Effects of sample history,time, and temperature on the sorption of monomer vapor by PVC

The sorption of vinyl chloride monomer (VCM) vapor by poly(vinyl chloride) (PVC) resin powders has been studied as a function of time, VCM pressure, temperature, and previous PVC history. In fine powders, sorption occurs in two separable stages, a rapid Fickean diffusion and a slow relaxation-controlled process. Hysteresis in interval sorption-desorption experiments and variable resorption behavior can be related to the relaxation-controlled process. The amount of VCM sorbed shows a pronounced variation with postpolymerization thermal history of the PVC sample; this effect seems to result from the freezing-in of differing “hole volumes” under varied conditions of glassification. The heat of VCM sorption by glassy PVC, ?4.3 kcal/mole, is nearly equal to the heat of condensation of VCM. The complex dependence of sorption behavior upon experimental variables can reasonably be interpreted as reflecting changes in hole volume with postpolymerization history.     

Thermodynamic picture of the glassy state gained from exactly solvable models

A picture for thermodynamics of the glassy state was introduced recently by us [Phys. Rev. Lett. 79, 1317 (1997); 80, 5580 (1998)]. It starts by assuming that one extra parameter, the effective temperature, is needed to describe the glassy state. This approach connects responses of macroscopic observables to a field change with their temporal fluctuations, and with the fluctuation-dissipation relation, in a generalized, nonequilibrium way. Similar universal relations do not hold between energy fluctuations and the specific heat. In the present paper, the underlying arguments are discussed in greater length. The main part of the paper involves details of the exact dynamical solution of two simple models introduced recently: uncoupled harmonic oscillators subject to parallel Monte Carlo dynamics, and independent spherical spins in a random field with such dynamics. At low temperature, the relaxation time of both models diverges as an Arrhenius law, which causes glassy behavior in typical situations. In the glassy regime, we are able to verify the above-mentioned relations for the thermodynamics of the glassy state. In the course of the analysis, it is argued that stretched exponential behavior is not a fundamental property of the glassy state, though it may be useful for fitting in a limited parameter regime.     

Topological characteristics of bonds in SiO2 and GeO2 oxide systems upon a glass-liquid transition

Using the Angell model of broken bonds (configurons), configuron clustering in a topologically disordered lattice (network) of amorphous SiO₂ and GeO₂ upon a glass-liquid transition is considered. It is shown that the glass-liquid transition is accompanied by the formation of a macroscopic (percolation) configuron cluster penetrating the entire bulk of the material and possessing fractal geometry. The glass-liquid (overcooled liquid) percolation phase transition in the amorphous substance is accompanied by a change in the Hausdorff dimension of the bond network structure for configurons from the three-dimensional Euclidean dimension in the glassy state to a fractal dimension of 2.55 ± 0.05 in the liquidlike state. Contrary to the kinetic character of the liquid-glass transition, the glass-transition temperature is a thermodynamic parameter of the amorphous substance, depending parametrically on the cooling rate.     

Atomic structure and phonons of a Pb ultrathin film on the Al(100) surface

The effect of high dynamic pressures on the electrical conductivity of the amorphous conducting carbon phase (glassy carbon) has been studied. The electrical conductivity of glassy carbon samples has been measured under the condition of shock compression and subsequent release wave. The history of the shock loading of glassy carbon has been calculated with the developed semiempirical equations of state. It has been shown the electrical conductivity of glassy carbon samples in the compression phase at a pressure of 45(5) GPa decreases abruptly by two orders of magnitude. In the relief phase, partially reversible change in the electrical conductivity of an amorphous carbon sample occurs. The recorded effect has been treated as a result of a partially reversible physicochemical transformation of shock-compressed amorphous carbon.     

