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1.
Several experimental techniques are currently used for the determination of the glass transition temperature, Tg. Thermally stimulated depolarization currents (TSDC) is a thermal analysis technique whose experimental results display a very clean glass transition signature and that, nevertheless, is seldom used as a technique for Tg determination. In the present work we explain how to get the glass transition temperature from TSDC data, and we compare the values obtained for a vast number of glass forming systems (with Tgs in a wide range between ?145 and +180 °C and fragilities between m = 15 and m = 100), with the values obtained by differential scanning calorimetry (DSC) and dielectric relaxation spectroscopy (DRS). We conclude that the Tg determination by TSDC is direct, accurate and reproducible and that the obtained values correlate very well with those obtained by DSC and DRS. This general survey thus suggests TSDC as a valuable alternative technique for determining Tg.  相似文献   

2.
The technique of Electron Spin Resonance (ESR) is shown to be useful in the study of dynamics of solid polymer electrolytes (SPE). Through the ESR of the nitroxide radical (2,2,6,6-tetramethyl-1-piperidine-1-oxyl; TEMPO) dispersed in the SPE PEG46LiClO4 temperature dependence of correlation time is found. The glass transition temperature Tg is estimated to be −51 °C from the measurement of T50G, the temperature at which the extrema separation 2AZZ becomes 50G and is found to be close to that measured using DSC (−51.7 °C). Tg for pure PEG-2000, which could not be measured from DSC because of its high crystallinity, is determined to be −72 °C by spin probe ESR. Paper presented at the 2nd International Conference on Ionic Devices, Anna University, Chennai, India, Nov. 28–30, 2003.  相似文献   

3.
Abstract

Healing of symmetric interfaces of amorphous anionically polymerized high‐ and ultrahigh‐molecular weight (HMW and UHMW, respectively) polystyrene (PS) in a range of the weight‐average molecular weight M w from 102.5 (M w/M n = 1.05) to 1110 kg/mol (M w/M n = 1.15) was followed at a constant healing temperature, T h, well below the glass transition temperature of the polymer bulk [T g‐bulk = 105–106°C as measured by differential scanning calorimeter (DSC)]. The bonded interfaces were shear fractured in tension on an Instron tester at ambient temperature. Autoadhesion at symmetric HMW PS–HMW PS and UHMW PS–UHMW PS interfaces was detected mechanically after healing at T h = 38°C for 107 hr, and even at 24°C (for longer healing times). The occurrence of autoadhesion between the surfaces of the UHMW PS with M w = 1110 kg/mol at 24°C implies that the glass transition temperature at the interface, T g‐interface, of this polymer was a least lower: by 82°C than its DSC T g‐bulk, by 30–40°C than the Vogel temperature, T —the lowest theoretical value of a kinetic T g‐bulk at infinite long time—and by 20°C than T 2 (a “true” thermodynamic T g‐bulk corresponding to a second‐order phase transition temperature). To our knowledge, this is the first observation of such nature, which gives further evidence of the lowering of the T g at polymeric surfaces and the persistence of this effect at early stages of healing of polymer–polymer interfaces.  相似文献   

4.
Abstract

In the enthalpy relaxation of poly(vinyl chloride), a decrease in enthalpy upon the isothermal ageing was measured using the differential scanning calorimetry method as a function of ageing time (tA) and ageing temperature. The range of the ageing temperature was from 56?°C (Tg ? 25?°C) to 72?°C (Tg ? 9?°C) where Tg denotes the glass transition temperature. The limiting value of the decrease in enthalpy was determined by applying a stretched exponential function to the measured enthalpy data. The relaxation function (?) was derived from the measured enthalpy and the construction of a master curve was tried by shifting the ? ? tA curves of the respective ageing temperatures horizontally. Although there was no agreement between the shift factors (aT) and the relaxation times of the ? ? tA curves, the superposition was successfully constructed and the aT values obtained for the poly(vinyl chloride) sample were found to be comparable to those reported for viscoelastic experiments over a broad temperature range above and below Tg carried out for different polymers. The origin of the decrease in enthalpy was briefly discussed in terms of the chain dynamics in the isothermal condition.  相似文献   

