NANOCRYSTALLINE CELLULOSE, ITS PREPARATION AND USES OF SUCH NANOCRYSTALLINE CELLULOSE

Abstract

The present invention relates to nanocrystalline cellulose, an efficient way of its preparation and to uses of such nanocrystalline cellulose. The present invention also relates to porous metal oxides having a chiral nematic structure which are prepared using nanocrystalline cellulose.

Claims

1. A method of preparing nanocrystalline cellulose, comprising the steps: a) providing cellulose fibers, b) subjecting said cellulose fibers to an acidic hydrolysis, c) stopping the hydrolysis by addition of a base.

2. The method according to claim 1, comprising the further step: d) isolating the nanocrystalline cellulose resulting from the performance of steps b)-c).

3. The method according to claim 2, wherein isolating the nanocrystalline cellulose in step d) is achieved by centrifugation and washing.

4. The method according to claim 3, wherein step d) is achieved by performing a first centrifugation on the product of step c), followed by a washing step and a further centrifugation step.

5. The method according to claim 4, wherein the washing step and the further centrifugation step are performed n-times, wherein n=1-10.

6. The method according to claim 1, wherein said hydrolysis in step b) is performed by the presence of a mineral acid.

7. The method according to claim 1, wherein said base that is added in step c) is selected from the group comprising metal hydroxides, metal oxides and NH.sub.3.

8. The method according to claim 1, wherein said hydrolysis in step b) is performed by the presence of a mineral acid, and wherein said base is i) a metal hydroxide or NH.sub.3, and wherein said metal hydroxide or NH.sub.3 is added in step c) in a molar ratio of base: mineral acid in a range of from 1:5 to 5:1, or ii) a metal oxide, wherein said metal oxide in step c) is added in a molar ratio of metal oxide: mineral acid in a range of from approximately 1:10 to 1:1.

9. The method according to claim 7, wherein said metal hydroxide is an alkali metal hydroxide or an earth alkali metal hydroxide or a metal hydroxide selected from Al(OH).sub.3, Zn(OH).sub.2, Mn(OH).sub.2 and Cu(OH).sub.2, and wherein said metal oxide has a general formula selected from MeO, MeO.sub.2 and Me.sub.2O.sub.3, wherein Me=metal and O=oxygen.

10. Nanocrystalline cellulose prepared by the method according to claim 1.

11. Nanocrystalline cellulose prepared by the method according to claim 7 wherein said base is a metal oxide or metal hydroxide, and/or is characterized by: a content of metal ions, wherein said content of metal ions is in the range of from 10 mg/g of sample nanocrystalline cellulose to 800 mg/g of sample nanocrystalline cellulose.

12. Nanocrystalline cellulose according to claim 10, further characterized by a chiral nematic structure, and a left handed pitch in the range of from 0.1 μm to 1 μm.

13. Use of the nanocrystalline cellulose according to claim 10, as a substrate, matrix or coating in an electronic or pharmaceutical application, as an additive in paper or food, as a coating in a medical or pharmaceutical application, or as a reinforcing agent/filler for enhancement of mechanical strength.

14. A method for preparing a porous metal oxide with chiral nematic structure, comprising the steps: a′) performing the method according to claim 1, wherein the base that is used is a metal oxide or a metal hydroxide, b′) casting the resultant nanocrystalline cellulose in a three-dimensional shape or as a thin film, wherein said thin film has a thickness in the range of from 50 nm to 500 μm, and c′) subjecting said casted shape or thin film to a heat treatment for annealing and for removal of the nanocrystalline cellulose, said heat treatment thus resulting in a porous metal oxide with chiral nematic structure.

15. A porous metal oxide prepared by a method comprising the steps: a′) performing the method according to claim 1, wherein the base that is used is a metal oxide or a metal hydroxide, b′) casting the resultant nanocrystalline cellulose in a three-dimensional shape or as a thin film, wherein said thin film has a thickness in the range of from 50 nm to 500 μm, c′) subjecting said casted shape or thin film to a heat treatment for annealing and for removal of the nanocrystalline cellulose, said heat treatment thus resulting in a porous metal oxide with chiral nematic structure characterized by an average pore size in the range of from 2-50 nm, and/or having a chiral nematic structure characterized by: a content of metal ions, wherein said content of metal ions is in the range of from 10 mg/g of sample nanocrystalline cellulose to 800 mg/g of sample nanocrystalline cellulose.

