Domain size dependence of glass transition temperature

The effect of particle size on glass transition temperatures is discussed. The phenomenon is treated in terms of Ehrenfest second-order thermodynamics and in addition related to free volume concepts. Consistent formulas are obtained and the order of magnitude of the effect is estimated.     

Enhanced translational diffusion of rubrene in sucrose benzoate

The translational diffusion of rubrene in the fragile molecular glass former, sucrose benzoate (SB) (fragility index m approximately 94), has been studied from T(g)+6 K to T(g)+71 K(T(g)=337 K) by using the technique of holographic fluorescence recovery after photobleaching. In the temperature range of the measurements, the translational relaxation functions were observed to decay exponentially, indicating that Fick's law of diffusion governs the translational motion of rubrene in sucrose benzoate. The value of the translational diffusion coefficient D(T) obtained from the 1e time of the translational relaxation function varied from 5.3 x 10(-15) cm2 s(-1) at 343 K to 5.0x10(-9) cm2 s(-1) at 408 K. The temperature dependence of D(T) for diffusion of rubrene in SB is compared with that of the viscosity and the dielectric relaxation time tau(D) of SB. The temperature dependence of D(T) is weaker than that of Teta for T<1.2T(g) but tracks the reciprocal of the dielectric relaxation time 1tau(D) for 1.05T(g)相似文献   

Breakdown of the Stokes-Einstein relationship: role of interactions in the size dependence of self-diffusivity

Einstein and others derived the reciprocal dependence of the self-diffusivity D on the solute radius r(u) for large solutes based on kinetic theory. We examine here (a) the range of r(u) over which Stokes-Einstein (SE) dependence is valid and (b) the precise dependence for small solutes outside of the SE regime. We show through molecular dynamics simulations that there are two distinct regimes for smaller solutes: (i) the interaction or Levitation effect (LE) regime for solutes of intermediate size and (ii) the D proportional, variant 1/r(u)(2) for still smaller solutes. We show that as the solute-solvent size ratio decreases, the breakdown in the Stokes-Einstein relationship leading to the LE regime has its origin in dispersion interaction between the solute and the solvent. These results explain reports of enhanced solute diffusion in solvents existing in the literature seen for small solutes for which no explanation exists.     

A multi-property fluorescent probe for the investigation of polymer dynamics near the glass transition

In addition to the commonly observed single molecule fluorescence intensity fluctuations due to molecular reorientation dynamics, a perylene bisimide-calixarene compound (1) shows additional on-off fluctuations due to its ability to undergo intramolecular excited state electron transfer (PET). This quenching process is turned on rather sharply when a film of poly(vinylacetate) containing 1 is heated above its glass transition temperature (T _g), which indicates that the electron transfer process depends on the availability of sufficient free volume. Spatial heterogeneities cause different individual molecules to reach the electron transfer regime at different temperatures, but these heterogeneities also fluctuate in time: in the matrix above T _g molecules that are mostly nonfluorescent due to PET can become fluorescent again on timescales of seconds to minutes.The two different mechanisms for intensity fluctuation, rotation and PET, thus far only observed in compound 1, make it a unique probe for the dynamics of supercooled liquids.

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On the breakdown of the Stokes-Einstein law in supercooled liquids

The wavevector-dependent shear viscosity, eta(k), is evaluated for a range of temperatures in a supercooled binary Lennard-Jones liquid. The mode coupling theory of Keyes and Oppenheim (Phys. Rev. A 1973, 8, 937) expresses the self-diffusion constant, D, in terms of eta(k). Replacing eta(k) with the usual viscosity, eta identical with eta(k = 0), yields the Stokes-Einstein law. It is found that the breakdown of the SE law in this system is well described by keeping the simulated k-dependence. Simply put, bath processes on all length scales (wavevectors) contribute to D, the system is much less viscous at finite k, and thus D exceeds the SE estimate based upon eta. The functional form of eta(k) allows for the estimation of a correlation length that grows with decreasing T.     