5.
H. Kumar  N. Chandel 《Phase Transitions》2016,89(11):1103-1118
In this communication, we report the results of calorimetric measurements on the samples of recently synthesized multi-component glassy alloys of Se78?xTe20Sn2Bix (0 ≤ x ≤ 6) system. For calorimetric study of glass transition kinetics, differential scanning calorimetry (DSC) technique has been used in non-isothermal mode. Peak glass transition temperature (Tg) is determined using the DSC scans. Kinetic parameters A and B of glass transition are determined using heating rate dependence of Tg. Activation energy of glass transition (Eg) has been calculated using Moynihan and Kissinger methods. Glass-forming ability and thermal stability are also determined using Hurby and Saad–Poulin relations, respectively.  相似文献   

6.
Differential scanning calorimetry (DSC) has been employed to investigate the glass transition activation energy E g, thermal stability and glass-forming ability (GFA) of Se90In10? x Sb x (x = 0, 2, 4, 6, 8, 10) chalcogenide glasses. DSC runs were performed at six different heating rates. Well-defined endothermic and exothermic peaks were obtained at glass transition and crystallization temperature. The dependence of glass transition temperature T g on heating rate (α), as well as composition of Sb, has been studied. From the dependence of glass transition temperature on heating rate, the E g has been calculated on the basis of the Kissinger [Anal. Chem. 29 (1957) p.1702] and Moynihan [J. Phys. Chem. 78 (1974) p.267] models. Thermal stability has been monitored through the calculation of temperature differences T cT g, the stability parameter S, and the enthalpy released during crystallization H c. The GFA has been investigated on the basis of the Hruby parameter H r, which is strong indicator of GFA. Results for GFA are in good agreement with fragility index F i calculations, indicating that Se90In6Sb4 is an excellent glass-former.  相似文献   

7.
The steady-state fluorescence (SSF) technique was used to study the sol-gel transition for the linear bulk polymerization of butyl methacrylate (BMA), carried out above the glass transition temperature of polybutylmethacrylate (PBMA) (T g?=?20°C). Pyrene (Py) was used as the fluorescence probe. The increase in Py intensity was monitored during free radical polymerization of BMA by using SSF technique. Changes in the viscosity of the pregel solutions due to gel formation dramatically enhance the fluorescent yield of aromatic molecules. This effect is used to monitor the sol-gel transition of BMA, as a function of time, at various temperatures. The results are interpreted in the view of percolation theory. The gel fraction exponent β?=?0.39?±?0.02 agreed the best with the static percolation values for the linear bulk BMA polymerization carried out above T g but weight average degree of polymerization exponent,?γ?deviated from the percolation results.  相似文献   

8.
Eu-doped B2O3–ZnO–PbO glass phosphor powders with spherical shape and fine size were directly prepared by spray pyrolysis. The glass phosphor powders prepared at a temperature of 1100°C had broad XRD peak at around 28°. One glass phosphor powder was formed from one droplet at the preparation temperature range from 900 to 1100°C. The mean size of the glass phosphor powders was 0.75 μm. The glass transition temperature (T g ) of the glass phosphor powders prepared by spray pyrolysis was 378.5°C. The excitation spectrum of the glass phosphor powders prepared at the optimum preparation temperature of 1100°C had bands at 362, 381, 392, 463, 525, and 532 nm. The glass phosphor powders had emission spectra with bands at 579, 614, and 653 nm. The glass phosphor powders with doping concentration of Eu of 7 wt% had the maximum photoluminescence intensity. The glass phosphor layer formed from the glass phosphor powders had high transparencies above 90%.  相似文献   

