WO2014070092A1

WO2014070092A1 - Functionalized cellulose nanocrystals, a method for the preparation thereof and use of functionalized cellulose nanocrystals in composites and for grafting

Info

Publication number: WO2014070092A1
Application number: PCT/SE2013/051276
Authority: WO
Inventors: Anna CARLMARK MALKOCH; Eva MALMSTRÖM JONSSON; Assya BOUJEMAOUI
Original assignee: Swetree Technologies Ab
Priority date: 2012-10-31
Filing date: 2013-10-31
Publication date: 2014-05-08

Abstract

The present invention relates to a cellulose nanocrystal functionalized with a group R, a method for preparation of cellulose nanocrystals that are functionalized with one or more groups R and use of functionalized cellulose nanocrystals in composites and for grafting. The aim of the invention is to offer more rapid and simplified methods to generate functional CNC with high degree of reactive groups on the surface. Furthermore, there is a need for functional CNCs with groups on the surface that can be used in composite materials and where the reactive group can work as coupling agents, initiators for polymer grafting or as crosslinkers. This is achieved by a method for preparation of cellulose nanocrystals that are functionalized with one or more groups R, wherein the method comprises the steps of: a) providing cellulose; b) treating the cellulose with an acidic water solution; c) removing the acidic water solution; d) adding a solution comprising an organic acid; e) reacting the mixture obtained in step d).

Description

Functionalized cellulose nanocrystals, a method for the preparation thereof and use of functionalized cellulose nanocrystals in composites and for grafting.

TECHNICAL FIELD

The present invention relates to functionalized cellulose nanocrystals, as well as methods to produce such cellulose nanocrystals. It covers also the use of the functionalized cellulose nanocrystals in composite materials.

BACKGROUND TO THE INVENTION

Cellulose is, as the major component in plants, the world's most common biopolymer. It is an

exceptionally interesting polymer in view of its abundance, renewability, high functionality and relatively high chain stiffness. Cellulose is a polydisperse, linear polysaccharide composed of repeating p-1 ,4-D- glucose units, i.e. anhydrous glucose units (for the repeating structure, see Fig. 1) where each monomer contains three hydroxyl groups, one primary and two secondary. The degree of polymerization (DP) is up to 14 000 in native cellulose. The many hydroxyl groups bring about strong intra- and inter-molecular hydrogen bonds, which are the origin to the three dimensional, supra-molecular semicrystalline structure, which is an effect of the individual polymer chains aggregation into structural components called microfibrils. Bundels of microfibrils make up the cellulose fiber (Klemm et al., Angewandte Chemie International Edition 2005, 36, 3358-3393).

In view of the desire to develop materials based on natural resources, cellulose has received

considerable attention as a highly functional, load-carrying and light-weight reinforcing material. However, as the field of nano-science has gained more and more interest due to the unique properties that can be obtained as a consequence of shrinking dimensions, the challenge on how to successfully disintegrate the cellulose fiber into sub-structures of smaller dimensions; for example the nanostructural components of cellulose, such as nanofibrillated cellulose (NFC) and cellulose nanocrystals (CNC); have attracted much interest in recent years. Cellulose nanocrystals (CNC) can be used for polymer reinforcement and nanocomposite formulation owing to their exceptionally high mechanical strength, such as a Young's modulus of 100-140 GPa (Biomacromolecules 2005, 6, 1055-1061), low density, chemical tenability, environmental sustainability, and anticipated low cost.

CNC is produced by acid-hydrolysis of the amorphous parts of cellulose which leaves mainly the crystalline part. CNCs can be produced using acids, enzymes, oxidizers, mechanical means or a combination of these in multiple steps. The idea is to remove most of the amorphous parts of the material to give cellulose structures of smaller sizes, relying on the slower hydrolysis kinetics for crystal structures (Samir et al., Biomacromolecules 2005, 6, 612-626.) These procedures require relatively pure cellulosic starting materials such as cotton, steam-exploded wood pulp and microcrystalline cellulose, or alkaline and bleaching agents as pretreatments. One challenge is that the essentially hydrophilic microfibrils readily absorb water, which reduces dimensional stability and, furthermore, bring about that the CNC are difficult to disperse appropriately in an organic matrix, which is often the case when producing biocomposites. To fully take advantage of CNC it often has to be surface modified, requiring an extra chemical step.

Covalent modification of cellulose is a way of changing its properties to enable cellulose to be utilized in different applications. During the production of CNC, water-based solutions are utilized, and in order to avoid agglomeration of the material, they are kept in water dispersions. However, many of the desired chemical modifications cannot be performed in water. Therefore, tedious solvent exchange needs to be performed in order to covalently attach functional groups such as initiators for polymerizations, alkene- groups for crosslinking etc, limiting the usage and commerciality of these materials.

Leung et al., Small 201 1 , 7(3) 302-305, has shown that ammonium persulfate will oxidize cellulose from different sources for production of CNC. During this process the OH-groups on the CNC will be oxidized to a degree of less than 20 % of the accessible OH-groups on the CNC. The formed CNC will have a width of 3 to 6 nm and a length of 88 to 150 nm in a one-step procedure. The reaction time with ammonium persulfate takes about 16 hours.

Stable suspensions of CNC can be formed by hydrolysis of cellulose using sulfuric acid or hydrochloric acid followed by mechanical disintegration. During sulfuric acid hydrolysis, sulfate ester groups are introduced randomly on the surface resulting in nonflocculating suspensions. For composite applications, these sulfate groups are problematic due to the decreased thermal stability after drying. Efforts to overcome this limitation are documented in the literature, and include acetylation of the CNC surface using mixtures of acetic acid and anhydride, use of surfactant and coupling agents, polymer grafting, and acylation by drying aqueous emulsion.

Braun and Dorgan, Biomacromolecules 2009, p 334-341 present a mixed acid system comprised of a small amount of hydrochloric acid and acetic acid or butyric acid in water for generating esterified CNC. They estimated that approximately 50 % of the surface hydroxyl groups are esterified on the cellulose nanocrystal. The generated esterified CNC have the dimensions of width (diameter) of 25 to 50 nm and a length of 170 to 280 nm.

In the US patent application US2008/01 18765 it is presented how cellulose can be functionalized by acetic acid and butyric acid.

Morandi et al, Langmuir, 2009, 25(14), 8280-8286, describe the preparation of CNC followed by esterification with 2-bromoisobutyryl bromide (BiB) in a post-functionalization step. Similarly, Harrisson, et al., Biomacromoleules 201 1 , 12, 1214-1223, also present a method for producing 2-bromoisobutyryl- functionalized CNC in post-functionalization step. Attachment of the 2-bromoisobutyryl-functional group is a common approach to introduce an initiating function on a surface for the reversible-deactivation radical polymerization (RDRP) denoted Atom Transfer Radical Polymerization (ATRP). However, BiB is a hazardous chemical which reacts rapidly with water. Hence, this reaction has to take place in organic solvent, requiring the CNCs to be subjected to tedious solvent exchange, a process that could take several days, prior to the reaction with BiB. Furthermore, in this approach the CNCs are produced separately after which the 2-bromoisobutyryl functionality is introduced through esterification reaction with acid halide.

