Self-diffusion of supercooled o-terphenyl near the glass transition temperature

Self-diffusion coefficients for the low molecular weight glass former o-terphenyl have been measured near Tg by isothermally desorbing thin film bilayers of deuterio and protio o-terphenyl in a vacuum chamber. We observe translational diffusion that is about 100 times faster at Tg + 3 K than the Stokes-Einstein prediction. Predictions from random first order transition theory and a dynamic facilitation approach are in reasonable agreement with our results; in these approaches, enhanced translational diffusion is associated with spatially heterogeneous dynamics. Self-diffusion controls crystallization in o-terphenyl for most of the supercooled liquid regime, but at temperatures below Tg + 10 K, the reported crystallization rate increases suddenly while the self-diffusion coefficient does not. This work and previous work on trisnaphthylbenzene both find a self-diffusion-controlled crystal growth regime and an enhancement in self-diffusion near Tg, suggesting that these phenomena are general characteristics of fragile low molecular weight glass formers. We discuss the width of the relaxation time distributions of o-terphenyl and trisnaphthylbenzene as they relate to the observation of enhanced translational diffusion.     

Isothermal desorption measurements of self-diffusion in supercooled o-terphenyl

Isothermal desorption of o-terphenyl thin-film bilayers was used to measure self-diffusion coefficients of supercooled o-terphenyl near the glass transition temperature (Tg=243 K). Diffusion coefficients from 10(-15.5) to 10(-12) cm2 s(-1) were obtained between 246 and 265 K. Protio and deuterio o-terphenyl were sequentially vapor deposited, then annealed to simultaneously diffuse and desorb the sample in a vacuum chamber. During the desorption of the bilayer, the concentration of each isotope was detected by a mass spectrometer, which revealed the extent of interfacial broadening. In these experiments, isotopic interdiffusion is indistinguishable from self-diffusion and the measured interfacial broadening is consistent with Fickian diffusion. The samples prepared under several different deposition conditions yielded the same self-diffusion coefficients, indicating that the experiments were conducted in the equilibrium supercooled liquid state.     

Interfacial effects on vitrification of confined glass-forming liquids

Mesoporous silica phases, with uniform pores of dimensions in the 2-30 nm range, offer a uniquely well-defined environment for the study of the effects of two-dimensional spatial confinement on the properties of glass-forming liquids. We report observations by differential scanning calorimetry of the vitrification of o-terphenyl (OTP), salol, and glycerol in hexagonal mesoporous silica (MCM-41 and SBA-15) in a wide range of pore sizes from 2.6 to 26.4 nm. In agreement with previous studies, where a controlled porous glass is used as a solid matrix, the glass transition temperature for o-terphenyl diminishes with decreasing pore size. In contrast to OTP, glycerol shows a gradual increase in glass transition temperature, while in salol a slight reduction of glass transition temperature is observed, followed by an increase, which results in glass transition temperature indistinguishable from that of the bulk for the smallest pores. These results are discussed in terms of liquid-surface interactions in an interfacial layer, monitored by Fourier-transformed infrared spectroscopy in the study. The hydrogen bonding with silica surface silanols dominates the glass transition trends observed in salol and glycerol.     

Nitrogen nuclear spin flips in nitroxide spin probes of different sizes in glassy o-terphenyl: possible relation with α- and β-relaxations

The pulsed electron-electron double resonance (ELDOR) technique was employed to study nitroxide spin probes of three different sizes dissolved in glassy o-terphenyl. A microwave pulse applied to the central hyperfine structure (hfs) component of the nitroxide electron paramagnetic resonance spectrum was followed by two echo-detecting pulses of different microwave frequency to probe the magnetization transfer (MT) to the low-field hfs component. The MT between hfs components is readily related to flips in the nitrogen nuclear spin, which in turn are induced by molecular motion. The MT on the time scale of tens of microseconds was observed over a wide temperature range, including temperatures near and well below the glass transition. For a bulky nitroxide, it was found that MT rates approach dielectric α (primary) relaxation frequencies reported for o-terphenyl in the literature. For small nitroxides, MT rates were found to match the frequencies of dielectric β (secondary) Johari-Goldstein relaxation. The most probable motional mechanism inducing the nitrogen nuclear spin flips is large-angle angular jumps, between some orientations of unequal occupation probabilities. The pulsed ELDOR of nitroxide spin probes may provide additional insight into the nature of Johari-Goldstein relaxation in glassy media and may serve as a tool for studying this relaxation in substances consisting of non-rigid molecules (such as branched polymers) and in heterogeneous and non-polar systems (such as a core of biological membranes).     

