Segment relaxation in PBT/PC and PA6/ABS blends as studied by thermally stimulated depolarization currents

The segment relaxation in two series of binary, finely dispersed poly(butylene terephthalate)/polycarbonate (PBT/PC) and polyamide-6/acrylonitrile-buta-diene-styrene (PA6/ABS) blends was studied by the method of thermally stimulated depolarization currents (TSDC) both in normal mode (global TSDC spectra) and in thermal-sampling mode (TSDC-TS). The resulting temperature dependencies and distribution functions of segment relaxation activation energy E_asr and the influence of annealing on the relaxation behavior of the mixed phases are discussed, considering the phase morphology. Common to all blends under study are lower E _asrp (the most probable value of E _asr), narrower E _asr distribution functions, and broader temperature ranges of the glass transitions in both phases of the blend compared to those of the initial components. The relationships are in good agreement with the hypothesis on the spontaneous fractionation of polymers in blends and on the breakdown of the cooperative segment mobility regions caused by the interactions between the molecular chains of different polymers. In finely dispersed small particles of the PBT-rich phase (particle diameter ≥ 0.5 μm), a degeneration of the cooperative segment (a) relaxation in a noncooperative segment (β) relaxation caused by the solution of PC molecules in PBT was detected.     

油酸甘油酯体系的低场核磁共振弛豫特性研究

甘油酯体系低场核磁共振（LF-NMR）弛豫特性的研究有助于脂质重构进程的快速监测及产物分析.本文主要对不同酯化度的单一或混合油酸甘油酯体系的LF-NMR弛豫特性进行了研究.结果表明,油酸甘油酯呈现三个弛豫峰;随酯化程度增加或温度升高,单组分和多组分弛豫时间均增大,各峰面积比例有一定改变,且酯化程度越高,弛豫时间的变化幅度越大;对二元和三元油酸甘油酯体系而言,随三油酸甘油酯（GTO）比例增至40%,单组分和多组分弛豫时间均增大,S₂₂峰面积比例增加,而S₂₃减小;三元混合体系的LF-NMR弛豫特性经主成分分析（PCA）后,混合体系在得分图上随GTO比例及二油酸甘油酯/单油酸甘油酯配比（GDO/GMO）的变化而呈规律性分布.     

Structural relaxation of Zr60Al15Ni25 amorphous ribbon: experimental evidence of the electrical resistivity

In order to study the structural relaxation of the Zr₆₀Al₁₅Ni₂₅ amorphous ribbon, the electrical resistivity was experimentally investigated. The changes in the resistivity before glass transition temperature were observed. Two temperature points (423?K and 573?K) were chosen for cyclic heating experiments. The results showed that both irreversibility and reversibility of structural changes existed in amorphous alloys, which were related to the selected temperature and cycle times. Based on the scattering mechanism of electron conduction in metal, the structural defects model was used to explain the changes of electrical resistivity. The sample was in a highly disorder state after experienced thermal cycling of high temperature (573?K). The number and kind of atoms may be changed to increase the crystallisation range.     

Thermal evolution of residual stress in IN718 alloy subjected to laser peening

The thermal relaxation behaviors of residual stresses induced by laser peening (LP) in IN718 alloy were investigated using an integrated numerical simulation and experimental approach. LP and heat treatments (HT) were carried out after which the X-ray diffraction (XRD) technique was employed in measuring the residual stresses. Micro-structures were observed using an optic microscope (OM) and transmission electron microscope (TEM). Dislocations induced by LP were also observed by TEM and characterized using the XRD technique. The effects of the applied temperature and the exposure time on residual stress and micro-structures were investigated. The results show that the extent of the residual stresses relaxation increased accordingly with the increase in the applied temperature. The relaxation rate was initially high and tended to stabilize for a longer exposure time. Grain size evolution during the process was subsequently discussed. Furthermore, a conceivable mechanism of residual stresses thermal relaxation behavior was obtained.     

