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981.
Liliana C. Tomé Susana C. M. Fernandes Denilson Silva Perez Patrizia Sadocco Armando J. D. Silvestre Carlos Pascoal Neto Isabel M. Marrucho Carmen S. R. Freire 《Cellulose (London, England)》2013,20(4):1807-1818
Thin nanocomposite films of thermoplastic starch, chitosan and cellulose nanofibers (bacterial cellulose or nanofibrillated cellulose) were prepared for the first time by solvent casting of water based suspensions of the three polysaccharides. The role of the different bioploymers on the final properties (thermal stability, transparency, mechanical performance and antimicrobial activity) of the films was related with their intrinsic features, contents and synergic effects resulting from the establishment of interactions between them. Thermoplastic starch displays an important role on the thermal stability of the films because it is the most stable polysaccharide; however it has a negative impact on the mechanical performance and transparency of the films. The addition of chitosan improves considerably the transparency (up to 50 % transmittance for 50 % of chitosan, in respect to the amount of starch), mechanical performance and antimicrobial properties (at least 25 % of chitosan and no more than 10 % of cellulose nanofibers are required to observe bacteriostatic or bactericidal activity) but decrease their thermal stability. The incorporation of cellulose nanofibers had the strongest positive impact on the mechanical properties of the materials (increments of up to 15 and 30 MPa on the Young′s modulus and Tensile strength, respectively, for films with 20 % of BC or NFC). Nonetheless, the impact in thermal stability and mechanical performance of the films, promoted by the addition of chitosan and cellulose nanofibres, respectively, was higher than the expected considering their percentage contents certainly because of the establishment of strong and complex interactions between the three polysaccharides. 相似文献
982.
Monika Österberg Maria Soledad Peresin Leena-Sisko Johansson Tekla Tammelin 《Cellulose (London, England)》2013,20(3):983-990
A simple, solvent-free and low cost method to activate the surface of nanofibrillated cellulose films for further functionalization is presented. The method is based on the oxidative properties of UV radiation and ozone, to effectively remove contaminants from nanocellulosic surface, which remains clean and reactive for at least a week. The efficiency of the method is demonstrated by X-ray photoelectron spectroscopy (XPS) and contact angle measurements. In clear contrast to previous results on nanoscaled cellulose the relative atomic concentration of non-cellulosic carbon atoms was only 4 %, and water completely wetted the surface within seconds. After activation, neither chemical degradation nor morphological changes on cellulose were observed. This surface activation is essential for further functionalization of the film in dry state or nonpolar media. The surface activation was confirmed by silylation and a four times higher degree of substitution was achieved on the activated sample compared to non-activated reference film, as monitored with XPS. 相似文献
983.
Natalia Ferraz Anastasiya Leschinskaya Farshad Toomadj Bengt Fellström Maria Strømme Albert Mihranyan 《Cellulose (London, England)》2013,20(6):2959-2970
The membrane and solute diffusion properties of Cladophora cellulose and polypyrrole (PPy) functionalized Cladophora cellulose were analyzed to investigate the feasibility of using electroactive membranes in hemodialysis. The membranes were characterized with scanning electron microscopy, ζ-potentiometry, He-pycnometry, N2 gas adsorption, and Hg porosimetry. The diffusion properties across the studied membranes for three model uremic toxins, i.e. creatinine, vitamin B12 and bovine serum albumin, were also analyzed. The characterization work revealed that the studied membranes present an open structure of weakly negatively charged nanofibers with an average pore size of 21 and 53 nm for pristine cellulose and PPy-Cladophora cellulose, respectively. The results showed that the diffusion of uremic toxins across the PPy-Cladophora cellulose membrane was faster than through pure cellulose membrane, which was related to the higher porosity and larger average pore size of the former. Since it was found that the average pore size of the membranes was larger than the hydrodynamic radius of the studied model solutes, it was concluded that these types of membranes are favorable to expand the Mw spectrum of uremic toxins to also include conditions associated with accumulation of large pathologic proteins during hemodialysis. The large average pore size of the composite membrane could also be exploited to ensure high-fluxes of solutes through the membrane while simultaneously extracting ions by an externally applied electric current. 相似文献
984.
Celso Santi Junior Adriane Maria Ferreira Milagres André Ferraz Walter Carvalho 《Cellulose (London, England)》2013,20(6):3165-3177
The porosity of lignocellulosic materials is a key feature for the enzymatic hydrolysis of the constituent polysaccharides, being affected by its drying and lignin content. Here we evaluated the influence of both parameters in the porosity of sugarcane bagasse, using raw and chlorite-delignified samples. A fraction of these samples was air dried at room temperature and the remainder one was kept wet. All the samples were subjected to porometry (solute exclusion technique), determination of water retention value and assessment of enzymatic saccharification of polysaccharides. Samples with higher lignin contents had lower porosities and exhibited worse enzymatic conversions of polysaccharides. Mild drying collapsed only the smaller pores, which already were inaccessible to enzymes, and therefore did not affect the efficiencies of saccharification. Our results show that the lignin content plays an important role in the porosity of the material and that its removal improves the enzymatic saccharification of the constituent polysaccharides. 相似文献
985.