Orientational and translational dynamics in room temperature ionic liquids

The authors investigate the dynamics of a series of room temperature ionic liquids, based on the same 1-butyl-3-methylimidazolium cation with different anions, by means of broadband (10(-6)-10(9) Hz) dielectric spectroscopy and depolarized light scattering in the temperature range from 400 K down to 35 K. Typical ionic conductivity is observed above the glass transition temperature Tg. Below Tg the authors detect relaxation processes that exhibit characteristics of secondary relaxations, as typically observed in molecular glasses. At high temperatures, the characteristic times of cation reorientation, deduced from the light scattering data, are approximately equal to the electric modulus relaxation times related to ionic conductivity. In the supercooled regime and close to Tg, the authors observe decoupling of conductivity from structural relaxation. Overall, room temperature ionic liquids exhibit typical glass transition dynamics, apparently unaltered by Coulomb interactions.     

Interdependence of primary and Johari-Goldstein secondary relaxations in glass-forming systems

We report evidence from broadband dielectric spectroscopy that the dynamics of the primary alpha- and secondary Johari-Goldstein (JG) beta-processes are strongly correlated in different glass-forming systems over a wide temperature T and pressure P range, in contrast with the widespread opinion of statistical independence of these processes. The alpha-beta mutual dependence is quantitatively confirmed by (a) the overall superposition of spectra measured at different T-P combinations but with an invariant alpha-relaxation time; (b) the contemporary scaling of the isothermal-pressure and isobaric-temperature dependences of the alpha-and beta-relaxation times as plotted versus the reduced variable Tg(P)/T where Tg is the glass transition temperature. These novel and model-independent evidences indicate the relevance of the JG relaxation phenomenon in glass transition, often overlooked by most current theories.     

Influence of glass transition temperature of crosslinked unsaturated polyester resin/styrene formulations on the final conversion after an isothermal curing at 100°C

SMC (sheet molding compound) is a composite based on fibers‐reinforced unsaturated polyester (UP) resin molded usually at 140°C to 170°C under a pressure of 60 to 100 bars. In order to develop new SMC formulations that can be molded at lower temperature (100°C) for economic and environmental reasons, the formulation of the composite had to be completely modified, both to allow a rapid reaction at 100°C, but also to avoid a vitrification phenomenon due to the fact that the glass transition temperature (Tg) of the SMC parts becomes, during the molding process, higher than the mold temperature. In this paper, the relation between the molding temperature, the glass transition temperature, and the final conversion of UP resin/styrene formulations has been underlined. The Tg of the cured resin was decreased by two different ways. The first way involved the reduction of the crosslinking density of the UP resin by using a blend of two resins, a pure maleic and a more flexible one. This blend allows to adjust the Tg over a temperature range from 197°C (Tg of the pure UP resin) to 75°C (Tg of the pure flexible resin). The second way consisted in the addition of butyl methacrylate (BuMA), a reactive plasticizer, to the formulation, allowing a decrease of the final material's Tg from 197°C to 130°C by replacing 35 wt% of styrene by BuMA. These two methods allow to obtain a final conversion of 99% after 8 minutes of molding at 100°C.     

Anisotropic pseudorotation of the photoexcited triplet state of fullerene C60 in molecular glasses studied by pulse EPR

Spin-polarized echo-detected electron paramagnetic resonance (EPR) spectra and the transversal relaxation rate T2(-1) of the photoexcited triplet state of fullerene C60 molecules were studied in o-terphenyl, 1-methylnaphthalene, and decalin glassy matrices. The model is composed of a fast (correlation time approximately 10(-12) s) pseudorotation of (3)C60 in a local anisotropic potential created by interaction of the fullerene molecule with the surrounding matrix molecules. In simulations, this potential is assumed to be axially symmetric around some axis of a preferable orientation in a matrix cage. The fitted value of the potential was found to depend on the type of glass and to decrease monotonically with a temperature increase. A sharp increase of the T2(-1) temperature dependence was found near 240 K in glassy o-terphenyl and near 100 K in glassy 1-methylnaphthalene and decalin. This increase probably is related to the influence on the pseudorotation of the onset of large-amplitude vibrational molecular motions (dynamical transition in glass) that are known for glasses from neutron scattering and molecular dynamics studies. The obtained results suggest that molecular and spin dynamics of the triplet fullerene are extremely sensitive to molecular motions in glassy materials.     