There is a need for more rapid and simplified methods to generate functional CNC with high degree of reactive groups on the surface. Furthermore, there is a need for functional CNC with groups on the surface that can be used in composite materials and where the reactive group can work as coupling agents, initiators for polymer grafting or as crosslinkers.

SUMMARY OF THE INVENTION

A mixed acid system with a small amount of hydrochloric acid and water in an organic acid is a viable alternative to previously utilized methods for surface modification of cellulosic nanocrystals. The present invention provides cellulose nanocrystals (CNC) with a high degree of functionalization, a method for their preparation and their use.

The cellulose nanocrystals, the method for their preparation and their use according to the present invention are defined in the appended claims.

In a first aspect the present invention relates to a cellulose nanocrystal wherein the cellulose nanocrystal is functionalized with a group R, wherein the group R is selected from the group consisting of C₂_₈ alkyl carbonyl that is halogenated in a secondary position; thiolated C₂_₈ alkyl carbonyl; tosylated C₂_₈ alkyl carbonyl; benzenesulfonyloxylated C₂_₄ alkyl carbonyl; mesylated C₂.₈ alkyl carbonyl; aryl-X-C^s-alkyl- carbonyl, which is optionally substituted; aryl-X-C^-alkyl-aryl-carbonyl. which is optionally substituted; C^ ^-alkyl-X-C^s-alkyl-carbonyl, which is optionally substituted; and C^o-alkyl-X-C^-alkyl-aryl-carbonyl, which is optionally substituted; wherein X is -0-C(=S)-S-, -S-C(=S)-0-, -N-C(=S)-S-, -S-C(=S)-N-, -S- C(=S)-S-, -S-C(=S)- or -C(=S)-S.

Another aspect of the present invention is a method for preparation of cellulose nanocrystals from native cellulose by the use of any organic acid, wherein the organic acid is selected from the group consisting of C_1-8 alkyl carboxylic acid; C₂.₈ alkyl carboxylic acid that is halogenated in a secondary position; thiolated C₂.₈ alkyl carboxylic acid; tosylated C₂.₈ alkyl carboxylic acid; benzenesulfonyloxylated C₂.₄ alkyl carboxylic acid; mesylated C₂.₈ alkyl carboxylic acid; C₂.₈ alkenyl carboxylic acid; C₂.₈ alkynyl carboxylic acid; benzoic acids that are halogenated in one or more positions; aryl-X-C^-alkyl-carboxylic acid, which is optionally substituted; aryl-X-C^-alkyl-aryl-carboxylic acid, which is optionally substituted with CN or C_1-4 alkyl; C^o-alkyl-X-C^-alkyl-carboxylic acid, which is optionally substituted; and C^o-alkyl-X-C^- alkyl-aryl-carboxylic acid, which is optionally substituted; wherein X is -0-C(=S)-S-, -S-C(=S)-0-, -N- C(=S)-S-, -S-C(=S)-N-, -S-C(=S)-S-, -S-C(=S)- or -C(=S)-S. The cellulose nanocrystals are formed simultaneously with the functionalization (esterification) of the hydroxyl groups on the cellulose.

Thus, the present invention relates to a method for preparation of cellulose nanocrystals that are functionalized with one or more groups R, wherein the method comprises the steps of:

a) providing cellulose;

b) treating the cellulose with an acidic water solution;

c) removing the acidic water solution;

d) adding a solution comprising an organic acid;

e) reacting the mixture obtained in step d);

wherein each R group is selected from the group consisting of C_1-8 alkyl carbonyl; C₂_₈ alkyl carbonyl that is halogenated in a secondary position; thiolated C₂_₈ alkyl carbonyl; tosylated C₂_₈ alkyl carbonyl;

benzenesulfonyloxylated C₂_₄ alkyl carbonyl; mesylated C₂.₈ alkyl carbonyl; C₂.₈ alkenyl carbonyl; C₂.₈ alkynyl carbonyl; benzoyl that is halogenated in one or more positions, aryl-X-C^s-alkyl-carbonyl, which is optionally substituted; aryl-X-C^s-alkyl-aryl-carbonyl, which is optionally substituted; C^o-alkyl-X-C^- alkyl-carbonyl, which is optionally substituted; and C^o-alkyl-X-C^-alkyl-aryl-carbonyl, which is optionally substituted; wherein X is -0-C(=S)-S-, -S-C(=S)-0-, -N-C(=S)-S-, -S-C(=S)-N-, -S-C(=S)-S-, -S-C(=S)- or -C(=S)-S. BRIEF DESCRIPTION OF THE FIGURES

Figure 1 presents the repeating unit of cellulose.

Figure 2 presents removal of amorphous parts of the cellulose-based materials results in micro- or nanosized cellulose structures with a higher degree of crystallinity.

Figure 3 presents the calibration curve of Ellman^'s reagent used for UV analysis. Wavelength 412 nm. Figure 4 depicts dispersions (0.5 mg/ml) of freeze-dried cellulose nanocrystals prepared from filter paper using (from the left to the right) 3-mercaptopropionic acid, acrylic acid, 2-propynoic acid and 4-pentenoic acid.

Figure 5 presents FTIR spectra of functionalized cellulose nanocrystals from (a) filter paper (b) sulfite pulp and (c) sulfate pulp.

Figure 6 presents FTIR spectra of functionalized cellulose nanocrystals from filter paper sulfite pulp and sulfate pulp prepared with 3-mercaptopropionic acid.

Figure 7 shows AFM images of CNC obtained by acid hydrolysis with different acids.

DETAILED DESCRIPTION OF THE INVENTION

A cellulose fiber consists of amorphous parts and crystalline parts. The amorphous parts can be removed by acidic hydrolysis. The crystal material obtained after acidic hydrolysis of cellulose fibers can be termed cellulose nanocrystal (CNC or CNCs in plural) or nanocrystalline cellulose (NCC or NCCs in plural), or cellulose nanowhiskers (CNW). In this application these terms are used interchangeably and refer to the crystalline part derived from cellulose fibers. The cellulose nanocrystal according to the present invention is functionalized with one or more groups R, wherein each group R is selected from the group consisting of C₂_₈ alkyl carbonyl that is halogenated in a secondary position; thiolated C₂_₈ alkyl carbonyl; tosylated C₂_₈ alkyl carbonyl; benzenesulfonyloxylated C₂-4 alkyl carbonyl; mesylated C₂_₈ alkyl carbonyl; aryl-X-C^s-alkyl-carbonyl, which is optionally substituted; aryl-X-C^-alkyl-aryl-carbonyl, which is optionally substituted; C^o-alkyl-X-C^-alkyl-carbonyl, which is optionally substituted; and C^o-alkyl-X-C^-alkyl-aryl-carbonyl, which is optionally substituted; wherein X is -0-C(=S)-S-, -S-C(=S)-0-, -N-C(=S)-S-, -S-C(=S)-N-, -S-C(=S)-S-, -S-C(=S)- or -C(=S)-S. The group R forms an ester bond together with an available OH-group on the cellulose nanocrystal to which the group R is attached. This is illustrated in Figure 2.

