Use of electrospinning to directly fabricate three-dimensional nanofiber stacks of cellulose acetate under high relative humidity condition

Unique structure-controllable three-dimensional (3D) nanofiber stacks of cellulose acetate (CA) were fabricated successfully by simply increasing relative humidity (RH) during the electrospinning process. It is found that once the RH exceeding 60 %, 3D flocculent nanofiber stacks would grow on the flat plate collector toward the needle tip without using special assisting apparatus or preparing special electrospinning solution. Compared with those obtained at low RH, the as-prepared nanofibers fabricated under high RH condition exhibited similar nanofiber diameter, density and porosity, and more importantly, 3D flocculent structures instead of typical two-dimensional (2D) electrospun non-woven mats, which would contribute to a significant improvement on the hydrophilicity. It is believed that rapid solidification of CA during the jet process and strong charge repulsion among CA nanofibers play important roles in the formation of 3D nanofibrous structure. Furthermore, these 3D flocculent nanofiber scaffolds exhibited better cytocompatibilities with human MG-63 cells than common 2D nanofibrous mats. Thus a facile and effective approach was presented to prepare 3D nanofiber stacks with tunable and reproducible properties for biodegradable scaffold applications.     

Conductivity percolation in loosely compacted microcrystalline cellulose: An in situ study by dielectric spectroscopy during densification

The present study aims at contributing to a complete understanding of the water-induced ionic charge transport in cellulose. The behavior of this transport in loosely compacted microcrystalline cellulose (MCC) powder was investigated as a function of density utilizing a new type of measurement setup, allowing for dielectric spectroscopy measurement in situ during compaction. The ionic conductivity in MCC was found to increase with increasing density until a leveling-out was observed for densities above approximately 0.7 g/cm3. Further, it was shown that the ionic conductivity vs density followed a percolation type behavior signifying the percolation of conductive paths in a 3D conducting network. The density percolation threshold was found to be between approximately 0.2 and 0.4 g/cm3, depending strongly on the cellulose moisture content. The observed percolation behavior was attributed to the forming of interparticulate bonds in the MCC and the percolation threshold dependence on moisture was linked to the moisture dependence of particle rearrangement and plastic deformation in MCC during compaction. The obtained results add to the understanding of the density-dependent water-induced ionic transport in cellulose showing that, at given moisture content, the two major parameters determining the magnitude of the conductivity are the connectedness of the interparticluate bonds and the connectedness of pores with a diameter in the 5-20 nm size range. At densities between approximately 0.7 and 1.2 g/cm3 both the bond and the pore networks have percolated, facilitating charge transport through the MCC compact.     

Relaxation phenomena and electrical conductivity of some polymeric films

Electrical conduction in sandwich samples of polyacrylonitrile (PAN) or copolymer of acrylonitrile with butadiene between silver has been studied, measuring the dependence of current on the applied field, temperature and time. The conduction mechanism depends on the polymer type. A polarization contribution is suggested in the conduction mechanism at high temperatures, besides Schottky emission in the case of PAN and the simple carrier jump model in the case of NBR at room temperature. The temporal current variation is explained in terms of dipole orientation. The mobility and charge carrier density are influenced by the applied field, temperature and film thickness.     

Dynamic mechanical analysis of RIM nylon-6

The β relaxation process was shown to occur at different temperatures and exhibit a different activation energy for dry and wet samples prepared by reaction injection molding (RIM) of Nylon-6. By employing a low operating frequency and controlled low moisture levels, it became possible to distinguish the two β relaxation processes, β₁ and β₂, in one sample by dynamic mechanical spectroscopy. It was shown that while β₁ was associated with moisture in the sample, β₂ was related to a structural mechanism independent of moisture. The effect of moisture content on the chain-chain association (α relaxation process) and chain-water association (β₁, relaxation process) in nylon-6 was assessed. The annealing mechanism of RIM Nylon-6 at 85°C was probed by both dynamic mechanical spectroscopy and x-ray diffraction.     