热处理条件对R-PET/LLDPE-g-MA共混物中PET玻璃化转变行为的影响

对回收聚对苯二甲酸乙二酯(R-PET)/LLDPE-g-MA马来酸酐改性的线性低密度聚乙烯共混物进行不同条件的热处理, 采用差示扫描量热仪(DSC)研究共混物基体PET的玻璃化转变行为. 结果表明, 当热处理温度低于PET的玻璃化转变温度(Tg)时, PET的玻璃化转变区域出现热焓松弛现象. 随着热处理温度的增加, PET的Tg逐渐升高; 在50~70 ℃下热处理48 h后, PET的Tg逐渐稳定. 当热处理温度高于PET的Tg而低于100 ℃时, PET的玻璃化转变区域出现2个热流转变, FTIR分析表明, PET分子构象开始发生变化. 当热处理温度为100 ℃时, DSC曲线上PET的玻璃化转变消失, PET的结晶度明显增加, 说明PET开始冷结晶的温度在90~100 ℃之间.     

双氰胺固化环氧树脂的研究

<正> 双氰胺做为环氧树脂的固化剂,其贮存期可以达到一年以上,这种固化物的机械和介电性能优异,因此,广泛用于制造层压等电工材料方面。国外许多人都曾对双氰胺与环氧树脂的固化反应历程和固化物的性能做过研究,指出了在固化过程中化学结构的变化     

Molecular dissipation phenomena of nanoscopic friction in the heterogeneous relaxation regime of a glass former

Nanoscale sliding friction involving a polystyrene melt near its glass transition temperature Tg (373 K) exhibited dissipation phenomena that provide insight into the underlying molecular relaxation processes. A dissipative length scale that shows significant parallelism with the size of cooperatively rearranging regions (CRRs) could be experimentally deduced from friction-velocity isotherms, combined with dielectric loss analysis. Upon cooling to approximately 10 K above Tg, the dissipation length Xd grew from a segmental scale of approximately 3 A to 2.1 nm, following a power-law relationship with the reduced temperature Xd approximately TR-phi. The resulting phi=1.89+/-0.08 is consistent with growth predictions for the length scale of CRRs in the heterogeneous regime of fragile glass formers. Deviations from the power-law behavior closer to Tg suggest that long-range processes, e.g., the normal mode or ultraslow Fischer modes, may couple with the alpha relaxation, leading to energy dissipation in domains of tens of nanometers.     

Dynamical heterogeneity in glassy o-terphenyl: manifestation of environment changes in photoisomerization kinetics of probe molecules

A new method for the investigation of dynamical heterogeneity in glassy matrixes is presented and illustrated by the example of o-terphenyl (OTP). UV-vis absorption spectroscopy has been used to monitor the cis-trans isomerization kinetics of probe molecules in glassy OTP. The dependence of isomerization quantum yield on light intensity has been established. This dependence is shown to be due to the change in the local environment of the probe molecules. The simple model is suggested to estimate the time required for the environment to change, tauex. The tauex values from 2.6 x 10(2) to 1.9 x 10(5) s have been obtained for environments of molecules of 1-naphthylazomethoxybenzene (NAMB) in OTP over a temperature range from 244 to 204 K (Tg+1 to Tg-39 K). The temperature dependence of exchange time has a non-Arrhenius character. As the temperature decreases, an increase in exchange time slows down. The activation energy of the relaxation process is 54 kJ/mol over the range of 224-239 K.     

