The degradation of cellulose to lmw samples with $\overline {DP} _{{\rm w}} $ varying from 15 to 130 is investigated. Cellulose samples prepared from the hydrolysis of regenerated cellulose fibers in dilute HCl possess $\overline {DP} _{{\rm w}} $ = 50. Applying homogenous degradation of microcrystalline cellulose in H3PO4 at RT for 3 weeks, samples with $\overline {DP} _{{\rm w}} $ = 35 and a PDI of 1.58 are obtained. Decreasing the hydrolysis temperature to 8 °C results in lmw cellulose with $\overline {DP} _{{\rm w}} $ > 70. Fractionation in DMA/LiCl provides samples with $\overline {DP} _{{\rm w}} $ = 12 to 130, together with a narrow molecular weight distribution. Detailed structural analysis by 2D NMR spectroscopy reveals that the prepared lmw celluloses are suitable as mimics for cellulose.
A systematic study of the effects of , flow rate, voltage, and composition on the morphology of electrospun PLGA nanofibers is reported. It is shown that changes of voltage and flow rate do not appreciably affect the morphology. However, the of PLGA predominantly determines the formation of bead structures. Uniform electrospun PLGA nanofibers with controllable diameters can be formed through optimization. Further, multi‐walled carbon nanotubes can be incorporated into the PLGA nanofibers, significantly enhancing their tensile strength and elasticity without compromising the uniform morphology. The variable size, porosity, and composition of the nanofibers are essential for their applications in regenerative medicine.
PET/PEN blends were prepared over the full composition range via a melt mixing process under various processing conditions. This resulted in transesterification reactions and formation of copolymer structures with various average sequence block lengths and degree of randomness (RD) determined by 1H NMR. It was seen that with an increase in time and temperature of mixing copolymer content (TEN%) and RD increased, whereas the , values were decreased. The differences in the extent of transreactions arising from different processing histories showed their systematic influence on rheological characteristics. Moreover due to progress of transreactions during the rheological measurements, convergence was seen in all the rheological characteristics at terminal zones in the high frequency regions. Similar convergence in the copolymer structural parameters was also obtained by NMR analysis. An increase in TEN% led to a systematic increase in viscosity of the blends. A decrease in the , values results in an increase in elasticity and relaxation time due to improvement of blend interface with increase in extent of copolymer formation.
The viscosity functions of long‐chain branched metallocene‐catalyzed ethene homopolymers and copolymers (LCB‐mPE) were described by an extended Carreau‐Yasuda model. The two characteristic relaxation times, λ1 and λ2, and the slope of the viscosity function between these two characteristic relaxation times can be correlated to the zero shear‐rate viscosity, η0, and the molar mass . The characteristic relaxation times, λ1 and λ2 (describing the main curvatures of the viscosity function) exhibit a power law dependency on the molar mass, . The parameterization of the viscosity function can be used for a molecular characterization and flow simulations of various kinds.
The fluidization characteristics of anthraquinone hydrogenation catalysts were investigated in a liquid–solid fluidized bed. The effects of the initial bed conditions such as particle size, bed depth‐to‐column diameter ratio and liquid density and viscosity on the fluidization behaviour, bed expansion and applicability of the Richardson–Zaki equation were studied. The results reveal a strong particle size effect on the Richardson–Zaki (R‐Z) expansion index which in general decreased as the particle diameter increased. One type of particles exhibited two distinct bed expansion behaviours, depending mainly on the bed depth‐to‐column diameter ratio, with an experimentally established boundary at . This behaviour could be attributed to increasing wall friction and a tendency to exhibit slugging. The dependence of the Richardson–Zaki exponent on the liquid dynamic viscosity confirms the classic result . 相似文献
Turn Bak : We present rationally designed scaffolds that mimic the spatial projection of the i, i+4, i+7, and i+11 residues of an α‐helix. A library of biphenyl derivatives was shown by competition fluorescence polarization and ITC to mimic Bak and disrupt the Bak/Bcl‐xL protein–protein interaction. 15N HSQC experiments confirmed that the surface of Bcl‐xL normally occupied by Bak was the target area of our new synthetic inhibitors.
Chemical modification of EVOH in the molten state at 185 °C by a grafting from process of poly(ε‐caprolactone) in batch was studied. 1H NMR was used to characterize the structure evolutions of PCL grafts. In addition to grafting reactions, dynamic covalent transesterification reactions between EVOH residual alcohols and the polyester grafts led to a redistribution of the PCL grafts length. up to 27 and SR up to 80% were obtained. Experiments made in a corotating mini twin‐screw extruder also confirmed these results. The effect of the alcohol to caprolactone ratio and catalyst concentration (SnOct2) on kinetic evolution showed that few minutes were necessary to complete the polymerization. A kinetic model was proposed and adequate conditions for the synthesis by reactive extrusion were defined.
An experimental correlation between the non‐linear behaviour of commercial polyethylene melts in LAOS flow, and the pressure fluctuations associated with melt flow instabilities developed in capillary rheometry are presented. Polyethylene melts with enhanced non‐linear behaviour under LAOS conditions present larger pressure fluctuations during capillary extrusion, and consequently, larger surface distortions on the extrudate. The combination of both methods can be a tool to predict the development of melt flow instabilities in the extrusion process of polyethylene melts, and can elucidate their correlation with material structural properties ( , MWD and topology).
A set of amorphous poly[ethylene‐co‐(1,4‐cyclohexanedimethylene terephthalate)] (PECT) copolymers containing 25 and 30% of 1,4‐cyclohexane dimethylene (CHDM) units and small amounts of branching agent pentaerythritol (PER) is investigated. The level of long chain branching was estimated by analyzing the positive deviation from law. Branching also produced melt elasticity enhancement which is desirable for certain processing methods. Capillary extrusion experiments at 180 °C generated flow‐induced crystallization in PECT containing 25% of CHDM. Crystallization increased with the amount of PER added, which was explained by the favorable effect of branching to increase elongational rate at the entrance of the capillary. Linear and branched PECTs containing 30% of CHDM did not crystallize.