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21.
The use of isopropanol (IPA) as the reaction solvent for the preparation of high molecular weight segmented polyether-urea copolymers based on cycloaliphatic diisocyanates was investigated. Reactivity of IPA with bis(4-isocyanatohexyl)methane (HMDI) and isophorone diisocyanate (IPDI) was studied between 0 and 40 °C using in-situ FTIR spectroscopy. HMDI, which has secondary isocyanate groups, shows a very slow reaction with a large excess of IPA at 0 and 23 °C. Analysis of the kinetic data indicates an activation energy of 51 kJ/mol for the reaction between HMDI and IPA. As expected, IPDI, which has both a primary and a secondary isocyanate (NCO) group, reacts faster with IPA compared with HMDI, which only has secondary NCO groups. However, the rate of reaction of IPDI with IPA at 0 °C is extremely slow (approximately 1% consumption of isocyanate in 60 min) thus allowing the use of IPA as the reaction solvent for polyether-urea synthesis. Preparation of high molecular weight, high-strength HMDI and IPDI based polyether-urea segmented copolymers in IPA has been demonstrated. Thermal analysis and stress-strain analyses were used to characterize the products. 相似文献
22.
23.
Nanocomposites consisting of thermoplastic polyurethane–urea (TPU) and silica nanoparticles of various size and filler loadings were prepared by solution blending and extensively characterized by Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), thermal analysis, tensile tests, and nanoindentation. TPU copolymer was based on a cycloaliphatic diisocyanate and poly(tetramethylene oxide) (PTMO-2000) soft segments and had urea hard segment content of 20% by weight. TPU/silica nanocomposites using silica particles of different size (29, 74 and 215 nm) and at different loadings (1, 5, 10, 20 and 40 wt. %) were prepared and characterized. Solution blending using isopropyl alcohol resulted in even distribution of silica nanoparticles in the polyurethane–urea matrix. FTIR spectroscopy indicated strong interactions between silica particles and polyether segments. Incorporation of silica nanoparticles of smaller size led to higher modulus and tensile strength of the nanocomposites, and elastomeric properties were retained. Increased filler content of up to about 20 wt. % resulted in materials with higher elastic moduli and tensile strength while the glass transition temperature remained the same. The fracture toughness increased relative to neat TPU regardless of the silica particle size. Improvements in tensile properties of the nanocomposites, particularly at intermediate silica loading levels and smaller particle size, are attributed to the interactions between the surface of silica nanoparticles and ether linkages of the polyether segments of the copolymers. 相似文献
24.
N. N. Ziyatdinov G. M. Ostrovskii I. I. Emel’yanov 《Theoretical Foundations of Chemical Engineering》2016,50(2):178-187
The approach to the problem of designing optimal heat-exchange networks for the reconstruction and synthesis of distillation columns is discussed. It has been proposed to reduce the original discrete-continuous nonlinear programming problem to the modified linear programming assignment problem that takes into account a number of features of separation systems. 相似文献
25.
Montmorillonite modified with distearyldimethyl ammonium chloride (C18-QAC) (Nanofil-15) (NF15) was incorporated into polydimethylsiloxane-urea
(silicone-urea, PSU) copolymers. PSU was obtained by the reaction of equimolar amounts of aminopropyl terminated polydimethylsiloxane
(PDMS) oligomer (<Mn> = 3,200 g/mol) and bis(4-isocyanatohexyl)methane (HMDI). A series of PSU/NF15 nanocomposites were prepared by solution blending
with organoclay loadings ranging from 0.80 to 9.60% by weight, corresponding to 0.30 to 3.60% C18-QAC. Colloidal dispersions
of organophilic clay (NF15) in isopropanol were mixed with the PSU solution in isopropanol and were subjected to ultrasonic
treatment. Composite films were obtained by solution casting. FTIR spectroscopy confirmed that the organoclay mainly interacted
with the urea groups but not with PDMS. XRD analysis showed that nanocomposites containing up to 6.40% by weight of organoclay
had fully exfoliated silicate layers in the polymer matrix, whereas 9.60% loading had an intercalated structure. Physicochemical
properties of nanocomposites were determined. PSU/NF15 nanocomposites displayed excellent long-term antibacterial properties
against E. coli. 相似文献
26.