The effect of viscosity on the kinetics of redox reactions in highly viscous silicate liquids

The kinetics of the temperature dependent redox reaction between chromium and manganese (Cr(6+) + 3Mn(2+)?Cr(3+) + 3Mn(3+)) in highly viscous silicate melts were studied by UV-vis-NIR spectroscopy at temperatures in the range from 25 to 800 °C. At high temperatures, the reaction is in equilibrium. During cooling, it is continuously shifted to the right. During cooling from Tg+50 K to Tg (Tg = glass transition temperature), a further decrease in the Cr(6+) concentration was obtained which, however, was less pronounced if larger cooling rates were applied. In this temperature range, the kinetics plays an important part. Finally, at a certain temperatures below Tg, the equilibrium was frozen. The temperature, the equilibrium is frozen in decreases with decreasing cooling rate. It also decreases with the glass transition temperature of the respective composition. The activation energies increase with the activation energies of the viscosity of the respective melt. The redox reaction is controlled by the viscosity, i.e., the rearrangement of the glass network and not by diffusion. The reason is a drastic change in the coordination spheres during the reaction which leads to a high inner reorganization energy according to Marcus' Theory.     

Photoinduced intramolecular charge separation in a polymer solid below the glass transition temperature

Photoinduced intramolecular charge separation (CS) in a polar polymer glass, cyanoethylated pullulan (CN-PUL), was studied below the glass transition temperature (Tg=395 K). A series of three carbazole (Cz: donor)-cyclohexane (S: spacer)-acceptor (A: acceptor) molecules (Cz-S-A) was used as intramolecular donor-acceptor dyads. The photoinduced CS rate was evaluated by the fluorescence decay measurement at temperatures from 100 to 400 K. The CS rate (kCS) increased above 200 K even far below Tg where micro-Brownian motions of the whole polymer chain are frozen. Below 200 K, on the other hand, kCS showed weak dependence on temperature. The temperature dependence of kCS is discussed in terms of the dielectric relaxation time of the polymer matrix. Consequently, CS below Tg was well explained by a thermally nonequilibrium electron transfer (ET) formula above 200 K and by a two-mode quantum-mechanical ET formula below 200 K. The increase in kCS above 200 K is mainly caused by a thermally activated low-frequency matrix mode originating from the side-chain relaxation of polar cyano groups. The weak temperature dependence of kCS can be explained by a nuclear-tunneling effect caused by a high-frequency matrix mode (variant Planck's over 2piomegH=250 cm-1) and an intramolecular vibrational mode (variant Planck's over 2piomegaQ=1300 cm-1). The high-frequency mode of the polymer matrix was attributed to a vibrational or librational motion of polar groups in the CN-PUL glassy solid.     

Effect of physical aging on the Johari-Goldstein and alpha relaxations of D-sorbitol: a study by thermally stimulated depolarization currents

The relaxations in amorphous D-sorbitol have been studied by thermally stimulated depolarization currents during annealing at 255 K, which is 17 K below its calorimetric glass transition temperature Tg=272 K. As the glass structurally relaxes on aging, the features of the alpha relaxation and of the Johari-Goldstein (JG) relaxation change with time. For the alpha relaxation (i) the dielectric strength decreases; (ii) the activation energy decreases; and (iii) the relaxation time increases. For the JG relaxation the dielectric strength also decreases but with a different time dependence, and there is no evidence for any modification of the kinetic features of the mobility. The amplitude of response to aging is higher for the higher temperature motional components of the Johari-Goldstein relaxation compared with the lower temperature ones.     

Identification of dielectric and structural relaxations in glass-forming secondary amides

Dielectric relaxation dynamics of secondary amides is explored in their supercooled state near the glass transition temperature Tg by investigating N-ethylacetamide and its mixtures with N-methylformamide. All the samples are found to exhibit giant dielectric permittivities, reaching over 500 in N-methylformamide-rich mixtures around Tg. For both the neat and binary systems, the predominant relaxation peak is of the Debye-type throughout the viscous regime, which is an unexpected feature for a glass former with intermediate fragility. The present results combined with the earlier reported high-temperature data reveal that the dielectric strength delta epsilon(D) of the Debye relaxation extrapolates to zero at frequencies of 10(10)-10(11) Hz, which is about two orders of magnitude lower than the phonon frequency limit typical of the structural relaxation. This Debye process is remarkably similar to the dielectric behavior of many monohydroxy alcohols, which implies a common nature of purely exponential relaxation dynamics in these liquids. Based on the dielectric properties, we conclude that the Debye relaxation in the secondary amides is not a direct signature of the primary or alpha-relaxation, the latter being obscured at low temperatures due to the relatively low permittivity and close spectral proximity to the Debye peak. As in the case of monohydroxy alcohols, dielectric polarization and structure fluctuate on different time scales in secondary amides. The Kirkwood-Fr?hlich correlation factors for Debye-type liquids are also discussed.     

