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排序方式: 共有71条查询结果,搜索用时 15 毫秒
1.
Ranjit Prasad Vanja Pasanovic‐Zujo Rahul K. Gupta Ferenc Cser Sati N. Bhattacharya 《Polymer Engineering and Science》2004,44(7):1220-1230
Shear and extensional rheological measurements were conducted in conjunction with laser light scattering (LLS) on ethylene‐vinyl acetate copolymer (EVA) nanocomposites. The materials were prepared by melt‐mixing EVA and commercially acquired layered silicates. Wide Angle X‐Ray Scattering (WAXS) was used to ascertain the degree of layer swelling. This could be attributed to the intercalation of polymer chains into the interlayer of the silicates. The nanocomposites prepared were determined to be predominantly intercalated in nature. In shear rheological tests, the nanocomposites exhibited an increase in viscoelastic properties compared to the pure EVA. The extent of this property enhancement was not as pronounced as had been reported in many instances with respect to other polymer nanocomposites. This could be attributed to the absence of a network structure normally observed in an exfoliated system. The extensional rheological tests showed an increase in extensional flow properties. This was confirmed by the LLS, which indicated that the filled systems had higher deformability than the unfilled one. Polym. Eng. Sci. 44:1220–1230, 2004. © 2004 Society of Plastics Engineers. 相似文献
2.
Polymeric nanocapsules are attractive devices with a number of potential applications. In the present contribution we describe a method for nanocapsule preparation which is based on the formation of nanosized templates (mesoglobules, prepared from thermo-responsive poly(methoxydiethyleneglycol methacrylate)s, PDEGMA). These mesoglobules were coated with a cross-linked shell formed by pseudo-seeded radical polymerization of either N-isopropylacrylamide or 2-hydroxyethylmethacrylate in the presence of a cross-linking agent. Dissolution and removal of templates were achieved by extensive dialysis against water at temperatures below the LCST of PDEGMA. The obtained nanocapsules were visualized by transmission electron microscopy and their dimensions were determined by dynamic light scattering. The differences in the morphology of the nanocapsules were attributed to the different structures of the cross-linked membranes. 相似文献
3.
Formation of mesoglobules in aqueous media from thermo-sensitive poly(ethoxytriethyleneglycol acrylate) 总被引:1,自引:1,他引:0
Natalia Toncheva-Moncheva Philip Dimitrov Christo B. Tsvetanov Barbara Robak Barbara Trzebicka Andrzej Dworak Stanislav Rangelov 《Polymer Bulletin》2011,67(7):1335-1346
A series of six poly(ethoxytriethyleneglycol acrylate) (PETEGA) homopolymers were synthesized by atom transfer radical polymerization,
reversible addition-fragmentation transfer polymerization, and anionic polymerization in order to cover a molecular weight
range from 7,000 to 40,000 Da. The polymers exhibited a lower critical solution temperature (LCST) behavior in water, which
was observed by the occurrence of a cloud point (CP) at around 35 °C. The transmittance of visible light versus temperature
dependence overlapped during the cooling and the heating cycles, showing almost a complete lack of hysteresis. Moreover, instead
of the occurrence of an uncontrolled macroscopic phase separation, stable colloidal aggregates (mesoglobules) of narrow distribution
in particle size were formed in water at temperatures above the LCST of PETEGA at 1 g L−1 solutions. The dimensions of the mesoglobules ranged from 91 to 235 nm, and particle size was not influenced by the molecular
weight of PETEGA. Temperature changes caused considerable variations of the mesoglobules dimensions, which were smaller at
higher temperatures. The addition of an anionic surfactant simultaneously increased the CP values by 4–6 °C and lowered the
dimensions of the mesoglobules. 相似文献
4.
Planar laser-induced fluorescence measurements were used to investigate the expansion dynamics of a femtosecond laser-induced plasma. Temporally and spatially resolved measurements were performed to monitor the atoms that were ablated from a silicon target. A dye laser (lambda = 288.16 nm) was used to excite fluorescence signals. The radiation of an off-resonant transition (Si 390.55 nm) was observed at different distances from the target surface. This allowed easy detection of the ablated Si atoms without problems caused by scattered laser light. Abel inversion was applied to obtain the radial distribution of the Si atoms. The atom distribution in the plasma shows some peculiarities, depending on the crater depth. 相似文献
5.
A femtosecond laser system was used in combination with a time-of-flight mass spectrometer (TOF-MS) for in-depth profiling of semiconductor and metal samples. The semiconductor sample was a Co-implanted (10(17) ions/cm3) silicon wafer that had been carefully characterized by other established techniques. The total depth of the shallow implanted layer was 150 nm. As a second sample, a thin film metal standard had been used (NIST 2135c). This standard consisted of a silicon wafer with nine alternating Cr and Ni layers, each having a thickness of 56 and 57 nm, respectively. An orthogonal TOF-MS setup was implemented. This configuration was optimized until a sufficient mass resolution of 300 (m/delta m) and sensitivity was achieved. The experiments revealed that femtosecond-laser ablation TOF-MS is capable of resolving the depth profiles of these demanding samples. The poor precision of the measurements is discussed, and it is shown that this is due to pulse-to-pulse stability of the current laser system. Femtosecond-laser ablation TOF-MS is shown to be a promising technique for rapid in-depth profiling with a good lateral resolution of various multilayer thin film samples. 相似文献
6.