16. The method, according to claim 6, wherein the mineral acid is sulfuric acid.

17. The method, according to claim 8, wherein (i) said metal hydroxide or NH.sub.3 is added in step c) in a molar ratio of base:mineral acid in a range of from 1:2 to 2:1, or ii) a metal oxide, wherein said metal oxide in step c) is added in a molar ratio of metal oxide: mineral acid in a range of from approximately 1:3 to 1:1.

18. The method, according to claim 11, wherein said content of metal ions is in the range of from 40 mg/g of sample nanocrystalline cellulose to 400 mg/g of sample nanocrystalline cellulose.

19. The method, according to claim 12, wherein said nanocrystalline cellulose is characterized by a left handed pitch of from 0.3 μm to 0.6 μm.

20. The method, according to claim 14, wherein said thin film has a thickness in the range of from 100 nm to 30 μm.

21. A method for packaging a product, wherein said method comprises the use of a material according to claim 10 as a packaging material, or as a coating on a packaging material.

22. The method, according to claim 21, used to package a food product.

23. The method, according to claim 21, wherein the material is used as a coating on a packaging material.

Description

[0045] In the following, reference is made to the figures wherein

[0046] FIG. 1 shows dispersions of the NCC crystallites in water viewed between cross polar filters. In the figure panel a) is the NCC dispersion prepared at 45° C., panel b) is the dispersion prepared at 60° C., and panel c) is the dispersion prepared at 70° C.

[0047] FIG. 2 shows images of the films of dried droplets of NCC dispersion on microscopic glass slides. The images a-d were taken at 90° and images e-h were taken at higher angles. In the figure, panels a) and e) are the film made from sample which was prepared at 45° C., panels b) and f) are the film made from sample which was prepared at 50° C., panels c) and g) are the film made from sample which was prepared at 60° C., and panels d) and h) are the film made from sample which was prepared at 70° C. These exemplary films have an average thickness of from 400 nm to 3 μm, or from 400 nm to 1 μm.

[0048] FIG. 3 shows microscope images of the dried NCC films obtained between crossed polarizers. Arrows indicate a typical liquid crystalline structure which was preserved in dried NCC films. FIG. 3a) shows a film obtained from the NCC nanoparticles prepared at 45° C., figure b) at 50° C., figure c) at 60° C. and figure d) at 70° C. Average film thickness of exemplary films is from 400 nm to 3 μm, or from 400 nm to 1 μm.

[0049] FIG. 4 shows XRD patterns of the NCC films made from the NCC dispersions which were washed five times. The average film thickness of exemplary films is from 15 μm to 30 μm.

[0050] FIG. 5 shows dispersions of NCC directly after the neutralization with metal oxides. From the left to the right: NCC dispersion neutralized with 5 g of ZnO, NCC dispersion neutralized with 4.96 g of CuO and NCC dispersion neutralized with 4.3 g of MnO. The greenish-blue color of the mixture of NCC dispersion indicates the formation of copper sulfate. Accordingly, pastel rose color of the Mn mixture indicates formation of manganese sulfate.

[0051] FIGS. 6, 7, 8 show NCC composites for different amounts of precursor, i.e. of metal oxide. The columns show different degrees of washing and centrifugation. The rows show the indicated amounts of precursors, i.e. metal oxides, used for neutralization. FIG. 6, 7, 8 show the results for ZnO, CuO and MnO, respectively.

[0052] FIG. 9 shows microscopic images of a ZnO—NCC composite before and after annealing of the composite. The columns show different degrees of washing and centrifugation. The rows show the state before (up) and after (down) annealing at 450° C. for 3 h. Images were taken between cross polarized filters.

