Holger Wondraczek

Water soluble photoactive cellulose derivatives: synthesis and characterization of mixed 2-[(4-methyl-2-oxo-2H-chromen-7-yl)oxy]acetic acid–(3-carboxypropyl)trimethylammonium chloride esters of cellulose

Photoactive derivatives of cellulose were prepared by a mild esterification of the biopolymer with 2-[(4-methyl-2-oxo-2H-chromen-7-yl)oxy]acetic acid via the activation of the carboxylic acid with N,N′-carbonyldiimidazole. Subsequently, modification with the cationic carboxylic acid (3-carboxypropyl)trimethylammonium chloride was carried out. Thus, water soluble polyelectrolytes decorated with high amounts of photochemically active chromene moieties were obtained. The structures of the novel polysaccharide esters and the polyelectrolytes were evaluated by means of NMR and IR spectroscopy. Moreover, the light triggered photodimerization of the chromene moieties of the photoactive polyelectrolytes was studied by means of UV–Vis spectroscopy in the dissolved state. The photochemistry observed may be used to control the properties of the new polysaccharide derivatives and are thus of interest in the design of smart materials.

Synthesis and characterization of aminocellulose sulfates as novel ampholytic polymers

Amino cellulose sulfate (ACS); namely 6-deoxy-6-(ω-aminoethyl) amino cellulose-2,3(6)-O-sulfate (AECS) and 6-deoxy-6-(2-(bis-N′,N′-(2-aminoethyl)aminoethyl)) amino cellulose-2,3(6)-O-sulfate (BAECS) were prepared by a three step synthesis starting with the functionalization of microcrystalline cellulose with p-toluenesulfonyl (tosyl) groups (degree of substitution, DSTos between 0.55 and 1.37). Subsequently the introduction of the sulfate moieties was carried out (DSSulf between 1.09 and 1.27) and the tosyl groups at position 6 were replaced by a nucleophilic substitution reaction. As nucleophilic agents 1,2-diaminoethane and tris-(2-aminoethyl)amine were applied, yielding AECS (DSAEA values between 0.41 and 0.86) and BAECS (DSBAEA values between 0.32 and 0.74), respectively. The ACS samples were characterized by means of elemental analysis, 13C-NMR-, FT-IR-, and UV–Vis spectroscopy. Moreover, the solubility of the samples in water at different pH values and the molecular weights of the samples in aqueous solution were studied.

Charge-Directed Fiber Surface Modification by Molecular Assemblies of Functional Polysaccharides

Molecular assemblies, namely, polyelectrolyte complexes (PECs) composed of negatively charged xylan-based derivatives and a novel positively charged cellulose derivative (CN(+)), were used for interfacial modification of wood fibers by charge directed self-assembly. The adsorption process was studied using polyelectrolyte titration and elemental analysis. X-ray spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) were used as advanced techniques for the characterization of the modified fiber surfaces. The measurements revealed an intense interaction between the pulp fibers and PECs, and provided essential information for a better understanding of the adsorption process. The information gathered on this paper might contribute to the basis for the development of new value added products by the use of underutilized biomass.

Nanoparticles from conventional cellulose esters: evaluation of preparation methods

Nano-scaled particles were obtained from two different cellulose acetates, cellulose acetate propionate, and cellulose acetate butyrate using the emulsification solvent evaporation procedure and the low energy methods of solvent displacement (dialysis and controlled precipitation). The relationship between the formulation parameters and the particle properties were evaluated in case of the emulsification-evaporation technique. For the solvent displacement procedures, the influence of the formulation parameters, and the intrinsic polymer properties like the hydrophilic-hydrophobic balance was evaluated. Comparing the methods, it could be shown that large amounts of small and uniform nanoparticles can be obtained by the emulsification solvent evaporation procedure. The solvent displacement techniques turned out to be very easy to use and to yield narrowly distributed particles as well.

Photoresponsive cellulose fibers by surface modification with multifunctional cellulose derivatives

Eucalyptus bleached kraft pulp fibers were modified by adsorption of novel bio-based multifunctional cellulose derivatives in order to generate light responsive surfaces. The cellulose derivatives used were decorated with both cationic groups (degree of substitution, DS of 0.34) and photoactive groups (DS of 0.11 and 0.37). The adsorption was studied by UV-vis spectroscopy, surface plasmon resonance (SPR) and time-of-flight secondary ion mass spectroscopy (ToF-SIMS). The adsorption isotherms followed the Freundlich model and it turned out that the main driving force for the adsorption was electrostatic interaction. Moreover, strong indications for hydrophobic interactions between the fibers and the derivatives and the derivatives themselves were found. ToF-SIMS imaging revealed an even distribution of the derivatives on the fiber surfaces. The modified fibers underwent fast photocrosslinking under UV-irradiation as demonstrated by light absorbance and fluorescence measurements. Thus, our results proved that the modified fibers exhibited light-responsive properties and can potentially be used for the manufacture of smart bio-based materials.

