3,117 research outputs found
The role of configurational entropy in chemical vitrification
No full textGlasses can be formed in numerous ways, involving very different microscopic processes. This article reviews some recent results on chemical vitrification, a process where the slowdown in the dynamics of a liquid is controlled by the irreversible formation of chemical bonds. Making a connection between the reduction in configurational entropy and the number of chemical bonds, the dynamics of vitrification in chemically reactive systems is explained in terms of their configurational restrictions in the same manner as in stable glass-forming liquids under cooling or compression
A simple analysis of Brillouin spectra from opaque liquids and its application to aqueous suspensions of poly-N-isopropylacrylamide microgel particles
No full textBrillouin spectroscopy is a powerful technique to probe the viscoelastic properties of materials. However, the phenomenon of multiple scattering makes getting information from opaque liquids quite difficult, thus limiting the use of this spectroscopy. In this paper we present a new method that greatly simplifies the problem of analyzing Brillouin spectra affected by multiple scattering from samples of moderate opacity. Our approach is based on the observation that multiple-scattered contributions broaden the spectrum acquired in external backscattering geometry, while preserving in the external side the information related to internally backscattered light. The new strategy avoids unnecessary approximations and requires minimum numerical effort to extract physical information. Here, we show the results of two Brillouin light scattering experiments performed on prototypical hard and soft colloidal systems. First, measurements on latex suspensions as a function of depth are used to validate the method and to derive new relations between the back-scattered and multiple-scattered components of the Brillouin spectrum. Second, measurements on poly-N-isopropylacrylamide (PNIPAM) microgels in water as a function of temperature are used as a testing ground to demonstrate the method's capabilities. Our analysis confirms that sound waves are extremely sensitive to the volume-phase transition of thermoresponsive particles. The presented approach, however, shows that a marked increase of attenuation is accompanied by only a moderate decrease of sound velocity. The study revises the viscoelastic properties of PNIPAM suspensions; more generally, it provides a new guideline in the characterization of moderately opaque media and fosters new theoretical investigations
Configurational and excess entropy in a fragile glass former and their relation with the structural relaxation
No full textA method to derive experimentally the configurational entropy that governs the structural relaxation process in supercooled (supercompressed) liquids is presented. Light scattering, dielectric, calorimetric and dilatometric measurements are used to test the method in o-terphenyl. The combined analysis of relaxation data as a function of temperature and pressure is found to be essential to quantify a realistic value of the non-structural thermal expansion and the configurational entropy of the system. The latter is estimated as a fraction of the excess entropy of the liquid over its stable crystalline phase. Our results indicate that the number of configurations connected to the structural relaxation contributes approximately 70% to the excess entropy
Comment on “Decrease in the configurational entropy during a melt's polymerization” [Chem. Phys. 305 (2004) 231]
No full textExploiting limited valence patchy particles to understand autocatalytic kinetics
No full textAutocatalysis, i.e., the speeding up of a reaction through the very same molecule which is produced, is common in chemistry, biophysics, and material science. Rate-equation-based approaches are often used to model the time dependence of products, but the key physical mechanisms behind the reaction cannot be properly recognized. Here, we develop a patchy particle model inspired by a bicomponent reactive mixture and endowed with adjustable autocatalytic ability. Such a coarse-grained model captures all general features of an autocatalytic aggregation process that takes place under controlled and realistic conditions, including crowded environments. Simulation reveals that a full understanding of the kinetics involves an unexpected effect that eludes the chemistry of the reaction, and which is crucially related to the presence of an activation barrier. The resulting analytical description can be exported to real systems, as confirmed by experimental data on epoxy-amine polymerizations, solving a long-standing issue in their mechanistic description
Configurational entropy and dynamics in chemical vitrification
