Complex macromolecular dynamics:: I. Estimation technique for time-resolved emission anisotropy ratio of chromophores incorporated into polymer chains

Time-resolved fluorescence emission anisotropy ratios of carbazolyl groups incorporated into polystyrene chains in polyethyleneoxide(PEO)/1,2-dichloroethane mixtures have been measured by the single photon counting method. The fluorescence depolarization method is very excellent to clarify various dynamical modes of polymer chains, and many theoretical and experimental researches have so far been reported in the field of polymer chain dynamics. However there are few reports about the dynamics on the polymer side chain, because the dynamical mechanism of the polymer side chain is very complicated. In this report we tried to analyze the dynamical modes of the polymer side chains by the fluorescence depolarization method. Five dynamical modes of a polymer chain based on the Wöessner model were estimated by our original analytical technique `χ²-map method'. The value of each mode of a polymer side chain was discussed above the overlap concentration (C^*) of PEO and the micro-environments were clarified in the vicinity of the chromophore attached to the polymer side chain.     

Relaxations IN amorphous and semi-crystalline polyesters

Thermally stimulated depolarization currents and differential scanning calorimetry are performed on thermoplastic polyesters to characterize both a and b relaxations. The influence on the different relaxations phenomena of the chemical structure (size of the naphthalene groups, presence of cyclohexane, length of the aliphatic group, ...) as well as the influence of the crystallinity are discussed. The three phases model with a crystalline part, a rigid amorphous part unable to relax and an amorphous phase able to relax at various temperatures depending on the distribution of the relaxation times is used to explain the evolution of the main α relaxation while the standard two-phases model is sufficient to explain the variations of the β relaxation mode. Elementary analysis of both α and β relaxations show that the β relaxation characterized by a continuous variation of activation energies as a function of temperature follows the activated state equation with a zero activation entropy while the cooperative a relaxation exhibits a prominent maximum of the activation energies at the glass transition temperature. This revised version was published online in July 2006 with corrections to the Cover Date.     

PZT压电陶瓷高温变形与失效的实验研究

随着工业设备集成化进程的加快,PZT压电陶瓷获得越来越广泛的应用,其工作环境也越来越复杂.例如,新一代节能环保型发动机中起致动作用的压电陶瓷叠层驱动器,工作时要承受力-热-电载荷的共同作用.因此,研究PZT压电陶瓷在高温下的变形与失效行为,对工业设计具有重要的指导意义.为此,本文利用自行搭建的压电陶瓷高温下电滞回线、蝶形回线和应力退极化曲线的测试装置,研究了PZT压电陶瓷在不同温度下的电致变形和应力退极化行为,得到了材料在不同温度下的电滞回线、蝶形回线、应力电位移曲线和应力应变曲线.实验结果表明,随着温度的升高,由电滞回线测得的剩余极化逐渐减小,蝶形回线逐渐变扁;由应力退极化产生的剩余极化和剩余应变也逐渐减小.极化量的改变是由材料的热释电效应(自发极化随温度的升高而减小)造成的,而应变量的变化是由晶胞畸变随着温度升高而减小所致.     

Elasticity and polarization of Ga<Subscript>x</Subscript>Al<Subscript>1-x</Subscript>N alloys subjected to uniaxial and biaxial compression

Elasticity and polarization of Ga_xAl_1-xN alloys subjected to uniaxial and homogeneous biaxial compression are calculated using first-principles methods. The uniaxial compression along the c-axis reduces Young’s modulus along the c-axis, and enhances bulk modulus and total polarization, whereas the biaxial compression in the plane perpendicular to the c-axis enhances bulk and Young’s moduli. It is also found that when the in-plane biaxial compression is applied by constraining the a-axis lattice constant to that of AlN, the bulk and Young’s moduli dramatically increase with increasing Ga concentration, and the total polarization could be suppressed, even annihilated, and finally enhanced by controlling Ga concentration.     

Plasmon‐enhanced polarized Raman spectroscopy for sensitive surface characterization

Local‐mode and localized surface plasmons generated on the silver thin film can selectively enhance the Raman signal from the surface. Further improvement of surface signal can be obtained by using the polarized Raman technique that results in a dramatic enhancement of the surface sensitivity by up to 25.4 times as compared to that without a silver coating. This technique will be very useful for Raman study on samples that suffer overlapping background signal. In this article, we show that it can be used to significantly improve the signal of thin strained‐Si layer on top of SiGe buffer layer. Copyright © 2008 John Wiley & Sons, Ltd.     

Advanced aspects of electromagnetic SERS enhancement factors at a hot spot

In this paper, we discuss some advanced theoretical aspects of electromagnetic enhancement factors (EFs) in surface‐enhanced Raman scattering (SERS). We focus in particular on the influence of surface selection rules (SSRs) on SERS EFs at hot spots, and the determination of SERS depolarization ratios. Both aspects could be viewed as secondary (compared to the overall magnitude of the SERS EF), but are nevertheless observable experimentally and crucial for a fundamental understanding of SERS. They also share the property that they cannot be studied within the commonly used |E | ⁴ approximation to the SERS EFs, and appropriate tools are developed here to make predictions beyond this approximation in the case of a SERS hot spot. In addition, theoretical estimates of different types of (previously defined) EFs are provided, and their origins discussed for the typical example of a SERS substrate dominated by SERS hot spots. Finally, experimental measurements of SERS depolarization ratios are presented to support the theoretical predictions. Copyright © 2008 John Wiley & Sons, Ltd.     