Novel method to detect the volumetric glass --> liquid transition at high pressures: glycerol as a test case

We report a novel method of detecting the glass --> liquid transition at high pressures, which comprises measuring the relative volume change incurred upon heating glassy samples into the liquid state. We show data on glycerol in the pressure range 0.050-1.00 GPa to demonstrate the viability of the method. The reversible glass --> liquid transition is observed by means of a kink in the relative volume change on heating the sample isobarically, which is attributed to the glass --> liquid transition temperature Tg. This kink can only be observed in the second and subsequent heating cycles since it is superposed by a compaction in the first heating cycle. The isobaric thermal expansivity beta, which is closely related to the first derivative of this curve, shows the features expected for a glass --> liquid transition, including a sharp rise of beta(glass) in a narrow temperature interval to beta(viscous liquid) and an accompanying overshoot effect. Both Tg and the size of the overshoot effect vary in accordance with theory upon changing the ratio of cooling to heating rates. From the shape of this curve the onset, inflection, overshoot peak, and endpoint of the glass --> liquid transition can be extracted, which can be employed to calculate the reduced glass transition width as a measure for the fragility of the liquid. Comparison with literature data allows quantifying the accuracy of the liquid's thermal expansivity beta to be at least +/-10%, while the error in beta is significantly larger for the expansivity of the glassy state. The reproducibility of the glass --> liquid transition temperature Tg is better than +/-2 K. Our glycerol data confirms literature studies showing a nonlinear increase of Tg with increasing pressure (approximately 35 K/GPa on average), which is accompanied by an increase in fragility.     

Heterogeneous dynamics and dynamic heterogeneities at the glass transition probed with single molecule spectroscopy

We studied the temperature dependence of the structural relaxation in poly(vinyl acetate) near the glass transition temperature with single molecule spectroscopy from Tg-1 K to Tg+12 K. The temperature dependence of the observed relaxation times matches results from bulk experiments; the observed relaxation times are, however, 80-fold slower than those from bulk experiments at the same temperature. We attribute this factor to the size of the probe molecule. The individual relaxation times of the single molecule environments are distributed normally on a logarithmic time scale, confirming that the dynamics in poly(vinyl acetate) is heterogeneous. The width of the distribution of individual relaxation times is essentially independent of temperature. The observed full width at half maximum (FWHM) on a logarithmic time axis is approximately 0.7, corresponding to a factor of about 5-fold, significantly narrower than the dielectric spectrum of the same material with a FWHM of about 2.0 on a logarithmic time axis, corresponding to a factor of about 100-fold. We explain this narrow width as the effect of temporal averaging of single molecule fluorescence signals over numerous environments due to a limited lifetime of the probed heterogeneities, indicating that heterogeneities are dynamic. We determine a loose upper limit for the ratio of the structural relaxation time to the lifetime of the heterogeneities (the rate memory parameter) of Q<80 for the range of investigated temperatures.     

Electric and dielectric properties of some gamma-irradiated cabal glasses

Electrical conductivity and dielectric properties have been studied for the glass system CaO---B₂O₃---Al₂O₃ in the temperature range 40–200°C. The substitution of 5% B₂O₃ by CaO or replacing 5% CaO by Na₂O or MgO cause a decrease in the conductivity, but the decrease obtained by soda is greater than that of magnesia. The activation energies of the tested glasses were calculated. All the glasses investigated showed a dielectric constant almost independent of temperature at fixed frequency. The effect of subjecting the glass to a constant dose of gamma-rays changes both the electrical conductivity and dielectric constant. The experimental results were discussed in relation to the specific conduction mechanism in such glasses. Also the effect of varying glass composition or temperature on the mobility or migration of current carrier was considered. The possible creation of induced defects in glass on irradiation was evaluated.