[0053] FIG. 10 shows dip-coated Mn-Oxide films after annealing at 450° C. for 3 h. From left to right the different samples show a difference in color due to a change in concentration of Mn ions. Average film thickness of exemplary metal oxide films is from 100 nm to 1.5 μm.

[0054] FIG. 11 shows the appearance of the “end product” after the washing procedure. Figures a and b show the appearance of the dispersion and its gel-like properties indicated by placing the bottle up-side-down. Figures c and d show appearance of the dispersion placed between crossed polarized filters. In the figure d can be seen the preservation of the iridescence birefringence in the gel-like product (bottle is placed up-side-down).

[0055] FIG. 12 shows the change of the concentration of sulfate and sodium ions upon repetition of washing and centrifugation. b Observed decrease of the yield upon repetition of washing and centrifugation. Both graphs present behavior of the sample prepared through neutralization with NaOH.

[0056] FIG. 13 shows variations in yield depending on the type of the cation used in the neutralization after repeating the washing and centrifugation for three or four times. b Variation of the concentration of sulfate and cation in the gel-like product after third centrifugation procedure for various neutralization procedures.

[0057] FIG. 14 shows NCC films observed between cross polarizers showing iridescence birefringence.

[0058] FIG. 15a shows UV-Vis spectra of the different NCC composite films where arrow indicates the observed shoulder in the absorbance. b The calculated values of the helical pitch for the chiral nematic ordering of NCC in the composite films by using Vries' equation and the UV-Vis data.

[0059] FIG. 16a shows adsorption and desorption isotherm. b Pore size distribution obtained from adsorption isotherm by applying a NLDFT calculation model for silica with cylindrical pores.

[0060] Furthermore, reference is made to the following specific description and examples which are given to illustrate, not to limit the present invention:

EXAMPLE 1

Preparation of Nanocrystalline Cellulose in Accordance with Embodiments of the Present Invention

[0061] The current prior art preparation procedure of nanocrystalline cellulose (NCC) is based on acidic hydrolysis of the cellulose source and usually leads to a maximal yield of ca. 20%. Additionally further purification of the crystallites from the resins of sulfuric acid is based on time consuming dialysis process.

[0062] Within embodiments of the present invention, the present inventors have changed the synthesis procedure in the way that it allows a separation of cellulose nanocrystals from solution without significant loss of the product. Yields obtained according to the present invention are 80%-95%. This is achieved by direct addition of a desired base at the end of the hydrolysis process and exclusion of repeated dilution and dialysis steps using water. In certain embodiments, the base is added in equimolar amount to the sulfuric acid which was used in the hydrolysis. When the base is e.g. an alkali metal hydroxide, addition of the base leads to a neutralization process between the negatively charged sulfuric anions and positively charged base cations according to the equation:

2MeOH.sub.(aq.)+H.sub.2SO.sub.4(aq.).fwdarw.2Me.sup.++SO.sub.4.sup.2−+H.sub.2O  eq. 1

and stops the hydrolysis reaction.

[0063] The presence of the cations in the dispersion allows the separation of cellulose nanocrystals by simple centrifugation without significant loss of the product. After the centrifugation of NCC crystals from the neutralized dispersion, the upper liquid part is discarded, and the lower jelly like part is diluted with water and left for stirring for ca. 15 minutes in order to wash out salt ions. This is followed by another centrifugation. The repetition of the whole washing process up to three times leads to the yield of separated NCC particles of approximately 90% of the starting materials. Depending on the desired purity of the NCC crystals, the washing process can be repeated.

[0064] The final obtained dispersions of NCC can be diluted to the desired concentration of NCC nanoparticles in the solvent. The dispersions, placed between the crossed polar filter, show typical iridescent birefringence patterns, see also FIG. 1.

[0065] The solutions after the desired washing procedure can be dried by placing the solution in the Petri dish or coating the microscopic slide with the solution by some coating technique such as dip or spin coating. Upon drying the dispersion, the formed films show the same iridescent birefringence patterns which were found in NCC dispersions, see also FIG. 2. The average film thickness is of exemplary films is in a range from 50 nm to 500 μm, preferably from 100 nm to 30 μm. The thickness can be adjusted depending on the intended use and application.