Particles in model filled rubber: Dispersion and mechanical properties

We have been able to design model filled rubbers with exactly the same chemical structure but different filler arrangements. From these model systems, we show that the particle arrangement in the elastomeric matrix controls the strain softening at small strain amplitude known as the Payne effect, as well as the elastic modulus dependence on the temperature. More precisely, we observed that the Payne effect disappears and the elastic modulus only weakly depends on the temperature when the particles are well separated. On the contrary, samples with the same interfacial physical chemistry but with aggregated particles show large amplitudes of the Payne effect and their elastic modulus decreases significantly with the temperature. We discuss these effects in terms of glassy bridge formation between filler particles. The observed effects provide evidence that glassy bridges play a key role on the mechanical properties of filled rubbers.     

Nonequilibrium tensile deformation of amorphous polymers in the rubbery state: Temperature,strain rate,and strain effects. I

We analyze nonequilibrium tensile deformational behavior of an amorphous uncrosslinked random copolymer of styrene and acrylonitrile stretched above its Tg at various temperatures and constant strain rates. The variable investigated is the force required to pull the specimen to a certain strain level. Our results suggest the deformational behavior in the rubbery state can be incorporated in a universal scheme where strain, strain rate, and temperature effects are all superposed, e.g., the effect of strain is identical to the effect of temperature and strain rate. This unexpected result is true within two contiguous but distinct regimes of strain, on both sides of ca. 40% strain, suggesting that non-equilibrium extension at constant pulling speed in the rubbery state occurs by at least two successive mechanisms. Our results also reveal that the physical parameter which should be used to achieve a true superposition of strain rate and temperature data is not as simple as has long been commonly proposed. For instance, we cannot use force alone nor the true stress, nor even the stress divided by strain. The best results are obtained when the function used is a complex function of force and strain. We introduce and explore in depth in this paper and future ones a new method to analyze the data. This method does not impose the choice of the functions which yield superposition and it provides an empirical mean for finding these functions. We have applied our method to other classical elastomers (PIB and SBR) stretched under similar conditions: we find the same conclusions, in particular the same small strain-middle strain transition separating two regimes of deformational behavior in the rubbery state.     

Time-temperature superposition and relaxational behavior in polymeric glasses

It is shown that a free-volume treatment of the relaxational behavior of a polymer can be extended into the glassy region. A modified form of the WLF equation is derived in which the temperature is replaced by T_e, a parameter related to the “frozen” free volume in the glass and defined such that Te = T at equilibrium. Measurements of the isothermal volume contraction of polystyrene and poly(methyl methacrylate) between ?20 and + 95°C are used to estimate the “frozen” free volume and to calculate the temperature dependence of log(a_T) below Tg. The calculated shift factors are compared to experimental values for the glassy state, and good agreement is obtained by selecting an arbitrary, but reasonable, equilibrium glass volume-temperature curve. The slope of this equilibrium glass curve is smaller than the experimental volume curve at some finite cooling rate. The data indicate that the glass is not an “iso-free volume” state and that the relaxation mechanisms in the glass are controlled primarily by the free volume, at least in the vicinity of Tg. A quantitative definition of the role of free volume in the glassy state requires evaluation of the quantities ? log a_T/?V)_T,P and (? log a_T/?T)_v,p; sufficient data of this type are not presently available.     

Isothermal Compression Behaviors of 6 Polymers up to 2 Kbar

The applicabilities of universal equation of state and Birch-Murnaghan equation of state to isothermal compressions of 6 polymers in the liquid, glassy and crystalline states have been examined up to 2 Kbar at various temperatures. The results show that the universal equation of state is very accurate for fitting the isothermal P - V data of polymers in the liquid, glassy and crystalline states, and that the information of the pressure-induced transition is obtained, and that the Birch-Murnaghan equation can also be used but it is not as good as the universal equation of state.     