Effect of the Content and Ordering of the sp2 Free Carbon Phase on the Charge Carrier Transport in Polymer-Derived Silicon Oxycarbides

The present work elaborates on the correlation between the amount and ordering of the free carbon phase in silicon oxycarbides and their charge carrier transport behavior. Thus, silicon oxycarbides possessing free carbon contents from 0 to ca. 58 vol.% (SiOC/C) were synthesized and exposed to temperatures from 1100 to 1800 °C. The prepared samples were extensively analyzed concerning the thermal evolution of the sp² carbon phase by means of Raman spectroscopy. Additionally, electrical conductivity and Hall measurements were performed and correlated with the structural information obtained from the Raman spectroscopic investigation. It is shown that the percolation threshold in SiOC/C samples depends on the temperature of their thermal treatment, varying from ca. 20 vol.% in the samples prepared at 1100 °C to ca. 6 vol.% for the samples annealed at 1600 °C. Moreover, three different conduction regimes are identified in SiOC/C, depending on its sp² carbon content: (i) at low carbon contents (i.e., <1 vol.%), the silicon oxycarbide glassy matrix dominates the charge carrier transport, which exhibits an activation energy of ca. 1 eV and occurs within localized states, presumably dangling bonds; (ii) near the percolation threshold, tunneling or hopping of charge carriers between spatially separated sp² carbon precipitates appear to be responsible for the electrical conductivity; (iii) whereas above the percolation threshold, the charge carrier transport is only weakly activated (E_a = 0.03 eV) and is realized through the (continuous) carbon phase. Hall measurements on SiOC/C samples above the percolation threshold indicate p-type carriers mainly contributing to conduction. Their density is shown to vary with the sp² carbon content in the range from 10¹⁴ to 10¹⁹ cm⁻³; whereas their mobility (ca. 3 cm²/V) seems to not depend on the sp² carbon content.     

Effect of humidity and solvent vapor phase on cellulose esters films

In the present study the effect of relative humidity (RH) during spin-coating process on the structural characteristics of cellulose acetate (CA), cellulose acetate phthalate (C-A-P), cellulose acetate butyrate (CAB) and carboxymethyl cellulose acetate butyrate (CMCAB) films was investigated by means of atomic force microscopy (AFM), ellipsometry and contact angle measurements. All polymer solutions were prepared in tetrahydrofuran (THF), which is a good solvent for all cellulose esters, and used for spin-coating at RH of (35 ± 5)%, (55 ± 5)% or (75 ± 5)%. The structural features were correlated with the molecular characteristics of each cellulose ester and with the balance between surface energies of water and THF and interface energy between water and THF. CA, CAB, CMCAB and C-A-P films spin-coated at RH of (55 ± 5)% were exposed to THF vapor during 3, 6, 9, 60 and 720 min. The structural changes on the cellulose esters films due to THF vapor exposition were monitored by means of AFM and ellipsometry. THF vapor enabled the mobility of cellulose esters chains, causing considerable changes in the film morphology. In the case of CA films, which are thermodynamically unstable, dewetting was observed after 6 min exposure to THF vapor. On the other hand, porous structures observed for C-A-P, CAB and CMCAB turned smooth and homogeneous after only 3 min exposure to THF vapor.     

羧甲基纤维素取代均一性的研究

羧基纤维素(钠盐,以下简称CMC)是一种重要的水溶性聚电解质。本文在传统溶媒法的基础上,采用乙醇作溶剂的两段加碱法新工艺制备CMC。研究表明,在低浴比(乙醇与纤维素质量比为2.04～2.45)条件下,采用新工艺制得的CMC具有较好的取代均一性和良好的性能:并用酶降解的方法,通过还原值测定,对CMC的取代基沿分子链的分布进行了表征;此外,对CMC的一些性能进行了测定,同时对其热稳定性进行了研究。     

Effects of drying and pressing on the pore structure in the cellulose fibre wall studied by 1H and 2H NMR relaxation

A 1H and 2H NMR relaxation method was used to investigate the influence of drying and pressing on the pore size and pore size distribution in the cellulose fibre wall. The investigation was made in the moisture interval in which cellulose fibres normally shrink, i.e. from a moisture ratio of about 1.5 g water/g fibre to dry fibres. When the moisture content of a fibre sample was decreased by drying or pressing, the pores decreased in size and the pore size distribution became narrower. It was found that there were only small differences at a given moisture content between the pore size distributions of samples prepared by drying and by pressing. The results also indicate that the pore shrinkage in cellulose fibres during pressing or drying is a process in which the cell wall pores of a wet cellulose fibre successively shrink as the moisture content decreases. It was observed that, at low moisture contents, pressing and drying resulted in different 1H NMR spin-lattice relaxation profiles. This is discussed in terms of morphology differences in the fibre matrix. The mobility of the protons in the solid phase influences the liquid 1H NMR spin-lattice relaxation in heterogeneous systems through magnetization transfer. We have also studied the effects of hornification in recycled pulps     

