arXiv subjects

Riccardo Casalini

Publications and source records attributed to Riccardo Casalini.

8 recordsLinked to original sources

arXiv2026

Modeling the Slow Arrhenius Process (SAP) in Polymers

Amorphous glass-forming polymers exhibit multiple relaxation processes, including the structural α-relaxation associated with the glass transition and faster secondary relaxations that typically follow Arrhenius behavior. Recently, a distinct slow Arrhenius process (SAP) has been observed at frequencies well below the α-process. Although Arrhenian in its temperature dependence, the SAP involves much longer relaxation times and its microscopic origin remains unclear. Here, we extend the two-state, two-timescale (TS2) theory to describe both the α-relaxation and the SAP within a unified framework. We propose that the SAP represents the high-temperature limit of an α-like process in a coarse-grained fluid of dynamically correlated clusters. With renormalized interaction energies and coordination parameters, the same model quantitatively reproduces both α and SAP data across multiple polymers without additional adjustable parameters and explains the observed Meyer-Neldel compensation behavior. The theory further predicts that the SAP should deviate from Arrhenius behavior at sufficiently low temperatures, transitioning to Vogel-Fulcher-Tammann-Hesse-like dynamics, thereby offering a physically transparent interpretation of cluster-scale relaxation in glass-forming polymers.

cond-mat.soft↗

arXiv2025

General Two-Parameter Model of Alpha-Relaxation in Glasses

In the vicinity of the glass transition, the characteristic relaxation time (e.g., the alpha-relaxation time in dielectric spectroscopy) of a glass-former exhibits a strongly super-Arrhenius temperature dependence, as compared to the classical Arrhenius behavior at high temperatures. A comprehensive description of both regions thus requires five parameters. Here, we demonstrate that many glass-formers exhibit a universal scaling, with only two material-specific parameters setting the timescale and the temperature scale; the other three being universal constants. Furthermore, we show that the master curve can be described by the recently developed two-state, two-(time) scale (TS2) theory (Soft Matter 2020, 16, 810) and regress the universal TS2 parameters. We also show the connection between the TS2 model and the Hall-Wolynes elastic relaxation theory.

cond-mat.dis-nn↗

arXiv2025

Universality of Dynamic and Thermodynamic Behavior of Polymers near their Glass Transition

Describing the dynamics and thermodynamics of amorphous materials near the glass transition is a major challenge in soft-matter physics and polymer engineering. Here, we show that the dependence of the dielectric alpha-relaxation time on temperature can be captured by a universal equation with only two independent parameters, Tg and fragility, m. This is similar in spirit to the ideas of van Krevelen and Bicerano, and can be related to the Boyer-Spencer and Simha-Boyer rules for the volumetric thermal expansion via a modified free-volume approach such as Sanchez-Lacombe "Two-state, two-(time)scale" (SL-TS2) theory. The model is compared to experimental data for nine amorphous polymers with varying values of Tg and m, and a good qualitative and quantitative agreement is found. We also derive the relationship between the experimental and computational (high-cooling-rate) Tg, and compare our model prediction for the Tg-shift between the two with the simulation results of Afzal et al., finding a good qualitative and semi-quantitative agreement. The results could serve as a guidance for future simulation studies.

cond-mat.soft↗

arXiv2025

New Insights into the Dependence of Glass Transition Temperature and Dynamic Fragility on Molecular Weight in Oligomers and Polymers

In an earlier preprint (V. V. Ginzburg, O. V. Gendelman, R. Casalini, and A. Zaccone, arxiv:2409.17291), we demonstrated that the dynamic (relaxation time) and volume equations of state for many amorphous polymers are near-universal -- each material is characterized by two governing parameters -- the glass transition temperature, T_{g}, and the elementary relaxation time, /tau_{el}. The dynamic fragility, m, is uniquely related to /tau_{el}. The earlier analysis was based on the data for high polymers (degree of polymerization, N > 200, and molecular weight, Mw > 20,000 g/mol). Here, we investigate the dependence of T_{g} and /tau_{el} on N (or, equivalently, on the molecular weight, M). We consider the dielectric spectroscopy data for homologous series of four polymers: polystyrene (PS), poly-(2-vinylpyridine) (P2VP), poly(dimethylsiloxane) (PDMS), and polybutadiene (PB). It is shown that the relaxation time curves for various oligomers can be successfully collapsed onto the same master curve as the high polymers. The glass transition temperature and the dynamic fragility are found to be linear functions of 1/M, in agreement with the Fox-Flory equation. The scaling results are interpreted based on the SL-TS2 theory, ensuring that the model description is consistent with the Boyer rules for the coefficients of thermal expansion above and below T_{g}, as well as the thermodynamical scaling for the pressure dependence of the relaxation time.