[0066] The presence of the iridescence birefringence pattern indicates that the cellulose crystals, both in the dispersion (FIG. 1) and in dried films (FIGS. 2 and 3), form a chiral nematic phase. The pitch of the chiral nematic phase is directly influenced by the presence of the ions in solution. Since the NCC fibrils (“whiskers”) bear the negatively charged sulfuric esters, the presence of the cations in its surrounding packs the fibrils more closely together due to the electrostatic interactions. This behavior leads to a decrease of the pitch. However, upon subsequent repetition of the washing with water, cations are increasingly getting removed which leads to the increase of the pitch.

[0067] The change of the pitch alters the wavelength of the reflected light from shorter to longer wavelength. Therefore, a desired reflection wavelength can be adjusted by the repetition of the washing procedure. The desired wavelength can also be transferred into dried product upon subsequent drying of the dispersion or by preparation of the NCC thin films with an appropriate coating technique.

[0068] In order to confirm that the obtained films indeed have the crystalline structure X-ray diffraction measurements were carried out. The XRD diffractograms show broad diffraction peaks typical for nanocrystalline cellulose, see also FIG. 4. The broad peaks indicate the presence of crystallites in the nanometer range.

EXAMPLE 2

Embodiment: Preparation of Nanocrystalline Cellulose (NCC)

[0069] In an exemplary synthesis, 2 g of cotton linters were placed in an Erlenmeyer bottle. 20 ml of 65% sulfuric acid was added to the bottle with cotton linters and everything was placed in the water bath at 60° C. and vigorously stirred for three hours. Four different temperatures, namely 45, 50, 60 and 70° C., were tested for the hydrolysis procedure. 15.15 g of NaOH were dissolved in 100 ml water. After three hours, the Erlenmeyer bottle with hydrolyzed cotton linters was removed from the water bath and 30 ml of water were added into the flask in order to adjust the viscosity of the dispersion. If necessary, such dilution step can be avoided if the base that is used is approximately diluted. In any case, the method according to the present invention does not use repeated dilution steps with water to stop the hydrolysis. In order to stop the hydrolysis, the diluted dispersion was directly mixed with NaOH solution. After the final dispersion was cooled down to the room temperature, the dispersion was transferred into centrifuge bottles and centrifuged at 4000 rpm for 15 minutes. The dispersion was separated into a transparent supernatant and a bottom jelly-like part. The transparent supernatant was decanted and the bottom jelly-like part was transferred into a glass vessel and was further washed. The washing was performed in such that the jelly-like part was transferred into a glass vessel, and 100 ml of water were added. The dispersion was left under vigorous stirring for ca. 15 min. After this, the centrifugation procedure was repeated again as described above. The whole washing process including centrifugation step was repeated 3 times.

[0070] The samples prepared at 45, 50, 60 and 70° C. show that a high yield of the NCC crystals with respect to the starting material can be achieved. This was not possible to achieve when the neutralization step had been omitted. All samples show a typical iridescence birefringence pattern when placing the bottles with samples between cross polarizers. This indicates the presence of chiral nematic ordering in the samples. Upon dilution of the samples to 3% of NCC in comparison to the dried product, the samples still kept the iridescence birefringence pattern, see also FIG. 1.

[0071] By drying a drop of each sample on microscopic glass and by placing a microscopic glass with dried NCC between the cross polarizer the iridescence pattern is still preserved. This indicates that the chiral nematic ordering remains upon drying the sample. The color of the sample changes depending on the angle of the view. When viewed at 90° C., the color is slightly blue but changes into brown-yellowish at sharper angles, see also FIG. 2. The average film thickness of exemplary films was 400 nm to 1 μm.

[0072] An investigation of the dried NCC on microscopic slides between the crossed polar filters in optical microscope shows a formation of tactoids in dried films. The tactoids are typical for liquid crystal ordering, and the presence of tactoids in dried films indicates that the chiral nematic structure is preserved, see also FIG. 3. The average film thickness of exemplary films was 400 nm to 1 μm.