9.
We present spectroscopic ellipsometry measurements on thin films of polymer nanocomposites consisting of gold nanoparticles embedded in poly(styrene). The temperature dependence of thickness variation is used to estimate the glass transition temperature, T g . In these thin films we find a significant dependence of T g on the nature of dispersion of the embedded nanoparticles. Our work thus highlights the crucial role played by the particle polymer interface morphology in determining the glass transition in particular and thermo-mechanical properties of such nanocomposite films.  相似文献   

10.
A differential scanning calorimeter (DSC) investigation (heating rate 10 K/min) is presented on the multiple transition (relaxation) spectra of PMMA: Tβ < Tg; Tg, Tll > Tg; and T > Tll, as a function of tacticity. Specimens are characterized by fractional triad content: isotactic (it-), Xii; syndiotactic (st-), Xss; and atactic (at-), Xis. Values for the seven specimens are it-, 1.0;, at-, 0.495 to 0.750; st-, 0.958. Results on Tβ were inconclusive. Our Tg results clarify some discrepancies in the prior literature.

Linear least squares regression analyses give: Tg (°C) = 56.6 + 76.6 Xss (our data) Tg (°C) = 49.1 + 87.3 Xss (our data plus selected literature data)

Extrapolated Tg 's for Xss = 1 are 133.2°C and 136.1°C, respectively, in contrast to Thompson's extrapolated value of 160°C. Similarly Tg(°C) = 99.5 + 71.6 (1?Xii) for our DSC data. The extrapolated Tll for Xss = 1 is 171.1°C. The intensity of Tll is high for st-and it-, with a broad minimum over the at-region. A second liquid state process, T > Tll, occurs at 149°C for it-, but is above the measured range for at-and st-. Tll (at-) from DSC compares favorably with reported literature values by a variety of techniques. Tll and T at all tacticities agree well with those from a companion study by the thermally stimulated current (TSC) technique on the same group of specimens, as reported elsewhere. This includes the influence of tacticity on intensity. These cross-comparisons by a variety of methods indicate that neither Tll nor T is an artifact of our DSC technique. The Frenkel segment-segment contact hypothesis is favored to explain the molecular origin of Tll Sequences longer than triads may be needed for improved correlations of Tg and T with tacticity.  相似文献   

11.
The characteristics of a BaO–Al2O3–B2O3–SiO2–La2O3 glass ceramic prepared by spray pyrolysis were studied. Glass powders with spherical shape and amorphous phase were prepared by complete melting at a preparation temperature of 1 500°C. The mean size and geometric standard deviation of the powders prepared at the temperature of 1 500°C were 0.6 μm and 1.3. The glass powders had similar composition to that of the spray solution. The glass transition temperature (T g) of the glass powders was 600.3°C. Two crystallization exothermic peaks were observed at 769.3 and 837.8°C. Densification of the specimen started at a sintering temperature of 600°C, in which Ba4La6O(SiO4)6 as main crystal structure was observed. Complete densification of the specimen occurred at a sintering temperature of 800°C. The specimens sintered at temperatures above 800°C had main crystal structure of BaAl2Si2O8.  相似文献   

12.
Fluorescence intensity measurements of chromophore-doped or -labeled polymers have been used for the first time to determine the effects of decreasing film thickness on glass transition temperature, T g, the relative strength of the glass transition, and the relative rate of physical aging below T g in supported, ultrathin polymer films. The temperature dependence of fluorescence intensity measured in the glassy state of thin and ultrathin films of pyrene-doped polystyrene (PS), poly(isobutyl methacrylate) (PiBMA), and poly(2-vinylpyridine) (P2VP) differs from that in the rubbery state with a transition at T g. Positive deviations from bulk T g are observed in ultrathin PiBMA and P2VP films on silica substrates while substantial negative deviations from bulk T g are observed in ultrathin PS films on silica substrates. The relative difference in the temperature dependences of fluorescence intensity in the rubbery and glassy states is usually reduced with decreasing film thickness, indicating that the strength of the glass transition is reduced in thinner films. The temperature dependence of fluorescence intensity also provides useful information on effects of processing history as well as on the degree of polymer-substrate interaction. In addition, when used as a polymer label, a mobility-sensitive rotor chromophore is demonstrated to be useful in measuring relative rates of physical aging in films as thin as 10 nm. Received 21 August 2001  相似文献   