Effect of humidity on the supramolecular structure of cotton, studied by quantitative spin probing

The effect of water content on the physicochemical properties of the amorphous regions in cotton were investigated by measuring the electron paramagnetic resonance (EPR) of TEMPOL nitroxide radicals, deposited in cotton at different loadings, as a function of the relative humidity (RH) and temperature. Three different components contribute differently to the experimental EPR spectra, corresponding to (a) mobile radicals absorbed in the bulk amorphous region, (b) slow moving radicals adsorbed on the crystallite surfaces in cotton, and (c) aggregated radicals. These components were analyzed by means of computer-aided simulations of the line shapes and simplified line width methods. Polarity and mobility parameters were extracted from the analysis of the spectra. For all loadings and temperatures, the polarity suddenly dropped when the water content fell below approximately 3 wt %, i.e., when water was removed from the bulk amorphous regions. At the lowest loading (2 x 10(-5) mol kg(-1)), the spectra were independent of the RH, and only mobile radicals were observed. At intermediate loading (10(-4)-10(-3) mol kg(-1)) both mobile (fast) and adsorbed (slow) moving radicals were present, the fraction of which depended on the RH. The mobility of the adsorbed and mobile radical signals was smaller at higher loadings, indicating microdomains of different character. The temperature dependence of the rotational correlation times provided the activation energies, which were much lower than in liquids. An equilibrium exists between the mobile and the adsorbed radicals. The temperature dependence of the equilibrium constant, K, gave the enthalpy and the entropy of the adsorption process. At low RH, the enthalpy and the entropy values indicated a simple adsorption process. At 10(-3) mol kg(-1), the values were independent of the RH, but at low loadings the values increased with the increase in the RH, which suggested a displacement of adsorbed water by the radicals at high water content. At loadings above 10(-3) mol kg(-1), signals from radicals strongly interacting via spin exchange were observed, which are assigned to aggregated radicals; simulation of the spectra gave an activation energy of 13 kJ mol(-1) for the spin exchange process. These effects are rationalized on the basis of microdomains of different character within cotton, reflecting the variation in pore sizes (0.5-8 nm) and the relaxation behavior of the cellulose chains.     

Physico-Chemical Stability of Warfarin in Ground Mixtures with β-Cyclodextrin or Microcrystalline Cellulose under Storage Conditions

The effects of grinding and the excipients used on the solid state stability of warfarin in ground mixtures with β-cyclodextrin or microcrystalline cellulose have been investigated by using x-ray powder diffractometry, IR spectrophotometry and differential scanning calorimetry. The grinding process did not result in warfarin-excipient interaction, but caused a crystalline form of warfarin to transfer to an amorphous state in the presence of β-cyclodextrin. The stability of this amorphous state of warfarin in tablets made from the ground mixtures stored at 37°C and 75% RH (relative humidity) was also studied. There was only a small amount of amorphous warfarin in tablets that recrystallized under storage conditions. The recrystallization of amorphous warfarin might be due to moisture adsorption. β-cyclodextrin possesses an ability to prevent the recrystallization of large amounts of amorphous warfarin in tablets stored at 37°C and 75% RH.     

The water vapour sorption behaviour of three celluloses: analysis using parallel exponential kinetics and interpretation using the Kelvin-Voigt viscoelastic model

The objective of this study was to examine the water vapour sorption behaviour of three celluloses, which were originally derived from cotton fibers, using a dynamic vapour sorption apparatus, including analyses of the sorption rate and hysteresis occurring in the isotherm run. Cotton linter, α-cellulose, and microcrystalline cellulose (MCC), respectively attained equilibrium moisture contents of 14.2, 20.6, and 16.9% at a relative humidity (RH) of 95%. All three cellulosic materials exhibited sorption hysteresis to varying degrees throughout the full RH range; the MCC and α-cellulose displayed the lowest and highest total hysteresis, respectively. The sorption kinetics were analysed in terms of the parallel exponential kinetics (PEK) model, with excellent fits to the data being obtained. The PEK data is further interpreted on the basis of two Kelvin-Voigt elements operating in series. Clear differences in behaviour were seen between the α–cellulose and the other two celluloses in this study. The relative importance of capillary condensation and matrix viscoelasticity with respect to sorption hysteresis is discussed.     

