Martin Gericke

Ionic liquids--promising but challenging solvents for homogeneous derivatization of cellulose.

In the past decade, ionic liquids (ILs) have received enormous interest as solvents for cellulose. They have been studied intensively for fractionation and biorefining of lignocellulosic biomass, for dissolution of the polysaccharide, for preparation of cellulosic fibers, and in particular as reaction media for the homogeneous preparation of highly engineered polysaccharide derivatives. ILs show great potential for application on a commercial scale regarding recyclability, high dissolution power, and their broad structural diversity. However, a critical analysis reveals that these promising features are combined with serious drawbacks that need to be addressed in order to utilize ILs for the efficient synthesis of cellulose derivatives. This review presents a comprehensive overview about chemical modification of cellulose in ILs. Difficulties encountered thereby are discussed critically and current as well as future developments in this field of polysaccharide research are outlined.

Rheological Properties of Cellulose/Ionic Liquid Solutions: From Dilute to Concentrated States

Steady state shear flow of different types of cellulose (microcrystalline, spruce sulfite and bacterial) dissolved in 1-ethyl-3-methylimidazolium acetate was studied in a large range of concentrations (0-15%) and temperatures (0-100 degrees C). Newtonian flow was recorded for all experimental conditions; these viscosity values were used for detailed viscosity-concentration and viscosity-temperature analysis. The exponent in the viscosity-concentration power law was found to be around 4 for temperatures from 0 to 40 degrees C, which is comparable with cellulose dissolved in other solvents, and around 2.5-3 for 60-100 degrees C. Intrinsic viscosities of all celluloses decreased with temperature, indicating a drop in solvent thermodynamic quality with heating. The data obtained can be reduced to a master plot of viscosity versus (concentration x intrinsic viscosity) for all celluloses studied in the whole temperature range. Mark-Houwink exponents were determined: they were lower than that for cellulose dissolved in LiCl/N,N-dimethylacetamide at 30 degrees C and close to theta-value. Viscosity-inverse temperature plots showed a concave shape that is dictated by solvent temperature dependence. The values of the activation energies calculated within Arrhenius approximation are in-line with those obtained for cellulose of comparable molecular weights in other solvents.

Interaction of Ionic Liquids with Polysaccharides, 8 – Synthesis of Cellulose Sulfates Suitable for Polyelectrolyte Complex Formation

Water-soluble CS with different DS were synthesized by homogeneous conversion of cellulose (microcrystalline cellulose and spruce sulfite pulp) with different sulfating agents in the ionic liquids BMIMCl, AMIMCl and EMIMAc. N,N-Dimethylformamide was used as a dipolar aprotic co-solvent in order to improve the miscibility of the reaction mixture. The CS prepared by the new homogeneous reaction pathway were studied towards the formation of PEC capsules. CS obtained from SSP formed mechanically stable PEC capsules with PolyDADMAC. Exploiting this method, the microencapsulation of glucose oxidase was studied and enzyme activity tests were performed with encapsulated GOD.

Studies on the tosylation of cellulose in mixtures of ionic liquids and a co-solvent.

The tosylation of cellulose in ionic liquids (ILs) was studied. Due to the beneficial effect of different co-solvents, the reaction could be performed at 25°C without the need of heating (in order to reduce viscosity) or cooling (in order to prevent side reactions). The effects of reaction parameters, such as time, molar ratio, and type of base, on the degree of substitution (DS) with tosyl- and chloro-deoxy groups as well as on the molecular weight were evaluated. Products with a DStosyl≤1.14 and DSCl≤0.16 were obtained and characterized by means of NMR- and FT-IR spectroscopy in order to evaluate their purity and distribution of functional groups within the modified anhydroglucose unit (AGU). Tosylation of cellulose in mixtures of IL and a co-solvent was found to result in predominant substitution at the primary hydroxyl group. Size exclusion chromatography (SEC) revealed only a moderate degradation of the polymer backbone at a reaction time of 4-8h. Finally, the nucleophilic displacement (SN) of tosyl- and chloro-deoxy groups by azide as well as recycling of the ILs was studied.

Physicochemical design of the morphology and ultrastructure of cellulose beads

Cellulose was dissolved in NaOH-urea-water and beads were prepared by coagulation into nitric acid as well as saline solution. Morphology and ultrastructure of the beads were modified by controlling the molarity of the acid (0-10M) and temperature (5-50°C) of the coagulation media and the cellulose concentration (3-7%). The beads were characterized by optical image analysis (shape, volume, and size distribution) and weight (total porosity). Cross-sections of CO2 critical point dried beads were studied by field emission scanning electron microscopy (FE-SEM) and specific surface areas of 336-470m(2)g(-1) were determined from nitrogen adsorption isoterms. Pore size distribution was analyzed using solute exclusion technique. Our results demonstrate that the ultrastructure can be controlled by alteration of the coagulation conditions. Changes in size, shape and surface area were substential. Also generation of micro- (⩽2Å), meso-, or macropores (⩾50Å) can be favored.