Metallic behavior of cyclotron relaxation time in two-dimensional systems

Cyclotron resonance of two-dimensional electrons is studied at low temperatures down to 0.4 K for a high-mobility Si/SiGe quantum well which exhibits a metallic temperature dependence of dc resistivity ρ. The relaxation time τ(CR) shows a negative temperature dependence, which is similar to that of the transport scattering time τ(t) obtained from ρ. The ratio τ(CR)/τ(t) at 0.4 K increases as the electron density N(s) decreases, and exceeds unity when N(s) approaches the critical density for the metal-insulator transition.     

Behavior of multi-walled carbon nanotubes on the porosity and microstructure of cement-based materials

The porosity and microstructure of a Portland cement-multi-walled carbon nanotube composite were investigated. Multi-walled carbon nanotubes (CNTs), up to 1 wt.% of cement, synthesized by infusion chemical vapor deposition, and Portland cement type I (PC) were used to produce pastes with a water to cement ratio of 0.5. Mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM) were used to characterize Portland cement-CNTs systems. MIP analysis of the results indicates that total porosity of the mixes with CNTs was found to decrease with increasing CNTs content. Moreover, an important effect of additional CNTs was a reduction in the number of mesopores, while SEM technique showed dispersion of CNTs between the hydration phases of Portland cement pastes.     

Investigation of Relaxation Process of Nylon 1212 Using Dielectric Relaxation Spectroscopy

The relaxation processes of α-form nylon 1212 from 50°C up to 160°C were studied by dielectric relaxation spectroscopy (DRS) in a wide frequency range of 63 Hz to 5 MHz. The α relaxation, the electrode relaxation, and the conductivity relaxation of nylon 1212 were observed and analyzed in detail using permittivity and modulus formalism. Electrode polarization and dc conductivity were the origin of high dielectric permittivity values at low frequencies and high temperatures. The strength of the imaginary part of the electric modulus of conductivity relaxation M″ _max was nearly independent of temperature. The distribution of local conductivity and relaxation time became broader with decreasing temperature.     

聚甲基丙烯酸甲酯/镓纳米复合物的动力学弛豫行为

通过原位自由基聚合方法制备了聚甲基丙烯酸甲酯(PMMA)以及含镓(Ga)质量分数为11.3%和13.5%的PMMA/Ga纳米复合材料.在玻璃化转变温度及以上温区,利用能量耗散技术研究PMMA/Ga纳米复合物的动力学弛豫行为,发现随着Ga含量的增加,复合物的α弛豫峰移向高温但峰高降低.此外,还定量研究了Ga含量对PMMA/Ga复合物的α' 弛豫的影响,并作出了相应的解释. 关键词： 相对能量耗散 玻璃化转变 力学弛豫     

Solving the initial condition of the string relaxation equation of the string model for glass transition: part-I

The string model for the glass transition can quantitatively describe the universal α-relaxation in glassformers,including the average relaxation time,the distribution function of the relaxation time,and the relaxation strength as functions of temperature.The string relaxation equation(SRE) of the model,at high enough temperatures,simplifies to the well-known single particle mean-field Debye relaxation equation,and at low enough temperatures to the well-known Rouse-Zimm relaxation equation that describes the relaxation dynamics of linear macromolecules.However,its initial condition,necessary to the further model predictions of glassy dynamics,has not been solved.In this paper,the special initial condition(SIC) of the SRE,i.e.for straight strings and the dielectric spectrum technique that is one of the most common methods to measure the glassy dynamics,was solved exactly.It should be expected that the obtained SIC would benefit the solution of the general initial condition of the SRE of the string model,i.e.for stochastically spatially configurating strings,as will be described in separate publications.     