Thus, the cellulose nanocrystal according to the present invention may be functionalized with one or more reactive groups, such as secondary chloride, secondary bromide, secondary iodide, thiol, tosyl, mesyl, dithio groups and trithio groups, which are bound by alkyl ester bonds to the cellulose nanocrystal.

The group R may be selected from the group consisting of C₂_₄ alkyl carbonyl halogenated in a secondary position, thiolated C₂.₄ alkyl carboxylic acids; tosylated C₂.₄ alkyl carbonyl; benzenesulfonyloxylated C₂.₄ alkyl carbonyl; mesylated C₂.₄ alkyl carbonyl; phenyl-C(=S)-S-C .₈-alkyl carbonyl, optionally substituted with CN or C_1-4 alkyl: phenyl-C(=S)-S-C .₈-alkyl benzoyl, optionally substituted with CN or C_1-4 alkyl; and C -2o alkyl-S-C(=S)-S-C .₈-alkyl carbonyl, optionally substituted with CN or C_1- alkyl. Alternatively, the group R may be selected from the group consisting of C₂.₄ alkyl carbonyl halogenated in a secondary position, thiolated C₂.₄ alkyl carbonyl; tosylated C₂.₄ alkyl carbonyl; and mesylated C₂.₄ alkyl carbonyl.

The functional cellulose nanocrystal may be functionalized with a covalently linked group R that together with an available OH-group on the cellulose nanocrystal to which it is attached may form a group that is selected from the group consisting of 2-bromopropanoate, 2-chloropropanoate, 2-iodopropanoate, 2-bromobutanoate, 2-chlorobutanoate, 2-iodobutanoate, 3-mercaptopropanoate, 4-cyano-4- (phenylcarbonothioylthio)pentanoyl, 2-(dodecylthiocarbonothioylthio)-2-methylpropionyl, 4-cyano-4- [(dodecylsulfanylthiocarbonyl)sulfanyl]pentanoyl, 2-((phenylcarbonothioyl)thio)acetyl, 4- ((phenylcarbonothioyl)thio)butanoyl, and 4-(((phenylcarbonothioyl)thio)methyl)benzoyl. Alternatively, the group R together with an available OH-group on the cellulose nanocrystal to which it is attached may form the group 2-bromopropanoate, 2-chloropropanoate, or 3-mercaptopropanoate; or the group R together with an available OH-group on the cellulose nanocrystal to which it is attached forms 2-bromopropanoate. The functional cellulose nanocrystal according to the present invention may have a width of 5 to 90 nm, or from 5 to 20 nm; and length between 150 to 1000 nm, or from 150 to 400 nm.

An advantage with the cellulose nanocrystals according to the present invention is that they can be used for grafting, for example with controlled polymerization such as atom transfer radical polymerization (ATRP). Grafting with ATRP provides uniform growth of the grafted polymer chain, which enables control of the molecular weight and dispersity, resulting in well-defined grafts. Another advantage of this method is that the polymerization can be restricted to the surface initiated polymerization, and hence no free polymer is formed. Acid-hydrolysis using an acid containing an ATRP initiator, such as 2-bromopropionic acid and 2-chloropropionic acid renders cellulose nanocrystals with ATRP-initiators covalently attached to the cellulose nanocrystal.

Grafting with RAFT gives the same advantages as ATRP, control of the molecular weight and dispersity, resulting in well-defined grafts. However, RAFT-polymerization can be performed in large range of solvents (including water), within a wide temperature range, is suitable for use with many different monomers and does not require highly rigorous removal of oxygen and other impurities. RAFT polymerization uses thiocarbonylthio compounds, such as dithioesters, thiocarbamates, and xanthates, to mediate the polymerization via a reversible chain-transfer process.

Another advantage with the cellulose nanocrystals according to the present invention is that they may be freeze-dried and further they may again be well re-dispersed in water after having been freeze-dried.

The present invention also relates to a method for preparation of cellulose nanocrystals that are functionalized with one or more groups R, wherein the method comprises the steps of:

a) providing cellulose;

b) treating the cellulose with an acidic water solution;

c) removing the acidic water solution;

d) adding a solution comprising an organic acid;

e) reacting the mixture obtained in step d) by raising the temperature to between 30 °C and 150 °C; optionally in a closed container;

f) stopping the reaction in step e);

g) purification of the functional cellulose nanocrystals obtained in step e);

wherein each R group is as previously defined herein.

As used herein the term "C^s alkyl" refers to a straight or branched, saturated aliphatic chain of 1 to 8 carbon atoms and includes but is not limited to methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, t-butyl, n-pentyl, n-hexyl, and the like. The term "G₂__s a!keny!" refers to a straight or branched aliphatic chain of 2 to 8 carbons comprising one double bond. The term "C₂_₈ alkynyl" refers to a straight or branched aliphatic chain of 2 to 8 carbons comprising one tripple bond. The term "aryl" refers to an aromatic monocyclic ring or aromatic bicyciic ring system containing 6 to 10 carbon atoms. Examples of aryl include phenyl, naphthyl and azulenyl. The term "C_1-8 alkyl carboxylic acid," as used herein, means a straight or branched C_1-8 alkyl, as defined before, with a carboxylic acid group, thus containing from 2 to 9 carbon atoms. The term "C_1-8 alkyl carbonyl" means a straight or branched C_1-8 alkyl with a carbonyl group, thus containing from 2 to 9 carbon atoms. Terms such as "C₂-8 a!keny! carboxylic acid", "C₂-s alkenyl carbonyl", "C₂._s alkynyl carboxylic acid" and "C₂,₈ alkynyl carbonyl" are to be construed accordingly. The cellulose used in step a) of the method according to the present invention may be chosen from native cellulose, bacterial cellulose, cotton, paper, sulfite pulp or sulfate pulp. The acidic solution in step b) may be an inorganic acid, such as hydrochloric acid (HCI), alkyl sulfonic acid, aryl sulfonic acid or sulfuric acid, for example in a concentration of 1 -5 N, 1 -4 N, 1 -3 N, or 2-3 N (normality concentration). The time for the treatment in step b) is somewhat dependent on the concentration of the acid used in the acidic water solution in step b). The treatment in step b) may be performed for from 5 to 20 minutes, or from 5 to 15 minutes, or from 5 to 10 minutes or for at least 5 minutes, for example at a temperature above room temperature, such as at a temperature of at least 90 °C. The heating may be performed by microwave irradiation, heating plate, oil bath, induction, or a combination of these.