No full textIt is common practice to form a glass starting from a liquid in a metastable state, by cooling or compressing the system so as to avoid crystallization ('physical vitrification'). However, there exist in nature and technology different ways to form a glass. The hardening of natural and synthetic resins and the formation of most of the materials used in engineering plastics and high-performance composites are based on 'chemical vitrification', a process involving progressive polymerization of initially liquid monomers via the formation of irreversible chemical bonds. Explaining the similarity observed in the slowing down of the dynamics in physical and chemical vitrification constitutes a challenge to general understanding of the glass transition and may disclose its universal nature. Here we use relaxation data from several techniques to show that the similarity between the dynamic behaviours of physical and chemical glass formers originates in a similar evolution of their configurational restrictions and in a similar dynamics-to-thermodynamics correlation. In particular, we derive a relation between relaxation properties and extent of reaction in step polymerization, in remarkable agreement with experimental results
Chemical and physical aggregation of small-functionality particles
No full textThe number of potentially relevant materials resulting from the aggregation of elementary units with a
finite functionality continues to increase. The growth of branched clusters and networks may proceed
through the formation of reversible (physical) or irreversible (chemical) bonds. The kinetics of bond
formation is sensitive both to the intrinsic rate of the bonding process, controlled by the chemistry of
the system, and to the encounter rate of clusters, controlled by cluster diffusion. In this Highlight we
review a series of our recent numerical simulation studies designed to investigate the connections
between chemical and physical aggregation and the crossover from a chemically controlled to
diffusion-controlled regime. It is shown that in the chemically controlled limit, it is possible to formally
correlate elapsed time during irreversible aggregation with equilibrium temperature in reversible
aggregation. The diffusion-controlled regime sets in well-beyond percolation and the effect of
diffusion can be described by introducing a single additional time scale, related to the average diffusion.
time. This concept can be readily generalized to interpret the experimental data
Correlation between structural relaxation and distribution of particle clusters in glass-forming epoxy-amine mixtures undergoing step polymerization
No full textThe evolution of the structural relaxation during the step polymerization process of four different formulations of an epoxy-amine mixture-diglycidyl ether of Bisphenol A with diethylenetriaminehas been studied by means of broadband dielectric spectroscopy. Step polymerization progressively turns the liquid into a glass and offers an efficient means to study the effect of the formation of clusters of bonded particles on the structural dynamics. Specifically, we investigate how changes in the distribution of particle clusters reflect in the slowdown and broadening of the relaxation process. We find that the average cluster size diverges as the system freezes at the glass transition and relates to the structural relaxation time in a manner formally similar to that predicted for the size of the cooperatively rearranging regions within the Adam-Gibbs model for glass-forming liquids. This result confirms the one previously obtained by photon correlation spectroscopy on the same systems and indicates it is independent of the experimental technique. Moreover, we observe that the low-frequency broadening of the relaxation function on approaching the glass transition is connected to the increasing polydispersity of the system. We quantify this polydispersity by the variance sigma of the cluster size distribution or by the steepness a of the distribution tail, and we find, over a wide range of these parameters, that the low-frequency power-law exponent m of the relaxation function is linear vs log a and log sigma
Clustering and Cooperative Dynamics in a Reactive System
No full textWe study the dependence of the dynamics on the size of particle clusters that grow by stepwise aggregation in a reactive epoxy-amine mixture. The data reveal the cluster property involved in the glasslike arrest and its quantitative link with the structural relaxation time. We find that the number-average cluster size x(n) governs the formation of a glassy phase as distinct from a gel phase, and that x(n) correlates to the size of the "cooperatively rearranging regions" postulated by the Adam-Gibbs model for glass forming liquids. These results suggest that the step polymerization process generates clusters that behave much like dynamical heterogeneities observed in supercooled liquids
Physical and chemical vitrification: the role of configurational entropy
No full textGlasses can be formed in numerous ways, involving very different microscopic processes. Here we report a dielectric and photon-correlation study of chemical vitrification, a process where the slowdown in the dynamics of a liquid system is controlled by the irreversible formation of chemical bonds. We find that vitrification dynamics in chemically reactive systems can be explained in terms of their configurational restrictions in the same manner as in stable glass-forming liquids under cooling or compression
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