Glass Transition Regime Research Articles

Correlative activation free energy (CAFE) model: Application to viscous processes in inorganic oxide glass-formers

We explore the concept of correlative activation free energy (CAFE) for the analysis and interpretation of viscosity data of a collection of 847 inorganic oxide glass formers that exhibit various degrees of non-Arrhenius behavior. The CAFE model formalism is strictly based on transition state theory and accounts for the variation in the activation barrier height for the viscous dissipation due to the structural evolution the system undergoes upon traversing the glass transition regime. Thus, fitting parameters are meaningful in a statistical thermodynamic context. Compared to the VFT and MYEGA equations, fits using the CAFE model are more robust when extrapolating to infinite temperature because the latter encodes non-enthalpic contributions to the rate coefficient more realistically. The CAFE model-based analysis reveals a strong connection between melt fragility and the degree of change in the potential energy landscape with temperature. Accordingly, while the average ground-state potential energy of the glass forming liquid gradually increases with temperature, the energy of the activated state remains relatively invariant. Our analysis also allows one to estimate the number of atoms involved in the viscous relaxation process, which ranges from approximately 10 to 50 atoms for the oxide glass formers studied. We observe that thermally activated dynamic phenomena exhibited by glassy networks, such as the mixed alkali effect, persist into the supercooled liquid state.

Dynamic mechanical properties in natural rubber/styrene‐butadiene rubber blends: The local strain behavior in the glass transition region

AbstractA study of the local strains developed in vulcanized blends of natural rubber (NR) and styrene butadiene rubber (SBR), prepared by solution mixing, is presented. A semi‐EV system based on sulfur and accelerator was used for curing unfilled compounds at 160°C and 170°C. Samples were characterized by dynamic mechanical properties between −80°C and 20°C. Special interest was placed on the glass transition region of the vulcanized immiscible blends, where the NR and SBR phases are rubbery and glassy respectively. From the study of loss modulus, this research demonstrates how the local strain in the NR phase changes as a function of the amount of SBR in the blend. Assuming a coalescence behavior of the SBR glassy phase above a certain concentration, differences in the local stiffness of each phase in the glass transition regime were found. While in the glassy SBR phase the stiffness slightly varies with the NR/SBR content, a notorious increase was obtained in the rubbery NR phase at higher SBR content. The local strain on the NR phase of the blends shows a slight decrease for the samples vulcanized at 170°C compared with those vulcanized at 160°C.

Physics-Informed Data-Driven Models for Predicting Time- and Temperature-Dependent Viscoelastic Material Behaviors of Optical Glasses

In glass transition regime, glass exhibits a viscoelastic property. This is a time-dependent property inherited from a nonequilibrium and non-crystalline state of materials. The property, furthermore, varies significantly with temperature. Understanding the complex time- and temperature-dependent viscoelastic material behaviors of glass is highly essential in numerous glass processing such as hot forming of glass. In conventional studies, characterization of the viscoelastic responses is prerequisite, requiring intensive experiments conducted over the entire temperature range of the glass transition. Instead, this work introduces an alternative data-driven approach using machine learning, indicating that the time- and temperature-dependent viscoelasticity in the temperature range from glass transition to softening temperatures can be predicted. Different supervised machine learning algorithms were implemented to understand the complexity of the intrinsically non-linear relationships of the existing viscoelastic data, while the most robust and reliable machine learning model was eventually determined by comparing the prediction performance of the selected learning algorithms. Furthermore, the models were developed by employing physics-informed descriptors using the Mauro-Yue-Ellison-Gupta-Allan (MYEGA) viscosity model. We demonstrate that the physics-informed descriptors are beneficial to enhance the extrapolation capability of the predictive model for two extreme temperature ranges of glass viscosity e.g., at the softening temperature and in the vicinity of glass transition temperature, while the dimensionality of the model is reduced. The promising result of this study allows glass manufacturers to bypass the costly experimental characterization required for understanding the thermo-viscoelastic material behaviors in glass processing.

