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排序方式: 共有345条查询结果,搜索用时 12 毫秒
1.
LH Olde Damink PJ Dijkstra MJ van Luyn PB van Wachem P Nieuwenhuis J Feijen 《Canadian Metallurgical Quarterly》1996,17(7):679-684
Bacterial collagenase was used to study the susceptibility of dermal sheep collagen (DSC) cross-linked with a mixture of the water-soluble carbodiimide 1-ethyl-3-(3-dimethyl aminopropyl)-carbodiimide hydrochloride and N-hydroxysuccinimide (E/N-DSC) towards enzymatic degradation. Contrary to non-cross-linked DSC (N-DSC), which had a rate of weight-loss of 18.1% per hour upon degradation, no weight loss was observed for E/N-DSC during a 24 h degradation period. The tensile strength of the E/N-DSC samples decreased during this time period, resulting in partially degraded samples having 80% of the initial tensile strength remaining. The susceptibility of E/N-DSC samples towards enzymatic degradation could be controlled by varying the degree of cross-linking of the samples. Ethylene oxide sterilization of E/N-DSC samples made the material more resistant against degradation compared with non-sterilized E/N-DSC samples. This may be explained by a decrease of the adsorption of bacterial collagenase onto the collagen owing to reaction of ethylene oxide with remaining free amine groups in the collagen matrix. 相似文献
2.
Ion exchange of HZSM-5 samples with alkali metal cations, using metal chloride solutions, results in partially exchanged zeolites, MHZSM-5, M = Li, Na, K or Cs. The degree of exchange is found to increase with increasing ionic radius of the cations. The catalytic properties of the alkalized zeolites were evaluated using the reaction conditions under which the catalytic activity of the HZSM-5 samples in terms of n-hexane cracking is proportional to the aluminium content. From the residual catalytic activity exhibited by the Na-, K- and CsHZSM-5 samples it is concluded that each of the larger Na+, K+ and Cs+ ions is influencing more than one AlO
4
–
tetrahedron, implying that the aluminium sites in ZSM-5 are not isolated. The ion-exchange results are then interpreted in terms of non-isolated aluminium sites. The ion-exchange and catalytic properties of the zeolites as a function of aluminium content are also discussed. 相似文献
3.
Together with the development of information systems research, there has also been increased interest in non-linear relationships between focal constructs. This article presents six Partial Least Squares-based approaches for estimating formative constructs’ quadratic effects. In addition, these approaches’ performance is tested by means of a complex Monte Carlo experiment. The experiment reveals significant and substantial differences between the approaches. In general, the performance of the hybrid approach as suggested by Wold (1982) is most convincing in terms of point estimate accuracy, statistical power, and prediction accuracy. The two-stage approach suggested by Chin et al (1996) showed almost the same performance; differences between it and the hybrid approach – although statistically significant – were unsubstantial. Based on these results, the article provides guidelines for the analysis of non-linear effects by means of variance-based structural equation modelling. 相似文献
4.
A new principle for an adaptive line driver is presented. This type of line driver can adapt its output impedance automatically to the applied load. This results in automatically corrected output impedance for different cables with terminations. Also, the line-driver output impedance becomes insensitive to process variations. As an example, a line driver for analog video signals has been designed. The circuit operates from a 2.4-V supply in a 0.35-μm CMOS technology. The realized circuit adapts between 38 and 85 Ω loads, has total harmonic distortion of <-50 dB at 1.2 Vpp for 0-10 MHz, 0.09-mm2 area, and 9-mW static power consumption 相似文献
5.
M.R. ten Breteler J. Feijen P.J. Dijkstra F. Signori 《Reactive and Functional Polymers》2013,73(1):30-38
Hetero-telechelic, low-molecular-weight polylactides (PLAs) were prepared by the zinc-catalyzed ring-opening polymerization of l-lactide or d-lactide using functional initiators and subsequent reaction with termination reagents, yielding –OH, –COOH, –NH2 and –SH as functional chain ends. Structural characterization was performed by molecular weight analysis, NMR spectroscopy and MALDI-TOF mass spectrometry. The thermal behavior of the species was investigated by DSC, which revealed that the modification of the hydroxyl terminus lowered the number of lactic acid units within a PLA chain that can effectively participate in crystallization. Both the Tm and ΔHm values for these polymers were lower compared to those of PLAs of comparable chain length with no modification of the hydroxyl end-group. In stereocomplexes prepared from equimolar amounts of the hetero-telechelic PLLAs and PDLAs, this suppressive effect on Tm and ΔHm was also observed. Modification of the hydroxyl ends to produce –COOH, –NH2 and –SH end groups reduced the stereocomplex Tm values to ~10–20 °C. The lower limit for the crystallization of the stereocomplexes was found at a DP of 5. 相似文献
6.
van der Laan J.M.; Teplyakov A.V.; Kelders H.; Kalk K.H.; Misset O.; Mulleners L.J.S.M.; Dijkstra B.W. 《Protein engineering, design & selection : PEDS》1992,5(5):405-411
The crystal structure of a serine protease from the alkalophilicstrain Bacillus alcalophilus PB92 has been determined by X-raydiffraction at 1.75 Â resolution. The structure has beensolved by molecular replacement using the atomic model of subtilisinCarlsberg. The model of the PB92 protease has been refined toan R-factor of 14.0% and contains 1882 protein atoms, two calciumions and 188 water molecules. The overall folding of the polypeptide chain closely resembles that of the subtilisins. Furthermore,almost all of the secondary structure elements found in subtilisinCarlsberg are also present in the PB92 protease. The major differencesbetween the two structures are located around the deletion regions(residues 37 and 158161 in subtilisin Carlsberg) andin two loops which are known to be the most variable parts ofsubtilisin structures. Flexibility of one of these loops (residues126130 in the PB92 protease) is believed to account forthe inducedfit mechanism of substrate binding. 相似文献
7.