Eduardo González‐Olivares Lina M. Gallego‐Berrío Betsabé González‐Yañez Alejandro Rojas‐Palma 《Mathematical Methods in the Applied Sciences》2015,38(18):5183-5196
In this work, a modified Holling–Tanner predator–prey model is analyzed, considering important aspects describing the interaction such as the predator growth function is of a logistic type; a weak Allee effect acting in the prey growth function, and the functional response is of hyperbolic type. Making a change of variables and time rescaling, we obtain a polynomial differential equations system topologically equivalent to the original one in which the non‐hyperbolic equilibrium point (0,0) is an attractor for all parameter values. An important consequence of this property is the existence of a separatrix curve dividing the behavior of trajectories in the phase plane, and the system exhibits the bistability phenomenon, because the trajectories can have different ω ? limit sets; as example, the origin (0,0) or a stable limit cycle surrounding an unstable positive equilibrium point. We show that, under certain parameter conditions, a positive equilibrium may undergo saddle‐node, Hopf, and Bogdanov–Takens bifurcations; the existence of a homoclinic curve on the phase plane is also proved, which breaks in an unstable limit cycle. Some simulations to reinforce our results are also shown. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
986.
987.
This study's aim is to analyze heart rate dynamics in subjects with diabetes by measures of heart rate variability (HRV). The correlation of chaotic global parameters in the two cohorts is able to assess the probability of cardiac failure and other dynamical diseases. Adults (46) were divided into two equal groups. The autonomic evaluation consisted of measuring HRV for 30 min in supine position in absence of any physical, sensory, or pharmacological stimuli. Chaotic global parameters are able to statistically determine which series of electrocardiograph interpeak intervals in short time‐series are diabetic and which are not. The chaotic forward parameter that applies all three parameters is suggested to be the most appropriate and robust algorithm. This was decided after tests for normality; followed by one‐way analysis of variance (ANOVA1); (P < 0.09) and Kruskal–Wallis technique (P < 0.03). Principal component analysis implied two components represent 99.8% of total variance. Therefore, diabetes is a disease which reduces the chaotic response and, as such may be termed a dynamical condition such as are cardiac arrest, asthma, and epilepsy. © 2014 Wiley Periodicals, Inc. Complexity 20: 84–92, 2015 相似文献
988.
Three azobenzene pH indicators with amino[bis(ethanesulfonate)] substituents were synthesized and studied by UV–visible absorption spectroscopy in aqueous solution. The indicators exhibit brilliant and distinct colour changes with transitions between pH 1 and 4. Significant changes were observed in the UV–visible spectra on titration with acid with pKa values ranging from 2.2 to 2.8. The indicators demonstrate individual changes in colour as a function of pH. These novel pH indicators complement the existing library of azobenzene indicator dyes and may be useful for environmental situations with high proton concentrations. 相似文献
989.
Dipeptides comprising a d-phenylglycyl moiety coupled to the l-enantiomer of 2-amino butyric acid, norvaline, norleucine, and homocysteine were successfully synthesized from d-phenylglycine amide and the racemate of the corresponding unnatural amino acid. The reaction is catalyzed by an immobilized form of penicillin G acylase in an aqueous medium. The dipeptides were subsequently converted into the corresponding enantiopure diketopiperazines in overall isolated yields of 22–33%. 相似文献
990.
Net charge changes in the calculation of relative ligand‐binding free energies via classical atomistic molecular dynamics simulation 下载免费PDF全文
The calculation of binding free energies of charged species to a target molecule is a frequently encountered problem in molecular dynamics studies of (bio‐)chemical thermodynamics. Many important endogenous receptor‐binding molecules, enzyme substrates, or drug molecules have a nonzero net charge. Absolute binding free energies, as well as binding free energies relative to another molecule with a different net charge will be affected by artifacts due to the used effective electrostatic interaction function and associated parameters (e.g., size of the computational box). In the present study, charging contributions to binding free energies of small oligoatomic ions to a series of model host cavities functionalized with different chemical groups are calculated with classical atomistic molecular dynamics simulation. Electrostatic interactions are treated using a lattice‐summation scheme or a cutoff‐truncation scheme with Barker–Watts reaction‐field correction, and the simulations are conducted in boxes of different edge lengths. It is illustrated that the charging free energies of the guest molecules in water and in the host strongly depend on the applied methodology and that neglect of correction terms for the artifacts introduced by the finite size of the simulated system and the use of an effective electrostatic interaction function considerably impairs the thermodynamic interpretation of guest‐host interactions. Application of correction terms for the various artifacts yields consistent results for the charging contribution to binding free energies and is thus a prerequisite for the valid interpretation or prediction of experimental data via molecular dynamics simulation. Analysis and correction of electrostatic artifacts according to the scheme proposed in the present study should therefore be considered an integral part of careful free‐energy calculation studies if changes in the net charge are involved. © 2013 The Authors Journal of Computational Chemistry Published by Wiley Periodicals, Inc. 相似文献