Universal features of annealing and aging in compaction of granular piles

This paper links the nonequilibrium glassy relaxation behavior of otherwise athermal granular materials to those of thermally activated glasses. Thus, it demonstrates a much wider universality among complex glassy materials out of equilibrium. Our three-dimensional molecular dynamics simulations, fully incorporating friction and inelastic collisions, are designed to reproduce experimental behavior of tapped granular piles. A simple theory based on a dynamics of records explains why the typical phenomenology of annealing and aging after a quench should extend to such granular matter, activated by taps, beyond the more familiar realm of polymers, colloids, and magnetic materials that all exhibit thermal fluctuations.

Hybrid mixture theory-based modeling of moisture transport coupled with quality changes in strawberries and carrots

Hybrid mixture theory-based multiscale fluid transport equations were solved for drying of strawberries and carrots to predict the moisture transport during the process. The fluid transport equation was coupled with the selected product quality and nutritional attributes to predict changes in stress distribution, shrinkage, color parameters, and ascorbic acid/β-carotene content. The fluid transport equation includes a non-Fickian term that incorporates the effect of glass transition on moisture transport. A transformation between Eulerian and Lagrangian coordinates was used to include the moving volume effects due to shrinkage. Good agreements between the experimental and the predicted values of moisture content (RMSE ≤ 0.095, RMSE ≤ 0.579), color parameters (R 2 ≥ 0.958, R 2 ≥ 0.934), and volume changes (R 2 ≥ 0.871, R 2 ≥ 0.865) were obtained for strawberries and carrots, respectively. Drying in the vicinity of glass transition regime led to sharper moisture profiles, indicating non-Fickian transport. However, drying at temperatures higher than glass transition temperature of strawberries and carrots resulted in Fickian characteristics-round and smooth drying profiles for drying at 70 °C. Drying at higher temperature caused higher loss of quality attributes; it resulted in samples with lower color parameters and ascorbic acid or β-carotene content. Higher drying temperature resulted in greater shrinkage and deformation throughout the structure, leading to cracks and openings and faster degradation in quality attributes. The developed model would allow improving quality attributes and finding optimum processing conditions of strawberries and carrots.

Local volume as a robust structural measure and its connection to icosahedral content in a model binary amorphous system

Molecular dynamics simulations are performed for a model binary glass to investigate the temperature dependence of the thermally driven localized structural excitations mediating glass relaxation. This is done under fixed zero pressure by increasing the temperature from zero to values close to and beyond the glass transition regime. It is found that for a given ramp rate, fixed pressure simulations admit a similar amount of structural relaxation as fixed volume simulations (Acta Materialia 143 (2018) 205–213), but at a reduced temperature scale corresponding to a reduced glass transition temperature. Furthermore, analysis of the volume evolution under fixed pressure, and stress evolution under fixed volume, demonstrates an equivalence between NPT and NVT ensemble molecular dynamics simulations in achieving a particular structural state. The observed relaxation induces a local densification which may be quantitatively connected to the creation of icosahedral structure that minimizes both bond energy frustration and maximizes local atomic packing. Together these results establish a robust link between the structural state of an amorphous solid and its free volume content, both of which now directly relate to the degree of relaxation.

Imparting optical functionality to aromatic thermosetting copolyester by luminescent silicon nanoparticles cross-linked via in situ thermal polymerization reaction