13.
C. Dohare  A. Kumar 《Phase Transitions》2013,86(11-12):1064-1074
The calorimetric measurements have been made in glassy Se70Te30 and Se70Te28M2 (M?=?Ag, Cd, and Zn) alloys using non-isothermal differential scanning calorimetry technique to see the effects of Ag, Cd, and Zn additives on the glass transition kinetics of binary Se70Te30. From the heating rate dependence of glass transition temperature, T g, different kinetic parameters of glass transition have been evaluated. The composition dependence of glass transition temperature T g and the related activation energy (Et ) is also discussed.  相似文献   

14.
The glass transition temperature dependence to heating rate and therefore the activation energy (ΔH?) of the glass transition of (60-x)V2O5xNiO–40TeO2 oxide glasses with 0≤x≤20 (in mol%) were investigated at heating rates φ (=3 6, 9, 10 and 12 K/min) using differential scanning calorimetry (DSC). The heating rate dependence of Tg was used to investigate the applicability of different theoretical models describing the glass transition. Using the application of Moynihan and Kissinger et al. models to the present data, different values of (ΔH?) at each different heating-rate regions were obtained. The fragility parameter (mH?/R Tg) was ∼24.98 for x=10 mol%, suggesting that this glass may be considered as a rather strong glass (fragility index m∼>20 is an indication of fragile glass). Also the compositional dependence of Tg and ΔH? was investigated.  相似文献   

15.
Fine-sized BaO-ZnO-B2O3-SiO2 (BZBS) glass powders were directly prepared by high temperature spray pyrolysis. The hollow glass powders prepared at low preparation temperature of 1000 °C had a low density of 2.65 g/cm3. However, the densities of the BZBS powders obtained at preparation temperatures of 1200 and 1400 °C were each 3.92 and 4.13 g/cm3. The mean size of the BZBS glass powders prepared by spray pyrolysis at preparation temperature of 1400 °C was 0.98 μm. The glass transition temperature (Tg) of the prepared BZBS glass powders was 518.9 °C. The dielectric layers formed from the prepared BZBS glass powders with a dense structure had a clean surface and a dense inner structure without voids at the firing temperature of 580 °C. The transparencies of the dielectric layers formed from the prepared BZBS glass powders were higher than 90% within the visible range. PACS 42.70.Ce; 85.60.Pg; 71.55.Jv  相似文献   

16.
Two distinct single-chain particles of poly(N-isopropylacrylamide) (PNIPAM) in the state of loose coil and compact globule, have been prepared successfully below and above the lower critical solution temperature (LCST) in extreme dilute aqueous solution by the freeze-drying method, respectively. During the preparation of the compact globular single-chain sample, the surfactant of sodium n-dodecyl sulfate (SDS) was added into the system to prevent aggregation of globular single chains formed at a temperature above the LCST. After all the coil has been transformed into the compact globular particle, the SDS molecules were removed by dialysis. The glass transition temperature (Tg) of the two single-chain samples has been measured by differential scanning calorimetery (DSC) in comparison with that of bulk polymer. It was found that the Tg of the single-chain sample in compact-globule state was very near to that of the bulk polymer, whereas the Tg of the single-chain sample in loose-coil state was approximately 6 K lower than that of the bulk polymer. After treating the sample with repeated DSC cycles, the Tg of the single-chain sample in loose-coil state rose up successively near to that of the bulk polymer. These results have been explained in terms of the effect of entanglement on the mobility of the polymer segments in the two distinct single-chain samples.  相似文献   

17.
Free volume quantities proposed earlier by Boyer and Simha in connection with the glass transition are reformulated by taking into account the temperature dependence of the thermal expansivities α l and αg for the liquid and the glass, respectively. This necessitates an extrapolation of the liquid to temperatures below Tg which is performed by means of the reduced volume-temperature function established and given a theoretical foundation previously. For the glass, low temperature experimental data, encompassing all relaxations occuring below Tg, are required.