Synthesis and aqueous solution properties of aliphatic double chain end-capped poly(ethylene glycol)
Summary
A PEG-based amphiphilic polymer bearing two aliphatic double chain moieties was prepared. Its diluted aqueous solution properties
were studied by viscometry, dye solubilization and rheology. At concentrations lower than (6–8)×10−3 g/mL ring-shaped macromolecules were formed due to intrachain hydrophobic interactions. At concentrations about (6–8)×10−3 g/mL a formation of "flower" type aggregates took place. 1,6-Diphenyl-1,3,5-hexatriene solubilization was used to prove the
existence of hydrophobic domains and a critical aggregation concentration of 7×10−3 g/mL was determined. The peculiar viscosity profile and the rheopectic behavior of the amphiphilic polymer were attributed
to the existence of aggregates and to a temporary network formation during deformation, respectively.
Received: 13 February 2001/Revised version: 15 June 2001/Accepted: 15 June 2001 相似文献
7.
The orthogonal frequency division multiplexing (OFDM) transmission scheme is currently experiencing increased popularity due
to advances in very large scale integration technology. It is used for a variety of broadband systems such as asymmetric digital
subscriber lines, very-high-speed digital subscriber lines, digital video, and audio broadcasting, and wide local area network
standards such as IEEE 802.11a, IEEE 802.11 g, and ETSI Hiperlan/2. However, propagation impairments can cause severe degradation
in bit error rates (BER) for coherent detection. We derive a semi-analytical method to evaluate BER of a quadrature phase
shift keying (QPSK)-OFDM system in Nakagami, m < 1 fading and additive noise where pilot-assisted linear channel estimation and channel equalization are used. This allows
modeling of more severe fading environments than can be depicted by a Rayleigh distribution. Numerical simulations are used
to validate the proposed methods. The techniques developed can be applied to other channel estimation techniques, modulation
schemes and the performance evaluation of equalized single carrier narrowband systems. 相似文献
8.
The hyper-singular traction boundary integral equation method (BIEM) has been developed to analyze the dynamic behavior of two-dimensional finite bi-material plates with one or more interface cracks under uniform time-harmonic tension. The multi-region BEM technique is employed. Fracture parameters and scattered wave field far from the crack-tips are computed. The numerical results show the dependence of these dynamic characteristics on the frequency of the applied load, on the Dundurs bi-material constants, on the crack length and on the existence of other neighboring cracks. 相似文献
9.
Perry TD Klepac-Ceraj V Zhang XV McNamara CJ Polz MF Martin ST Berke N Mitchell R 《Environmental science & technology》2005,39(22):8770-8775
Biologically produced exopolysaccharides (EPS) affect calcite dissolution and precipitation. In this study, natural alkaliphilic microbial isolates were collected from biofilms on historic limestone. The isolates were screened for their ability to produce significant quantities of EPS in cultures. The most productive isolates were identified by 16S rRNA sequence analysis as a close relative of Bacillus cereus. EPS with different chemical structures were harvested from the isolates. Isothermal titration calorimetry (ITC) was used to quantify the thermodynamics of binding by the harvested EPS to calcite. The binding was described by a Langmuir adsorption isotherm. Characterization of the EPS showed that binding strength to calcite depended on the chemical nature of the polymer. 相似文献
10.
Hollow polymeric nanocapsules with a thermosensitive membrane are prepared and characterized. They reversibly change their dimensions during temperature variations below and above the transition of the membrane. The nanocapsules were prepared by three steps: (i) well-defined mesoglobules prepared from an LCST polymer (hydroxyl end functionalized poly(2-isopropyl-2-oxazoline), PiPOZ-OH) were coated with a thermo-sensitive cross-linked shell formed via seeded radical copolymerization of N-isopropylacrylamide (NIPAM) and N,N′-bis-methylene acrylamide to produce core–shell nanoparticles (ii), which were subjected to extensive dialysis below the LCSTs of both the core-forming PiPOZ-OH and shell-forming PNIPAM to remove the core (iii). The use of a core-forming polymer of low molecular weight (<8900 g mol−1), narrow dispersity (<1.15) and relatively low Tg (52–68 °C) is beneficial as far as the effectiveness of the removal of the cores is concerned. The inherent immiscibility between PiPOZ-OH and PNIPAM as well as the specific raspberry-like structure of the core–shell particles also contributed for enhancement of the core removal effectiveness. 相似文献