| 1. |
- Chen, Pan, et al.
(författare)
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Alternative hydrogen bond models of cellulose II and IIII based on molecular force-fields and density functional theory
- 2015
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Ingår i: Cellulose. - : Springer Science and Business Media LLC. - 0969-0239 .- 1572-882X. ; 22:3, s. 1485-1493
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Tidskriftsartikel (refereegranskat)abstract
- Alternative hydrogen-bond structures were found for cellulose II and IIII based on molecular dynamics simulations using four force fields and energy optimization based on density functional theory. All the modeling results were in support to the new hydrogen-bonding network. The revised structures of cellulose II and IIII differ with the fiber diffraction models mainly in the orientation of two hydroxyl groups, namely, OH2 and OH6 forming hydrogen-bond chains perpendicular to the cellulose molecule. In the alternative structures, the sense of hydrogen bond is inversed but little difference can be seen in hydrogen bond geometries. The preference of these alternative hydrogen bond structures comes from the local stabilization of hydroxyl groups with respect to the beta carbon. On the other hand when simulated fiber diffraction patterns were compared with experimental ones, the current structure of cellulose II with higher energy and the alternative structure of cellulose IIII with lower energy were in better agreement.
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| 2. |
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| 3. |
- Chen, Pan, et al.
(författare)
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I alpha to I beta mechano-conversion and amorphization in native cellulose simulated by crystal bending
- 2018
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Ingår i: Cellulose. - : Springer. - 0969-0239 .- 1572-882X. ; 25:8, s. 4345-4355
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Tidskriftsartikel (refereegranskat)abstract
- The bending of rod-like native cellulose crystals with degree of polymerization 40 and 160 using molecular dynamics simulations resulted in a deformation-induced local amorphization at the kinking point and allomorphic interconversion between cellulose I alpha and I beta in the unbent segments. The transformation mechanism involves a longitudinal chain slippage of the hydrogen-bonded sheets by the length of one anhydroglucose residue ( 0.5 nm), which alters the chain stacking from the monotonic (I alpha) form to the alternating I beta one or vice versa. This mechanical deformation converts the I alpha form progressively to the I beta form, as has been experimentally observed for ultrasonication of microfibrils. I beta is also able to partially convert to I alpha-like organization but this conversion is only transitory. The qualitative agreement between the behavior of ultrasonicated microfibrils and in silico observed I alpha -> I beta conversion suggests that shear deformation and chain slippage under bending deformation is a general process when cellulose fibrils experience lateral mechanical stress.
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| 4. |
- Chen, Pan, et al.
(författare)
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Quantifying the Contribution of the Dispersion Interaction and Hydrogen Bonding to the Anisotropic Elastic Properties of Chitin and Chitosan
- 2022
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Ingår i: Biomacromolecules. - : American Chemical Society (ACS). - 1525-7797 .- 1526-4602. ; 23:4, s. 1633-1642
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Tidskriftsartikel (refereegranskat)abstract
- The elastic tensors of chitin and chitosan allomorphs were calculated using density functional theory (DFT) with and without the dispersion correction and compared with experimental values. The longitudinal Young's moduli were 114.9 or 126.9 GPa for alpha-chitin depending on the hydrogen bond pattern: 129.0 GPa for beta-chitin and 191.5 GPa for chitosan. Furthermore, the moduli were found to vary between 17.0 and 52.8 GPa in the transverse directions and between 2.2 and 15.2 GPa in shear. Switching off the dispersion correction led to a decrease in modulus by up to 63%, depending on the direction. The transverse Young's moduli of a-chitin strongly depended on the hydroxylmethyl group conformation coupled with the dispersion correction, suggesting a synergy between hydrogen bonding and dispersion interactions. The calculated longitudinal Young's moduli were, in general, higher than experimental values obtained in static conditions, and the Poisson's ratios were lower than experimental values obtained in static conditions.
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| 5. |
- Chen, Pan, et al.