[0073] In order to investigate the influence of the amount of metal ions on the formation of NCC in the chiral nematic phase, the samples were prepared in the same way as described above, but the washing and centrifugation step was repeated five times instead of three. This was performed in order to remove more ions from the solution.

[0074] The crystallinity of the dried NCC which were washed and centrifuged five times was confirmed with XRD measurements, see also FIG. 4. The XRD measurements were performed on the dried films of NCC. The films were prepared such that dispersions which contained 3% NCC were placed in a Petri dish over night to dry. The XRD measurements were performed on the formed NCC films in the range of 5 till 45° 2θ. The XRD diffractograms showed at the presence of a broad peak positioned at 22.5° 2θ and two very close peaks at 15 and 17° 2θ. These peaks are typical for cellulose in crystalline form indicating that the samples are in nanocrystalline from. The average film thickness of exemplary films was from 15 μm to 30 μm.

[0075] The iridescent birefringence of the samples which were washed and centrifuged five times, in comparison to the samples which were washed and centrifuged only three times, was also examined. The samples after placing them into Petri dish and drying to form films preserve the chiral nematic ordering which was confirmed by placing the samples on microscopic glass slides between the cross polar filters. Samples viewed at 90° shows almost white color and upon rotation of the sample towards higher angles, the colors of the samples do not change significantly. This means that the reflection wavelength of the samples changed by increasing the frequency of the washing step in comparison to the samples which were washed only three times. This behavior is a result of the increased pitch since these NCC dispersions contain fewer ions than the NCC dispersions which were washed three times. The removal of ions is believed to lead to a less dense packing of NCC fibrils (“whiskers”). The increase in pitch as a consequence changes the wavelength of the reflected light.

[0076] The yield of the obtained NCC crystals slightly decreases with increasing the amount of washing steps. This indicates that upon removal of cations which hold the NCC crystals together, the electrostatic repulsions become predominant and lead to a stronger dispersion of NCC crystals. However, even after five times of washing it was possible to separate the NCC crystallites from the dispersion with centrifugation. In comparison, the samples which were prepared without any neutralization were impossible to separate with repeated washing and centrifugation steps.

EXAMPLE 3

Preparation of Nanocrystalline Cellulose-Metal Oxide-Composite Materials and/or Preparation of Porous Metal Oxides in Accordance with Embodiments of the Present Invention

[0077] According to this aspect of the present invention, it is also possible to produce a nanocrystalline cellulose-metal oxide-composite material which can subsequently be transformed into a porous oxide having a chiral nematic structure upon annealing. This can be achieved if the isolation of the nanocrystalline cellulose (NCC) and the preparation of the composite material are performed concomitantly in one pot.

[0078] In EXAMPLES 1 and 2 above it was shown that the successful separation of NCC after the hydrolysis can be achieved by addition of a desired base at the end of the hydrolysis process to stop the hydrolysis process. This addition of a base leads to a neutralization process between the negatively charged sulfuric ions and positively charged base cations, e.g. according to the equation (when the base is MeOH)

2MeOH.sub.(aq.)+H.sub.2SO.sub.4(aq.).fwdarw.2Me.sup.++SO.sub.4.sup.2−+H.sub.2O  eq. 1

and this stops the hydrolysis. Furthermore, the same neutralization reaction can be achieved by using metal oxides as a base which can react with sulfuric acid leading to the same type of the neutralization reaction according to the equation

MeO+H.sub.2SO.sub.4(aq.).fwdarw.Me.sup.2++SO.sub.4.sup.2−+H.sub.2O  eq. 2

MeO.sub.2+2H.sub.2SO.sub.4(aq.).fwdarw.Me.sup.4++2SO.sub.4.sup.2−+2H.sub.2O  eq. 3

Me.sub.2O.sub.3+3H.sub.2SO.sub.4(aq.).fwdarw.2Me.sup.3++3SO.sub.4.sup.2−+3H.sub.2O  eq. 3

which also leads to the end of the hydrolysis, see also FIG. 5. In this reaction any type of metal oxide which can follow one of the above reactions or any similar neutralization reaction can be used. In an exemplary experiment the inventors used ZnO, CuO and MnO.

