The determination of the glass transition temperature by thermally stimulated depolarization currents. Comparison with the performance of other techniques

Several experimental techniques are currently used for the determination of the glass transition temperature, T_g. 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 T_g 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 T_gs 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 T_g 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 T_g.     

Spin probe ESR studies of PEGxLiClO4 polymer electrolyte systems

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 T_g is estimated to be −51 °C from the measurement of T_50G, the temperature at which the extrema separation 2A_ZZ becomes 50G and is found to be close to that measured using DSC (−51.7 °C). T_g 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.     

Autoadhesion of High‐Molecular‐Weight Monodisperse Glassy Polystyrene at Unexpectedly Low Temperatures

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.     

Enthalpy Relaxation Study of Poly(Vinyl Chloride) Films Based on the Time-Temperature Superposition Principle

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 (t_A) and ageing temperature. The range of the ageing temperature was from 56?°C (T_g ? 25?°C) to 72?°C (T_g ? 9?°C) where T_g 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 ? ? t_A curves of the respective ageing temperatures horizontally. Although there was no agreement between the shift factors (a_T) and the relaxation times of the ? ? t_A curves, the superposition was successfully constructed and the a_T 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 T_g 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.     

Dynamic mechanical characterization of relaxations in poly(oxymethylene), miscible blends,and oriented filaments

Multifrequency dynamic mechanical analysis (DMA) data were obtained for molded poly(oxymethylene) (POM) and its blends from-150°C to 150°C. Because of the high crystallinity, the assignment of the glass transition in POM has been controversial in the literature. Low and high glass transition temperature (T _g) phenolated compounds, including poly(vinyl phenol), were found to be miscible with POM. The shift of the β transition in the POM blends favors an assignment of the β transition detected at ?3°C(1 Hz), not the ?80°C γ transition, as the T _g in semicrystalline POM because the latter is invariant with diluent. The peak at the β transition in pure POM is weak and can only be seen clearly by DMA measurements on samples that have not “aged” at ambient temperature. This is further evidence that the β transition arises from a cooperative glass-transition-like motion. The γ transition is not influenced by aging because it is due to a concerted localized main chain motion. The β transition of an oriented POM filament can be seen in the DMA flexural loss spectrum at-18°C (1 Hz), but not in a tensile loss spectrum. The broad a relaxation was detected at about 110°C (1 Hz) in molded POM and its blends, while it was shifted to about 135°C in the higher crystallinity, oriented system. The α peak is also independent of diluent, consistent with a crystalline origin for this transition, as was proposed earlier.     

Dependence of Tg and T ll on tacticity of PMMA by differential scanning calorimetry

A differential scanning calorimeter (DSC) investigation (heating rate 10 K/min) is presented on the multiple transition (relaxation) spectra of PMMA: T_β < T_g; T_g, T_ll > T_g; and T_lρ > T_ll, 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 T_β were inconclusive. Our T_g results clarify some discrepancies in the prior literature.

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

Extrapolated T_g 's for Xss = 1 are 133.2°C and 136.1°C, respectively, in contrast to Thompson's extrapolated value of 160°C. Similarly T_g(°C) = 99.5 + 71.6 (1?Xii) for our DSC data. The extrapolated T_ll for Xss = 1 is 171.1°C. The intensity of T_ll is high for st-and it-, with a broad minimum over the at-region. A second liquid state process, T_lρ > T_ll, occurs at 149°C for it-, but is above the measured range for at-and st-. T_ll (at-) from DSC compares favorably with reported literature values by a variety of techniques. T_ll and T_lρ 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 T_ll nor T_lρ is an artifact of our DSC technique. The Frenkel segment-segment contact hypothesis is favored to explain the molecular origin of T_ll Sequences longer than triads may be needed for improved correlations of T_g and T_lρ with tacticity.     

Glass transition,thermal stability and glass-forming ability of Se90In10− x Sb x (x = 0, 2, 4, 6, 8, 10) chalcogenide glasses

Differential scanning calorimetry (DSC) has been employed to investigate the glass transition activation energy E _g, thermal stability and glass-forming ability (GFA) of Se₉₀In_{10? 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 _c–T _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.     

The Slow Molecular Mobility in Semicrystalline Poly(Ethylene Oxide)

The thermally stimulated depolarization current (TSDC) technique has been used to study the slow molecular mobility in the amorphous part of the semicrystalline polymer, poly(ethylene oxide) (PEO). Experiments were carried out in the temperature range that includes the glassy state, the glass transformation region and the rubber state. The dipole moments in the polymeric main chain originated a broad and low intensity secondary relaxation in the temperature region from ?130°C up to the glass transition region; the activation energy of the motional modes of this secondary relaxation was in the range between 35 and 100 kJ mol^?1. The glass transition temperature of the PEO, provided by the TSDC technique, was T_g = ?53°C, and the fragility index was found to be m = 43. A strong relaxation above T_g was observed, whose molecular origin was discussed. The thermal behavior of the PEO was also characterized by differential scanning calorimetry.     

A fluorescence study on the critical exponents for the linear polymerization of butyl-methacrylate

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 (P_y) was used as the fluorescence probe. The increase in P_y 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.     

Eu-doped B2O3–ZnO–PbO glass phosphor powders with?spherical shape and fine size prepared by spray pyrolysis

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%.     