Charge carrier mobility and concentration as a function of composition in AgPO3-AgI glasses

Conductivity data of the xAgI(1 - x)AgPO(3) system (0 ≤ x ≤ 0.5) were collected in the liquid and glassy states. The difference in the dependence of ionic conductivity on temperature below and above their glass transition temperatures (T(g)) is interpreted by a discontinuity in the charge carrier's mobility mechanisms. Charge carrier displacement occurs through an activated mechanism below T(g) and through a Vogel-Fulcher-Tammann-Hesse mechanism above this temperature. Fitting conductivity data with the proposed model allows one to determine separately the enthalpies of charge carrier formation and migration. For the five investigated compositions, the enthalpy of charge carrier formation is found to decrease, with x, from 0.86 to 0.2 eV, while the migration enthalpy remains constant at ≈0.14 eV. Based on these values, the charge carrier mobility and concentration in the glassy state can then be calculated. Mobility values at room temperature (≈10(-4) cm(2) V(-1) s(-1)) do not vary significantly with the AgI content and are in good agreement with those previously measured by the Hall-effect technique. The observed increase in ionic conductivity with x would thus only be due to an increase in the effective charge carrier concentration. Considering AgI as a weak electrolyte, the change in the effective charge carrier concentration is justified and is correlated to the partial free energy of silver iodide forming a regular solution with AgPO(3).     

Promotion of low-humidity proton conduction by controlling hydrophilicity in layered metal-organic frameworks

We controlled the hydrophilicity of metal-organic frameworks (MOFs) to achieve high proton conductivity and high adsorption of water under low humidity conditions, by employing novel class of MOFs, {NR(3)(CH(2)COOH)}[MCr(ox)(3)]·nH(2)O (abbreviated as R-MCr, where R = Me (methyl), Et (ethyl), or Bu (n-butyl), and M = Mn or Fe): Me-FeCr, Et-MnCr, Bu-MnCr, and Bu-FeCr. The cationic components have a carboxyl group that functions as the proton carrier. The hydrophilicity of the cationic ions was tuned by the NR(3) residue to decrease with increasing bulkiness of the residue: {NMe(3)(CH(2)COOH)}(+) > {NEt(3)(CH(2)COOH)}(+) > {NBu(3)(CH(2)COOH)}(+). The proton conduction of the MOFs increased with increasing hydrophilicity of the cationic ions. The most hydrophilic sample, Me-FeCr, adsorbed a large number of water molecules and showed a high proton conductivity of ~10(-4) S cm(-1), even at a low humidity of 65% relative humidity (RH), at ambient temperature. Notably, this is the highest conductivity among the previously reported proton-conducting MOFs that operate under low RH conditions.     

CuInS2量子点阻变效应

采用改进的热分解法制备了具有半导体效应的CuInS₂量子点,量子点尺寸均匀、大小为4.2 nm。组装的Au/CuInS₂/FTO阻变存储器件表现出典型的双极性阻变特点,其开态电压为-3.8 V,关态电压为4 V,ON/OFF开关比约为10³。对器件的I-V特性曲线线性拟合发现,器件的阻变机制在高阻态时表现为空间限制电荷（SCLC）传导机制,在低阻态时表现为欧姆传导机制。器件的阻变特性主要是由于电荷被CuInS₂薄膜中的缺陷产生的势阱捕获导致。通过调节陷阱势垒高度引起电荷在陷阱中移动,导致导电通路的产生和断裂,使器件处于高阻态和低阻态。     

On the origins of sudden adhesion loss at a critical relative humidity: examination of bulk and interfacial contributions

The origins for abrupt adhesion loss at a critical relative humidity (RH) for polymeric adhesives bonded to inorganic surfaces have been explored using a model poly(methyl methacrylate) (PMMA) film on glass. The interfacial and bulk water concentrations within the polymer film as a function of D 2O partial pressure were quantified using neutron reflectivity. Adhesion strength of these PMMA/SiO 2 interfaces under the same conditions was quantified using a shaft loaded blister test. A drop in adhesion strength was observed at a critical RH, and at this same RH, a discontinuity in the bulk moisture concentration occurred. The moisture concentration near the interface was higher than that in the bulk PMMA, and at the critical RH, the breadth of the interfacial water concentration distribution as a function of distance from the SiO 2/PMMA interface increased dramatically. We propose a mechanism for loss of adhesion at a critical RH based upon the interplay between bulk swelling induced stress and weakening of the interfacial bond by moisture accumulation at the PMMA/SiO 2 interface.     

