Correlation between the a relaxation and the excess wing for polychlorinated biphenyls and glycerol

Data are presented for three glass formers, each having an excess wing in the low temperature dielectric loss spectra. Two polychlorinated biphenyls, whose α relaxations have equivalent temperature dependences, exhibit excess wings that are clearly different. Comparison of the spectra for glycerol at atmospheric pressure and at P=0.9 GPa reveals a different response of the α relaxation and the excess wing. These findings cannot be reconciled with the notion that the excess wing is an inherent part of the α relaxation. Interpretation of the spectra as a superposition of distinct α and β processes, however, is consistent with the observed behavior. This revised version was published online in August 2006 with corrections to the Cover Date.     

Local segmental relaxation in bidisperse polystyrenes

Dynamic mechanical results are reported for segmental relaxation of monodisperse polystyrenes (PSs) with molecular weights of 0.7, 3, 18, and 104 kg/mol and bidisperse PSs created from blending pairs of these materials. The data for the monodisperse polymers confirm previous findings; namely, there is an increase in the glass‐transition temperature normalized temperature dependence of the segmental relaxation times (fragility) with increasing molecular weight, along with a breakdown of the correlation between the fragility and the breadth of the relaxation function. For both the monodisperse and bidisperse PSs, the glass‐transition temperature is a single function of the average number of chain ends, independent of the nature of the molecular weight distribution. It is also found that these materials exhibit fragilities that uniquely depend on the number‐average molecular weight, that is, on the concentration of chain ends. In blends with linear PS, cyclic PS with a low molecular weight behaves as a high polymer, similar to its neat behavior, reflecting the overriding importance of chain ends. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 2604–2611, 2004     

Characterisation of structural relaxation phenomena in polymeric materials from thermal analysis investigations

Measurements have been performed on poly(ethylene terephthalate)glycol/montmorillonite nanocomposites with different filler contents using differential scanning calorimetry (DSC) and temperature modulated differential scanning calorimetry (TMDSC). According to the strong-fragile concept proposed by Angell, we have determined the values of the fragility index m. In a second time, we have calculated the average size of a cooperative rearranging region (CRR) z(T _g) at the glass transition according to the definition proposed by Solunov. However, z(T _g is a dimensionless quantity and then only allows a comparative study between different samples. To calculate the average number of monomer units by CRR noted N _α, we have used the method developed by Donth. The results show that the presence of montmorillonite in PET_g matrice implies modifications on structural relaxation phenomena. Furthermore, we have shown that z(T _g and N _α values have the same evolution in function of filler content.     

Studying the Glass Transition by DSC and TMDSC

First, the principal features of the glass transformation process in polymers are reviewed, and then it is shown how they are manifest in conventional DSC, and the quantitative analysis of typical DSC data is discussed in terms of the Tool-Narayanaswamy-Moynihan (TNM) model. Subsequently, the way in which the glass transition is manifest in Temperature Modulated DSC is presented, and the effects of both experimental and material parameters are discussed. In conclusion, the two techniques are compared in terms of the information they provide about the glass transformation process. This revised version was published online in July 2006 with corrections to the Cover Date.     

A model for the fragility of the melts

The concept of fragility has been used widely to characterize the temperature dependence of the viscosity of glass forming materials. However, the physical background that determines the degree of fragility is still not well understood. In the present study an expression for the fragility is derived based on a simple model of the melt. According to the model, the fragility is determined by the relaxation of structural units that form the melt, and is described in terms of the bond strength (E ₀), coordination number (Z ₀), and their fluctuations (ΔE and ΔZ). It is shown that a strong system is characterized by large value of total bond strength (Z ₀ E ₀) and small value of its fluctuation ((ΔZ)²(ΔE)²). On the other hand, a fragile system is characterized by small value of total bond strength and large value of its fluctuation. This revised version was published online in August 2006 with corrections to the Cover Date.     

TOPEM,a new temperature modulated DSC technique

TOPEM is a new temperature modulated DSC technique, introduced by Mettler-Toledo in late 2005, in which stochastic temperature modulations are superimposed on the underlying rate of a conventional DSC scan. These modulations consist of temperature pulses, of fixed magnitude and alternating sign, with random durations within limits specified by the user. The resulting heat flow signal is analysed by a parameter estimation method which yields a so-called ‘quasi-static’ specific heat capacity and a ‘dynamic’ specific heat capacity over a range of frequencies. In a single scan it is thus possible to distinguish frequency-dependent phenomena from frequency-independent phenomena. Its application to the glass transition is examined here.     

