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New Cellulose Derivatives from Wood for High Value Products
Monica Ek*1, Sigbritt Karlsson1, Pedro Fardim2, Sirkka Liisa Maunu3, Pertti Nousiainen4,
Matti Siika-Aho5, Pernilla Walkenström6

Abstract
Various pre-treatment methods to increase the accessibility of cellulose materials to swelling and reactive agents at different hierarchical levels, has been developed. It was demonstrated that enzymes are very efficient at increasing the cellulose reactivity. A new method to measure the reactivity and accessibility of the hydroxyl groups on cellulose was developed. Reversible addition-fragmentation chain transfer (RAFT) agents were synthesized and used further in esterification reactions with cellulose samples to prepare cellulose based graft-copolymers. MALDI-TOFMS was evaluated for characterization of cellulose derivatives. Methods to prepare electrospun fibres, casings, sponges and beads, were developed with target applications in pharmaceuticals and foods.

Introduction
A high accessibility is an essential prerequisite for a homogeneous or designed substitution of cellulose material. The accessibility of cellulose in wood pulps to solvent and reactants is usually limited. Only the surfaces and the amorphous regions of the cellulose fibrils are available for reactions.
Commercial cellulose derivatives often contain inhomogeneities in their structures resulting in poor performance of the products. To obtain more homogeneous cellulose derivatives it is also important to find homogeneous modification conditions and to use controlled techniques to derivate cellulose. Pre-treated cellulose and chosen cellulose derivatives were therefore modified further using living radical polymerisation reactions. Hydroxyl groups in starting materials were substituted in a way that facilitates the further polymerisation reactions. As a result a series of cellulose based graft-copolymers were prepared.

The properties of cellulose derivatives are influenced by several parameters related to the chemical structure and the molecular size. Therefore it is important to determine the molar mass and molar mass distribution, the degree of substitution (DS) and the substitution pattern along the cellulose polymer in order to foresee the performance of the material in a specific application.

In this project1 the chemical structure was investigated by MALDI-TOFMS, NMR spectroscopy and various chromatographic techniques, whereas the molar mass and molar mass distribution was determined by size-exclusion chromatography.

The project1 was divided into four subprojects with the overall objectives:

  • Preparations of cellulose derivatives with controlled chemical and supramolecular structure for sophisticated pharmaceutical and fine chemical applications
  • Development of knowledge about improvement of the accessibility of cellulose
  • Use of new controlled free-radical polymerization reactions on various cellulose based materials to develop new cellulose derivatives
  • Characterization of new cellulose derivatives by chemical and physical means (MALDI-TOFMS, SEC, NMR etc.)
  • Application of the new derivatives from wood into a series of high-value products

RESULTS AND DISCUSSION

Cellulose accessibility
Various methods to analyse accessibility and reactivity have been evaluated, i.e. Fock method2, Iodine Sorption3, alkali solubility (R10)4 and swelling coefficient5. A new method to evaluate the results of the pre-treatments is under development.

Chemical, mechanical and enzymatic pre-treatments to increase the accessibility of cellulose materials to swelling and reactive agents have been investigated. By the Fock's method, a micro-scale process similar to the viscose process, an increase in cellulose yield after enzymatic treatment with endoglucanases was demonstrated Figure 1.

ek_fig1

Figure 1. Reactivity according to Fock versus viscosity of dissolving pulp samples: (p, ------ ) treated with endoglucanase with different incubation times (from the left in the figure 90, 30, 10 minutes and the reference), (n, − ∙ − ∙) treated with endoglucanase with different enzyme dosage (from the left in the figure 50, 30, 5, 0.5, 0 .05 and 0 ECU/g dry weight pulp), (,——) treated by acid hydrolysis for 30, 15, 8, 2 and 0 minutes (from the left in the figure)6

Additional endoglucanases, other proteins and commercial enzymes were also studied. The best results in terms of increased water accessibility and alkali solubility were obtained using endoglucanases, both purified and commercial products. The Swelling co-efficient was clearly correlated with the measured R10 values. The hydrolysis rate, estimated based on soluble products, was less than 1.2 % of pulp with purified enzymes and less than 2.5 % with commercial preparations. In terms of the accessibility, the drying history of pulp had a noticeable effect, and pre-treatments gave clear improvement in the accessibility.

New method for determination of accessibility
Water-soluble, natural hemicelluloses such as galactoglucomannan (GGM) have a linear polymer back-bone. They have been found to adsorb onto polymers of analogous structure, e.g. cellulose. This attraction property may be utilised for determination of the free sites or the accessibility of cellulose pulp fibres. The GGM method (Figure 2) has been tested on different pulps and preliminary results indicates that it is a sensitive method which can be used for determination of accessibility.

ek_fig2
Figure 2. Overview of the experimental path way.