Polyelectrolyte Synthesis and in Situ Complex Formation in Ionic Liquids

For the first time, polyelectrolyte complex (PEC) capsules were prepared from a water insoluble polyanion, namely cellulose sulfates (CSs) with a degree of substitution (DS) below 0.2 in ionic liquids (IL). Capsules prepared via interaction with the polycation poly(dimethyldiallyammonium chloride) were free of residual IL and possessed an outer shell and a hollow inner core that made them ideal containers for enzyme mediated reactions. Due to the reestablished hydrogen bond system of the low substituted CS, the capsules showed increased stability, compared to the products obtained by application of the common aqueous preparation. Encapsulation of glucose oxidase demonstrated that the steps of CS preparation, PEC capsule formation, and encapsulation could be combined in a single pot, with the elimination of time and cost consuming isolation and purification steps.

Semi-Synthetic Polysaccharide Sulfates as Anticoagulant Coatings for PET, 1 – Cellulose Sulfate

In the present study, blood-compatible PET surfaces were prepared by coating with anticoagulant cellulose sulfates that were synthesized homogeneously in ionic liquids. The adsorption behavior of polysaccharides on PET films was investigated using QCM-D. It was demonstrated that pre-coating with different amino-group-containing polysaccharides improves the affinity toward cellulose sulfate. Moreover, the effect of different degrees of sulfation on the adsorption process was evaluated. Based on these results, several layer-by-layer coated PET foils were prepared that showed significantly improved blood compatibility compared to the initial untreated material.

Homogeneous tosylation of agarose as an approach toward novel functional polysaccharide materials

The homogeneous tosylation of agarose was studied with respect to the effects of reaction parameters, such as reaction medium, time, and molar ratio, on the reaction course, the degree of substitution (DS) with tosyl/chloro deoxy groups, and the molecular structure. Tosyl agaroses (TOSA) with DS tosyl ≤ 1 .81 could be obtained in completely homogeneous reactions by using N,N-dimethylacetamide (DMA)/LiCl or 1,3-dimethyl-2-imidazolidinone (DMI) as solvents. The products were characterized by FT-IR and NMR spectroscopy and it was demonstrated that two types of substitution pattern can be achieved: (i) non-preferential substitution at position 6 of the 1 → 3-linked β-d-galactose unit (G-6) and position 2 of the 1 → 4-linked 3,6-anyhdro-α-L-galactose unit (LA-2) and (ii) regioselective tosylation at G-6, depending on whether the reaction is performed with or without LiCl. Finally, the nucleophilic displacement reaction of TOSA was studied using azide and ethylenediamine as representative nucleophiles. Novel deoxy-agarose derivatives were obtained that showed an interesting solubility behavior and will be used for creating functional polysaccharide materials.

Ionic Liquids as Solvents for Homogeneous Derivatization of Cellulose: Challenges and Opportunities

The chapter provides a comprehensive overview of the chemical derivatization of cellulose in ionic liquids (ILs). Different types of chemical reactions, including esterification, etherification, and grafting reactions, that have been performed in these novel type of polysaccharide solvents are discussed separately regarding efficiencies and unique characteristics. With respect to the use of ILs in technical scale, specific limitations and open questions are discussed such as the chemical reactivity of certain ILs, their high viscosity and hydrophilicity, and the need to develop efficient recycling strategies. Finally, an outlook on the development of task-specific ILs and IL/co-solvent systems as reaction media for cellulose is presented.

Chitosan–Cellulose Multifunctional Hydrogel Beads: Design, Characterization and Evaluation of Cytocompatibility with Breast Adenocarcinoma and Osteoblast Cells

Cytocompatible polysaccharide-based functional scaffolds are potential extracellular matrix candidates for soft and hard tissue engineering. This paper describes a facile approach to design cytocompatible, non-toxic, and multifunctional chitosan-cellulose based hydrogel beads utilising polysaccharide dissolution in sodium hydroxide-urea-water solvent system and coagulation under three different acidic conditions, namely 2 M acetic acid, 2 M hydrochloric acid, and 2 M sulfuric acid. The effect of coagulating medium on the final chemical composition of the hydrogel beads is investigated by spectroscopic techniques (ATR–FTIR, Raman, NMR), and elemental analysis. The beads coagulated in 2 M acetic acid displayed an unchanged chitosan composition with free amino groups, while the beads coagulated in 2 M hydrochloric and sulfuric acid showed protonation of amino groups and ionic interaction with the counterions. The ultrastructural morphological study of lyophilized beads showed that increased chitosan content enhanced the porosity of the hydrogel beads. Furthermore, cytocompatibility evaluation of the hydrogel beads with human breast adenocarcinoma cells (soft tissue) showed that the beads coagulated in 2 M acetic acid are the most suitable for this type of cells in comparison to other coagulating systems. The acetic acid fabricated hydrogel beads also support osteoblast growth and adhesion over 192 h. Thus, in future, these hydrogel beads can be tested in the in vitro studies related to breast cancer and for bone regeneration.