[0079] The nanocrystalline cellulose can be isolated using centrifugation while at the same time the mixture can also be used for the preparation of a NCC/metal oxide composite material which is subjected to further annealing and allows the preparation of metal oxide with chiral nematic structure. The amount of metal cations in this dispersion can be tuned either by decreasing the amount of metal oxide used for the neutralization or by applying one or several washing procedures. As above, the washing procedure consists of adding water to the jelly-like NCC which contains metal, agitating it for some time and repeating the centrifugation. The supernatant is discarded while the lower jelly like part which contains NCC and metal oxide precursor is collected.

[0080] The final jelly like part can be either casted to form films having an average thickness of from 1 μm to 50 μm in one embodiment or from 15 μm to 30 μm in another embodiment (and metal oxide-NCC composites can be obtained, see also FIGS. 6, 7, 8) or further diluted to a desired concentration and used as coating material solution for the preparation of thin films on substrates, FIG. 10, with average film thicknesses of from 50 nm to 5 μm, preferably from 100 nm to 5 μm, more preferably from 400 nm to 3 μm. Microscopic images of NCC composite materials placed between cross polarized filters clearly show a preservation of the chiral nematic structure. Subsequently, both casted materials and thin films can be transferred into an oven for annealing and removal of the NCC through combustion, resulting in a porous metal oxide with chiral nematic structure.

[0081] The iridescent color of the composite material can be tuned by adding various amount of metal oxide/metal hydroxide for the neutralization and by repetition of the washing procedure, see FIGS. 6, 7 and 8.

[0082] Upon coating of the glass slides with a diluted jelly like NCC solution and annealing them at 450° C. it is possible to obtain metal oxide films which differentiate in color, see FIG. 9.

EXAMPLE 4

Experimental Part

[0083] The NCC was isolated from cotton linters according to the modified acid hydrolysis. In a typical reaction 20 mL of 65% sulfuric acid (0.185 mol) was added to 2 g of cotton linters. The mixture was transferred into a preheated water bath at 50° C. and vigorously stirred at 50° C. for 3 hours. After 3 hours the mixture was very viscous and had brownish color. The mixture was taken out from the heating bath and 30 mL of water was added to the mixture in order to decrease the viscosity. Subsequently, the diluted mixture was added to 100 mL of 3.7 M NaOH solution to stop the hydrolysis reaction. The amount of NaOH was calculated to correspond to the equimolar amount of hydroxyl groups required for the neutralization of hydrogen ions from sulfuric acid used in the hydrolysis. The neutralized mixture was stirred for 20 minutes and subsequently centrifuged at 4000 rpm for 15 minutes in order to separate the NCC. The supernatant was decanted and the gel-like NCC sediment was collected and further washed. The washing was performed in the way that to the gel-like sediment 100 ml of water was added, the whole mixture was stirred for 15 minutes and again centrifuged. After the third centrifugation the sediment was collected and used for further investigation.

[0084] For the neutralization with NaOH, KOH and NH4OH the same amount of the base was used for the neutralization. In the case of the oxides, in order to ensure that the whole amount of the oxides will undergo the reaction with the sulfuric acid a bit lower amount of the oxide was used for the neutralization, namely 0.150 moles.

Instrumentation

[0085] Optical microscopy was used to investigate the homogeneity of the samples and the presence of the chiral nematic ordering of NCC. The images were obtained by using Olympus BX51 microscope equipped with Olympus XC50 camera.

[0086] The mass concentration of the present ions, after the isolation procedure, was investigated by ion chromatography of the gel-like NCC sediment. For preparation 100 μl, 200 μl, 500 μl or 1 ml of the sample was diluted in 100 ml, depending on the concentration of the ions. 12 ml of this solution was used for the ion chromatography measurements. The ion chromatography measurements were performed using Metrohin 820 IC separation center with ASupp4-250 separation column for anions and C4-150 separation column for cations.