An advantage with the treatment step b) is that the time required for successful acid hydrolysis during the reaction in step e) can be significantly reduced. Another advantage with this method is that the treatment in step b) reduces the total volume of cellulose, and thus less amount of the organic, functional acid is needed in step d).

The acidic water solution in step c) may be removed by filtration optionally without further drying.

The functional acid used in step d) of the method of the present invention may have a low pKa, such as a pKa less than 5, less than 4.7, less than 4.4, less than 4, less than 3.6, or less than 3.2.

The organic acid used in step d) of the method according to the present invention may be an organic acid, such as a carboxylic acid, such as an organic acid selected from the group consisting of C_1-8 alkyl carboxylic acids, C₂_₈ alkyl carboxylic acids that are halogenated in a secondary position, thiolated C₂_₈ alkyl carboxylic acids, tosylated C₂_₈ alkyl carboxylic acids; benzenesulfonyloxylated C₂_₄ alkyl carboxylic acids; mesylated C₂.₈ alkyl carboxylic acids; C₂.₈ alkenyl carboxylic acids; C₂.₈ alkynyl carboxylic acids; benzoic acids that are halogenated in one or more positions; aryl-X-C^s-alkyl-carboxylic acid, which is optionally substituted; aryl-X-C^s-alkyl-aryl-carboxylic acid, which is optionally substituted; C^o-alkyl-X- C^-alkyl-carboxylic acid, which is optionally substituted; and C^o-alkyl-X-C^-alkyl-aryl-carboxylic acid, which is optionally substituted; wherein X is -0-C(=S)-S-, -S-C(=S)-0-, -N-C(=S)-S-, -S-C(=S)-N-, -S- C(=S)-S-, -S-C(=S)- or -C(=S)-S. The organic acid used in step d) of the method according to the present invention may be an organic acid selected from the group consisting of:

C_1-8 alkyl carboxylic acids, such as ethanoic acid, propanoic acid, butanoic acid, pentanoic acid, hexanoic acid, heptanoic acid, or octanoic acid;

C₂_4 alkyl carboxylic acids that is halogenated in a secondary position, such as 2-bromopropanoic acid, 2- chloropropanoic acid, 2-bromobutanoic acid, or 2-chlorobutanoic acid; thiolated C₂_₄ alkyl carboxylic acids, such as 3-mercaptopropionic acid;

tosylated C₂_₄ alkyl carboxylic acids, such as 4-methyl benzenesulfonyloxy ethanoic acid, 1 -[4-methyl benzenesulfonyloxy] propanoic acid, 2-[4-methyl benzenesulfonyloxy] propanoic acid, 1 -[4-methyl benzenesulfonyloxy] butanoic acid, 2-[4-methyl benzenesulfonyloxy] butanoic acid, or 3-[4-methyl benzenesulfonyloxy] butanoic acid;

benzenesulfonyloxylated C₂_₄ alkyl carboxylic acids, such as 3-benzenesulfonyloxy propanoic acid, 2- benzenesulfonyloxy propanoic acid, 4-benzenesulfonyloxy butanoic acid, 2-benzenesulfonyloxy butanoic acid, or 3-benzenesulfonyloxy butanoic acid;

mesylated C₂_₄ alkyl carboxylic acids, such as 3-methylsulfonyl propanoic acid, 2-methylsulfonyl propanoic acid, 4-methylsulfonyl butanoic acid, 2-methylsulfonyl butanoic acid, or 3-methylsulfonyl butanoic acid;

C₂_8 alkenyl carboxylic acids, such as acrylic acid, methacrylic acid, (£/Z)-2-pentenoic acid, (£/Z)-3- pentenoic acid, 4-pentenoic acid, (£/Z)-2-methyl-2-butenoic acid, 3-methyl-2-butenoic acid, 2-methyl-3- butenoic acid, 3-methyl-3-butenoic acid, 2-methylenebutanic acid, (£/Z)-2-hexenoic acid, (£/Z)-3- hexenoic acid, 5-hexenoic acid, (£/Z)-2-methyl-2-pentenoic acid, (£/Z)-2-ethyl-2-butenoic acid, 2,3- dimethyl-2-butenoic acid, 3-methyl-2-methylenebutanoic acid, (£/Z)-4-methyl-2-pentenoic acid, (E/Z)-2- heptenoic acid, (£ Z)-5-heptenoic acid, (£/Z)-3-heptenoic acid, (E/Z)- 3,4-dimethyl-2-pentenoic acid, (£ Z)-4-heptenoic acid, 3-methyl-2-methylenepentanoic acid, 6-heptenoic acid, (£/Z)-2-isopropyl-2- butenoic acid, (£/Z)-2,3-dimethyl-2-pentenoic acid, (£/Z)-4-methyl-2-hexenoic acid, (£ Z)-2,3-dimethyl-2- pentenoic acid, (£ Z)-2-methyl-2-hexenoic acid, (£ Z)-2-ethylidenepentanoic acid, (£/Z)-2-ethyl-2- pentenoic acid, 2-ethyl-3-methyl-2-butenoic acid, (£/Z)-2-ethyl-2-pentenoic acid, (£/Z)-2-octenoic acid, (£ Z)-3-octenoic acid, (£/Z)-4-octenoic acid, (£/Z)-5-octenoic acid, (£/Z)-6-octenoic acid, 7-octenoic acid, (£ Z)-2-ethyl-2-hexenoic acid, (£/Z)-2-methyl-2-heptenoic acid, (£/Z)-2-ethyl-2-hexenoic acid, (E/Z)-2- propyl-2-pentenoic acid, 2-methyleneheptanoic acid, (£/Z)-2-propyl-2-pentenoic acid, (E/Z)-2- ethylidenehexanoic acid, 2-(propan-2-ylidene)pentanoic acid, (£/Z)-2-ethyl-3-methyl-2-pentenoic acid,

(£ Z)-2-ethyl-3-methyl-2-pentenoic acid, 3-ethyl-2-methyl-2-pentenoic acid, 2-(propan-2-ylidene)pentanoic acid, 3-methyl-2-methylenehexanoic acid, 3-ethyl-2-methylenepentanoic acid, (£/Z)-2-isopropyl-2- pentenoic acid, 2-isopropyl-3-methyl-2-butenoic acid, (£ Z)-2-ethylidene-3-methylpentanoic acid, (E/Z)- 3,4-dimethyl-2-hexenoic acid, or (£/Z)-2,3,4-trimethyl-2-pentenoic acid;