Two polymer series are examined in detail, namely, poly(methacrylates) and poly(vinyl) alkyl ethers, where αg has been measured between at least 30°K and Tg. Results for poly(methylacrylate) and poly(styrene) are also given. The systematic decrease in the product (αl - αg) · T|T=Tg with increasing length of the side chain noted previously is considerably reduced but not eliminated when the appropriately corrected expression is substituted instead. However, the free volume fraction related to the quantity αlT|T=Tg remains more nearly invariant in the polymers analyzed.

An alternative treatment is discussed which considers an occupied volume expanding below Tg by a mechanism of thermal vibrations solely. Experimental and theoretical means of obtaining this quantity arc suggested.  相似文献   

18.
A. Dahshan  H.H. Amer  K.A. Aly 《哲学杂志》2013,93(11):1435-1449
The effect is reported of varying cadmium concentration on the glass transition, thermal stability and crystallization kinetics of Ge20Se80? x Cd x (x = 2.5, 5, 7.5 and 10 at. %) glasses. Differential scanning calorimetry results under non-isothermal conditions for the studied glasses are reported and discussed. The values of the glass transition temperature (Tg ) and the peak temperature of crystallization (Tp ) were found to be dependent on heating rate and Cd content. From the heating rate dependence of Tg and Tp , the values of the activation energy for glass transition (Eg ) and the activation energy for crystallization (Ec ) were evaluated and their composition dependence discussed. The thermal stability of the glasses was evaluated using various thermal stability criteria such as ΔT, Hg and S. The stability calculations emphasize that the thermal stability decreases with increasing Cd content.  相似文献   

19.
The glass transition in Ge x Se1−x ) (0·1 ⩽x ⩽ 0·25) glasses has been investigated using the photoacoustic (PA) technique. It is found that the PA amplitude and phase undergo anomalous changes at the glass transition temperatureT g. The amplitude has critical minimum and phase has maximum values atT g. The variation of the thermal diffusivity, determined by measuring the frequency dependence of the PA amplitude and phase, with temperature shows sharp decrease near the glass transition temperature. The temperature dependence of the optical energy gap also has been measured and it shows a decrease with temperature for all compositions, the rate of decrease being higher for temperatures greater thanT g.  相似文献   

20.
An ultraquenching technique was used to prepare thin (ca. 1000 Å) amorphous films of polypivalolactone and poly(4-methyl-pentene-1). These films were characterized by electron microscopy, electron diffraction, and dynamic mechanical analysis. Other ultraquenched films of these polymers were crystallized by annealing for various times in the vicinity of their glass transition temperatures. Electron microscopy and electron diffraction were used to follow the reorganization of their structures.

Evidence for a double Tg in polypivalolactone (PPVL) was found, with crystallization of annealed, ultraquenched films occurring just above Tg (L) = 270°K. A Tg (U) = 340°K was noted. When the disordered glass was annealed above Tg (L), polypivalolactone crystallized into the a crystal form, which is composed of antiparallel chain segments, suggesting a chain-folded crystallization mechanism.

Poly(4-methyl-pentene-1) (P4MP1) gave evidence for Tg (L) = 220°K and Tg (U) = 325°K by dynamic mechanical analysis. However, morphology and electron diffraction showed that significant crystallization of ultraquenched polymer did not occur until Tg (U) was reached. X-ray data also supported this conclusion, which is explained by the lower density of the crystal phase of P4MP1 (compared to amorphous material) below 320°K. Long-term annealing of films at Tg (U) resulted in the formation of single-crystal structures, again indicative of a mechanism of chain-folded crystallization from the glass.  相似文献   

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