Using a continuous space rod‐bead model and an off‐lattice Monte Carlo technique we investigate interfacial properties between two incompatible polymers of different stiffnesses. The interfacial tension is determined by using virial theorem and analyzing the spectrum of capillary waves. Detailed interfacial profiles for segment and chain densities and orientations are obtained. The simulation results agree with mean field approaches for not too large stiffness disparities and show a marked tendency towards a plateau at higher stiffness disparities where the differences to mean field approaches increase. 相似文献
A predictive CG model based on a conventional freely rotating chain was developed to describe semiflexible polymers on a relatively large length/time scale. Parameterization of the model requires only two material properties such as, the Kuhn length and coil density. The diameter of spherical “beads” employed in the model is used as an effective parameter that needs to be determined from preliminary data. Once determined for a particular solvent system, this parameter can then be used to model general solvent systems on a parameter‐free basis. Comparison with SANS data on dilute conjugated polymer solutions reveals that the CG polymer model can well describe material properties ranging from local rodlike segments to bulk interchain aggregates.
The distribution function P(S) of the radius of gyration S, the corresponding elastic free energy A(S) and the mean force were computed from simulations based on the wormlike chain (WLC) model. The relation of the S‐conjugated elastic functions to the analogous functions based on the chain vector R and their connection to the statistical‐mechanics ensembles was elucidated. Simulation data revealed that available analytical functions for P(S) fail to predict the behavior of semiflexible chains. When the power‐law function P(S) was used instead, the exponents sizeably raised with stiffness at chain expansion. The exponents deduced from elastic compression of a chain agreed fairly with the scaling exponents for chain confinement into a sphere.
The thermal characteristics of a series of degradable polymers have been investigated using thermogravimetry and differential
scanning calorimetry. While the results of the thermogravimetry experiments suggest that the thermal stability of the polymers
should not pose any problems at the temperatures that can be expected in a commercial composting process (60°C), phase changes
associated with some of the polymers investigated may cause problems in the interpretation of data from composting degradation
studies. Several biodegradable polymers were observed to have melt transitions at temperatures similar to those found in a
composting environment. Consequently, under the controlled composting conditions used to evaluate biodegradable polymers,
degradation of a polymer may be inferred, while actually the polymer has merely undergone a phase change.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
The thermal behaviour of polyaniline plasticized by an oligoester of phosphoric acid was investigated by means of thermogravimetry,
either alone or coupled with Fourier transform infrared spectroscopy, and differential scanning calorimetry. It was found
that additional thermal transitions, resulting from the dopant, occur in the temperature range 200-250°C. The emission of
volatile products, as evidenced by the Gram-Schmidt chromatograms, was suppressed by the plasticizer used. This stabilization
was probably due to a decreased possibility of diffusion in the three-dimensional honeycomb polymer-plasticizer structure.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
It is shown that the obtained hardness and electrophilicity values of the short length oligomers for a heterocyclic conductive polymer from density functional theory method are quadraticly correlated to those obtained from semiempirical method. Therefore these quantities for all oligomers (up to 15 repeating units) are predicted using the obtained quadratic relations. Both of these predicted quantities for different oligomers are fitted to a new exponential model (Y = Y∞ ed/n) to estimate the electronic properties for the considered conjugated polymers. The calculated band gaps from this model show better agreement with the reported experimental data than those predicted by the previous models. Because of a wide range of variations, the electrophilicity could be a better index for investigating the doping effect in polymers than the hardness.