C₂.₈ alkynyl carboxylic acids, such as propiolic acid, 2-butynoic acid, 2-pentynoic acid, 2-hexynoic acid, 2- heptynoic acid, 2-octynoic acid, 4-methyl-2-pentynoic acid, 4-methyl-2-hexynoic acid, 6-methyl-2- heptynoic acid, 4-ethyl-2-hexynoic acid, 4-methyl-2-heptynoic acid, 2-methyl-3-heptynoic acid, 2-methyl- 3-hexynoic acid, 2-methyl-3-pentynoic acid, 2-methyl-3-butynoic acid, 2,5-dimethyl-3-hexynoic acid, 5- methyl-3-heptynoic acid, 5-methyl-3-hexynoic acid, 3-octynoic acid, 3-heptynoic acid, 3-hexynoic acid, 3- pentynoic acid, 3-butynoic acid, 4-pentynoic acid, 4-hexynoic acid, 4-heptynoic acid, 4-octynoic acid, 2- methyl-4-pentynoic acid, 2,3-dimethyl-4-pentynoic acid, 2-methyl-4-hexynoic acid, 3-methyl-4-hexynoic acid, 3-ethyl-4-hexynoic acid, 6-methyl-4-heptynoic acid, 3-ethyl-2-methyl-4-pentynoic acid, 4-pentynoic acid, 3-ethynylhexanoic acid, 2-ethyl-4-hexynoic acid, 2,3-dimethyl-4-hexynoic acid, 3-ethyl-4-hexynoic acid, 2-propyl-4-pentynoic acid, 2-ethyl-4-pentynoic acid, 2-isopropyl-4-pentynoic acid, 2-methyl-4- heptynoic acid, 3-methyl-4-heptynoic acid, 3-isopropyl-4-pentynoic acid, 5-hexynoic acid, 5-heptynoic acid, 5-octynoic acid, 2-methyl-5-hexynoic acid, 3-methyl-5-hexynoic acid, 4-methyl-5-hexynoic acid, 2,3- dimethyl-5-hexynoic acid, 2,4-dimethyl-5-hexynoic acid, 3,4-dimethyl-5-hexynoic acid, 2-methyl-5- heptynoic acid, 3-methyl-5-heptynoic acid, 4-methyl-5-heptynoic acid, 2-ethyl-5-hexynoic acid, 4-ethyl-5- hexynoic acid, 3-ethyl-5-hexynoic acid, 6-heptynoic acid, 6-octynoic acid, 2-methyl-6-heptynoic acid, 3- methyl-6-heptynoic acid, 4-methyl-6-heptynoic acid, 5-methyl-6-heptynoic acid, or 7-octynoic acid;

benzoic acids that are halogenated in one, two, three, four or five positions, such as 3-bromobenzoic acid, or 4-bromobenzoic acid;

phenyl-C(=S)-S -Ci.₈-alkyl-carboxylic acid, optionally substituted with CN or Ci_-4 alkyl, such as 2- [(phenylcarbonothioyl)thio]acetic acid, 4-[(phenylcarbonothioyl)thio]butanoic acid, or 4-Cyano-4- (phenylcarbonothioylthio)pentanoic acid;

phenyl-C(=S)-S-Ci-8-alky1-benzoic acid, optionally substituted with CN or Ci_-4 alkyl, such as 4- ([(phenylcarbonothioyl)thio]methyl)benzoic acid;

Ci-2o-alkyl-S-C(=S)-S-Ci.₈-alkyl-carboxylic acid, optionally substituted with CN or Ci_-4 alkyl, such as 2-(dodecylthiocarbonothioylthio)-2-methylpropionic acid, or 4-Cyano-4- [(dodecylsulfanylthiocarbonyl)sulfanyl]pentanoic acid.

Alternatively, the functional acid used in step d) may be 2-bromopropanoic acid, 2-chloropropanoic acid, 2-bromobutanoic acid, 2-chlorobutanoic acid or 3-mercaptopropionic acid; or the functional acid used in step d) may be 2-bromopropanoic acid or 3-mercaptopropionic acid. The functional acid in step d) may be used neat.

When using organic acids that are crystalline in step d) an organic solvent may be added such as, dimethylformamide (DMF), N-methyl pyrrolidone (NMP), 1 ,3-dimethyl-3,4,5,6-tetrahydro-2(1 H)- pyrimidinone (DMPU), dimethyl sulfoxide (DMSO), dimethyl acetamide (DMAC), acetonitrile,

tetrahydrofuran (THF).

With the method according to the present invention the CNC may be functionalized with acrylate groups by using acrylic acids. Alkene functionalities can be introduced using acids containing alkene

functionalities, such as 4-pentenoic acid. These groups can subsequently be utilized as coupling agents or as cross linkable groups; clickable-groups and/or cross linking groups can also be introduced using acids containing alkyne functionalities, such as 2-propynoic acid; thiol-groups can be introduced using acids containing thiol functionalities, such as 3-mercaptopropionic acid.

The temperature in step e) may be raised to between 30 °C and 150 °C, or to between 70 °C and 150 °C, or to between 90 °C and 110 °C, or to 100 °C, or just above the boiling point of water.

The time for reaction in step e) may be no longer than 6 hours, or no longer than 5 hours, or no longer that 4 hours, or no longer than 3 hours, or no longer than 2 hours, or no longer than 1 hour, or no longer than 30 minutes. The reaction in step e) may be stopped by diluting the functional acid with water and/or lowering the temperature to 30 °C or below. The dilution of the functional acid for stopping the reaction may be made by water or an acidic water solution, wherein the acidic water solution may comprise hydrochloric or sulfuric acid.

The purification in step g) may be performed by removal of the reaction solution including any residual reactants and degradation products, for example by a method comprising filtration or centrifugation.

The relative amount of reagents required in the method for preparation of cellulose nanocrystals that are functionalized with one or more groups R according to the present invention may be readily determined in the light of the foregoing description and the examples herein.

An advantage with the present method is that the production of functional cellulose nanocrystals may be made in-situ from native cellulose by the use of the corresponding organic acids. The hydrolysis of the cellulose chains will simultaneously hydrolyze the amorphous cellulose part of native cellulose and perform an esterification of the hydroxyl groups on the formed cellulose nanocrystal.

The method provides a versatile and easy-to use procedure to obtain CNC with a specific surface functionality that can be used for post-functionalization or the attachment/grafting of polymers. An advantage with using acid-hydrolysis in the method according to the present is that elaborate solvent exchange steps, centrifugation, utilizations of organic solvents and immobilizations steps can be omitted and that the need for work-up may be limited to washing.

Another advantage with the method according to the present invention is that it enables the preparation of a cellulose nanocrystal that is functionalized with a group that may act as an ATRP-initiator, such as 2-bromopropanoate, without using 2-bromoisobutyryl bromide (BiB) and therefore organic solvents and time-consuming solvent exchanges in connection therewith can be avoided.