Segmental Dynamics in net -poly(methyl methacrylate)- co -poly(n-butyl acrylate) Copolymer Networks

Dynamic mechanical spectroscopy and differential scanning calorimetry investigations of segmental dynamics are reported for net-poly(methyl methacrylate)-co-poly(n-butyl acrylate) copolymer networks. Three characteristic temperatures, namely, Vogel (T_∞), glass transition (T _g ), and crossover (T _c ), were used to define cooperativity range and a new reduced temperature parameter (Solidness, S). The results showed that broadness of the α -dispersion (glass transition) and cooperativity length scale at the glass transition temperature decreased with increasing butyl acrylate content and T _g -scaled temperature dependence of the relaxation time (fragility). However, the cooperativity range (T _c –T_∞), decreased with increasing fragility index. Furthermore, the solidness at T _g (S(T _g )) was nearly independent of chemical structure of the samples. Finally, a correlation was found between two measures of cooperativity length scale in the glass transition region, namely, average volume of cooperatively rearranging regions, V _CRR , and the number of basic units in an act of rearrangement in the glass transition region, Z(T _g ), determined from two completely independent experimental techniques.     

Confinement effects on glass transition temperature,transition breadth,and expansivity: Comparison of ellipsometry and fluorescence measurements on polystyrene films

Using ellipsometry, we characterized the nanoconfinement effect on the glass transition temperature (T _gof supported polystyrene (PS) films employing two methods: the intersection of fits to the temperature (Tdependences of rubbery- and glassy-state thicknesses, and the transition mid-point between rubbery- and glassy-state expansivities. The results demonstrate a strong effect of thickness: T_g(bulk)-T_g(23 nm) = 10 ^°\ensuremath T_{{\rm g}}({\rm bulk})-T_{{\rm g}}(23{\,\mbox{nm}})= 10 ^{\circ} C. The T -range needed for accurate measurement increases significantly with decreasing thickness, an effect that arises from the broadening of the transition with confinement and a region below T _g where expansivity slowly decreases with decreasing T . As determined from expansivities, the T _g breadth triples in going from bulk films to a 21-nm-thick film; this broadening of the transition may be a more dramatic effect of confinement than the T _g reduction itself. In contrast, there is little effect of confinement on the rubbery- and glassy-state expansivities. Compared with ellipsometry, T _g ’s from fluorescence agree well in bulk films but yield lower values in nanoconfined films: T _g(bulk) - T _g(23 nm) = 15^° C via fluorescence. This small difference in the T _g confinement effect reflects differences in how fluorescence and ellipsometry report “average T _g ” with confinement. With decreasing nanoscale thickness, fluorescence may slightly overweight the contribution of the free-surface layer while ellipsometry may evenly weight or underweight its contribution.     

Effect of nanoparticle dispersion on glass transition in thin films of polymer nanocomposites

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.     

Preparation and characteristics of?a?BaO–Al2O3–B2O3–SiO2–La2O3 glass ceramic via spray pyrolysis

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.     

Confinement and processing effects on glass transition temperature and physical aging in ultrathin polymer films: Novel fluorescence measurements

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     

Metal-induced effects on the glass transition kinetics of glassy Se70Te30 alloy

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 (E_t ) is also discussed.     

The Evolution of Interfacial Strength as a Way to Characterize a Low-Temperature Limit of the Surface Glass Transition of an Amorphous Polymer

The evolution of autoadhesive strength, σ, with healing temperature, T _h, at the symmetric amorphous polystyrene (PS)?PS interfaces of the samples with vitrified bulk has been used to characterize a low-temperature limit of the surface glass transition temperature T _g ^surface(low). The existence of a linear relationship between the square root of σ and T _h has been found for both polydisperse and monodisperse polymers. By the extrapolation of straight lines σ ^1/2 ? T _h to σ ^1/2 = 0, the values of T _g ^surface(low) have been determined and compared with those of a high-temperature limit of T _g ^surface, T _g ^surface(high), measured earlier. The differences between T _g ^surface(low) and T _g ^surface(high) have been found to be insignificant, 10–20°C. Using an average value of the shift of T _g ^surface(low) with healing time, t _h, the quasi-equilibrium value of the surface glass transition temperature of amorphous PS T _∞ ^surface has been estimated to be 10–15°C.     

The glass transition temperature of amorphous poly(ethylene terephthalate) by thermally stimulated currents

The relation between the temperature T_α of the dipolar relaxation, obtained by the technique of thermally stimulated currents (TSC) and the glass transition temperature T_g has been studied in amorphous poly(ethylene terephthalate) samples. The temperature T_α depends fundamentally on the polarization temperature T_p, the polarization time t_p, and the heating rate v. For each heating rate a maximum T_α, T_M, was obtained for an optimum polarization temperature T_po. The value of T_po is 70°C, independent of the heating rate, and very close to the glass transition temperature obtained by differential scanning calorimetry (69°C). The resulting value for T_M coincides with T_po in the limits of null heating rate and null isothermal polarization time, and, consequently, T_M gives the value of the glass transition temperature for each heating rate as a function of the isothermal dipolar contribution on polarizing at the temperature T_po.     

The glassy state of the amorphous V2O5–NiO–TeO2 samples

The glass transition temperature dependence to heating rate and therefore the activation energy (ΔH^?) of the glass transition of (60-x)V₂O₅–xNiO–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 T_g 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 (m=ΔH^?/R T_g) 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 T_g and ΔH^? was investigated.     

Spherical shape BaO-ZnO-B<Subscript>2</Subscript>O<Subscript>3</Subscript>-SiO<Subscript>2</Subscript> glass powders prepared by spray pyrolysis

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 (T_g) 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