Combined relaxation study of poly(vinyl chloride) blends with chlorinated poly(ethylene), hydroxyl-terminated poly(butadiene), and ethylene-propylene-diene terpolymer

A number of blends based on suspension poly(vinyl chloride) and stabilizers with poly(ethylene) chlorinated in a fluidized-bed reactor containing 21.8% chlorine, hydroxyl-terminated poly(butadiene), and ethylene-propylene-diene terpolymer have been studied using such methods as thermally stimulated current depolarization and dynamic mechanical analysis. Some dielectric and thermodynamic parameters (τ_max, τ_o, E_a, ΔH^*, ΔS_E^*, ΔG^*, μ_eff) have been determined. Blends containing randomly chlorinated poly(ethylene) exhibited dipole–dipole interactions between the macromolecules of poly(vinyl chloride) which decreased at the expense of the long sequences of nonchlorinated methylene groups. Simultaneously, an increased physical interaction between poly(vinyl chloride) and the additives was observed in blends containing chlorinated poly(ethylene) and/or hydroxyl-terminated poly(butadiene), and ethylene-propylene-diene terpolymer. On the basis of the data of dynamic mechanical analysis obtained a heterogeneous structure of the blends is suggested. The development of a boundary interfacial layer with a proper region of relaxation proves the formation of compatible structures between the components. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1595–1608, 1998     

Moisture effect on the low- and high-temperature dielectric relaxations in nylon-6

The effect of water sorption on the dielectric relaxation processes of nylon-6 samples with water concentrations ranging from the dry to the water-saturated polymer has been studied by thermally stimulated depolarization currents experiments in a broad temperature range, from 77 to 365 K. The strengths of the low-temperature modes, γ- and β-peaks, are affected in opposite ways by the water concentration, h, as the first one shows a decrease in intensity and the second one grows as h increases. The precise determination of the relaxation parameters is made by the decomposition in elementary Debye processes and best fitting to the experimental profile of the complex peak. For h < 3%, the reorienting energies are almost independent of the water content, and the most significant intensity variations occur. The firmly bound water is held responsible for these effects. As for the higher temperature zone besides the α-peak, which is the dielectric manifestation of the glass transition, intermediate temperatures modes are observed at high h values and are originated by the loosely bound water, while the highest temperature peak is attributed to a Maxwell–Wagner interfacial polarization. The characteristic parameters of the α-mode are determined and related to the plasticization effect of water. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 2879–2888, 1997     

Correlation between dipolar TSDC and AC dielectric spectroscopy at the PVDF glass transition

The α_a-mode (associated to the dynamic glass transition) in PVDF-α has been studied by Thermally Stimulated Depolarization Currents (TSDC) and Dielectric Spectroscopy (DS) techniques. The distribution of relaxation parameters, reorientation energies, characteristic temperature, and preexponential factors of the Vogel–Tammann–Fulcher relaxation times have been precisely determined by using the Simulated Annealing Direct Signal Analysis applied to a partially discharged TSDC α_a peak. This distribution has been used to predict the variation of the dielectric loss, ε″(ω, T), in the temperature and frequency range where the DS measurements were made on the same material. The simulated ε′(T, ω) for various ω, are compared to the experimental values. The width of the peak is always too low, due to the restricted distribution used for the generation of the curves. A relaxation map including the TSDC results is used to determine the relaxation time variation. In the limited frequency range where the AC DS experiments are performed (10² ≤ f ≤ 10⁵ Hz) a master curve is drawn and the exponents of the frequency dependence are found at low and high frequency; also, a fitting to the Havriliak–Negami distribution is successfully performed. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35 : 2483–2493, 1997     

Water sorption and thermally stimulated depolarization currents in nylon-6

The influence of water on the low temperature dielectric relaxations of nylon-6 has been investigated in the range from dry to water-saturated polymer by means of sorption experiments and thermally stimulated depolarization currents (TSDC). Three different ranges of water sorption are observed and their reflection in the TSDC spectra is examined. It is found that position and intensity of the TSDC maxima exhibit characteristic and different behavior in each of these sorption ranges. The critical concentrations are 1.5 ± 0.5% and 6.0 ± 0.5% water content. An interpretation of the experimental results is given with respect to the molecular origins of the low-temperature dielectric relaxations and the mechanisms of water sorption in nylons. The most important idea is that water exists as firmly bound, loosely bound, and clustered water, depending on its concentration. An attempt is made to determine relations between these different states of water in nylon-6 and some characteristic features of the low-temperature TSDC spectrum. © 1996 John Wiley & Sons, Inc.