Eijsink Vincent G.H.; Dijkstra Bauke W.; Vriend Gerrit; van der Zee J.Rob; Vettman Oene R.; van der Vinne B.; van den Burg B.; Kempe S.; Venema G. 《Protein engineering, design & selection : PEDS》1992,5(5):421-426
Cavities in the hydrophobic core of the neutral protease ofBacillus stearothermophilus were analyzed using a threedimensionalmodel that was inferred from the crystal structure of thermolysin,the highly homologous neutral protease of B.thermoproteolyticus(85% sequence identity). Sitedirected mutagenesis wasused to fill some of these cavities, thereby improving hydrophobicpacking in the protein interior. The mutations had small effectson the thermostability, even after drastic changes, such asLeu284Trp and Met168Trp. The effects on T50, the temperatureat which 50% of the enzyme is irreversibly inactivated in 30min, ranged from 0.0 to +0.4°C. These results can be explainedby assuming that the mutations have positive and negative structuraleffects of approximately the same magnitude. Alternatively,it could be envisaged that the local unfolding steps, whichrender the enzyme susceptible towards autolysis and which arerate limiting in the process of thermal inactivation, are onlyslightly affected by alterations in the hydrophobic core. 相似文献
8.
Zhong Zhiyuan Schneiderbauer Stefan Dijkstra Pieter J. Westerhausen Matthias Feijen Jan 《Polymer Bulletin》2003,50(3):175-182
Summary
Single-site calcium initiators containing chelating tmhd
(H-tmhd = 2,2,6,6-tetramethylheptane-33-dione) ligands
[(THF)Ca(tmhd)]2[-N(SiMe3)2](-tmhd)
(2) and
[(THF)Ca(tmhd)]2[-OCH(Me)Ph](-tmhd)
(3) have been synthesized and
applied for the ring-opening polymerization of L-lactide and
-caprolactone. Both 2 and
3 were highly reactive and
promoted a fast polymerization of L-lactide and -caprolactone
to high monomer conversions under mild conditions (THF as a
solvent, room temperature). More importantly, results showed
that the ring-opening polymerizations of lactides and lactones
initiated by either 3 or
2 in the presence of equivalent
2-propanol are living, to provide polymers and block copolymers
of controlled molecular weights and tailored end-groups. The
polymerizations were first-order in monomer up to high
conversions, in which the in
situ initiating system 2/2-propanol revealed no induction period
and much faster polymerization kinetics as compared to
3. 相似文献
9.
P. T. S. Dijkstra R. J. Gaymans D. J. Van Dijk J. Hutink 《Polymer Engineering and Science》2003,43(9):1613-1623
The deformation behavior of isotactic polypropylene (PP) as a function of strain rate was investigated at 50°C in uniaxial tension. Injection molded dogbone specimens were tested at high strain rates, ε = 10?1 – 102 s?1, and the local deformation in the neck was studied using fast tensile videometry. A strong elastic recoil was observed after fracture in this strain rate range with local elastic strains as high as ?e = 2.0 – 3.2. The recoil is very fast and takes place within 1 ms. The elastic fraction of the strain at break was found to increase with the local strain rate. The elasticity further depends on strain and temperature. The elastic deformation behavior is part of the known transition from ductile cold drawing behavior to brittle fracture that occurs with strain rate or temperature. The elasticity in PP is thought to be due to a decrease in crystallinity, resulting in a discontinuous crystalline structure comparable to that of thermoplastic elastomers. 相似文献
10.
Niels J. Sijbrandi Ad J. Kimenai Edwin P.C. Mes René Broos Georg Bar Martin Rosenthal Yaroslav I. Odarchenko Dimitri A. Ivanov Jan Feijen Pieter J. Dijkstra 《Polymer》2012,53(19):4033-4044
Segmented poly(ether ester amide)s comprising glycine or β-alanine extended bisoxalamide hard segments are highly phase separated thermoplastic elastomers with a broad temperature independent rubber plateau. These materials with molecular weights, Mn, exceeding 30 × 103 g mol?1 are conveniently prepared by polycondensation of preformed bisester–bisoxalamides and commercially available PTHF diols. FT-IR revealed strongly hydrogen bonded and highly ordered bisoxalamide hard segments with degrees of ordering between 73 and 99%. The morphology consists of fiber-like nano-crystals randomly dispersed in the soft polymer matrix. The micro-structural parameters of the copolymers were addressed by simultaneous small- and wide-angle X-ray scattering. It is shown that the crystals have strictly identical thickness, which is close to the contour length of the hard segment. The long dimension of the crystals is identified with the direction of the hydrogen bonds. The melting transitions of the hard segments are sharp, with temperatures up to 170 °C. The studied polymers have an elastic modulus in the range of 139–170 MPa, a stress at break in the range of 19–31 MPa combined with strains at break of higher than 800%. The segmented copolymer comprising the β-alanine based bisoxalamide hard segment with a spacer of 6 methylene groups has a melting transition of 141 °C which is higher than the melting transition of its glycine analogue of 119 °C. Likewise, the fracture stress increased from 22 to 31 MPa when the glycine ester group in the hard segment was replaced with β-alanine. The improved thermal and mechanical properties of the latter polymers is related to the crystal packing of the β-alanine based hard segments in the copolymer compared to the packing of the hard segments comprising glycine ester groups. 相似文献