Layer flexibility in two-dimensional coordination polymers (2D-CPs) contributes to several functional materials as it results in anisotropic structural response to external stimuli. Chemical modification is a common technique for modifying layer structures. This study demonstrates that crystal morphology of a cyanide-bridged 2D-CP of type [Mn(salen)]2[ReN(CN)4] ( 1 ) consisting of flexible undulating layers significantly impacts the layer configuration and assembly. Nanoplates of 1 showed an in-plane contraction of layers with a longer interlayer distance compared to the micrometer-sized rod-type particles. These effects by crystal morphology on the structure of the 2D-CP impacted the structural flexibility, resulting in dual-functional changes: the enhancement of the sensitivity of structural transformation to water adsorption and modification of anisotropic thermal expansion of 1 . Moreover, the nanoplates incorporated new adsorption sites within the layers, resulting in the uptake of an additional water molecule compared to the micrometer-sized rods. 相似文献
AbstractHigh performance and high temperature polymers are a class of polymeric materials exhibiting high thermal stability and their resistance to fire makes them valuable assets for many applications. Those applications include as typical examples high temperature gas separation membranes, automotive and aerospace industry as well as the construction industry. The high performance polymers have been synthesized since the early 1960s, and have developed rapidly over the past few decades. Most high performance polymers comprise a highly aromatic backbone, linear chains, and strong inter-chain interactions. This review deals mostly with commercial polymeric materials. Studies regarding their thermal behavior, degradation mechanism and their reaction to fire have been synthetically combined in order to bring out potential insight concerning the effect of the thermal decomposition and thermal behavior on the fire properties of those polymers. 相似文献
Films ≈350 μm of poly(vinyl-alcohol) composites, containing copper (Cu), aluminium (Al) and iron (Fe), metallic powder very fine, were prepared by a casting method. Thermal conductivity, phonon velocity, mean free path and specific heat were studied. The pure sample of PVA has a lower values of thermal conductivity than that which are doped with metals. For all samples the thermal conductivityK increases up to a certain temperatureTgg (120–160°C) and then decreases with temperature. The specific heat increase with temperature up to ≈120°C and above 120°C is nearly independent on temperature. The pure sample of PVA has small values of mean free path (L)≈0.2 Å at room temperature, but for PVA+ metalsL≈2.0 Å. The phonon velocity of pure PVA is larger than that of PVA containing metals. 相似文献
Branched poly(methoxy‐PEG acrylate) and thermally responsive poly(methoxy‐PEG acrylate)‐block‐poly(N‐isopropylacrylamide) are synthesized by RAFT polymerization. After reduction, these polymers are fluorescently labeled by reacting the free thiol groups with N‐(5‐fluoresceinyl)maleimide. As shown by DLS, the labeled copolymer poly(methoxy‐PEG acrylate)‐block‐poly(N‐isopropylacrylamide) forms nanoparticles at body temperature (37 °C) due to the presence of the thermosensitive poly(N‐isopropylacrylamide). These materials were used as bioprobes for imaging HUVECs in vitro and chick embryo CAM in vivo. Both labeled polymer and nanoparticles are biocompatible and can be used as efficient fluorescent bioprobes.
The application of Boltzmann statistics to a complete distribution of molecular conformation energies of simplified homo‐ and copolymer models gives meaningful information about temperatures at which phase transitions take place in the bulk. We have calculated in the conformation statistical distribution (CSD) approximation Helmholtz free energy variation versus temperature δF = δU–TδS, where U and S are, respectively, the internal molecular energy and the Gibbs statistical entropy of the considered polymeric model. The deepest minima correspond to glass‐transition temperature (Tg) and melting temperature (Tm) of modelled polymers, while the remaining peaks are related to some other transitions, the existence of which is also experimentally proven. The adopted method is able to give Tg and Tm as a function of the molecular weight of polymers. Some indications can also be achieved about the instability of polymers. The same procedure has been applied to copolymers and blends and has given acceptable results for Tg and Tm as functions of the material microstructure and composition. Other thermal and mechanical properties, such as moduli, mobilities, chemical resistance to oxidation, physical tendency to miscibility, have been directly or indirectly estimated. 相似文献
This paper reports the synthesis and study of some Co(II) coordination polymers with the following acids as ligands: 2,8-dimethylphenoxyphosphinic,
diphenyloxyphosphinacetic, diphenylthiophosphinacetic and dihexylphosphinic acids. The study was performed by means of chemical
analysis, gel chromatography, IR spectroscopy, ESR, X-ray diffraction, thermogravimetry and electric resistance measurements.