Syntheses and detailed structure characterization of dextran carbonates

Dextran alkyl carbonates were synthesized applying ethyl chloroformate, butyl chloroformate, butyl fluoroformate and 1H-imidazole-1-carboxylates. The influence of the reaction conditions on the reaction efficiency and the substitution pattern was studied in detail. The structure of the products obtained was clearly described by means of NMR- and IR-spectroscopy.

Enhanced Dewatering of Polyelectrolyte Nanocomposites by Hydrophobic Polyelectrolytes

We demonstrate that increasing the hydrophobic environment around the charge center of a polyelectrolyte (PE) not only decreases the water content of an adsorbed PE layer but can even dewater up to ~50% of an initially hydrated substrate. The results of this work are expected to yield new stratagies to dewater PE systems and have potential applications in mineral recovery, paper manufacturing, and biomedical materials. Adsorption of a series of cationically derivatized dextran polyelectrolytes onto sulfated nanocrystalline cellulose (SNC) has been studied using quartz crystal microbalance with dissipation monitoring (QCM-D) and surface plasmon resonance (SPR). Synthesized samples of (N,N-dimethylamino)ethyldextran (DMAE-Dex), (N,N-diethylamino)ethyldextran (DEAE-Dex), and (N,N-diisopropylamino)ethyldextran (DIAE-Dex) had degrees of substitution (DS) ranging from 0.05 to 0.82. DMAE-Dex, DEAE-Dex, and DIAE-Dex all showed decreasing adsorption onto SNC and decreasing water content of the adsorbed film with increasing DS. Additionally, DEAE-Dex and DIAE-Dex films adsorbed onto SNC contained less water than DMAE-Dex films with the same DS. Interestingly, QCM-D results for high DS DIAE-Dex adsorbed onto SNC revealed mass loss, whereas SPR results clearly showed DIAE-Dex adsorbed. These observations were consistent with dehydration of the SNC substrate. This study indicates that the water content of the substrate could be tailored by controlling the DS and hydrophobic character of the adsorbed polyelectrolytes.

Preparation of reactive fibre interfaces using multifunctional cellulose derivatives.

Cellulose fibres have poor reactivity and limited potential for surface engineering with advanced chemical functionalization in water. In this work, cellulose fibres were decorated with azide functions by charge-directed self-assembly of a novel water-soluble multifunctional cellulose derivative yielding reactive fibres. Propargylamine and 1-ethynylpyrene were utilized as a proof of concept that alkyne molecules may react with the azide functions of the reactive fibres via copper(I)-catalyzed azide-alkyne Huisgen cycloaddition (CuAAc) reaction in mild conditions. Chemical characterization of the fibres was carried out using classical techniques such as Raman-, fluorescence-, and UV-vis spectroscopy. Among other techniques, time-of-flight secondary ion mass spectrometry (ToF-SIMS), X-ray spectroscopy (XPS), two-photon microscopy (TPM), and inductively coupled plasma mass spectrometry (ICP-MS) were useful tools for additional characterization of the fibres decorated with amino- or photoactive groups. The information gathered in this work might contribute to the basis for the preparation of reactive cellulose-based interfaces with potential application in CuAAc reactions.

Studies on the structure of coumarin-modified dextran nanoparticles by fluorescence spectroscopy.

The photophysical and photochemical characteristics of nano-scaled particles obtained via solvent displacement from coumarin-modified dextrans were studied by means of absorption- and fluorescence-spectroscopy. The environment-dependent fluorescence emission of the pendant 4-methyl-7-alkoxy coumarin groups was exploited as a probe to gain information about the inner structure of the polysaccharide based nanoparticles. Time-resolved fluorescence measurements showed that the particles had two domains of different polarity and it could be confirmed that the core of the nano-assemblies contained water. Moreover, preliminary experiments were carried out demonstrating the possibility to control the morphology of the nanoparticles by the light induced 2π+2π cycloaddition of the coumarin substituents.

Tailored Fabrication of Transferable and Hollow Weblike Titanium Dioxide Structures

The preparation of weblike titanium dioxide thin films by atomic layer deposition on cellulose biotemplates is reported. The method produces a TiO2 web, which is flexible and transferable from the deposition substrate to that of the end application. Removal of the cellulose template by calcination converts the amorphous titania to crystalline anatase and gives the structure a hollow morphology. The TiO2 webs are thoroughly characterized using electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy to give new insight into manufacturing of porous titanium dioxide structures by means of template-based methods. Functionality and integrity of the TiO2 hollow weblike thin films were successfully confirmed by applying them as electrodes in dye-sensitized solar cells.