Glass transition temperatures of stereoblock,isotactic and atactic polypropylenes of various chain lengths

The effect of sample molecular weight on the glass transition temperature has been examined for isotactic, stereoblock and atactic polypropylene samples. Asymptotic values of T₉ (∞) (isotactic) = 272°K, T₉ (∞) (atactic) = 266°K, and T_g (∞) (stereoblock) = 266°K were found. Deviation from T_g (∞) was observed when M?_n was below 10⁴; the dependence of T_g on M?_n has been discussed in relation to the Gibbs-DiMarzio treatment of the glass transition. The possible effects of both tacticity and crystallinity on T_g were examined; comparison of the data obtained with those of other workers was made. It was concluded that molecular order in polypropylene samples could affect T_g significantly and that this was particularly obvious in short chain stereoblock fractions.     

Effect of in impurity on crystallization kinetics of (Se.7Te.3)100−xInx system

The effect of In impurity on the crystallization kinetics and the changes taking place in the structure of (Se₇Te₃) have been studied by DTA measurements at different heating rates (α=5 deg·min^?1, 10 deg·min^?1, 15 deg·min^?1 and 20 deg·min^?1). From the heating rate dependence of the values ofT _g,T _c andT _p, the glass transition activation energy (E _t) and the crystallization activation energy (E _c) have been obtained for different compositions of (Se₇Te₃)_100?xIn_x (0≤×≤20). The variation of viscosity as a function of temperature has been evaluated using Vogel-Tamman-Fulcher equation. The crystallization data are analysed using Kissinger's and Matusita's approach for nonisothermic crystallization. It has been found that for samples containing In=0, 10, 15, 20 at%, three dimensional nucleation is predominant whereas for samples containing In=5 at%, two dimensional nucleation is the dominant mechanism. The compositional dependence ofT _g and crystallization kinetics are discussed in terms of the modification of the structure of the Se?Te system.     

Moisture absorption and diffusion in an underfill encapsulant at T > T g and T < T g

Moisture absorption and diffusion behavior of an underfill material used for electronic packaging with a glass transition temperature (T _g) slightly above room temperature have been investigated by the sorption thermogravimetric analysis technique. It has been found that moisture diffusion in this material follows the Fick’s diffusion model, and moisture absorption–desorption is reversible and repeatable. Based on moisture-induced mass gain versus time curve, the diffusion constant can be determined. It was found that below T _g, moisture diffusivity exhibits an Arrhenius temperature dependence, which changes to a different Arrhenius temperature dependence as the temperature increases to T > T _g. The change in diffusivity from T < T _g to T > T _g is accompanied by a significant decrease in the energy barrier for moisture diffusion. Results shed light on the change in moisture diffusion in polymer-based materials in the glassy and the rubbery state.     

Comparison of electrical conductivity and thermal properties of borosilicate glass with and without simulated radioactive waste

Electrical conductivity and percentage linear thermal expansion of the borosilicate glass (BSG) and simulated waste-loaded borosilicate glass (BSGW) were measured in the temperature range of 300–780 K and compared. Pronounced increase in electrical conductivity was observed around glass transition temperature (T _g) of BSG and BSGW. The activation energy (E _a) of electrical conduction determined from the measured data for BSG and BSGW is 0.961 ± 0.005 and 0.960 ± 0.005 eV, respectively. The % average linear thermal expansion of BSGW showed a slight decreasing trend compared with pristine BSG. The average coefficient of thermal expansion determined from dilatometry data is 12.87 ± 0.24 × 10^?6 and 11.94 ± 0.23 × 10^?6 K^?1 for BSG and BSGW, respectively. The T _g measured by dilatometry is 806 ± 24 K for BSG and 790 ± 23 K for BSGW, respectively. The T _g measured by DTA was found to be 820 ± 7 and 805 ± 5 K for BSG and BSGW, respectively, for heating cycle. The T _g values obtained from DSC measurements are 805 ± 5 and 803 ± 5 K for BSG and BSGW, respectively. The T _g of BSGW showed a slight decrease compared with that of BSG. The values obtained by DSC examination also showed the lowering of T _g values for the waste-loaded composition. The lowering of T _g may be attributed to the interaction of glass-forming agents and simulated waste elements.     

