Kari Pirkkalainen

Amorphous Characteristics of an Ultrathin Cellulose Film

Swelling behavior and rearrangements of an amorphous ultrathin cellulose film (20 nm thickness) exposed to water and subsequently dried were investigated with grazing incidence X-ray diffraction, neutron reflectivity, atomic force microscopy, and surface energy calculations obtained from contact angle measurements. The film swelled excessively in water, doubling its thickness, but shrunk back to the original thickness upon water removal. Crystallinity (or amorphousness) and morphology remained relatively unchanged after the wetting/drying cycle, but surface free energy increased considerably (ca. 15%) due to an increase in its polar component, that is, the hydrophilicity of the film, indicating that rearrangements occurred during the films exposure to water. Furthermore, stability of the films in aqueous NaOH solution was investigated with quartz crystal microbalance with dissipation monitoring. The films were stable at 0.0001 M NaOH but already 0.001 M NaOH partially dissolved the film. The surprising susceptibility to dissolve in dilute NaOH was hypothetically attributed to the lack of hierarchical morphology in the amorphous film.

Specific enzymatic tailoring of wheat arabinoxylan reveals the role of substitution on xylan film properties.

To increase understanding of the applicability of agro biomass by-products as biodegradable film formers, the effect of wheat arabinoxylan (WAX) fine structure on film properties was studied by applying specific enzyme modifications. WAX was selectively modified to mimic the natural variations of different arabinoxylans, particularly the degree of mono and disubstitution of α-L-arabinofuranosyl (Araf) units in β-D-xylopyranosyl (Xylp) backbone residues. The resulting modified WAX samples had similar arabinose-to-xylose (Ara/Xyl) ratios, but they differed in the number of unsubstituted Xylp units. The substitution of WAX was found to affect, in particular, tensile strength, crystallinity, and oxygen permeability properties of the films, as statistically significant decreases in tensile strength and oxygen permeability took place after WAX de-branching. An increase in the number of unsubstituted Xylp units decreased the temperature of relaxation of small-scale molecular motions of WAX (β-relaxation) and increased the degree of crystallinity of the films.

Coordination number in ideal spherical nanocrystals

The effect of very small crystal sizes on the average coordination number (ACN) was evaluated by computer calculations. For well ordered materials, the experimentally measured ACN is often very near the theoretical maximum, i.e. the infinite lattice limit. However, even in perfectly ordered crystals the coordination number has a significantly lowered value if the crystals are nanosized. For very small nanocrystals, the ACN drops down to 70–80% of the infinite lattice limit and such variation can be detected experimentally. A quick and accurate way of estimating the theoretical ACN from the first four coordination shells of simple cubic, face-centred cubic, body-centred cubic and hexagonal close-packed spherical nanocrystals is introduced.

Structure of nickel nanoparticles in a microcrystalline cellulose matrix studied using anomalous small-angle X-ray scattering

Nickel nanoparticles were synthesized by adding aqueous nickel salt into a microcrystalline cellulose matrix. The NiII ions were reduced with either sodium borohydride, NaBH_4, or potassium hypophosphite, KH_2PO_2, in water or aqueous NH_3 medium. The mass fraction of Ni in the samples was between 3.7 and 8.9%. X-ray absorption spectra at the Ni K-edge showed that Ni was partially oxidized only in a sample reduced with NaBH_4. Wide-angle X-ray scattering results showed that nickel was in nanocrystalline or amorphous form in the samples. Upon heating fcc Ni, hcp Ni, NiO, Ni_3P and other Ni–P phases formed depending on the reduction parameters. Using anomalous small-angle X-ray scattering the nanometre-scale particle size distributions of the Ni particles were determined. A large fraction of particles less than 15 nm in size were observed in the samples reduced in aqueous ammonium compared with the samples reduced in water. Particles reduced in aqueous ammonium had a large ferromagnetic component.

Simultaneous X-ray diffraction and X-ray fluorescence microanalysis on secondary xylem of Norway spruce

Secondary xylem of Norway spruce was studied by X-ray microanalysis. Average dimensions of cellulose crystallites, fraction of oriented cellulose, mean microfibril angle, and nutrient element concentrations of K, Ca, Mn, and Zn were simultaneously determined using microfocused synchrotron radiation and a combination of X-ray diffraction and X-ray fluorescence spectroscopy techniques. The variation of these quantities in the microscopic size scale was noticeable, and similar between samples taken from the same annual ring. The mean microfibril angle and the nutrient concentrations of Ca, Mn, and Zn showed a correlation. The mean values of the structural parameters and their variation as a function of the annual ring were similar as reported in previous studies on Norway spruce.

Small-angle x-ray scattering study on the structure of microcrystalline and nanofibrillated cellulose

The effects of different solvents on the structure of microcrystalline and nanofibrillated cellulose (MCC, NFC) were studied using small-angle x-ray scattering (SAXS). MCC was immersed in water, ethanol, and acetone, and NFC was immersed only in water and ethanol, but studied also in the form of foam-like water-NFC-gel in wet, air-dried and re-wet states. The solvent affected the average chord length, which reveals the typical length scale of the structure of the sample: 2.4 ± 0.1 nm was obtained for MCC-water, 2.5 ± 0.1 nm for re-wet NFC-gel, 1.6 ± 0.1 nm for MCC-ethanol, 1.2 ± 0.1 nm for NFC-ethanol, and 1.3 ± 0.1 nm for MCC-acetone. The specific surface of cellulose increased strongly when MCC and NFC were immersed in the solvents compared to dry cellulose. The specific surface of cellulose was determined to be larger for NFC-water than MCC-water, and slightly larger for dry NFC powder than for dry MCC, which can be explained by the fact that the width of cellulose crystallites perpendicular to the cellulose chain direction was slightly larger in MCC than in NFC on the basis of wide-angle x-ray scattering results.

Cellulose as a nanoreactor for the synthesis of nickel nanoparticles

The formation of cellulose-nickel nanocomposites has been studied by WAXS, X-ray absorption spectroscopy, X-ray photoelectron spectroscopy, and scanning electron spectroscopy. Changes in the experimental conditions make it possible to widely vary the size of nickel and its oxide nanoparticles in the bulk and on the surface of cellulose fibers and to monitor the localization of these nanoparticles with the structure of cellulose remaining unchanged. The use of cellulose allowed preparation of nickel nanoparticles stabilized in the matrix.