(författare)
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Small Angle Neutron Scattering Shows Nanoscale PMMA Distribution in Transparent Wood Biocomposites
- 2021
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Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6984 .- 1530-6992. ; 21:7, s. 2883-2890
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Tidskriftsartikel (refereegranskat)abstract
- Transparent wood biocomposites based on PMMA combine high optical transmittance with excellent mechanical properties. One hypothesis is that despite poor miscibility the polymer is distributed at the nanoscale inside the cell wall. Small-angle neutron scattering (SANS) experiments are performed to test this hypothesis, using biocomposites based on deuterated PMMA and "contrast-matched" PMMA. The wood cell wall nanostructure soaked in heavy water is quantified in terms of the correlation distance d between the center of elementary cellulose fibrils. For wood/deuterated PMMA, this distance d is very similar as for wood/heavy water (correlation peaks at q approximate to 0.1 angstrom(-1)). The peak disappears when contrast-matched PMMA is used, indeed proving nanoscale polymer distribution in the cell wall. The specific processing method used for transparent wood explains the nanocomposite nature of the wood cell wall and can serve as a nanotechnology for cell wall impregnation of polymers in large wood biocomposite structures.
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| 6. |
- Chen, Pan, et al.
(författare)
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Translational Entropy and Dispersion Energy Jointly Drive the Adsorption of Urea to Cellulose
- 2017
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Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1520-6106 .- 1520-5207. ; 121:10, s. 2244-2251
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Tidskriftsartikel (refereegranskat)abstract
- The adsorption of urea on cellulose at room temperature has been studied using adsorption isotherm experiments and molecular dynamics (MD) simulations. The immersion of cotton cellulose into bulk urea solutions with concentrations between 0.01 and 0.30 g/mL led to a decrease in urea concentration in all solutions, allowing the adsorption of urea on the cellulose surface to be measured quantitatively. MD simulations suggest that urea molecules form sorption layers on both hydrophobic and hydrophilic surfaces. Although electrostatic interactions accounted for the majority of the calculated interaction energy between urea and cellulose, dispersion interactions were revealed to be the key driving force for the accumulation of urea around cellulose. The preferred orientation of urea and water molecules in the first solvation shell varied depending on the nature of the cellulose surface, but urea molecules were systematically oriented parallel to the hydrophobic plane of cellulose. The translational entropies of urea and water molecules, calculated from the velocity spectrum of the trajectory, are lower near the cellulose surface than in bulk. As urea molecules adsorb on cellulose and expel surface water into the bulk, the increase in the translational entropy of the water compensated for the decrease in the entropy of urea, resulting in a total entropy gain of the solvent system. Therefore, the cellulose urea dispersion energy and the translational entropy gain of water are the main factors that drive the adsorption of urea on cellulose.
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| 7. |
- Chen, Yu, et al.
(författare)
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The thermodynamics of enhanced dope stability of cellulose solution in NaOH solution by urea
- 2023
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Ingår i: Carbohydrate Polymers. - : Elsevier BV. - 0144-8617 .- 1879-1344. ; 311, s. 120744-
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Tidskriftsartikel (refereegranskat)abstract
- The addition of urea in pre-cooled alkali aqueous solution is known to improve the dope stability of cellulose solution. However, its thermodynamic mechanism at a molecular level is not fully understood yet. By using molecular dynamics simulation of an aqueous NaOH/urea/cellulose system using an empirical force field, we found that urea was concentrated in the first solvation shell of the cellulose chain stabilized mainly by dispersion interaction. When adding a glucan chain into the solution, the total solvent entropy reduction is smaller if urea is present. Each urea molecule expelled an average of 2.3 water molecules away from the cellulose surface, releasing water entropy that over-compensates the entropy loss of urea and thus maximizing the total entropy. Scaling the Lennard-Jones parameter and atomistic partial charge of urea revealed that direct urea/cellulose interaction was also driven by dispersion energy. The mixing of urea solution and cellulose solution in the presence or absence of NaOH are both exothermic even after correcting for the contribution from dilution.
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| 8. |
- Han, Xiao, et al.