Structural inhomogeneity in cotton cellulose upon its interaction with water

Comparative analysis of the characteristics of supramolecular structures of dry and swollen cotton fibers makes it possible to differentiate structural regions accessible to water. The revealed features of water desorption from cotton cellulose (exceptionally low rates at the final stages, presence of residual moisture) are related to the removal of water, which is localized in the regions of the crystalline phase disorganized upon drying. The fact of incomplete moisture removal from cotton fibers at < 325 K is interpreted from the stand-point of the frozen molecular mobility in the microsurroundings of sorption sites, which are located at the defects of crystallites, at the final stage of the desorption process. A marked contribution from the recrystallization of disorganized regions in the surface layer of crystallites to the thermal effect of the interaction between water and cotton cellulose at low water content is established.     

The pressure–volume–temperature relationship of cellulose

Pressure–volume–temperature (PVT) measurements of α-cellulose with different water contents, were performed at temperatures from 25 to 180 °C and pressures from 19.6 to 196 MPa. PVT measurements allowed observation of the combined effects of pressure and temperature on the specific volume during cellulose thermo-compression. All isobars showed a decrease in cellulose specific volume with temperature. This densification is associated with a transition process of the cellulose, occurring at a temperature defined by the inflection point T _t of the isobar curve. T _t decreases from 110 to 40 °C with pressure and is lower as moisture content increases. For isobars obtained at high pressures and high moisture contents, after attaining a minimum, an increase in volume is observed with temperature that may be related to free water evaporation. PVT α-cellulose experimental data was compared with predicted values from a regression analysis of the Tait equations of state, usually applied to synthetic polymers. Good correlations were observed at low temperatures and low pressures. The densification observed from the PVT experimental data, at a temperature that decreases with pressure, could result from a sintering phenomenon, but more research is needed to actually understand the cohesion mechanism under these conditions.     

On the electron donor and the electrophilic substitution activating abilities of substituents in uracil

QTAIM properties for uracil and 18 derivatives containing the substituents -NH(2), -OH, -OCH(3), -SH, -F, -Cl, -CH(3) -NO(2), and -Li in position 5 or 6 were computed on MP2/6-31++G//MP2/6-31G charge densities. The results indicate that -OH, -OCH(3), and -NH(2) groups are really retrieving charge from the ring. Also, the activating ability of the substituent groups, usually considered as the variation of electron population at the carbon where the electrophilic attack takes place, C, was studied. The study shows that the activating ability is reflected by the variation of pi charge or quadrupole moment at C, and also by the variation of the Laplacian of the charge density in the secondary charge concentration points around C (SCC-C). They indicate a similar, but not exactly equal, graduation of activating ability. The relative behavior of the substituents is basically the same as in benzene, though benzene has more tendency to concentrate charge in the SCC-C regions than uracil, where this tendency is larger for 6- than for 5-derivatives. sigma(+/-)(R) Taft parameters are found to display good correlations with the above indicated activating indexes. Finally, the resonance model predicts most of the main variations displayed by QTAIM atomic pi electron populations of derivatives with regard to uracil, but there are still some significant variations of the pi electron charge that it cannot predict.     

Improved Conductivity Induced by Photodesorption in SnO2 Thin Films Grown by a Sol-Gel Dip Coating Technique

Thin films of undoped and Sb-doped SnO2 have been prepared by a sol-gel dip-coating technique. For the high doping level (2–3 mol% Sb) n-type degenerate conduction is expected, however, measurements of resistance as a function of temperature show that doped samples exhibit strong electron trapping, with capture levels at 39 and 81 meV. Heating in a vacuum and irradiation with UV monochromatic light (305 nm) improves the electrical characteristics, decreasing the carrier capture at low temperature. This suggests an oxygen related level, which can be eliminated by a photodesorption process. Absorption spectral dependence indicates an indirect bandgap transition with Eg 3.5 eV. Current-voltage characteristics indicate a thermionic emission mechanism through interfacial states.