Controlled synthesis
To obtain homogeneous cellulose derivatives it is important to find homogeneous modification conditions and to use controlled techniques to derivatise cellulose. Preparation of CMC was chosen as a model derivative since it is one of the most important and widely applied cellulose ethers. CMC, ethyl hydroxyethyl cellulose (EHEC) and different pretreated celluloses were used in graft-copolymerization. During the work the focus of CMC synthesis was on a heterogeneous slurry process because it is the most relevant from an industrial point of view. Various pre-treatments and viscosities of the pulps were used in order to obtain controlled structures.

MALDI-TOFMS and high-performance liquid chromatography were used to characterize the CMC products with regard to the DS and monomer composition. A comparison between the DS values measured by HPLC and MALDI-TOFMS obtained similar values for both methods.

Reversible addition fragmentation chain transfer polymerization (RAFT) makes possible the tailoring of macromolecules with sophisticated architectures including graft copolymers and was therefore used in this project to build up side chains on different cellulose based backbones as seen in Figure 37. The synthesis consists of a conventional free radical polymerization of a substituted monomer in the presence of a suitable chain-transfer agent. Various RAFT-agents with carboxylic acid function were prepared and used in esterification reactions. The macroRAFT-agents were characterised with ATR-FTIR, 1H, 13C and/or 13C CPMAS NMR. The results were used to confirm that RAFT-agent was successfully bound to the backbone and the amount was analysed by elemental analyses.

Different monomers were polymerized in a controlled manner by grafting with the technique using the prepared macroRAFT-agents. Hydrophilic and hydrophobic grafts were polymerized successfully. The products, new cellulose based copolymers, were characterized using ATR-FTIR, 1H, 13C and/or 13C CPMAS NMR.

ek_fig3

Figure 3. 13C CPMAS NMR spectra of CMC, RAFT-agent and CMC-macroRAFT-agent (from bottom to top, respectively).

Properties of new derivatives
MALDI-TOFMS of CMC was refined and can now be used for a rapid evaluation of the DS of multiple samples, e.g. in a screening process (Enebro and Karlsson8). MALDI-TOFMS of enzymatically hydrolyzed CMCs was performed and valuable information on the chemical structure of the substrate, as well as the enzyme selectivity, could be obtained.

An approach for detecting variations in the substitution pattern along the cellulose backbone was developed. Two CMCs, with similar chemical composition (e.g. similar DS and glucose content) and molar mass, displayed differences in the technical properties. These variations could be related to the variations in the substitution pattern as detected by the described method.

Applications of new derivatives

Electrospun fibres
Electrospinning of nanofibers based on various cellulose derivatives, mainly commercially available, have been conducted9. Different derivatives have been electrospun and the influence of the molecular weight (Mw), degree of substitution (DS) and distribution of the side-groups on the backbone (block or random) on the electrospinning process as well as on the obtained fibres have been investigated.

Encapsulation of a model drug in the nanofibers during the electrospinning has been evaluated. SEM images show pure HPMC-nanofibers as well as fibres of HPMC in mixture with a model drug substance. A comparison shows that the fibre morphology is highly influenced. Furthermore, the different solvent used for the mixed system, i.e. water/ethanol and dichloromethane/ethanol also influences the morphology of the final fibres.

Cellulosic casings
New derivatives have been developed in order to substitute some or all of the cellulose xanthate that is currently used for production of fibrous casings. As the casings are in contact with food they cannot contain any harmful chemicals or substances, which must be taken into consideration in developing work. The primary test to find out proper coagulant agents for cellulose derivatives have been carried out in a limited range. Suitable solvents and coagulants are found for some derivatives.

The studies cover laboratory trials to form sheets with and without viscose solution onto base paper, and also by measuring the properties of elaborated films.

Cellulosic sponges, membranes and beads
A technology for manufacturing of cellulosic beads with tailored properties was developed using viscose and was further evaluated for different derivatives. Size, shape, surface morphology, porosity and degree of crystallization can be tuned to design innovative functionalities for bead applications in drug delivery and encapsulation. These innovative functionalities for beads can open new doors for further utilization in drug delivery and encapsulation. In the area of sponge making, progress was made by producing sponges with tailored pore sizes.

Conclusions
Various methods to increase the accessibility of cellulose materials to swelling and reactive agents at different hierarchical levels have been studied. Among mechanical, chemical, and enzymatic pre-treatments it was found that monocomponent cellulases, cellulase mixtures and other cellulose acting proteins were very effective pre-treatment methods. The best results were obtained using endoglucanases, both purified and commercial products. A new method to measure the reactivity or accessibility of the hydroxyl groups of the cellulose has been developed.