Physical ageing in poly(vinyl acetate) 1. Enthalpy relaxation

The enthalpy changes ΔH_∞ between a poly(vinyl acetate) glass formed by rapid cooling and the corresponding fully relaxed glass have been estimated at four temperatures below the glass transition. The values obtained were different to those expected by extrapolating liquid behavior below the glass transition and were found to agree well with the predictions of a simple expression for the combined main chain conformational and free volume contributions to enthalpy. Conformational contributions from the side chain alone were also considered but were not required to obtain agreement with experiment. It can be concluded that the side chains remained mobile below the glass transition and do not contribute to the heat capacity discontinuity at T_g. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 1107–1116, 1997     

Influence of the molecular mass on the segmental relaxation times of polystyrene determined by DSC

Journal of Thermal Analysis and Calorimetry - The segmental dynamics of narrow fractions (áMwñ/áMnñ ≈1.05) of polystyrene with molecular masses ranging from 4000 to 600000...     

Effect of chemical structure on the isobaric and isochoric fragility in polychlorinated biphenyls

Pressure-volume-temperature data, along with dielectric relaxation measurements, are reported for a series of polychlorinated biphenyls (PCB), differing in the number of chlorine atoms on their phenyl rings. Analysis of the results reveals that with increasing chlorine content, the relaxation times of the PCB become governed to a greater degree by density rho relative to the effect of temperature T. This result is consistent with the respective magnitudes of the scaling exponent gamma yielding superpositioning of the relaxation times measured at various temperatures and pressures, when plotted versus rho(gamma)/T. While at constant (atmospheric) pressure, fragilities for the various PCB are equivalent, the fragility at constant volume varies inversely with chlorine content. Evidently, the presence of bulkier chlorine atoms on the phenyl rings magnifies the effect which the density has on the relaxation dynamics.     

La55Al25Ni10Cu10大块金属玻璃结构弛豫过程中的焓变

用铜模吸铸法制备直径为4 mm的La55Al25Ni10Cu10大块金属玻璃非晶棒。采用差示扫描量热仪（DSC）研究了金属玻璃结构弛豫行为,并运用Stretched-Exponential弛豫方程对焓弛豫动力学进行描述。结果表明：La55Al25Ni10Cu10金属玻璃在玻璃转变温度Tg以下不同时间等温退火中会发生焓弛豫,用Stretched-Exponential弛豫方程能很好地描述焓弛豫动力学行为,得到Kohlrausch指数β=0.78,弛豫激活能E=60.8kJ.mol^-1。La55Al25Ni10Cu10金属玻璃形成液体的脆性指数为54,其玻璃形成液体属于强液体。     

Sample encapsulation on glass transition of methylmetacrylate copolymers: TMDSC and DSC studies

The use of modulated temperature differential scanning calorimetry (TMDSC) and differential scanning calorimetry (DSC) in the measurement of the glass transition temperature (T _g) in polymer-water systems presents several important problems. These include the presence of water evaporation endotherms, partial water evaporation during scanning, changes in pan integrity due to vapour pressure developing in the pan headspace during analysis, and possible interaction between water and polymer at high temperatures. As a result, in most of the cases, only apparent T_g values can be obtained. In this study, TMDSC and DSC were used to determinate the thermal behaviour of methylmethacrylate copolymer-water systems. The samples were previously equilibrated at different relative humidities (RH) from 0 to 97% RH. Three different pan arrangements were used. In addition, thermogravimetric analysis (TG) was carried out to determine the initial amount of water in the sample. None of the pan arrangements was entirely suitable for the study of these systems. When sealed pans were used, the plastification effect of water was observed. Some evidence of degradation was also observed in which water and methylmethacrylate appeared to play roles.This revised version was published online in November 2005 with corrections to the Cover Date.     

Gas chromatographic separation of polychlorinated biphenyls on a new apolar capillary column

Eighty one polychlorinated biphenyls (PCBs) and nineteen chlorinated pesticide standards have been analyzed on a newly developed apolar high resolution gas chromatography (HRGC) column. Emphasis was placed on the determination of PCB indicator congeners which are part of national regulations, and of toxic coplanar PCB congeners. The new column enables almost unambiguous quantification of the PCB indicator congeners (PCB 138 can be separated from both PCB 163 and PCB 164; PCB 28 and PCB 31 were also separated and no coelution was observed for PCB 52, PCB 118, and PCB 180). The new column furnished better results than CP-Sil 8 for the analysis of PCB indicator congeners in a sample of seal blubber.     

Bioremediation of polychlorinated biphenyls degradation capabilities in field lysimeters

The degradation of 4-chlorobiphenyl (4CB) was compared in field lysimeters containing 60 Kg of soil contaminated with 5–10 mg/Kg of polychlorinated biphenyls.Alcaligenes A5, a bacterium carrying a plasmid for 4CB degradation, was inoculated into three lysimeters. When compared to an untreated control, soil samples from water, mineral, and yeast extract treated lysimeters with and without a bacterial inoculum exhibited greater than 10-fold increases in the rate of [1-¹⁴ C-acetate incorporation into lipids and¹⁴CO₂ production from [U-¹⁴C-4-chlorobiphenyl. Gene probe analyses for the 4CB plasmid and most-probable-number enumerations demonstrated the presence of biodegradative populations in lysimeters and the probable survival of the addedAlcaligenes A5.     