cond-mat.soft↗

arXiv2024

Insight into properties of sizable glass former from volumetric measurements

Sizable glass formers feature numerous unique properties and potential applications, but many questions regarding their glass transition dynamics have not been resolved yet. Here we analyzed structural relaxation times measured as a function of temperature and pressure in combination with the equation of state obtained from pressure-volume-temperature (PVT) measurements. Despite evidence from previous dielectric studies indicating a remarkable sensitivity of supercooled dynamics to compression, and contrary to intuition, our results demonstrated the temperate proof for the almost equivalent importance of thermal energy and free volume fluctuations in controlling reorientation dynamics of sizable molecules. The found scaling exponent equals 3.0 and Ev/Ep ratio of 0.6 were typical for glass-forming materials with relaxation dynamics determined by both effects with a minor advantage of thermal fluctuations involvement. It shows that the high values of key parameters characterizing the sensitivity of the glass transition dynamics to pressure changes, i.e. activation volume and dTg/dP, are not a valid premise for a remarkable contribution of volume on glass transition dynamics.

cond-mat.soft↗

arXiv2022

Combined Description of Pressure-Volume-Temperature and Dielectric Relaxation of Several Polymeric and Low-Molecular-Weight Organic Glass-Formers using 'SL-TS2' Mean-Field Approach

We apply our recently-developed mean-field 'SL-TS2' (two-state Sanchez-Lacombe) model to simultaneously describe dielectric alpha-relaxation time and pressure-volume-temperature (PVT) data in four polymers (polystyrene, poly(methylmethacrylate), poly(vinyl acetate) and poly(cyclohexane methyl acrylate)) and four organic molecular glass formers (ortho-terphenyl, glycerol, PCB-62, and PDE). Previously, it has been shown that for all eight materials, the Casalini-Roland thermodynamical scaling, /tau_{/alpha} = f(TV_{sp}^{/gamma}) (where T is temperature and V_{sp} is specific volume), (Casalini, R.; Roland, C. M. Phys. Rev. Lett. 2014, 113 (8), 85701), is satisfied. It has also been previously shown that the same scaling emerges naturally (for sufficiently low pressures) within the 'SL-TS2' framework (Ginzburg, V. V. Soft Matter 2021, 17, 9094). Here, we fit the ambient pressure curves for the relaxation time and the specific volume as functions of temperature for the eight materials and observe a good agreement between theory and experiment. We then use the Casalini-Roland scaling to convert those results into 'master curves', thus enabling predictions of relaxation times and specific volumes at elevated pressures. The proposed approach can be used to describe other glass-forming materials, both low-molecular-weight and polymeric.

cond-mat.soft↗

arXiv2019

On the pressure dependence of the thermodynamical scaling exponent gamma

Since its initial discovery more than fifteen years ago, the thermodynamical scaling of the dynamics of supercooled liquids has been used to provide many new important insights in the physics of liquids, particularly on the link between dynamics and intermolecular potential. A question that has long been discussed is whether the scaling exponent gamma is a constant or it depends on pressure. Here we offer a simple method to determine the pressure dependence of gamma using only the pressure dependence of the glass transition and the equation of state. Using this new method we find that for the six non-associated liquids investigated gamma always decreases with increasing pressure. Importantly in all cases the value of gammaremains always larger than 4. Liquids having gamma closer to 4 at low pressure show a smaller change in gamma with pressure. We argue that this result has very important consequences for the experimental determination of the functional form of the repulsive part of the potential in liquids.

cond-mat.soft↗

arXiv2019

On the experimental determination of the repulsive component of the potential from high pressure measurements: what is special about twelve?

In this paper we present an overview of results in the literature regarding the thermodynamical scaling of the dynamics of liquids and polymers as measured from high-pressure measurements. Specifically, we look at the scaling exponent gamma, and argue that it exhibits the limiting behavior gamma=4 in regimes for which molecular interactions are dominated by the repulsive part of the intermolecular potential. For repulsive potentials of the form U(r)= r^n, gamma has been found to be related to the exponent n via the relation gamma = n/3. Therefore, this limiting behavior for gamma would suggest that a large number of molecular systems may be described by a common repulsive potential U(r)~ r^n with n~12.

cond-mat.soft↗