Physical aging of glassy PMMA/toluene films: Influence of drying/swelling history

Gravimetry experiments in a well-controlled environment have been performed to investigate aging for a glassy PMMA/toluene film. The temperature is constant and the control parameter is the solvent vapor pressure above the film (i.e. the activity). Several experimental protocols have been used, starting from a high activity where the film is swollen and rubbery and then aging the film at different activities below the glass transition. Desorption and resorption curves have been compared for the different protocols, in particular in terms of the softening time, i.e. the time needed by the sample to recover an equilibrium state at high activity. Non-trivial behaviors have been observed, especially at small activities (deep quench). A model is proposed, extending the Leibler-Sekimoto approach to take into account the structural relaxation in the glassy state, using the Tool formalism. This model well captures some of the observed phenomena, but fails in describing the specific kinetics observed when aging is followed by a short but deep quench.     

Thermodynamic response of Mie-Grüneisen materials at high pressures

Expressions are derived to give entropy and temperature in a material which obeys the Mie-Grüneisen equation, based on the assumption that both the Hugoniot pressure and the Grüneisen parameter may be expanded as infinite power series in strain. The resulting equations are used to obtain specific expressions for isothermal compression and for isentropic release from a Hugoniot state. A method is suggested for determining the Grüneisen parameter (as a function of strain) from isothermal compression data. Certain limitations involving possible phase transitions are pointed out.     

Yielding behavior of glassy polymers. III. Relative influences of free volume and kinetic energy

The free-volume model for interpreting the initial yielding behavior of glassy polymers is extended to include kinetic energy influences. Molecular mobility is assumed to be determined by the product of the probabilities of attaining sufficient local free volume to allow molecular rearrangement, and kinetic energy to overcome restraining forces. Yielding then is initiated at the point where the free-volume increase resulting from the dilational component of the applied stress is sufficient to bring the local molecular mobility to that characteristic of the unstressed polymer at T_g.

Expressions are derived for the temperature and strain-rate dependences of the initial yield strain (defined as the proportional limit on the tensile stress-strain curve) and compared with experimental data for poly(methyl methacrylate). The extended model is found to afford no substantial improvement over a simple free-volume model, indicating that relaxational processes in the glassy state-at least in the range T_g to (T_g - 100°C)-are governed principally by freevolume changes.     

Is the configurational entropic model able to predict the final equilibrium state reached by Se glasses after very long ageing durations?

This work compares the equilibrium states reached by glassy selenium (g-Se) after several ageing durations at temperatures lower than the glass transition temperature T _g, with the forecasts given by the configurational entropic model. The comparison is carried out through experimental data collected both on glassy samples after short-term ageing and on glassy samples older than 20?years, kept at room temperature. It is shown that the configurational entropic model is not able to describe the behaviour of g-Se, both at short- and long-ageing terms with the same set of fitting parameters. For short-term ageing, the forecasts given by the entropic model are in good agreement with the experimental data; the hypothesis that the heat capacities corresponding to the equilibrium state and to the state extrapolated from the liquid differ by an amount δ is therefore properly justified. For long-term ageing, the amount δ vanishes and the assumption does not hold any longer. Indeed, experimental results support the idea that at least two separate mechanisms of relaxation coexist in the physical ageing of g-Se, one with very fast kinetics and the other with much slower kinetics.     

Effect of the temperature and way of preliminary torsion in the austenitic state on the shape memory effect in TiNi alloy

The effect of temperature and direction of preliminary torsion in the austenitic state on the degree of strain recovery upon heating of a TiNi alloy has been investigated. It is shown that an increase in the preliminary deformation temperature from 500 to 700 K leads to an increase in the degree of shape recovery upon heating of the material studied. In particular, a 20% strain at a temperature of 500 K decreases the recovery coefficient by 20%, whereas the same preliminary strain at 700 K deteriorates the shape recovery by only 4%. It is established that, applying preliminary torsion in the austenitic and martensitic states in opposite directions, one can obtain an increase in the shape memory strain with an increase in the preliminary plastic strain. Thus, at some plastic strains (λ _pl > 10%), the strain recovered upon heating may even exceed the strain set in the martensitic state.     