On the continuity of the diffusion behaviour at amorphous polymer–polymer interfaces on both sides of the bulk glass transition temperature

The diffusion behaviour at amorphous polystyrene (PS)–PS interfaces has been investigated over an interval of temperatures (T) from below to above the bulk glass transition temperature (T _g ^bulk) using the Arrhenius and Vogel-Fulcher approaches. No discontinuity in the variation of the logarithm of the diffusion coefficient versus 1/T has been observed when going through the PS T _g ^bulk over a broad interval of T, from T _g ^bulk???50 °C to T _g ^bulk?+?50 °C. The molecular mechanism of interdiffusion has been discussed.     

Argon sorption and partial molar volume in poly(ethyl methacrylate) above and below the glass transition temperature

Sorption and dilation isotherms for argon in poly(ethyl methacrylate) (PEMA) are reported for pressures up to 50 atm over the temperature range 5–85°C. At temperatures below the glass transition (T_g=61°C), sorption isotherms are well described by the dual-mode sorption model; and isotherms above T_g follow Henry's law. However, isotherms for dilation due to sorption are linear in pressure at all temperatures over the range investigated. Partial molar volumes of Ar in PEMA are obtained from these isotherms. The volumes are approximately constant above T_g (about 40 cm³/mol), whereas the volumes below T_g are smaller and dependent on both temperature and concentration (19–26 cm³/mol). By analyzing the experimental data according to the dual-mode sorption and dilation model, the volume occupied by a dissolved Ar molecule and the mean size of microvoid in the glass are estimated to be 67 129 ų, respectively. The cohesive energy density of the polymer is also estimated as 61 cal/cm³ from the temperature dependence of the dual-mode parameters.     

Is adhesion between amorphous polymers sensitive to the bulk glass transition?

Two bulk samples of one and the same or of different amorphous polymers were brought into contact and held for a chosen period of time at a constant healing temperature (T) over the interval of T from below the bulk glass transition temperature (T _g ^bulk) by ~50 °C to above T _g ^bulk by ~10 °C. As formed adhesive joints were shear-fractured in tension at room temperature, and lap-shear strength (σ) was measured as a function of T. It has been found that σ develops with T as logσ?~?1/T both at symmetric and asymmetric interfaces of polystyrene, poly (methyl methacrylate) and poly (2,6-dimethyl-1,4-phenylene oxide). This behaviour implies that there is no discontinuity in the evolution of σ when going through T _g ^bulk, and that this process is controlled by one and the same diffusion mechanism both below and above T _g ^bulk. The results obtained indicate that the contact layer of the polymers investigated is in the viscoelastic state at T well below T _g ^bulk and support the concept of a decrease in the T _g of a near-surface layer with respect to T _g ^bulk.     

Organic semiconductors: optical magnetic and electrical properties of 1-methyl-3-propylimidazolium 7,7,8,8-tetracyano-p-quinodimethane

Electrical conductivity and spectral properties (UV VIS absorption and especially the temperature dependence of IR absorption) are studied for the simple salt 1-methyl-3-propyl-imidazolium 7,7,8,8-tetracyano-p-quinodimethane (MPI⁺ TCNQ⁻. The temperature dependence of the IR absorption coefficient for the bands due to activation of the mode A_g is discussed in terms of changes in energy level population, geometry, and electron interactions. The temperature dependence of the absolute number of the TCNQ dimers in the triplet state is measured independently using the EPR method. The temperature dependences of the ground (singlet) state population of the TCNQ dimers and of the singlet—triplet energy separation J(T) = [(0.18 ± 0.01)-(2.5 ± 0.5) × 10⁻⁴ T] eV were determined from this measurement. These values lead to an unrealistically large value of the linear temperature expansion coefficient within the framework of the simple isolated model of dimers. Therefore, the weakly interacting model of dimers should be preferred. The activation energy of the triplet state exchange frequency. E_W = 0.27 eV, and the room temperature jumping frequency of the exciton along the TCNQ stacks, ν_j = 5 × 10¹⁰ s⁻¹, were determined from EPR measurements of the width of triplet lines.     