Aromatic thermosetting copolyester (ATSP) enables high- to low-k tunability as well as providing reliable thermomechanical performance from cryogenic to elevated temperatures, so it is a promising polymer system for silicon-based microelectronics and spacecraft applications. Here we reported on imparting strong photoluminescence to otherwise weak luminescent ATSP matrix by incorporating H-terminated Si nanoparticles (Sinps) to obtain luminescent polymer nanocomposites without degenerating the polymer backbone chain configuration. We employed photoluminescence and ultraviolet-visible (UV-Vis) spectroscopies along with electron microscopy analysis to characterize pre- and post-thermal polymerization processes. In the pre-polymerization stage, a size-dependent convection-like motion was observed in a liquid medium under thermal gradient, which caused the nanocomposites to accumulate and be trapped. Scanning electron microscope (SEM) images revealed the formation of 2-D nanosheets, as small as 50–100 nm, decorated with 10–15 nm size complexes of Sinps and oligomers. The post-polymerization analysis showed that the Sinps homogenously incorporated, with insignificant aggregation, into the polymerization process as a secondary cross-linker neither losing their red-luminescence functionality nor deteriorating the physical properties of the ATSP matrix. Chain relaxation characteristics, measured by dynamic mechanical analysis (DMA), in the glass transition regime likewise indicated the effective conjugation of the Sinps with the cross-linked polymer backbone. The ATSP–Sinps composite structure manifested bandgap imprints of both Sinps and ATSP in the ultraviolet–visible spectral region. Via superb thermomechanical properties enriched with luminescence, the ATSP–Sinps nanocomposite may potentially afford UV shielding to mitigate photo-degradation and enhance operational efficiency for photonics applications.

Dynamical heterogeneities and mechanical non-linearities: Modeling the onset of plasticity in polymer in the glass transition

In this paper we focus on the role of dynamical heterogeneities on the non-linear response of polymers in the glass transition domain. We start from a simple coarse-grained model that assumes a random distribution of the initial local relaxation times and that quantitatively describes the linear viscoelasticity of a polymer in the glass transition regime. We extend this model to non-linear mechanics assuming a local Eyring stress dependence of the relaxation times. Implementing the model in a finite element mechanics code, we derive the mechanical properties and the local mechanical fields at the beginning of the non-linear regime. The model predicts a narrowing of distribution of relaxation times and the storage of a part of the mechanical energy --internal stress-- transferred to the material during stretching in this temperature range. We show that the stress field is not spatially correlated under and after loading and follows a Gaussian distribution. In addition the strain field exhibits shear bands, but the strain distribution is narrow. Hence, most of the mechanical quantities can be calculated analytically, in a very good approximation, with the simple assumption that the strain rate is constant.

Thermally-activated stress relaxation in a model amorphous solid and the formation of a system-spanning shear event

Molecular dynamics simulations are performed to investigate the underlying thermally activated processes contributing to stress relaxation in a model binary amorphous system. Amorphous samples are first shear strained at zero kelvin and then the temperature is ramped up to the glass transition regime. Four orders of magnitude in the temperature ramp rate are considered. It is found that depending on the degree to which the amorphous solid is relaxed, significant atomic-scale scale activity is seen well below the glass transition regime, and that this is largely independent of the external load. The stress relaxation below the glass-transition temperature is mediated by spatially localized and thermally activated structural excitations. Over time, such thermally driven stochastic activity can collectively lead to a global gauge shearing, the location of which correlates with regions of low icosahedral content with lower coordination and higher density.

Thermal processing and enthalpy storage of a binary amorphous solid: A molecular dynamics study

Abstract

Role of Dynamical Heterogeneities on the Viscoelastic Spectrum of Polymers: A Stochastic Continuum Mechanics Model

Amorphous polymers in their glass transition regime can be described as a tiling of nanometric domains. Each domain exhibits its own relaxation time, which is distributed over at least four decades. These domains are known as dynamical heterogeneities. This article describes the mechanics of amorphous polymers using a stochastic continuum mechanics model that includes their heterogeneous dynamics. Solving this model both by finite elements and by using a self-consistent method, we find a viscoelastic relaxation spectrum quantitatively similar to an experimentally measured spectrum in a polymer. We show evidence that elastic couplings between domains control the stress relaxation after a step strain and result in a narrowing of the long-time region of the viscoelastic spectrum (as compared to that of dynamical heterogeneities).