The experimental results were used to propose the structural formulae of these compounds and to calculate the kinetic parameters
of the thermal decomposition reactions.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
Herein, two coordination polymers (CPs) [{Dy(Cl2An)1.5(CH3OH)} ⋅ 4.5 H2O]n ( 1 ) and [Dy(Cl2An)1.5(DMF)2]n ( 2 ), in which Cl2An is chloranilate (2,5-dihydroxy-1,4-benzoquinone dianion), exhibiting field-induced single-molecule magnet behavior with moderate barrier of magnetization reversal are reported. Detailed structural and topological analysis disclosed that 1 has a 3D network, whereas 2 has a 2D layered-type structure. In both CPs, magnetic measurements showed weak antiferromagnetic exchange interaction between the dysprosium centers and field-induced slow magnetic relaxation with barriers of 175(9)K and 145(7)K for 1 and 2 , respectively. Notably, the energy barriers of magnetization reversal of 1 and 2 are remarkable for metal–chloranilate-based 3D ( 1 ) and 2D ( 2 ) CPs. The temperature and field dependence of relaxation time indicate the presence of multiple relaxation pathways, such as direct, quantum tunneling of magnetization, Raman, and Orbach processes, in both CPs. Ab initio theoretical calculations reinforced the experimentally observed higher energy barrier in 1 as compared with 2 due to the presence of large transverse anisotropy in the ground state in the latter. The average transition magnetic moment between the computed low-lying spin–orbit states also rationalized the relaxation as Orbach and Raman processes through the first excited state. BS-DFT calculations were carried out for both CPs to provide more insight into the exchange interaction. 相似文献
Summary: This paper presents a systematic study of the compression of end‐anchored polymer layers by a variety of mechanisms. We treat layers in both good and Θ solvents, and in the range of polymer densities that is normally encountered in experiments. Our primary technique is numerical self‐consistent field (NSCF) theory. We compare the NSCF results for the different mechanisms with each other, and with those of the analytic SCF theory. For each mechanism, we calculate the density profiles, layer thicknesses, and free energies, all as functions of the degree of polymerization and surface coverage. The free energy and the deformation of each layer depend on the compression mechanism, and they can be very different from the ASCF theory. For example, the energy of compression can be as much as three times greater than the analytical SCF (ASCF) prediction, and it does not reduce to simple, universal functions of the reduced distance between the surfaces. The overall physical picture simplifies if the free energy is expressed in terms of the layer deformation, rather than the reduced surface separation. We also examine and quantify the interpenetration of layers, discuss why ASCF theory applies better to some compression mechanisms than others, and end with comments on the difficulties in extracting quantitative information from surface‐forces experiments.
Comparisons of forces of compression in a good solvent for the three different systems, as functions of D/nb. The lower three curves are for σ* = 3, and the upper three are for σ* = 23. 相似文献
The thermal mechanical properties and degradation behaviors were studied on fibers prepared from two high-performance, heterocyclic polymers, poly(p-phenylenebenzobisthiazole) (PBZT) and poly(p-phenylenebenzobisoxazole) (PBZO). Our research demonstrated that these two fibers exhibited excellent mechanical properties and outstanding thermal and thermo-oxidative stability. Their long-term mechanical tensile performance at high temperatures was found to be critically associated with the stability of the C—O or C—S linkage at the heterocyclic rings on these polymers' backbones. PBZO fibers with the C—O linkages displayed substantially higher thermal stability compared to PBZT containing C—S linkages. High resolution pyrolysis-gas chromatography/mass spectrometry provided the information of the pyrolyzates' compositions and distributions as well as their relationships with the structures of PBZT and PBZO. Based on the analysis of the compositions and distributions of all pyrolyzates at different temperatures, it was found that the thermal degradation mechanisms for both of these heterocyclic polymers were identical. Kevlar®-49 fibers were also studied under the same experimental conditions in order to make a comparison of thermo-oxidative stability and long-term mechanical performance at high temperatures with PBZO and PBZT fibers. The data of two high-performance aromatic polyimide fibers were also included as references.This revised version was published online in November 2005 with corrections to the Cover Date. 相似文献