Sonophotocatalytic degradation of methyl orange by carbon nanotube/TiO2 in aqueous solutions

In this study, carbon nanotubes (CNTs)/TiO2 composite were prepared and the sonophotocatalytic activity of CNTs/TiO2 nanoparticles was investigated, in which methyl orange (MO) was chosen as an object. The results indicate that the photocatalytic efficiency of CNTs/TiO2 remarkable increases in the presence of ultrasound, and the sonophotocatalysis process followed a first-order kinetics. The kinetic constant of CNTs/TiO2 for the MO degradation is 2.2 times higher than that of P25, which indicated that the sonophotocatalytic ability of CNTs/TiO2 is obviously higher than P25 powder.     

Effect of carbon nanotubes dispersion on morphology, internal structure and thermal stability of electrospun poly(vinyl alcohol)/carbon nanotubes nanofibers

The current work reports the effect of multi walled carbon nanotubes and single walled carbon nanotubes dispersion on morphological, structural and thermal degradation of electrospun poly(vinyl alcohol) (PVA)/carbon nanotubes (CNTs) dispersed in sodium dodecyl sulfate (SDS) (PVA/CNTs–SDS) composites nanofibers. (PVA/CNTs–SDS) nanocomposites fibers were elaborated using the traditional electrospinning process to disperse and align CNTs into the fibers, especially for low CNTs loading fraction: 0.3 and 0.7 wt%. The morphology of the electrospun fibers was studied using the scanning electronic microscopy. The average diameter of the fibers changes significantly after the incorporation of the CNTs in the PVA. Furthermore, Fourier transform infrared spectroscopy elucidated the effect of CNTs on the crystallization of the PVA which was confirmed by X-ray diffraction analysis. Thermogravimetric analysis showed that the thermal stability of the composite fibers depends on the loading fraction and on the type of carbon nanotubes.     

可激发气体振动弛豫时间的两频点声测量重建算法

振动弛豫时间是可激发气体分子内外自由度能量转移速率的宏观体现,它决定了声吸收谱峰值点对应的弛豫频率.本文给出了等温、绝热定压和绝热定容三种不同热力学过程下振动弛豫时间的相互关系;基于Petculescu和Lueptow[2005 Phys.Rev.Lett.94 238301]的弛豫过程合成算法,推导了单一压强下两频点声测量值的弛豫时间重建算法.该算法可应用于等温、绝热定压、绝热定容弛豫时间和弛豫频率的重建测量,并避免了弛豫时间传统声测量方法需要不断改变气体腔体压强的问题.仿真结果表明,对于室温下CO_2,CH_4,Cl_2,N_2和O_2组成的多种气体,重建的弛豫时间和弛豫频率与实验数据相符.     

Fast proton spin-lattice relaxation time in the rotating frame during the application of time averaged precession frequency

A relatively rapid phase alternation of the effective field in the time averaged precession frequency (TAPF) sequence results in averaging of the proton RF spin-lock field. The spin-locking of the proton magnetization becomes less efficient and thus shortens T(1rho)(H), the proton spin-lattice relaxation time in the rotating frame. The relaxation time also depends on the ratio of tau(1) and tau(2) intervals i.e. tau(1)/tau(2) and not only on the number of tau(c)=tau(1)+tau(2) blocks, i.e. the number of the phase transients. Experiments are performed on solid samples of ferrocene and glycine and for some time intervals, T(1rho)(H) is shortened by factors of 9-100 compared to the relaxation times obtained in the standard experiment.     

Dielectric relaxation study of dipropylsulfoxide/water mixtures

The complex dielectric spectra of dipropylsulfoxide (DPSO)/water mixtures in the whole concentration range have been measured as a function of frequency between 100 MHz and 20 GHz at four temperatures between 298.15 K and 328.15 K. The dielectric parameters, static dielectric constant (ε_s), relaxation time (τ) and relaxation strength (Δε) have been obtained by the least squares fit method. The relaxation in these mixtures can be described by two Debye functions, whereas for pure DPSO Cole-Davidson type is valid. The relaxation times of the mixtures show a maximum at about x(DPSO) ≈ 0.3. In the concentration range where a maximum appears, the interaction of DPSO with water is presumably the result of hydrogen bonding between water and the sulfonyl group of the sulfoxide molecule. The concentration and temperature dependent excess dielectric constant and effective Kirkwood correlation factor of the binary mixtures have been determined. The excess permittivity is found to be negative for all concentrations.     

