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- Bergenstråhle, Malin, 1977-, et al.
(författare)
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Force Pulling of Single Cellulose Chains at the Crystalline Cellulose-Liquid Interface : A Molecular Dynamics Study
- 2009
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Ingår i: Langmuir. - : American Chemical Society (ACS). - 0743-7463 .- 1520-5827. ; 25:8, s. 4635-4642
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Tidskriftsartikel (refereegranskat)abstract
- Pulling single cellulose molecules from a crystalline cellulose surface has been modeled by molecular dynamics (MD) simulations of the experimental procedure used in atomic force microscopy (AFM). Specifically, the aim of the study was to investigate cellulose interactions at desorption. Simulations were performed in both water and the organic solvent cyclohexane. Moreover, the effects of initial octamer conformation and orientation with respect to the surface chains were studied. A strong effect from the solvent was observed. In cyclohexane, normal forces of 200-500 pN and energies of 43.5 +/- 6.0 kJ/mol glucose unit were required to pull off the octamer. The normal forces in water were substantially lower, around 58 pN, and the energies were 18.2 +/- 3.6 kJ/mol glucose unit. In addition, the lateral components of the pull-off force were shown to provide information on initial conformation and orientation. Hydrogen bonds between the octamer and surface were analyzed and found to be an important factor in the pull-off behavior. Altogether, it was shown that MD provides detailed information on the desorption processes that may be useful for the interpretation of AFM experiments.
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| 2. |
- Hellgren, Niklas, et al.
(författare)
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Electronic structure of carbon nitride thin films studied by X-ray spectroscopy techniques
- 2005
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Ingår i: Thin Solid Films. - : Elsevier BV. - 0040-6090 .- 1879-2731. ; 471:02-jan, s. 19-34
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Tidskriftsartikel (refereegranskat)abstract
- Magnetron-sputtered carbon nitride thin films with different structures and compositions were analyzed by X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS), near-edge X-ray absorption fine structure spectroscopy (NEXAFS), as well as X-ray emission spectroscopy (XES). In all techniques, the carbon spectra are broad and featureless with little variation depending on growth conditions. The nitrogen spectra, on the other hand, show more distinct features, providing a powerful tool for structural characterization. By comparing the experimental spectra with calculations on different model systems, we are able to identify three major bonding structures of the nitrogen-N1: nitrile (C equivalent to N) bonds; N2: Pyridine-like N, i.e., N bonded to two C atoms; and N3: graphite-like N, i.e., N bonded to three C atoms as if substituted in a graphitic network, however, possibly positioned in a pentagon and/or with sp(3) carbon neighbors. The presence of N2 and N3 are best detected by XPS, while N1 is better detected by NEXAFS. The calculated XES spectra also give good indications how valence band spectra should be interpreted. Films grown at the higher temperatures ( greater than or equal to 350 degreesC) show a pronounced angular dependence of the incoming photon beam in NEXATS measurements, which suggests a textured microstructure with standing graphitic basal planes, while amorphous films grown at low temperatures show isotropic properties.
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| 4. |
- Mizuno, Hiroyasu, et al.
(författare)
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Friction measurement between polyester fibres using the fibre probe SPM
- 2006
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Ingår i: Australian journal of chemistry (Print). - 0004-9425 .- 1445-0038. ; 59, s. 390-393
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Tidskriftsartikel (refereegranskat)abstract
- An SPM has been used to measure frictional interactions between two crossed fibres for the first time. The preparation of the surfaces is briefly described, but the crucial element is that the fibre attached to the AFM cantilever is glued parallel to the long axis. The fibres consist of polyester and frictional forces were measured both in air and solutions of cationic surfactant C(14)TAB. The friction coefficients reduce markedly with increasing concentration of surfactant which is ascribed to the formation of a boundary lubricating film between the surfaces. On removal of the solution, the subsequent friction coefficient in air was reduced by more than a factor of two compared to its value before immersion.
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| 5. |
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| 6. |
- Nordgren, Niklas, 1975-, et al.
(författare)
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Adhesion Dynamics for Cellulose Nano-Composites
- 2009
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Ingår i: ACS applied Materials & Interfaces. - : American Chemical Society (ACS). - 1944-8252 .- 1944-8244. ; , s. 2098-2103
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Tidskriftsartikel (refereegranskat)abstract
- The efficiency of poly(ε-caprolactone) (PCL) as a matrix polymer for cellulose nano-composites has been investigated at the macromolecular contact level using atomic force microscopy (AFM) in colloidal probe configuration. Model cellulose micro-spheres grafted with PCL were prepared via ring-opening polymerization (ROP). Force measurements between the functionalized particles revealed the adhesion to be highly dependent on contact time due to a diffusion controlled mechanism. Moreover, an increase of the temperature to 60 °C (close to Tm for the PCL-graft) greatly enhanced the adhesion at the polymer-polymer interface demonstrating the importance of entanglements in the annealing of composite materials.
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| 10. |
- Nordgren, Niklas, 1975-
(författare)
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Interfacial Properties of Biomacromolecular Model Systems: Surface Forces and Nanotribology
- 2009
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The nanotribology, adhesion and related interfacial properties of biomacromolecular systems have been studied. The aim was to elucidate the role of physisorbed and chemically grafted bio-polymers implicated as mediators for cellulose based processing and material design. To that end, model surfaces were prepared and characterized by the versatile techniques quartz crystal microbalance with dissipation (QCM-D) and atomic force microscopy (AFM) in colloidal probe mode. A prerequisite for the latter in conducting quantifiable force and friction measurements lies in obtaining accurate values for the cantilever spring constants. An investigation of various recently proposed calibration methods shows that they return essentially the same result, but that the route of Sader et al. is the least strenuous and indeed most reliable. A novel approach for determining the required frictional detector sensitivity is proposed. Cellulose model surfaces with varying morphology were evaluated. For a given surface roughness the adsorption of xyloglucan, a nonionic polysaccharide, leads to a consistent decrease in the friction coefficients for all cellulose substrates, demonstrating that the effects on friction arising from either roughness or chemistry can be decoupled. Moreover, xyloglucan grafts on gold show biological enzymatic accessibility, and their native affinity towards cellulose is retained, even in an extended brush conformation. Chitosan, a cationic polysaccharide is also found to mediate lubrication and adhesion of cellulose in a solvent dependent manner, though the mechanisms are unlike those of xyloglucan. At low pH the most efficient lubrication is achieved due to highly charged chitosan species extending out in the aqueous media, forming a highly hydrated cushion. Conversely, at high pH the deswelling of the layer results in higher friction. The same trend is observed between synthetic dual-responsive polyionic grafts on gold where the polymer charge is decisive for the friction properties. Above the lower critical solution temperature where the grafts are fully collapsed and display an attractive force, the friction is slightly reduced due to a nanoscopic flattening at the interface. Finally, polymer grafts as matrix compatibilizers for bio-degradable cellulose based nano-composite applications were evaluated. Adhesion is greatly promoted by chain entanglements. The effect displays a dynamic diffusion based dependence of which the rate is significantly enhanced at a higher temperature close to the melting point of the polymeric layer. No such adhesion benefit was obtained between ungrafted cellulose and the matrix material.
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