Critical aspects of three-dimensional anisotropic spin-glass models

We study the $\pm J$ three-dimensional Ising model with a longitudinal anisotropic bond randomness on the simple cubic lattice. The random exchange interaction is applied only in the $z$ direction, whereas in the other two directions, $xy$ - planes, we consider ferromagnetic exchange. By implementing an effective parallel tempering scheme, we outline the phase diagram of the model and compare it to the corresponding isotropic one, as well as to a previously studied anisotropic (transverse) case. We present a detailed finite-size scaling analysis of the ferromagnetic - paramagnetic and spin glass - paramagnetic transition lines, and we also discuss the ferromagnetic - spin glass transition regime. We conclude that the present model shares the same universality classes with the isotropic model, but at the symmetric point has a considerably higher transition temperature from the spin-glass state to the paramagnetic phase. Our data for the ferromagnetic - spin glass transition line are supporting a forward behavior in contrast to the reentrant behavior of the isotropic model.

Breakdown of time-temperature superposition in a bead-spring polymer melt near the glass transition temperature.

The breakdown of the time−temperature superposition (TTS) near its glass transition temperature (T(g)) in simple bead−spring polymer melts with and without the chain angle potential was numerically investigated. The stress relaxation modulus at different temperatures G(t,T) was calculated by the Green−Kubo relation. The TTS of G (t,T) of bead−spring polymer melts worked well at temperatures sufficiently higher than its T(g). However, when the system temperature is approaching the glass transition regime, the breakdown of TTS is observed. At temperatures near the Tg,the temperature dependence of the shift factor (a(TB)), which is defined on the time scale between the bond relaxation and the chain relaxation regimes of a G(t) function, is significantly stronger than ones (a(TA)) defined by the time scale of the chain relaxation modes. The analysis of the van Hove function G(s)(r,t) and non-Gaussian parameter, α(2)(t), of the bead motions strongly suggests that the TTS breakdown is concerned with the dynamic heterogeneity. The effect of the chain stiffness on the temperature dependence of the shift factors was also investigated in this study. The stiffer chains melt has a stronger temperature dependence of the shift factors than the ones of the flexible chains melt. However, regardless of the chain stiffness, the stress relaxation modulus functions of the bead−spring polymer melts will begin to break down the TTS at a similar T(g)-normalized temperature (T/T(g)).

Molecular dynamics study of one-component soft-core system: thermodynamic properties in the supercooled liquid and glassy states.

Molecular dynamics simulations were performed to study the thermal properties of a supercooled liquid near the glass transition regime and of glasses in a one-component soft-core system with the pair potential φn(r) = ɛ(σ/r)(n), in which n = 12. The results are examined along a phase diagram, in which the compressibility factor defined by [Formula: see text] is plotted against the reduced density ρ* = ρ(ɛ/kBT)(3/n) (or the reduced temperature T* = ρ*(-n/3)). Similarly, a time-dependent dynamical compressibility factor can be plotted against the time-dependent reduced density [Formula: see text] (or the reduced time-dependent temperature). Analytical expressions of the specific heats CV and CP and of the entropy, S, were obtained as a function of [Formula: see text] or of the scaled potential U*. Even for a rapid cooling process, the CV values are found to be affected by non-equilibrium relaxations in the [Formula: see text] region, where [Formula: see text] is the given initial value of [Formula: see text]. The problem of the Kauzmann paradox is discussed using these expressions. The fluctuation of the time-dependent temperature, Tt*, which determines CV, is characterized by the spectra that are obtained by multitaper methods. The thermal fluctuation along the non-equilibrium relaxation under NVE conditions was also examined.

Nonconventional Spin Glass Transition in a Chemically Ordered Pyrochlore

We report on the study of unusual spin glass properties in the geometrically frustrated pyrochlore Tb(2)Mo(2)O(7), T(g) ~/= 24 K. The analysis of the nonlinear part of dc and complex susceptibilities, near the glass transition regime, suggests the existence of a statistical distribution of relaxation times in short-range ordered ferromagnetic clusters. In addition, the magnetic spins are not sufficiently frozen below the glass transition temperature, which is apparently responsible for the nonequilibrium scaling behavior of the static critical exponents of nonlinear susceptibilities. Our report is expected to shed new light in understanding the freezing properties of frustrated pyrochlores with short-range ferromagnetic interactions.