L. B. Boinovich A. M. Emel’yanenko 《Protection of Metals and Physical Chemistry of Surfaces》2010,46(4):403-410
A general thermodynamic approach is presented towards a triple point shift in arbitrary spatially limited systems with a curvilinear interface, including interfacial phases, small particles, and matter in pores. The suggested approach is based on using the coupling equations for vapor pressure over the bulk and spatially limited media together with the Clausius-Clapeyron relationships, and allows one to explain the effect of different physical-chemical parameters on the value and sign of the shift in the phase transition temperature. Besides, an explanation was offered within the developed theory for the difference in the melting points of different crystal faces, the presence of a nanometer melt layer for the substance in fine pores, and the formation of premolten or presolidified shell about nanoparticles. 相似文献
27.
Tamahkar Emel Özkahraman Bengi Özbaş Zehra İzbudak Burçin Yarimcan Filiz Boran Filiz Öztürk Ayça Bal 《Journal of Porous Materials》2021,28(3):741-750
Journal of Porous Materials - The antibacterial sponges with high macroporosity, high interconnectivity and high biocompatibility is a significant concern for wound healing applications. In this... 相似文献
28.
Emine Ubay Cokgor Gulsum Emel Zengin Didem Okutman Tas Seda Oktay Clifford Randall Derin Orhon 《Canadian Metallurgical Quarterly》2006,132(1):68-74
This study investigates the potential of primary sludge fermentation for the generation of readily biodegradable substrate. Experimental evaluation indicates that uncontrolled fermentation converted 22% of the initial volatile suspended solids in the sludge into soluble biodegradable chemical oxygen demand (COD). More than 85% of the soluble COD generated was associated with the formation of short chain volatile fatty acids (VFAs). The recoverable fraction of the fermented sludge supernatant may potentially increase the biodegradable COD content of the primary effluent by 5%. The VFA composition predominantly involved acetic and propionic acids as reported in the literature. Due to the high VFA content, activated sludge Model No. 1 could not predict the COD fractionation in the primary sludge; activated sludge Model No. 3 provided a better interpretation of the oxygen uptake profile through initial storage of the VFAs in the sludge. 相似文献
29.
The influence of temperature on the parameters of the band-to-band emission spectrum of a light-emitting diode based on single-crystal silicon was investigated; the unprecedentedly high stability against variations in temperature was observed for both the electroluminescence intensity at the peak of the spectral distribution (I EL m ) and the wavelength corresponding to this peak (λm). The internal quantum efficiency of the light-emitting diode at room temperature is estimated as no lower than 0.1%. The value of I EL m varies by no more than ~10% as the temperature is varied from 120 to 300 K. The value of λm remains virtually constant in the temperature range of 200–300 K. The unprecedentedly high stability of λm is related to interference effects in the oxide film through which the radiation of the light-emitting exits. It is shown that one of the important factors that govern the temperature stability of I EL m is a decrease in the lifetime of the minority charge carriers with decreasing temperature. 相似文献
30.
G. A. Emel’chenko A. N. Gruzintsev A. B. Kulakov É. N. Samarov I. A. Karpov A. N. Red’kin E. E. Yakimov C. Barthou 《Semiconductors》2007,41(2):176-179
The spectra of spontaneous and stimulated emission of ZnO nanorods grown by two low-temperature procedures are studied. Stimulated emission at 385 nm is observed at room temperature in CVD ZnO nanocrystals pumped by the nitrogen laser radiation at the wavelength 337 nm. The threshold pumping power density for lasing exitonic recombination is ~600 kW/cm2. 相似文献