Light-scattering photography of polymers near the glass transition

Laser light scattered from a block of polystyrene is observed to have a pattern of bright and dark regions called speckle. We observe that the speckle pattern has a lifetime consistent with that of the average relaxation lifetime 〈τ〉 of the intensity fluctuations of the medium as determined by photon correlation spectroscopy. In order to study the pattern of the scattered light, a series of photographs was taken at a variety of exposure times. For each series, the value of the average relaxation lifetime is defined by the temperature and pressure of the polystyrene sample. When the value of 〈τ〉 is short relative to the exposure time, the photograph displays only a random pattern of exposed grains. This is due to the large number of fluctuations that have occurred during the exposure time with random phases relative to one another. As the average relaxation time is increased at a constant exposure time, the speckle pattern appears when the value of 〈τ〉 becomes comparable to the exposure time. The phenomenon of laser speckle allows the time scale of the slowly relaxing fluctuations near the glass transition to be visualized. A digitized series of such pictures could be analyzed to obtain the average relaxation time for the fluctuations, just as in normal photon correlation spectroscopy where the intensity of one coherence area is measured as a function of time.     

Communication: crystallite nucleation in supercooled glycerol near the glass transition

Heterogeneity and solid-like structures found near the glass transition provide a key to a better understanding of supercooled liquids and of the glass transition. However, the formation of solid-like structures and its effect on spatial heterogeneity in supercooled liquids is neither well documented nor well understood. In this work, we reveal the crystalline nature of the solid-like structures in supercooled glycerol by means of neutron scattering. The results indicate that inhomogeneous nucleation happens at temperatures near T(g). Nevertheless, the thermal history of the sample is essential for crystallization. This implies such structures in supercooled liquids strongly depend on thermal history. Our work suggests that different thermal histories may lead to different structures and therefore to different length and time scales of heterogeneity near the glass transition.     

Small molecule diffusion in a rubbery polymer near Tg: Effects of probe size,shape, and flexibility

A novel experimental approach involving fluorescence nonradiative energy transfer (NRET) is employed to study the Fickian diffusion of small molecules in rubbery poly(isobutyl methacrylate) (PiBMA) films near the glass transition, using a formalism that directly relates the small molecule translational diffusion coefficient, D, to changes in the normalized nonradiative energy transfer efficiency, E_N. Values of D for pyrene, 1,3-bis-(1-pyrene) propane (BPP), 1,3-bis-(1-pyrene) decane (BPD), 9,10-bis-phenyl ethynyl anthracene (BPEA), diphenyl Disperse Red 4 (DPDR4), and decacyclene in PiBMA are measured over temperatures ranging from approximately T_g to T_g + 25°C. Among these chromophores, significant differences in both the magnitude and temperature dependence of D are observed which are attributed to differences in molecule shape and flexibility, as well as molar volume. Other factors being equal, chromophore flexibility was shown both to increase the magnitude of D and to decrease its dependence on temperature, as does an increase in aspect ratio. For BPD, these effects are attributed to the ability of the flexible molecule to diffuse in a piecewise manner, requiring the cooperative mobility of fewer polymer chain segments than a rigid molecule of the same molar volume. For BPEA and DPDR4, this deviation from D being dominated by molar volume effects is attributed the to high aspect ratio of these elongated molecules. © 1996 John Wiley & Sons, Inc.     

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.     

Nature of the breakdown in the Stokes-Einstein relationship in a hard sphere fluid

Molecular dynamics simulations of high density hard sphere fluids clearly show a breakdown of the Stokes-Einstein equation (SE). This result has been conjectured to be due to the presence of mobile particles, i.e., ones which have the propensity to "hop" distances that are integer multiples of the interparticle distance. We conclusively show that the sedentary particles, i.e., ones complementary to the "hoppers," obey the SE relationship to a good approximation, even though the fluid as a whole violates the SE equation at high densities. These results support the notion that the unusual diffusive behavior of supercooled liquids is dominated by the hopping particles.     

An EPR spin probe study of the glass transition in MBBA

EPR evidence is presented to support the literature contention that a small spherical probe will detect chain melting while a longer probe will detect the glass transition of MBBA. One finds DTA/DSC values for T_g from both probes when using the Grest-Cohen model near room temperature.     