[0087] The porosity and BET surface area (“Brunauer-Emmett-Teller” surface area) of NCC template material was investigated by the analysis of adsorption isotherms of N.sub.2 at 77 K using an Autosorb-iQ-MP.

[0088] The optical properties of the NCC films were investigated using UV-Vis measurements which were carried out with a Specord 50 Plus spectrophotometer in the wavelength range from 190 nm up to 1100 nm.

Calculations.

[0089] The chiral nematic structures reflect the light and the peak wavelength (,max) reflected by chiral nematic structure depends on the refractive index (navg) of the material and the helical pitch (P) according the Vries' expression:

λ.sub.max=n.sub.avgP  (7)

[0090] Using the refractive index of cellulose (1.54) it is possible to calculate the helical pitch from UV-Vis spectra.

[0091] In order to estimate the mass of the dried content in the gel-like obtained product, 3 ml of the gel-like sediment were weighted (m.sub.s) and left to dry in a plastic vessel. In order to ensure that the whole water evaporates from the sample (m.sub.water), the dried samples were put into a drying oven at 50° C. for 1 h and subsequently cooled down to the room temperature before weighting. The samples were weighted before and after drying and from the mass difference the mass of the dried sample (m.sub.d,s) was calculated according:

m.sub.d,s=m.sub.s−m.sub.water  (1)

[0092] The mass of the hydrolyzed cellulose product (m.sub.HCP) in dried sample was calculated using the mass of the dried sample (m.sub.d,s) and the concentration of the ions obtained by ion chromatography measurements according to the equation

[00001]$\begin{array}{cc}{m}_{\mathrm{HCP}}=m_{d,s}-m_s.Math.{.Math.}_{i=0}^n.Math.\frac{{\gamma }_{\mathrm{ion},\mathrm{IC},i}.Math.V_{\mathrm{IC}}}{m_{\mathrm{IC},s}}& \left(2\right)\end{array}$

where m.sub.HCP is the mass of the hydrolyzed cellulose product, m.sub.d,s and m.sub.s are the masses from the equation 1, γ.sub.ion,IC,i is the mass concentration of the ion i obtained through IC measurements, V.sub.IC is the volume of the bottle in which the sample i was diluted for the IC measurement and m.sub.IC,s is the mass of the pipetted gel-like NCC for the ion i in IC measurement.

[0093] The yield of the reaction was defined as:

[00002]$\begin{array}{cc}\mathrm{yield}=\frac{m_{\mathrm{HCP}}}{m_{\mathrm{cotton}.Math..Math.\mathrm{linters}}}& \left(3\right)\end{array}$

[0094] Where m.sub.HCP is the mass of the hydrolyzed cellulose product from equation 2 and m.sub.cotton linters is the starting mass of the cotton linters used for the hydrolysis.

EXAMPLE 5

Results

Appearance of the Cellulose Nanocrystal Dispersions

[0095] Colloidal dispersions containing ca. 3-4 wt. % of cellulose nanocrystals prepared by neutralization with NaOH have a turbid and gel-like appearance and placed between crossed polarizers show iridescence birefringence, FIG. 11.

Influence of Ions on the Isolation Process

[0096] The influence of the ions on the separation of the hydrolyzed product was studied in the sample which was neutralized with NaOH. Ion chromatography measurements were performed on the sediment gel-like product which was subsequently collected after each centrifugation procedure. The ion chromatography analysis of the isolated hydrolyzed product showed that directly after the synthesis, i.e. first centrifugation, the sediment gel-like product contains high concentration of ions, FIG. 12.