The estimation of phase modifications in polymers by the method of “molecular probes” in gas chromatography—VI. The study of the transitions in poly(ethylene terephthalate)

Multiple transitions have been identified by inverse gas-chromatography for fractions of poly(ethylene terephthalate) with $\text{M}$_n between 1.47 and 2.69 × 10⁴. Depending on the thermal treatment, at least two liquid-liquid transitions T_ll⁽¹⁾ and T_ll⁽²⁾ have been observed in addition to T_g and T_m; this feature is generally considered as characteristic of semi-crystalline polymers. The similarity of the dependence of T_g and T_ll on the molecular weight of the polymer probes suggests a common molecular origin of these phenomena; the fact that T_ll >T_g indicates that the T_ll transitions involve longer chain segments. The melting temperature decreases with increasing molecular weight, suggesting again that the crystallinity and/or the size distribution will enlarge as the molecular weight of the polymer is made higher.     

Thermal stability and crystallization kinetics of ternary Se–Te–Sb semiconducting glassy alloys

This paper presents the results of kinematical studies of glass transition and crystallization in glassy Se_85?x Te₁₅Sb _x (x = 2, 4, 6 and 8) using differential scanning calorimetry (DSC). From the dependence on heating rates of, the glass transition temperatures (T _g), and temperature of crystallization (T _p) the activation energy for glass transition (E _g) and the activation energy for crystallization (E _c) are calculated and their composition dependence can be discussed in term of the average coordination number and cohesive energy. The thermal stability of Se_85?x Te₁₅Sb _x was evaluated in terms of criterion ΔT = T _c ? T _g and kinetic criteria K(T _g) and K(T _p). By analyzing the crystallization results, the crystallization mechanism is characterized. Two (two- and three-dimensional growth) mechanisms are working simultaneously during the amorphous–crystalline transformation of the Se₈₃Te₁₅Sb₃ alloy while only one (three-dimensional growth) mechanism is responsible for the crystallization process of the chalcogenides Se_85?x Te₁₅Sb _x (x = 4, 6 and 8) glass. The phases at which the alloy crystallizes after the thermal process have been identified by X-ray diffraction.     

Magnetic and spectroscopic investigation of partially reduced vanadium pentoxides. I. α-CuxV2O5

The electron paramagnetic resonance (EPR) and magnetic susceptibility of both powdered and crystalline α-Cu_xV₂O₅ near the compositional limit have been studied as a function of temperature. The EPR data indicate that the paramagnetic species are isolated V⁴⁺ centers in a nearly axially symmetric ligand field. It is demonstrated that reasonably precise EPR parameters can be derived from powdered spectra. The narrowness of the EPR line suggests that the spectra are motionally narrowed via thermally activated hopping of the paramagnetic electron of V⁴⁺. Expressions are derived for the temperature-dependent magnetic moments of the octahedral ²T_2g term under the combined perturbation of spin-orbit coupling and an axially symmetric ligand field, and the susceptibility data are interpreted in terms of this model. It is found that the sixfold degenerate ²T_2g level is split into three levels with the lowest and highest levels possessing a magnetic moment and the intermediate level having no moment. This model is also consistent with the g-tensor and can account for the temperature dependence of the EPR linewidth.     