Dynamic measurement of stress optical behavior of three amorphous polymers

In the present study, rheo-optical and mechanical properties of three amorphous polymers, i.e., PS (polystyrene), PC(polycarbonate) and COC(cyclo olefin copolymer), widely used for optical products have been investigated. Accurate measurement of stress optical coefficients and elastic modulus data across the glass transition region are essential for predicting optical anisotropy in many injection molded optical products like pickup lenses and waveguide in LCD module since the final products have both flow and thermal history from the melt to glass. To obtain stress optical behavior in wide range of frequency and temperature including rubbery, glassy and glass transition regime, frequency sweep tests with extensional bar and shear sandwich tools were undertaken. As a result, glassy and melt extreme values of stress optical coefficient of PS and PC were evaluated as well as master plots in wide frequency region. The sign change of stress optical coefficient was shown clearly for PS as the frequency increased. On the other hand, the sign of stress optical coefficient over the whole frequency region is always positive for PC. For COC’s of different composition, even though the glass transition temperature can vary, the stress optical coefficient of COC’s with different composition stays almost constant at two extremes.

A comparative study of epoxy resin cured with a linear diamine and a branched polyamine

The molecular topology of amine curing agents is expected to greatly affect the curing reaction and properties of the epoxy resins, yet the exact influence still remains little understood. Herein we find out two representative aliphatic amines: linear propanediamine (PDA) and branched N,N,N′,N′-tetra(3-aminopropyl)-1,3-propanediamine (TAPA), and use them to cure diglycidyl ether of bisphenol A (DGEBA). The curing reaction, dynamic mechanical properties, and thermal stability of DGEBA/PDA and DGEBA/TAPA are systematically investigated and compared. Differential scanning calorimetry (DSC) confirms TAPA and PDA have very close reactivity much higher than that of commercial Jeffamine T-403 with the branched molecular structure. The curing kinetic analysis shows TAPA causes the higher isothermal conversion at the lower temperature (e.g., 40°C), the autocatalysis and diffusion-associated kinetics feature the isothermal reactions, and the extended Kamal model turns out to be able to well predict the curing rate. Then, the isoconversional analysis with the Vyazovkin methods demonstrates compared to PDA, TAPA leads to the lower effective activation energy at the very beginning due to its catalytic tertiary amino groups, but the reversed trend emerges in the deep-conversion stage, especially, in the glass-transition regime, owing to its flexible aliphatic molecular chains. Furthermore, the isothermal conversion at the higher temperatures is predicted from the nonisothermal experiments. Finally, dynamic mechanical analysis (DMA) shows cured DGEBA/TAPA exhibits the higher glass- and β-relaxation temperatures and crosslink density than DGEBA/PDA, and thermogravimetric analysis (TGA) reveals TAPA-cured epoxy has the excellent thermal stability with the higher char yield.

Experimental study on the effect of glass transition on moisture profiles and stress-crack formation during continuous and time-varying drying of maize kernels

The concept of glass transition can help understand the drying process and provide insight into the stress-cracking behaviour of maize (Zea mays) kernels. Thin-layer drying experiments using continuous and time-varying temperature and humidity profiles were conducted and the stress crack index was evaluated. Overlaying glass-transition regimes over the continuous drying profiles demonstrated a broader transition region at lower temperatures and a narrower transition region at higher temperatures. The time taken by maize kernels to pass through the glass-transition regime was directly related to the stress-crack formation. High temperature drying at 90 °C decreased the drying time but showed a higher cracking index. Low temperature drying at 30 °C had the longest drying time but this caused the least amount of cracking in the samples. Time-varying temperature profiles were also tested using a combination of temperatures in the glass-transition zone. Introduction of a high humidity step along with high temperature in the time-varying drying profile offered advantages in terms of drying time and reduced cracking in comparison to continuous single temperature drying at 70 °C. To extend the findings of this study, future studies should be conducted using additional drying conditions and computer simulations.