Another advantage with the method according to the present invention is that it enables the preparation of a cellulose nanocrystal that is functionalized with a group that may act as an RAFT-agent, such as 4- cyano-4-(phenylcarbonothioylthio)pentanoic acid, 2-(dodecylthiocarbonothioylthio)-2-methylpropionic acid, 4-cyano-4-[(dodecylsulfanylthiocarbonyl)sulfanyl]pentanoic acid, 2-((phenylcarbonothioyl)thio)acetic acid, 4-((phenylcarbonothioyl)thio)butanoic acid, 4-(((phenylcarbonothioyl)thio)methyl)benzoic acid and therefore organic solvents and time-consuming solvent exchanges in connection therewith can be avoided.

It was surprisingly found that CNC could be functionalized with 2-bromopropanoic acid although both carbonyl and Br are reactive groups that could be expected to be hydrolyzed. An advantage with the method of the present invention is that at least 50%, at least 60%, at least 70%, at least 80%, at least 90%, or close to 100% of the total number of accessible hydroxyl groups in the cellulose nanocrystal may be esterified with the group R. The present invention also relates to the use of a functional cellulose nanocrystal according to the present invention in composites and for grafting, for example grafting with radical polymerization, such as atom transfer radical polymerization (ATRP) or reversible addition fragmentation chain transfer (RAFT) polymerization. With a method according to the present invention several other functionalities can be incorporated directly onto the CNC's by using acid-hydrolysis.

Experimental

Materials and Instrumentation Dried sulfite and sulfate pulp were obtained from SweTree Technologies AB. Cotton, in the form of filter paper (Whatman #1) from Whatman. Filter paper sulfite pulp and sulfate pulp were ground using a household coffee grinder and used without further purification or drying. 3-mercaptopropionic acid (3- MPA), acrylic acid (AA), 4-pentenoic acid (4-PA), 2-propynoic acid (2-PyA), 2-bromopropanoic acid (2- BPA) and 37% hydrochloric acid (HCI) were used as received. Sonication of the CNC water dispersion after acid hydrolysis and subsequent washing was performed using an ultrasonic homogenizer. The disruption period was 5 s with 1 s intervals in an ice bath for duration of 30 min using an amplitude of 30%.

The quantification of the amount of thiol groups attached to the surface was performed using the Ellman ^'s reagent (http://en.wikipedia.org/wiki/Ellman's_reagent 2012-10-31). A solution of 3 mM 5,5'-dithiobis-(2- nitrobenzoic acid) (DTNB) phosphate buffer was prepared using 29.7 mg of DTNB in 25 ml phosphate buffer solution (pH=7). For the analysis, 10 ml of this DTNB phosphate buffer solution was added to 2 mg of CNC immobilized with 3-mercaptopropionic acid, covered with aluminum foil and mixed on a shaking device for 30min. The mixture was further diluted with phosphate buffer solution to obtain a concentration of 0.033 mg/ml and filtered using Teflon syringe filter to remove CNC that could influence the analysis. A Cary 100 UV/VIS spectrophotometer (Varian, Palo Alto, CA, USA) was used to record the absorbance increase for 412 nm, (extinction coefficient of 14.150 M^" cm^"1).

The calibration curve at wavelength 412 nm is presented in Figure 3. Characterization

Fourier transform infrared spectroscopy analysis (FTIR) of the CNC was conducted on a Perkin-Elmer Spectrum 2000 FT-IR equipped with a MKII Golden Gate, Single Reflection ATR system from Specac Ltd, London, U.K. All spectra were normalized against a specific ATR crystal adsorption. Atomic force microscopy analysis (AFM): the sample was applied on mica modified surface with poly L- lysine 1 % and the sample surface was scanned with a cantilever having a tip radius <10 μηι in tapping mode.

EXAMPLE 1 Synthesis of cellulose nanocrystals (CNC)

Cellulose nanocrystals (CNC) were prepared according to a modified procedure as described by Paillet and Dufresne (ref Michel Paillet and Alain Dufresne Macromolecules, Vol. 34, No. 19, 2001). Hydrochloric acid (37%, 15 ml_, 12 N) was added drop-wise to a suspension of cellulose fibers (2 g) in deionized water (45 ml_, 0 °C) cooled on an ice/water bath. The solution was subsequently refluxed for 1 .5 h. Thereafter the reaction was quenched by diluting it with a 10 times the reaction volume of deionized water and the suspension was filtered over glass filter funnel pore 4 and washed with deionized water until neutrality was reached (pH ~ 5). Finally, the HCI-CNC was dispersed in deionized water (250 ml_) by sonication for 30 min and the resulting aqueous dispersion was subsequently freeze-dried. The yield obtained was 60% w/wt. EXAMPLE 2

Synthesis of functional cellulose nanocrystals (CNC)

0.5 g of ground cellulose fibers (cellulose filter paper (FP) Whatman # 1 , sulfite pulp or sulfate pulp, respectively) were added to a 50 ml two-necked round bottomed flask equipped with a magnetic stirrer, and 1 1 .25 ml of deionized water was added. The flask was immersed in ice bath and HCI (3.75 ml, 37 %) was added drop-wise. After the addition of HCI, the reaction mixture was stirred in an oil-bath pre-heated to 1 10 °C for 15 min. Thereafter, the mixture was filtered over glass filter pore 1 and the solid residue was further treated with an excess of the desired organic acid of choice (10 ml) at 1 10 °C for 4 hours. Table 1 summarizes the organic acids used for the hydrolysis reaction and their pK_a values and boiling points, respectively. The hydrolysis was quenched by diluting the system with 10 times the liquid volume with deionized water thereafter the reaction mixture was filtered through a glass filter pore 4, and thoroughly washed with deionized water until a pH of 5 was reached. The obtained suspension was collected, diluted with deionized water to obtain a total volume of 250 ml, sonicated for 30 min using 30% as amplitude and subsequently freeze dried. Noteworthy, when acrylic acid was used for the hydrolysis reaction, an amount of methylhydroquinone (1 % wt/wt to the acrylic acid) was added to the mixture to avoid polymerization of the acrylate groups.

Table 1

3-mercaptopropionic acid (3-MPA) 4.34 217

acrylic acid (AA) 4.35 141

4-pentenoic acid (4-PA) 4.67 83 - 84 °C. @ 12.00 mm Hg

2-propynoic acid (2-PyA) 1 .88 140

EXAMPLE 3

Comparative example

2 g of cellulose filter paper (FP) Whatman # 1was blended in 100 ml deionized water for 5 min using a domestic blender. Thereafter, 200 ml hydrochloric acid was added (37%) and the reaction mixture was stirred at 50 °C for 1 day. The obtained cellulose substrate was washed until neutral pH then placed in 250 ml round bottomed flask in a mixture of 12 g 2-bromo-2-methylpropionic acid in 100 ml deionized water. The reaction mixture was stirred in an oil-bath pre-heated to 70 °C for 5 days. Thereafter, the mixture was diluted 10-fold then filtered over glass filter pore 1 to remove any left large fibers and the filtrate was subsequently filtered through a glass filter pore 4, and thoroughly washed with deionized water until a pH of 5 was reached. The obtained suspension was collected and dialyzed against deionized water for 4 days then dispersed by ultrasonication. Since 2-bromo-2-methylpropionic acid is a solid that is not stable in hot water the tertiary bromine group was hydrolyzed, which rendered a group inactive as an ATRP-initiator and thus the obtained CNC was not functionalized with 2-bromoisobutyryl comprising a tertiary bromine atom.