Anharmonic calculation of bandwidths and frequency shifts in crystalline CO2

The bandwidths and the anharmonic frequency shifts of the optical lattice phonons of crystalline CO₂ are calculated as a function of temperature using three different intermolecular potentials. A previously available potential (MOSMD) which reproduces correctly the structure, energy and harmonic frequencies of the CO₂ crystal provides anharmonic, calculated bandwidths about 10 times larger than the experimental values. An improved potential (PRC-1) is obtained by moving inside the OC bonds the oxygen interaction center and by increasing the molecular quadrupole. This potential predicts bandwidths and shifts of the right order of magnitude but still too large. The third potential (PRC-2) is obtained from the previous one by using a non-collinear distribution of negative charges. The negative charges are localized on exagons around the OC bond at a distance d = 0.3 Å. This potential reproduces correctly the experimental bandwidths and yields small anharmonic shifts. The temperature dependence of the bandwidths and shifts is correctly reproduced in terms of three-phonon decay processes for the T⁺_g and for the T⁻_g phonons. In the case of the E_g phonon four-phonon processes are required in order to fit the experimental data.     

DSC investigation of the polystyrene films filled with fullerene

Polystyrene composite films with different content of C₆₀?+?C₇₀ fullerene mix have been obtained from o-xylene solutions. The mass fraction of fullerene was varied from 0.01 to 0.1 mass%. The glass transition temperatures and specific heat capacities in range of 293?C423?K have been determined for the films by DSC method. The plasticization of the polymer is observed in thermal properties of the films under influence of small fullerene additions. The values of T _g and C _P decrease and thermal coefficient of heat capacity b increase as fullerene content increases up to 0.02 mass%. The effect of interaction between polymer and fullerene molecules on thermal properties becomes evident at higher fullerene content in range from 0.02 to 0.1 mass%. At this the values of T _g and C _P increase and b coefficient decrease with increasing content of fullerene. Concentration dependence of C _P and b values is less steep for polymer composite films in elastic state at temperatures above T _g. Molecular interactions in the composites are discussed in view of our-self and literature data.     

Determination of glass transition temperature of poly(ethylene glycol) by spin probe technique

The rotational relaxation data for free nitroxyl radicals in poly(ethylene glycol) samples of mol. wt 3000-22,000 gave for the glass-transition temperature (T_p) a value of ?60°, independent of molecular weight. The rotational activation energy was ～ 2 kJ/mole below T_g and ～ 10 kJ/mole above T_g. This indicated that the mechanism of motion below T_g differs from the above T_g. Evidently the rotating units of polymer are much smaller below T_g than above it. The rotational relaxation time (T) was found to be dependent on the molar volume of PEG (V) and to follow the empirical equations t = A exp (?kV₁) and t = B exp (?kV₁) where V_a and V₁ are the molar volumes (cm³/mol) when T < T_g and T > T_g respectively, and k ～ 1. Therefore the defects in which radicals are located are perhaps the dominant factor determining the dynamic state of probe radicals in polymers at low temperatures.     

Effect of nanoscale confinement on the glass transition temperature of free-standing polymer films: Novel,self-referencing fluorescence method

The effect of nanoscale confinement on the glass transition temperature, T_g, of freely standing polystyrene (PS) films was determined using the temperature dependence of a fluorescence intensity ratio associated with pyrene dye labeled to the polymer. The ratio of the intensity of the third fluorescence peak to that of the first fluorescence peak in 1-pyrenylmethyl methacrylate-labeled PS (MApyrene-labeled PS) decreased with decreasing temperature, and the intersection of the linear temperature dependences in the rubbery and glassy states yielded the measurement of T_g. The sensitivity of this method to T_g was also shown in bulk, supported PS and poly(isobutyl methacrylate) films. With free-standing PS films, a strong effect of confinement on T_g was evident at thicknesses less than 80–90 nm. For MApyrene-labeled PS with M_n = 701 kg mol⁻¹, a 41-nm-thick film exhibited a 47 K reduction in T_g relative to bulk PS. A strong molecular weight dependence of the T_g-confinement effect was also observed, with a 65-nm-thick free-standing film exhibiting a reduction in T_g relative to bulk PS of 19 K with M_n = 701 kg mol⁻¹ and 31 K with M_n = 1460 kg mol⁻¹. The data are in reasonable agreement with results of Forrest, Dalnoki-Veress, and Dutcher who performed the seminal studies on T_g-confinement effects in free-standing PS films. The utility of self-referencing fluorescence for novel studies of confinement effects in free-standing films is discussed. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2754–2764, 2008     