[0097] The concentration of ions is gradually decreasing by repeating the washing and centrifugation procedure. After the first centrifugation the concentration of sulfate ions is 540 mg and sodium ions is 275 mg per gram of the dried sample. After the second centrifugation the concentration of ions decreases to 390 mg for sulfate and 190 mg for sodium per gram of the dried sample. Finally, after the third centrifugation the concentrations of sulfate and sodium are 179 mg and 98 mg per gram of the dried sample, respectively. Taking into account the mass fraction of the present salts in the dried samples it is possible to derive the mass of the hydrolyzed cellulose product which is present in the dried sample. It can be seen (FIG. 12b) that directly after synthesis and centrifugation a high amount of the cellulose product is obtained which corresponds to 98% of the starting mass of the cotton linters. However, one should not correlate directly this product with nanocrystalline cellulose since it would be unrealistic to expect that cotton linters contain such high portion of the crystalline phase and in the same time neglect the presence of the hydrolyzed glucose units within the sample. Upon repetition of the washing and centrifugation procedure the mass of the hydrolyzed cellulose product is decreasing to 63% and 50% of the mass of starting cotton linters for the second and third centrifugation procedure, respectively. Obviously, upon repetition of the washing and centrifugation the concentration of ions decreases but also hydrolyzed glucose parts are removed from the sediment leaving more crystalline material in the gel-like product. Further washing procedure, for the sample prepared through neutralization with NaOH, leads to the significant loss of the NCC (FIG. 13a) what can be also observed by the turbidity of the supernatant after centrifugation.

[0098] Obviously, the concentration of sodium ions seems to be important factor for the separation of the NCC by centrifugation since they play a role of counter ions for NCC. In other words by reaching the minimal critical concentration of cation, further dilution of the NCC dispersion diminished the effect of the Coulomb interaction between the cations and negatively charged NCC leading to increased dispersibility of NCC in solution. The amount of the isolated product after third centrifugation is strongly affected by the type of the cation that was used in the neutralization step (FIG. 13). The highest yield, in the neutralization reaction with a monovalent cation, of 70% was obtained for KOH and, in the neutralization reaction with a divalent cation, of 78% with MnO. After the fourth washing, significant product loss was observed for all used monovalent cations (FIG. 13a). On the other hand, the samples obtained by neutralization with oxides (ZnO, CuO, MnO), do not show such significant product loss upon repetition of the washing procedure for the fourth time although the mass fraction of present ions is decreasing from approximately 30 wt. % to approximately 15 wt. % of the dried product. Even though all samples were processed in the same way, the concentration of the salts present after the washing procedures differs between the samples. The presence of the lowest mass fraction of salt (23 wt. % of the dried hydrolyzed product) was obtained for the neutralization with NH.sub.4OH. The highest mass fraction of the salt (42 wt. % of the dried product) was observed when the neutralization was performed with KOH (FIG. 13b). These results suggest that the type of cation used in the neutralization process, its charge density and interaction with sulfate groups, play a significant role in the isolation procedure since they are involved in the Coulomb interaction with negatively charged NCC crystallites.

Composite Films

[0099] Although the cellulose nanocrystals isolated in this way contain some amount of ions, by drying the dispersions flexible films can be obtained which between cross polarizers show a preservation of the iridescence birefringence, FIG. 14.

[0100] The UV-Vis spectra of all NCC films (NCC composites) show a broad shoulder located in the region 290 nm up to 350 nm, FIG. 15. Using the refractive index of cellulose and Vries' equation it is possible to calculate the helical pitch from UV-Vis spectra. We found that the pitch of our composite materials, calculated from the shoulder observed in UV-Vis spectrum is between 190 nm for Mn-NCC and 212 nm for K-NCC composite film, respectively.

[0101] The isolated cellulose nanocrystals can be successfully used as a template material for the preparation of porous oxides. For these purposes we have prepared a silica-NCC composite material by mixing an end gel-like product obtained through neutralization with NaOH and a tetraethylorthosilicate (TEOS). Upon drying and annealing such composite material at 500° C. a porous oxide can be formed. The adsorption measurement of N.sub.2 on such material shows type IV isotherms typical for mesoporous material with a pore size distribution around 10 nm and BET area of 110 m.sup.2/g, FIG. 16.

[0102] The features of the present invention disclosed in the specification, the claims, and/or in the accompanying drawings may, both separately and in combination thereof, be material for realizing the invention in various forms thereof.