Molecular dynamics study of structure and glass forming ability of Zr<Subscript>70</Subscript>Pd<Subscript>30</Subscript> alloy

In this study, the temperature effects on the structural evolution of theZr₇₀Pd₃₀ binary alloy in the glassy and liquid states werestudied using the molecular dynamics simulations based on the many-body type tight-bindingpotential. We considered the following properties in detail: the temperature dependence ofthe volume, the partial and total pair distribution functions and the simulated glasstransition temperature. The effects of the cooling rates on the glass transitiontemperature were examined. The Wendt-Abraham parameter was calculated to determine theglass transition temperature of Zr₇₀Pd₃₀ glassy alloy. The pair analysis technique ofHoneycutt-Andersen was applied to define local atomic arrangements produced from moleculardynamics simulations. The results show that the icosahedral ordering in glassy state hasbeen composed during quenching period, and the simulated glass transition temperature andthe total pair distribution functions are in good agreement with the experimental data.     

Thermodynamic study of fluid in terms of equation of state containing physical parameters

We introduce a simple condition for one mole fluid by considering the thermodynamics of molecules pointing towards the effective potential for the cluster.Efforts are made to estimate new physical parameter f in liquid state using the equation of state containing only two physical parameters such as the hard sphere diameter and binding energy.The temperature dependence of the structural properties and the thermodynamic behavior of the clusters are studied.Computations based on f predict the variation of numbers of particles at the contact point of the molecular cavity(radial distribution function).From the thermodynamic profile of the fluid,the model results are discussed in terms of the cavity due to the closed surface along with suitable energy.The present calculation is based upon the sample thermodynamic data for n-hexanol,such as the ultrasonic wave,density,volume expansion coefficient,and ratio of specific heat in the liquid state,and it is consistent with the thermodynamic relations containing physical parameters such as size and energy.Since the data is restricted to n-hexanol,we avoid giving the physical meaning of f,which is the key parameter studied in the present work.     

Phenomenological theory for the melting of a thin lubricant film between two atomically smooth solid surfaces

A thermodynamic model is developed for the melting of an ultrathin lubricant film squeezed between two atomically smooth solid surfaces. To describe the state of lubricant, an excess volume parameter is introduced; it appears due to the chaos in the structure of a solid body induced by melting. This parameter increases with the total internal energy upon melting. Thermodynamic melting and shear melting are described. The dependences of the friction force on the lubricant temperature and the shear rate of friction surfaces are analyzed. The calculated results are compared to the experimental data.     

Glass transition of the monodisperse Gaussian core model

We numerically investigate the dynamical properties of the one-component Gaussian core model in supercooled states. We find that nucleation is increasingly suppressed with increasing density. The system concomitantly exhibits glassy, slow dynamics characterized by the two-step stretched exponential relaxation of the density correlation and a drastic increase of the relaxation time. We also find a weaker violation of the Stokes-Einstein relation and a smaller non-Gaussian parameter than in typical model glass formers, implying weaker dynamic heterogeneities. Additionally, the agreement of the simulation data with the prediction of mode-coupling theory is exceptionally good, indicating that the nature of the slow dynamics of this ultrasoft particle fluid is mean-field-like. This fact may be understood as a consequence of the long-range nature of the interaction.     

Slow dynamics of supercooled hydration water in contact with lysozyme: examining the cage effect at different length scales

ABSTRACT

We study by computer simulation the dynamics of hydration water in solution with lysozyme upon approaching the glassy state of water. We calculate the self-density correlation function at different wavelengths to test the Mode Coupling Theory (MCT) of glassy dynamics at different length scales. The results show a strict and clear relation of the behaviour of the structural relaxation with the cage effect. We find a good agreement with the predictions of the MCT in the short and medium scale range, while at increasing length scales the interaction of water molecules with the protein's substrate induces deviations from the MCT behaviour, as found in previous studies. Besides at low temperatures the slow dynamics deviates from MCT due to hopping processes, similar to the bulk, as witnessed by a crossover from a fragile behaviour to a strong behaviour. We show that this deviation is evident at all length scales. Interestingly, we find that in the fragile region the confining cage decreases in radius with temperature while in the strong region it appears stable.