) (° C.) Char H-CNC 4.5 60 230 24 NH.sub.4-CNC 4.6 121 246 21 HdMe.sub.2Im/H-CNC 5.8 100 260 21 Na-CNC (as received) 6.5 131 281 22 Na-CNC (exchanged) 9.7 276 316 12 Me.sub.3Im-CNC 10.0 171 304 12 Me.sub.3Im/Na-CNC 8.0 178 284 14 HxMe.sub.2Im-CNC 12.0 203 331 9 HdMe.sub.2Im/Na-CNC 7.8 277 315 15 Me.sub.3PhP-CNC 12.6 195 341 12

[0046] A column containing cation exchange resin was loaded with MePh.sub.3PBr. NaCNC was exchanged and the effluent was collected in a sequence of consecutive equal volumes. As the ions are exchanged, a concentration gradient develops through the column. The initial vial has the highest percent of ions exchanged and each subsequent vial has a lower extent of exchange. As noted in Table 3, series of CNCs with variable levels of MePh.sub.3P.sup.+ were thus obtained.

TABLE-US-00003 TABLE 3 Cation analysis of MePh.sub.3P- CNC, prepared sequentially. MePh.sub.3P 0-8 mL 2.24 4.89 7.1 MePh.sub.3P 9-16 mL 1.50 4.77 6.3 MePh.sub.3P 17-24 mL 1.71 4.50 6.2 MePh.sub.3P 25-32 mL 2.05 3.92 6.0 MePh.sub.3P 33-40 mL 2.35 3.67 6.0 MePh.sub.3P 43-50 mL 2.75 3.21 6.0 MePh.sub.3P 51-58 mL 3.06 3.16 6.2 MePh.sub.3P 59-66 mL 3.22 2.89 6.1 MePh.sub.3P 67-74 mL 3.55 2.57 6.1 MePh.sub.3P 75-82 mL 3.51 2.73 6.2

[0047] Cation exchange resins were loaded with (100:1) NaCl:Rhodamine 6G and (100:1) MePh.sub.3PBr:Rhodamine 6G and packed in separate columns. Na-CNC was fed through the columns to produce fluorescent CNCs with overall surface energies close to Na-CNC and MePh.sub.3P—CNC, respectively. The exchange was confirmed using ICP analysis to determine the Na.sup.+ and S content and fluorescence spectra to determine the rhodamine 6G. The absorbance and fluorescence spectra for the exchanged CNCs are shown in FIG. 10.

[0048] The methods and advantages discussed here can be applied to other celluloses that contain multiple negative charges. Uncharged cellulose nanofibers (CNF) were reacted with NaIO.sub.4 to form dialdehyde cellulose nanofibers (DACNF). The DACNF were then reacted with NaClO.sub.2 to form dicarboxylic acid cellulose nanofibers (DCCNF). The DCCNF were filtered and rinsed with MePh.sub.3PBr, followed by ultra-pure water. The resulting nanofibers, now containing ionically bound MePh.sub.3P.sup.+ were solvent exchanged into N,N-dimethylformamide and blended with PS to form solvent-cast composites. Similar anionic celluloses can be formed using TEMPO, dicarboxylic acids, or chlorosulfonic acid. The technology described can be applied to these celluloses as well.

[0049] The methods and advantages described here can also be applied to other carbohydrates that contain multiple negative charges. Applicable polysaccharides include pectin (carboxylic groups), alginic acid (carboxylic groups), carrageenans (sulfate groups), glucosaminoglycans (such as chondroitin sulfate, heparin sulfate, or hyaluronic acid), and cellulose phosphate (phosphate groups). The cations in Na-alginate and i-carrageenan were exchanged with MePh.sub.3P.sup.+ and (100:1) MePh.sub.3P.sup.+:Rhodamine 6G. Although the thermal stabilities of these carbohydrates were not enhanced as much as the exchanged CNCs, their surface energies were modified, allowing for the preparation of well dispersed epoxy composites. The technology can also be applied to anionic mono- and di-saccharides, such as phytic acid, sugar phosphates, uronic acids, aldonic acids, and aldaric acids or nucleotides, such as ADP or ATP.

[0050] The embodiments of this invention shown in the drawings and described above are exemplary of numerous embodiments that may be made within the scope of the appended claims.

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