Various macro-RAFT agents were synthesized from modified cellulose materials and were further used as starting materials for graft polymerization. Various monomers were polymerized in a controlled manner to prepare side chains with a hydrophilic or hydrophobic character. Different starting materials required their own modification conditions as well as synthetic path ways, however new cellulose based copolymers were prepared successfully.

Valuable information regarding block-wise or random substitution pattern along the polymer backbone can be obtained by combining enzymatic hydrolysis by purified endoglucanases with MALDI-TOFMS and SEC/MALS/RI.

MALDI-TOFMS is a valuable tool for chemical structure characterization of cellulose derivatives. Improvements in the sample preparation procedure have made it possible to evaluate the DS in CMC as a compliment to more expensive and laborious methods.

Nanofibres of several commercial cellulose derivatives have been successfully produced by electrospinning. The spinnability of the derivatives was shown to be influenced by the material parameters (e.g. molar mass and substitution pattern) but also by other compounds, as seen when a model drug was incorporated into the fibre. Methods for developing beads from mixtures of viscose and CMC have been developed. The physicochemical properties and surface chemistry of the beads can be tailored, changing properties like swellability, porosity , size, shape, morphology and surface charge.

List of acronyms
ATR-FTIR – Attenuated Total Reflectance-Fourier Transform Infrared Spectroscopy
CMC – Carboxy Methyl Cellulose
CPMAS – Cross Polarisation Magic Angle Spinning
HPLC – High Performance Liquid Chromatography
HPMC – Hydroxy Propyl Methyl Cellulose
MALDI-TOFMS – Matrix Assisted Laser Desorption/Ionisation-Time of Flight Mass Spectroscopy.
NMR – Nuclear Magnetic Resonance
SEC – Size Exclusion Chromatography
SEC/MALS/RI – Size Exclusion Chromatography/Multi Angle Laser Light Scattering/Refractive Index


Acknowledgements
The authors would like to acknowledge financial support to the project New cellulose derivatives from wood for high value products from VINNOVA/Tekes programme on Wood Material Science and Engineering. All the participants in the project at universities, institutes and industrial companies are acknowledged for skilful laboratory work, many fruitful discussions as well as financial support, i.e from Astra Zeneca, Akzo Nobel Surface Chemistry , Domsjö Fabriker, CP Kelco, Vivoxide, Visko, Metsä Botnia and Borregaard.

Bibliographic references
1. Wood Material Science and Engineering Final Report 2007 www.woodwisdom.fi
2. Fock, W. Papier 1959, 13, 92-95
3. Schwertassek, K. 1931. Melliand Textil, 12, 457
4. SCAN-N 33:94 standard
5. Krässig, H.A., Cellulose, Polymer Monographs Vol 11, Gordon and Breach Science Publishers 1993
6. Engström, A-C., Ek, M. and Henriksson, G., "Improved Accessibility and Reactivity of Dissolving Pulp for the Viscose Process: Pretreatment with Monocomponent Endoglucanase", Biomacromolecules, 2006; 7(6); 2027-2031.
7. Hiltunen M, Maunu SL and Tenhu H, "New Cellulose Copolymers with Controlled Chemical Structure of Grafts via RAFT Polymerization", 14th International Symposium on Wood, Fibre and Pulping Chemistry (ISWFPC). Durban, South Africa 2007, Proceedings.
8. Enebro, J.; Karlsson, S. "Improved Matrix-assisted Laser Desorption/Ionisation Time-of -Flight Mass Spectrometry of Carboxymethyl Cellulose", Rapid Communications in Mass Spectrometry, 2006; 20: 3693-3698
9. Frenot, A.; Henriksson, M. W.; Walkenström, P. Journal of Applied Polymer Science 2007, 103, 1473-1482.

Contact Details
1 Monica Ek,* Associate professor, KTH, Sweden, monicaek@kth.se, corresponding author
1 Sigbritt Karlsson, Professor, KTH, Sweden, sigbritt@polymer.kth.se
2 Pedro Fardim, Professor, Åbo Academy University, Finland, pfardim@abo.fi
3 Sirkka Liisa Maunu, Professor, University of Helsinki, Finland, sirkka.maunu@helsinki.fi,
4 Pertti Nousiainen, Professor, Tampere University of Technology, Finland, pertti.nousiainen@tut.fi
5 Matti Siika-Aho, Senior Scientist, VTT Technical Research Center of Finland, Finland, Matti.Siika-Aho@vtt.fi
6Pernilla Walkenström, Associate professor, IFP Research, Sweden, pernilla.walkenstrom@ifp.se

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