EXAMPLE 4

Comparative example

5 g of ground cellulose fibers (cellulose filter paper (FP) Whatman # 1 were added to a 250 ml round bottomed flask equipped with a magnetic stirrer, and 1 12.5 ml of deionized water was added. The flask was immersed in ice bath and HCI (37.5 ml, 37 %) was added drop-wise. After the addition of HCI, the reaction mixture was stirred in an oil-bath pre-heated to 1 10 °C for 15 min. Thereafter, the mixture was filtered over a Durapore® membrane filter (pore seize: 0.22 μηι) and the solid residue was further treated with an excess of acetic acid (15 ml) at 1 10 °C for 4 hours. The hydrolysis was quenched by diluting the system with 4 times the liquid volume with deionized water thereafter the reaction mixture was centrifuged at 4400 rpm, and thoroughly washed and centrifuged with deionized water until a pH of 5 was reached.

Thus, by using pre-hydrolysis with HCI the amount of organic acid used for functionalization can be significantly reduced with respect to the amount of cellulose than without the use of pre-hydrolysis, as for example in Braun and Dorgan, Biomacromolecules 2009, p 334-341 . Results

All pulp sources functionalized according to Example 2 gave nanocrystalline cellulose, which showed flow birefringence observed through cross polarized filters. Figure 4 shows photographs of the dispersability (0.5 mg/ml) of freeze-dried cellulose nanocrystals prepared from filter paper using (from the left to the right) 3-mercaptopropionic acid, acrylic acid, 2-propynoic acid and 4-pentenoic acid. The functional CNC's have been re-dispersed in water.

Fourier transform infrared spectroscopy analysis (FTIR)

Normalized FT-IR spectra for the modified cellulose nanocrystals (Figures 5 a, b and c) show an increase in the intensity of the peak attributed to the carbonyl group at 1730 cm^"1 in the ester group which confirms the successful functionalization of CNC by acid hydrolysis. The abbreviations used in the spectra in Figures 5 a, b and c; 2-PyA, 4-PA, 2-BPA, AA, 3-MPA; stand for: 2-propynoic acid, 4-pentenoic acid, 2- bromopropanoic acid, acrylic acid, 3-mercaptopropionic acid, respectively. Figure 6 illustrates a comparison between FT-IR spectra for the three different CNC prepared with 3- mercaptopropionic acid from filter paper, sulfate pulp and sulfite pulp. The intensity of the peak attributed to the carbonyl group is higher for CNC from sulfite pulp while the lowest intensity is obtained for the CNC prepared from filter paper. This comparison indicates that the degree of substitution of available hydroxyl group depends on cellulose source used as starting material. This statement is further approved with the results of Ellman^'s analysis discussed in the next paragraph.

Analysis of the degree of surface functionalization by Ellman^'s reagent

Ellman^'s reagent is used to quantify the number or concentration of thiol groups in a sample and is more or less to be considered as a standard method in biochemistry. This analysis can be used to evaluate the thiol-containing initiator content attached on cellulose nanocrystals made with 3-MPA. Table 2 summarizes the results obtained for functionalized CNC prepared from filter paper (FP), sulfite pulp and sulfate pulp. The initiator content varied depending on the source of cellulose fibers. These results are in good agreement with FTIR results where a higher intensity for the carbonyl peak is obtained for sulfite pulp followed by sulfate pulp and then filter paper.

Table 2

(-) not calculated The calculation of the percentage of modified hydroxyl group was based on the following equation:

% modified OH groups = n_(thi0| _groups) / n_(avaNabie OH groups) x 100

It is assumed that the n_(avaii_abi_e OH groups) available for modification is similar to the one obtained for cotton linters determined by C ³ solid state NMR wherein only 7.1 % of hydroxyl group in cellulose are available for surface modification (Susanne Hansson, ARGET ATRP as a Tool for Cellulose Modification, KTH Royal Institute of Technology, 2012. ISBN: 978-91 -7501 -544-6). This number is equal to 1 .31 mmol of available hydroxyl groups. For samples prepared from sulfite and sulfate pulp, the percentage of modified hydroxyl groups could not be measured as there is no indication of how many hydroxyl groups are available in these starting materials. AFM images of functionalized CNC

AFM images of CNC obtained from filter paper by acid hydrolysis using hydrochloric acid, 2-propynoic acid, 2-bromopropionic acid, 3-mercaptopropionic acid, 4-pentenoic acid are presented from left to right in Figure 7. The micrograph size is 5x5 μητ The obtained CNC has different geometry, for example CNC prepared using 2-propynoic acid have higher aspect ratio as they are longer compared to the two other ones. The acid used for the acid hydrolysis has a great influence on the size and consequently the aspect ratio of the obtained cellulose nanocrystals.

Claims

1 . A cellulose nanocrystal functionalized with a group R, wherein the group R is selected from the group consisting of C₂_₈ alkyl carbonyl halogenated in a secondary position; thiolated C₂-₈ alkyl carbonyl; tosylated C₂_₈ alkyl carbonyl; benzenesulfonyloxylated C₂_₄ alkyl carbonyl; mesylated C₂_ 8 alkyl carbonyl; aryl-X-C^-alkyl-carbonyl, optionally substituted; aryl-X-C^-alkyl-aryl-carbonyl, which is optionally substituted; C^o-alkyl-X-C^-alkyl-carbonyl, which is optionally substituted; and C^o-alkyl-X-C^s-alkyl-aryl-carbonyl, which is optionally substituted; wherein X is

-0-C(=S)-S-, -S-C(=S)-0-, -N-C(=S)-S-, -S-C(=S)-N-, -S-C(=S)-S-, -S-C(=S)- or -C(=S)-S.

2. A cellulose nanocrystal according to claim 1 , wherein the group R forms an ester bond together with an available OH-group on the cellulose nanocrystal to which it is attached.

3. A cellulose nanocrystal according to claim 1 or 2, wherein the group R is selected from the group consisting of C₂.₄ alkyl carbonyl halogenated in a secondary position, thiolated C₂.₄ alkyl carboxylic acids; tosylated C₂.₄ alkyl carbonyl; benzenesulfonyloxylated C₂.₄ alkyl carbonyl;

mesylated C₂.₄ alkyl carbonyl; phenyl-C(=S)-S-C .₈-alkyl carbonyl, optionally substituted with CN or C_1-4 alkyl: phenyl-C(=S)-S-C .₈-alkyl benzoyl, optionally substituted with CN or C_1-4 alkyl; and C -2o alkyl-S-C(=S)-S-C .₈-alkyl carbonyl, optionally substituted with CN or C_1-4 alkyl.