Measuring the Glass Transition of Thermosets by Alternating Differential Scanning Calorimetry

The glass transition temperature of thermosets is determined by alternating differential scanning calorimetry (ADSC), which is a temperature modulated DSC technique. The different values of the glass transition obtained from heat flow measurements (total and reversible) and heat capacity (modulus of the complex heat capacity) are analysed and compared with the values obtained by conventional DSC. The effect of the sample mass on the values of T_g, heat capacity and phase angle has been analysed. The effect of the thermal contact between sample and pan has been studied using samples cured directly inside the pan and disc-shaped samples of different thickness. The results obtained for the thermal properties and the phase angle are compared and analysed. The modulus of the complex heat capacity enables the determination of the dynamic glass transition, T_g, which is frequency dependent. The apparent activation energy ofthe relaxation process associated with the glass transition has been evaluated from the dependence of T_g on the period of the modulation.This revised version was published online in November 2005 with corrections to the Cover Date.     

Bioanalytical methods applied to endocrine disrupting polychlorinated biphenyls, polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans. A review

The usual methods for determining polychlorinated dibenzo-p-dioxins (PCDD), polychlorinated dibenzofurans (PCDF) and polychlorinated biphenyls (PCB) are generally expensive and time consuming. This fact has favored the development of faster and cheaper techniques, based on immunoassays and bioassays. This paper reviews these bioanalytical methods and their analytical importance at the present moment.     

A new advanced method for heterogeneous catalysed dechlorination of polychlorinated biphenyls (PCBs) in hydrocarbon solvent

The dechlorination of PCBs with solid hydrazine hydrochloride catalysed by palladium, in an organic solvent, yields biphenyl in short reaction times. The catalyst system can be efficiently reused for several cycles. Ultrasonication of the heterogeneous catalysed reaction increases the dechlorination rate remarkably. The reactivity of the C---Cl bond on the PCB ring are in the order meta>paraortho.     

MTDSC at the Glass Transition Quantitative use of the phase lag correction The effects of long annealing times

A study of the effects of changing sample mass and purge gas on the phase lag and calculation of the out of phase (kinetic) component for polystyrene in MTDSC was undertaken. The results confirm those from an earlier study that suggested the kinetic signal is unchanged by altering these parameters thus it is probably a correct quantitative measure of this signal. The effects of long annealing times were also studied and it was shown that all the signals of MTDSC are affected in contrast to a previous study showing that for moderate annealing the reversing and kinetic signals are substantially invariant. Nevertheless the non-reversing signal remains useful for characterising relative enthalpy losses. This revised version was published online in July 2006 with corrections to the Cover Date.     

Glass Transition Temperature and Value of the Relaxation Time at Tg in Vitreous Polymers

Summary: Many works focused on glassy polymers determine values of glass transition temperature (T_g) and an overview of the literature shows that depending on the method used, values of T_g are found different for the same material. In this paper, a review of data collected on different materials are used and interpreted in term of molecular mobility characterized by relaxation time functions. By using three independent experimental procedures (dielectric, thermally depolarized current and calorimetric), we show that the value of the glass transition and the value of the relaxation time at T_g can be correctly determined. It is also shown that the assumption: τ _(Tg) = 100 s is constant, is not correct. The protocol proposed also allows the determination of the value of the fragility index “m” of the glass forming liquid with a great accuracy.     

基于位形熵模型的山梨醇玻璃焓松弛的量热分析

利用DSC技术考察了无定形山梨醇体系的焓松弛行为, 在10 K•min^－1的升温速率下测定了经历不同降温速率(0.5～20 K•min^－1)的山梨醇在玻璃化转变(T_g)前后的比热容[c_p(T)]. 利用基于位形熵演变的焓松弛现象学模型(GR模型)模拟了实验数据. 不论是否假设松弛过程存在一个亚稳极限态, 模型参数均能很好地重现经历不同热历史体系的升温c_p(T)曲线. 在物理意义明确的模型参数组中, 除了非指数参数随降温速率的增加而增加外, 其余均不随热历史的变化而变化. 拟合较低降温速率下c_p(T)曲线获得的GR模型参数的预测力明显好于在较大降温速率下获得的结果. 由于松弛时间对拟合过程中选择的“固定参数”的取值很敏感, 因此模型能否预测体系的比热容不能看成确定松弛时间的唯一依据. 在利用GR模型分析无定形山梨醇体系的脆度时, 如果选择极限假想温度作为T_g, 会导致计算结果明显小于文献值.