Contribution of Temperature Modulated DSC® to the Study of the Molecular Mobility in Glass Forming Pharmaceutical Systems

The temperature modulated differential scanning calorimetry (MDSC^®) technique has been used to characterise the low frequency molecular mobility of indomethacin and maltitol just above their respective calorimetric glass transition temperatureT _g. Analysis has been made using the concept of complex specific heat. Spectroscopic information are thus obtained through the temperature dependence of the isochronal real and imaginary parts C′ and C″. This gives access to the fragility index m and the stretched exponent β. The comparison with dielectric spectroscopy has been performed to check the coherence of spectroscopic information. Measurements on maltitol enable to demonstrate the useful complementarity of the technique when the low frequencies dielectric relaxations are occulted by the presence of conductors default.     

Dielectric properties of films of Ag-ED20 epoxy nanocomposite synthesized in situ. Temperature dependence of direct current conductivity

The influence of silver myristate used as a precursor of silver nanoparticles on the direct current conductivity σ _dc of epoxy polymer within the concentration range of ≤0.8 wt % was investigated. The value of direct current conductivity was determined on the basis of analysis of the frequency dependence of complex permittivity within the frequency range of 10^?2–10⁵ Hz. The temperature dependence of σ _dc is composed of two regions. The dependence corresponds to the Vogel-Fulcher-Tammann empirical law σ _dc = σ _dc0exp{?DT ₀/(T-T ₀)} (where T ₀ is the Vogel temperature and D is the strength parameter) at temperatures higher than the glass transition temperature T _g. At the same time, T ₀ does not depend on the concentration of nanoparticles. The Arrhenius temperature dependence characterized by activation energy about 1.2 eV is observed at temperatures lower than T _g. The observed shape of the temperature dependence is related to the change in the mechanism of conductivity after “freezing” of ionic mobility at temperatures lower than T _g. The value of σ _dc is increased as the concentration of nanoparticles is raised within the temperature range of T > T _g. The obtained dependence of σ _dc on silver myristate concentration is similar to the root one, indicating the absence of percolation within the studied range of concentrations.     

Bisphenol-A polycarbonate–diluent interactions

The effect of low-molecular-weight miscible additives on the sub-T_g (β) relaxation process in bisphenol-A polycarbonate (BPAPC) was studied using high-resolution carbon-13 solid-state NMR. The trend of the spin-lattice relaxation times T₁ at 50 MHz suggests that strong intermolecular interactions occur upon mixing when BPAPC is physically stiffened by the antiplasticizing diluent, diphenylphthalate. The values of ¹³C T₁ at 15 MHz in d-chloroform solutions for similar BPAPC-diluent mixtures suggest that diluent effects on the megahertz mobility of the polymer occur exclusively in the solid state. These results are explained using equilibrium thermodynamics, in the Ehrenfest sense, at the second-order glass transition temperature T_g. Theory predicts that the temperature dependence of the Flory–Huggins interaction parameter ?χ/?T changes abruptly as the polymer-diluent blends are cooled below T_g from the molten state. The difference between ?χ/?T in the liquid and glassy states is the major factor which determines the diluent concentration dependence of T_g. A method is developed to estimate the relative magnitudes of χ for polymerdiluent blends in the glassy state.     

Low-temperature behaviour of water in nafion membranes

Proton NMR measurements of T₁, T₁π and T₂ in “Nafion” perfluorosulphonate membranes, together with neutron- scattering and dielectric data, show that the aqueous phase in Nafion solidifies at a glass transformation whose temperature T_g is 168 K in water-saturated acid membranes. T_g is higher for the salt forms. Mo¨ssbauer measurements on Eu³⁺ Nafion confirm that the cations are present in an aqueous phase with T_g ≈ 220 K.