Constitutive modeling of the rate, temperature, and hydration dependent deformation response of Nafion to monotonic and cyclic loading

The elastic–plastic behavior of the polymer electrolyte membrane (PEM) Nafion is characterized via monotonic and cyclic uniaxial tension testing as a function of strain rate, temperature, and hydration. Dynamic mechanical analysis shows that, under dry (30%RH) conditions, the material begins to transition from the glassy to the rubbery state at 75 ° C, with a glass transition of 105 ° C. DMA reveals the fully hydrated state to be significantly more compliant than the dry state, with a glass transition beginning at 40 ° C. Large strain monotonic tensile tests find the rate-dependent stress–strain behavior to be highly dependent on temperature and hydration. The dry state transitions from an elastic–plastic behavior at 25 ° C to an increasingly more compliant behavior and lower yield stress as temperature is increased through the glass transition, until exhibiting a rubbery-like behavior at 100 ° C. At 25 ° C, the stress–strain behavior remains elastic-plastic for all hydrated states with the stiffness and yield stress decreasing with increasing hydration. Increasing hydration at all temperatures acts to decrease the initial elastic stiffness and yield stress. Unloading from different strains reveals the elastic-plastic nature of the behavior even for the elevated temperature and hydrated states. Cyclic loading-unloading-reloading excursions to different strains show significant nonlinear recovery at all strains past yield with a highly nonlinear reloading behavior which rejoins the initial loading path. A micromechanically motivated constitutive model consisting of an intermolecular resistance in parallel with an elastic network resistance is shown to be capable of capturing the rate, temperature, and hydration dependence of the monotonic stress-strain behavior. The intermolecular resistance captures the local intermolecular barriers to initial elastic deformation and also captures the thermally activated nature of yield; these intermolecular barriers are modeled to decrease with increasing temperature and hydration, in particular mimicking the reduction in these barriers as the material approaches and enters the glass transition regime, successfully capturing the strong temperature and hydration dependence of the stress-strain behavior. The highly nonlinear post-yield unloading and reloading suggest the development of a back stress during inelastic deformation which aids reverse plastic flow during unloading. Inclusion of a back stress which saturates after reaching a critical level provides an ability to capture the highly nonlinear cyclic loading stress response. Hence, the proposed model provides the capability to describe the complex evolution of stress and strain that occurs in PEM membranes due to the constrained hygrothermal cyclic swelling/deswelling characteristic of membranes in operating fuel cells.

Molecular Dynamics Study of Thermodynamic Scaling of the Glass-Transition Dynamics in Ionic Liquids over Wide Temperature and Pressure Ranges

Experimentally, superpositioning of dynamic properties such as viscosity, relaxation times, or diffusion coefficients under different conditions of temperature T, pressure P, and volume V by the scaling variable TV(gamma) (where gamma is a material constant) has been reported as a general feature of many kinds of glass-forming materials. In the present work, molecular dynamics (MD) simulations have been performed to study the scaling of dynamics near the glass-transition regime of ionic liquids. Scaling in the simulated 1-ethyl-3-methylimidazolium nitrate (EMIM-NO(3)) system has been tested over wide ranges of temperatures and pressures. TV(gamma) scaling of the dynamics is well described by master curves with gamma = 4.0 +/- 0.2 and 3.8 +/- 0.2 for cation and anion, respectively. Structures and Coulombic terms of the corresponding states are found to be quite similar. The temperature and pressure dependence of the pair correlation function show similar trends and therefore can be superpositioned onto the master curve. Although the behaviors with gamma = 4 might be expected from the relation, gamma = n/3, for the dynamics with the soft-core-type potential U = epsilon(sigma/r)(n), with n = 12, pair potentials used in the MD simulation have a more complex form, and not all the repulsive terms can play their roles in the heterogeneous structures determined by ion-ion interactions. Scaling is related to the common part of effective potentials related to the pair correlation functions, including the many-body effect in real space.