Interlaminar interface relaxation upon heating carbon fiber thermoplastic-matrix composite,studied by contact electrical resistivity measurement

The interlaminar interface of a crossply carbon fiber thermoplastic-matrix composite was found to relax upon heating at and above the glass transition temperature of the matrix. The relaxation caused the contact electrical resistivity of the interface to increase gradually and irreversibly.     

Complex permittivity and microwave absorption properties of carbon nanotubes/polymer composite: A numerical study

Based on an equivalent resistance-capacitance (RC) network, we investigate theoretically the complex permittivity and microwave absorption properties of carbon nanotubes (CNTs)/polymer composite in the frequency range of 50 MHz-3 GHz using the logarithmic mixing rule. Both the real and imaginary parts of the permittivities of CNTs and polymer are considered in detail. The simulated results show that the real and imaginary permittivities of the composite increase explicitly with increasing volume fraction of CNTs, and the latter is more sensitive. The calculated complex permittivity spectra of the composite are in good agreement with the available experimental data. In addition, a good linear relationship between microwave absorbance and frequency is found.     

Anomalous Curie-point relaxation in a Cr-containing amorphous alloy

Anomalous (negative) irreversible and reversible Curie-point relaxation was observed in amorphous Fe₇₆Cr₈B₁₅Cu₁ alloy. This observation is in line with the results previously found in a FeNiCrSiB alloy series where the Cr-concentration dependence of the normal (positive) reversible Curie-point relaxation could only be explained by assuming two contributions of different signs to the relaxation. The normal contribution is related to the ordering of Fe and Ni atoms while the anomalous one is associated with the Cr atoms. The role of Cr on the resistivity relaxation is also presented. Existing literature results on anomalous structural relaxation of metallic glasses are critically discussed.     

Spatial evolution of the generation and relaxation of excited carriers near the breakdown of the quantum Hall effect

We have measured the generation and relaxation of excited carriers along their drift direction near the breakdown of the quantum Hall effect (QHE). The dissipative resistivity ρ_xx(x) at current densities close to the critical value for the QHE breakdown was measured as a function of the distance x from the electron injection at x=0. By injecting “cold” electrons into constrictions at supercritical current levels, the evolution of the breakdown along the drift direction was monitored. After a smooth increase of the resistivity with the drifting distance, an avalanche-like rise towards a saturation value occurs. Drastic changes of the resistivity profiles with the applied current were found in a narrow range around the critical current. The observed behavior is attributed to impurity-assisted tunneling between Landau levels. By injecting hot electrons (excited in a periodic set of constrictions) into a region with subcritical current density, the relaxation process was analyzed. Inelastic relaxation lengths with typical values in the range from 0.3 to 4 μm were found, which agree within 10% with the elastic mean free path determined from the Hall mobility at zero magnetic field. We conclude that the energy relaxation process is triggered by scattering at impurity potentials.     

Dielectric relaxation study of ascorbic acid solutions in pure dimethylsulfoxide (or diethylsulfoxide) and in dimethylsulfoxide (or diethylsulfoxide)/water mixtures

The complex dielectric spectra of ascorbic acid solutions in pure dimethylsulfoxide (or diethylsulfoxide) and in dimethylsulfoxide (or diethylsulfoxide)/water mixtures have been measured at frequencies between 100 MHz and 20 GHz at 298.15 K.Two kinds of dielectric relaxation processes were observed in each solution. The low frequency relaxation (Debye term) is assigned to solute. The high frequency relaxation (Cole-Davidson term) is ascribed to practically unaffected solvent.From the results obtained it follows that relaxation time and relaxation strength in DESO containing solutions (i.e. binary AA/DESO and ternary AA/DESO/water systems) are greater than those in DMSO containing solutions.