1,721,022 research outputs found
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
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
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
Brillouin light scattering from shear waves in an epoxy resin through the glass transition
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Non-ergodicity in a locally ordered fragile glass former
No full textWe measure the dynamic structure factor of m-toluidine through inelastic X-ray scattering in the mesoscopic Q range between 1 and 10 nm(-1), where the static structure factor exhibits a prepeak resulting from a molecular spatial organization in nanometer size clusters, due to the formation of hydrogen bonds. We present experimental evidence of the square-root cusp in the temperature behavior of the non-ergodicity factor of in-toluidine which supports the view that the mode-coupling theory succeeds in describing the dynamics of supercooled molecular liquids, even for liquids with a local order, notwithstanding the cage formation is controlled by a mechanism different from the packing constraints typical of simple liquids. Moreover, we report on the behavior of the non-ergodicity factor in the low-temperature and wavevector regimes, finding a confirmation of the phenomenologic law proposed by Scopigno et al. [T. Scopigno, G. Ruocco, F. Sette; G. Monaco, Science 302 (2003) 849] which correlates the vibrational dynamics of the glass to the fragility of the supercooled liquid
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