Derivation of the Kissinger equation for non-isothermal glass transition peaks

A brief derivation of the Kissinger’s equation for analysis of experimental data of non-isothermal glass transition peaks based on the free volume model is given. This equation was applied successfully to Cu_0.3(SSe₂₀)_0.7 chalcogenide glass for different heating rates. For granted this model, the obtained glass transition activation energy, E _g must be constant throughout the whole glass transition temperature range. This required that T _g to be determined for three characteristic temperature points for each DSC curve.     

Inclusion of the concentration dependence of the diffusion coefficient in the sand equation

The applications of the Sand equation in potentiometry of electrode and membrane systems for precise measurements of the transition time (τ) have been determined. An approach was suggested for choosing the diffusion coefficient of electrolyte (D) in the case when the concentration changes from its value in the agitated solution (where D = D_b) to the nearly zero value at the surface (D = D₀ corresponds to an infinitely dilute solution), D_b and D₀ being substantially different. The Nernst–Planck–Poisson nonstationary equations were numerically solved in a one-dimensional system including an ion-exchange membrane and two adjacent diffusion layers (for the electrode–solution system, the result is a particular case). An effective value D_ef was found, whose substitution in the Sand equation gave τ identical to that obtained by numerical solution. The neglect of the concentration dependence D(с) can lead to a nonadequate determination of the ion transport numbers in the membrane.     

Small angle neutron scattering (SANS) studies of diffusion in bulk polystyrene near the glass transition temperature (Tg)

Ultraslow diffusion in bulk polymers has been measured by SANS. The experiment begins by measuring scattering from heterogeneous specimens containing domains of protonated-and deuterated-polymers at temperatures far below T_g. The samples are subsequently held [annealed] above T_g for a known time-interval, then cooled below T_g where SANS is measured again. Scattering changes, from before to after annealing, are analysed to obtain diffusion coefficients. The recent Summerfield - Ullman procedure is used to deconvolute portions of the scattering curve that decrease and increase with annealing time. Because of SANS sensitivity to small distances, the method yields D ≈ 10⁻¹⁸ to 10⁻¹⁵ cm²/s after annealing times of 1–24 h. Data analysis is complicated by “smearing effects” which produce apparent Q-dependent diffusion coefficients. Representative experimental results on polystyrene at 108°–130°C are discussed.     

Solute diffusion in relation to the glass transition temperature of solute-polymer blends

The transport of various dye solute molecules from amorphous donor polymer films to an amorphous acceptor polymer film has been investigated. Dye diffusion was studied by laminating dye-donor and dye-acceptor films under controlled pressure and temperature. The acceptor medium was kept constant, whereas a wide range of polymer structures were used as binders in the donor system. At constant dye concentration, the transport of the dye molecules from the donor sheet was found to be controlled by the glass transition temperature of the dye-polymer mixture in the donor film. This relationship was ubiquitous for all types of chromophore and polymer studied. The data was found to fit well with free volume considerations and the Williams-Landel-Ferry equation by using the glass transition as that of the dye-polymer blend in the donor matrix.     

Boson peak and fast relaxation process near the glass transition in polystyrene

Atactic polystyrene, both side group and main chain deuterated, was investigated by inelastic neutron scattering in a wide temperature range around the glass transition from 2 to 450 K. In the glass the Boson peak position is only very weakly influenced by the deuteration of the phenyl group. In the neighborhood of the glass transition temperatureT _g we find a fast relaxation process similar to other glasses. The onset of the fast relaxation in polystyrene, however, is observed already at temperaturesT _g — 200 K. Results from partially deuterated polystyrene suggest a change of the phenyl ring dynamics already far belowT _g.     

Physical aging and dye diffusion in polysulfone below the glass transition temperature

The effect of physical aging on the tracer diffusion coefficient D of camphorquinone in polysulfone is investigated. It is shown that if the sample is sufficiently annealed and physical aging is nearly complete, the temperature dependence of D will reflect the primary α-relaxation process of the host polymer. In the temperature range between T_g (=185°C) and 165°C, D is found to be a function of time, and the time dependence of D is given by D = At^?μ, with μ approximately equal to unity. © 1994 John Wiley & Sons, Inc.