(författare)
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Planar and uniplanar orientation in nanocellulose films : interpretation of 2D diffraction patterns step-by-step
- 2023
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Ingår i: Cellulose. - : Springer Nature. - 0969-0239 .- 1572-882X. ; 30:13, s. 8151-8159
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Tidskriftsartikel (refereegranskat)abstract
- X-ray diffraction (XRD) is widely used in cellulose structural characterization. The commonly used “powder” XRD assumes the sample is macroscopically isotropic. For cellulose fibrous samples, however, due to the high aspect ratio of the components, the structure is often anisotropic, and the texture affects the materials properties to a large extent. A simple setup of a point-focused X-ray beam and a two-dimensional detection of scattered X-ray is a practical tool to analyze the texture. We studied three types of cellulose nanofibril (CNF) films obtained by casting. 2,2,6,6-tetramethylpiper- idine-1-oxyl radical (TEMPO) oxidized one shows a high degree of (1–10) uniplanar orientation, whereas holocellulose CNF and enzyme-pretreated CNF showed planar orientation. In the planar orientation, the c-axis is preferentially oriented in the plane parallel to the film while within each fibril other crystallographic axis would be randomly distributed around the c-axis. Also, a clear peak can be detected at low angle corresponding to a d-spacing of 3–4 nm indicating a strong correlation perpendicular to the film at this length scale. This distance was the lowest for TEMPO-CNF and corroborates with the model of uniplanar orientation of rectangular cross-section. The numerically simulated azimuthal intensity distribution of hk0 reflections in the two types of texture agreed well with the experimental intensity distribution.
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| 9. |
- Li, Lengwan, et al.
(författare)
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Recyclable nanocomposites of well-dispersed 2D layered silicates in cellulose nanofibril (CNF) matrix
- 2022
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Ingår i: Carbohydrate Polymers. - : Elsevier BV. - 0144-8617 .- 1879-1344. ; 279, s. 119004-
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Tidskriftsartikel (refereegranskat)abstract
- Nanocomposites based on components from nature, which can be recycled are of great interest in new materials for sustainable development. The range of properties of nacre-inspired hybrids of 1D cellulose and 2D clay platelets are investigated in nanocomposites with improved nanoparticle dispersion in the starting hydrocolloid mixture. Films with a wide range of compositions are prepared by capillary force assisted physical assembly (vacuum-assisted filtration) of TEMPO-oxidized cellulose nanofibers (TOCN) reinforced by exfoliated nanoclays of three different aspect ratios: saponite, montmorillonite and mica. X-ray diffraction and transmission electron micrographs show almost monolayer dispersion of saponite and montmorillonite and high orientation parallel to the film surface. Films exhibit ultimate strength up to 573 MPa. Young's modulus exceeds 38 GPa even at high MTM contents (40-80 vol%). Optical transmittance, UV-shielding, thermal shielding and fire-retardant properties are measured, found to be very good and are sensitive to the 2D nanoplatelet dispersion.
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| 10. |
- Li, Lengwan, et al.
(författare)
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Residual Strain and Nanostructural Effects during Drying of Nanocellulose/Clay Nanosheet Hybrids : Synchrotron X-ray Scattering Results
- 2023
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Ingår i: ACS Nano. - : American Chemical Society (ACS). - 1936-0851 .- 1936-086X. ; 17:16, s. 15810-15820
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Tidskriftsartikel (refereegranskat)abstract
- Cellulose nanofibrils (CNF) with 2D silicate nanoplatelet reinforcement readily form multifunctional composites by vacuum-assisted self-assembly from hydrocolloidal mixtures. The final nanostructure is formed during drying. The crystalline nature of CNF and montmorillonite (MTM) made it possible to use synchrotron X-ray scattering (WAXS, SAXS) to monitor structural development during drying from water and from ethanol. Nanostructural changes in the CNF and MTM crystals were investigated. Changes in the out-of-plane orientation of CNF and MTM were determined. Residual drying strains previously predicted from theory were confirmed in both cellulose and MTM platelets due to capillary forces. The formation of tactoid platelet stacks could be followed. We propose that after filtration, the constituent nanoparticles in the swollen, solid gel already have a “fixed” location, although self-assembly and ordering processes take place during drying.
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