4. A cellulose nanocrystal according to any one of claims 1 to 3, wherein the group R together with an available OH-group on the cellulose nanocrystal to which it is attached forms a group that is selected from the group consisting of 2-bromopropanoate, 2-chloropropanoate, 2- bromobutanoate, 2-chlorobutanoate, and 3-mercaptopropanoate.

5. A cellulose nanocrystal according to any one of claims 1 to 4, where the group R together with an available OH-group on the cellulose nanocrystal to which it is attached forms a group that is selected from the group consisting of 2-bromopropanoate, 2-chloropropanoate, and 3- mercaptopropanoate.

6. A cellulose nanocrystal according to any of claim 1 to 5, wherein the group R together with an available OH-group on the cellulose nanocrystal to which it is attached forms 2- bromopropanoate.

7. A method for preparation of cellulose nanocrystals that are functionalized with one or more

groups R, wherein the method comprises the steps of

a) providing cellulose;

b) treating the cellulose with an acidic water solution;

c) removing the acidic water solution;

d) adding a solution comprising an organic acid; and

1 e) reacting the mixture obtained in step d);

wherein each group R is selected from the group consisting of C_1-8 alkyl carbonyl; C₂_₈ alkyl carbonyl that is halogenated in a secondary position; thiolated C₂_₈ alkyl carbonyl; tosylated C₂_₈ alkyl carbonyl; benzenesulfonyloxylated C₂_₄ alkyl carbonyl; mesylated C₂.₈ alkyl carbonyl; C₂.₈ alkenyl carbonyl; C₂.₈ alkynyl carbonyl; benzoyl that is halogenated in one or more positions, aryl- X-C^s-alkyl-carbonyl, which is optionally substituted ; aryl-X-C^-alkyl-aryl-carbonyl. which is optionally substituted; C^o-alkyl-X-C^-alkyl-carbonyl, which is optionally substituted; and C_{1 -2}o- alkyl-X-C^s-alkyl-aryl-carbonyl, which is optionally substituted; wherein X is -0-C(=S)-S-, -S- C(=S)-0-, -N-C(=S)-S-, -S-C(=S)-N-, -S-C(=S)-S-, -S-C(=S)- or -C(=S)-S.

The method for preparation of cellulose nanocrystals that are functionalized with one or more groups R, wherein the method comprises the steps of

a) providing cellulose;

b) treating the cellulose with an acidic water solution;

c) removing the acidic water solution;

d) adding a solution comprising a organic acid;

e) reacting the mixture obtained in step d) by raising the temperature to between 30 °C and 150 °C;

f) stopping the reaction in step e);

g) purification of the functional cellulose nanocrystals obtained in step e);

wherein each group R is as defined in claim 7.

The method according to claim 7 or 8, wherein the group R is selected from the group consisting of C_1-8 alkyl carbonyl; C₂.₄ alkyl carbonyl halogenated in a secondary position; thiolated C₂.₄ alkyl carbonyl; tosylated C₂.₄ alkyl carbonyl; benzenesulfonyloxylated C₂.₄ alkyl carbonyl; mesylated C₂.₄ alkyl carbonyl; phenyl-S-C(=S)-C .₈-alkyl carbonyl, optionally substituted with CN or C_1- alkyl: phenyl-C(=S)-S-C .₈-alkyl benzoyl, optionally substituted with CN or C_1-4 alkyl; and C_{1 -2}o alkyl-S- C(=S)-S-C .₈-alkyl carbonyl, optionally substituted with CN or C_1- alkyl.

The method according to any one of claims 7 to 9 wherein the cellulose is chosen from native cellulose, bacterial cellulose, cotton, paper, sulfite pulp or sulfate pulp.

1 1 . The method according to any one of claims 7 to 10 wherein the acidic solution in step b) is an inorganic acid.

12. The method according to any one of claims 7 to 1 1 , wherein the acidic water solution in step b) comprises hydrochloric, alkyl sulfonic acid, aryl sulfonic acid or sulfuric acid.

13. The method according to any one of claims 7 to 12, wherein the organic acid in step d) is

selected from the group consisting of C_1-8 alkyl carboxylic acids; C₂.₈ carboxylic acids that are

2 halogenated in a secondary position; thiolated C₂_₈ alkyl carboxylic acids; tosylated C₂_₈ alkyl carboxylic acids; benzenesulfonyloxylated C₂_₄ alkyl carboxylic acids; mesylated C₂_₈ alkyl carboxylic acid; C₂.₈ alkenyl carboxylic acid; C₂.₈ alkynyl carboxylic acids; benzoic acids that are halogenated in one; two; three; four or five positions; benzoic acids that are halogenated in one or more positions; aryl-X-C^s-alkyl-carboxylic acid, which is optionally substituted; aryl-X-C^- alkyl-aryl-carboxylic acid, which is optionally substituted; C^o-alkyl-X-C^-alkyl-carboxylic acid, which is optionally substituted; and C^o-alkyl-X-C^-alkyl-aryl-carboxylic acid, which is optionally substituted; wherein X is -0-C(=S)-S-, -S-C(=S)-0-, -N-C(=S)-S-, -S-C(=S)-N-, -S-C(=S)-S-, -S-C(=S)- or -C(=S)-S-.

14. A method according to any one of claims 7 to 13, wherein the organic acid is selected from the group consisting of alkyl carboxylic acids, C₂.₄ alkyl carboxylic acids that is halogenated in a secondary position; thiolated C₂.₄ alkyl carboxylic acids; tosylated C₂.₄ alkyl carboxylic acids; mesylated C₂.₄ alkyl carboxylic acids; C₂.₈ alkenyl carboxylic acids; C₂.₈ alkynyl carboxylic acids and phenyl-C(=S)-S-C .₈-alkyl carboxylic acids, optionally substituted with CN or C_1-4 alkyl: phenyl-C(=S)-S-C .₈-alkyl benzoic acids, optionally substituted with CN or C_1-4 alkyl; and C_1-2o alkyl-S-C(=S)-S-C .₈-alkyl carboxylic acids, optionally substituted with CN or C_1-4 alkyl,

15. A method according to any one of claims 7 to 14, wherein the organic acid is selected from the group consisting of 2-bromopropanoic acid, 2-chloropropanoic acid, 2-bromobutanoic acid, 2- chlorobutanoic acid; 3-mercaptopropionic acid, 4-pentenoic acid, and 2-propynoic acid.

16. A method according to any one of claims 7 to 15, wherein the organic acid is 2-bromopropanoic acid.

17. A method according to any one of claims 7 to 16, wherein the organic acid is used neat.