Search results for "THERMODYNAMICS"
showing 10 items of 2774 documents
Dielectric properties of a filled epoxy resin: Effect of thermal treatments
1982
Dielectric measurements have been performed at several frequencies on samples of a cycloaliphatic epoxy resin filled with aluminum hydroxide. The samples were thermally treated at three different temperatures for times up to 2,000 hours. At low aging temperatures and times an improvement of the characteristics is observed due perhaps to an assessment of the bulk of the polymer and more precisely to a change in the free volume. At high temperatures and times a thermo-oxidation involving layers deeper and deeper becomes the predominant mechanism responsible of the decrease in the properties with respect to the virgin material.
Monte Carlo simulation of the glass transition in polymer melts: An application of MCT
1995
Abstract This paper reviews the results of a large scale Monte Carlo simulation for the dynamics of a supercooled polymer melt. The dynamics of the melt was studied by means of the time evolution of the incoherent intermediate scattering function φs q(t), which was monitored over seven decades in time. In an intermediate time window it is possible to describe the decay of φs q(t) quantitatively in the framework of mode-coupling theory, provided the extended version of the theory is used.
Pressure-induced formation of diblock copolymer "micelles" in supercritical fluids. A combined study by small angle scattering experiments and mean-f…
2004
We developed a simple time-dependent mean-field theory to describe the phase separation kinetics of either homopolymers or AB-diblock copolymers in supercritical (SC) fluids. The model, previously used to describe the phase behavior of AB-block copolymers under the assumption of strong solvent selectivity for just one copolymer chain, has been extended to study the kinetics of the phase separation process. Time resolved small angle x-ray scattering (TR-SAXS) measurements have been performed on different AB-diblock copolymers containing a perfluorinated chain and dissolved in SC-CO2. The data obtained over a wide range of pressure and temperature confirm our theoretical predictions. Particul…
Low-temperature specific heat of orientational glasses
1992
This review summarizes specific heat data measured at low temperatures (T<1 K) on orientational glasses. Three species of mixed molecular crystals exhibiting orientational disorder are considered, namely (KBr)1−x (KCN) x , (NaCN)1−x (KCN) x (Rb)1−x (NH4) x H2PO4. For intermediate concentrations of the anisotropic components, glass-like excitations have been observed. It is demonstrated that with respect to thermal properties, orientational disorder leads to the same “universal” behaviours than for structural disorder, i.e. a specific heat which varies below 1 K and for times 10−4 s–10 s as:C p(T,t)∞T 1×ln(t). The variation of the glass-like anomaly with compositional disorder is also discus…
Correlations of the nonexponentiality and state dependence of mechanical relaxations with bond connectivity in Ge-As-Se supercooled liquids
1992
We have studied the mechanical responses of supercooled Ge-As-Se liquids to flexural strains and temperature steps. The departures from exponential relaxation correlate well with the variations in connectivity. The structural state dependence of the mechanical relaxation, detected in pure and weakly cross-linked Se, is suppressed completely at the rigidity percolation threshold {l angle}{ital r}{sub {ital c}}{r angle}, where the liquid fragility is a minimum. The shapes of the decay functions of samples with the same {l angle}{ital r}{sub {ital c}}{r angle} but different compositions are not universal at {ital T}{sub {ital g}} probably because of chemical effects near the binary edges of th…
Coil-Globule Collapse of Polystyrene Chains in Tetrahydrofuran-Water Mixtures.
2018
We study the coil and globule states of a single polymer chain in solution by performing molecular dynamics simulations with a united atom model. Specifically, we characterize the structural properties of atactic polystyrene chains with N = 20–150 monomers in tetrahydrofuran–water mixtures at varying mixing ratios. We find that the hydrophobic polymers form rather open coils when the mole fraction of water, XW, is roughly below 0.25, whereas the chains collapse into globules when XW ≳ 0.75. We confirm the theoretically expected scaling laws for the radius of gyration, Rg, in these regimes, i.e., Rg ∝ N3/5 and Rg ∝ N1/3 for good and poor solvent conditions, respectively. For poor solvent con…
Solidification behavior of the theta system 2-propanol/poly(n-butyl methacrylate)
1995
Thermoreversible gelation of the system 2-propanol/poly (n-butyl methacrylate) — as detected by D'SC or dielectric experiments — does not manifest itself in a straightforward manner in the dynamic-mechanical properties. Its occurrence can, however, be seen in many ways: i) For constant composition of the system and a reference temperature lower than Tgel, the storage modulus G′ is larger than the loss modulus G″ in the glass transition zone of the master curve and both vary in an almost parallel manner with the angular frequency ω over almost two decades (whereas this feature is normally found for other gelling systems within the rubber plateau or the flow region). ii) The entanglement mole…
Unusual crystallization kinetics in a hard sphere colloid-polymer mixture.
2008
We investigated the crystallization kinetics of a hard sphere colloid-polymer mixture at conditions where about 95% of solid coexists with about 5% of fluid. From time resolved Bragg and small angle light scattering, we find that the crystallite size increases with a power law of exponent alpha approximately 1/3 during both the conversion and the coarsening stage. This observation points to a single conserved order parameter for both stages and cannot be explained if the mixture is regarded as an effective one-component system. We alternatively suggest that--based on local geometric demixing--the polymer density takes the role of the conserved order parameter.
Escape transition of a compressed polymer mushroom under good solvent conditions
1999
The escape transition of a flexible polymer chain of chain length N, endgrafted at a hard wall and compressed by a piston of radius R, is studied by Monte Carlo simulation and by phenomenological arguments. In contrast to previous theories which have considered the transition as a function of a (fixed) height H of the piston above the wall, we consider the transition as a function of the conjugate variable, the force f acting on the piston. We find that the transition (which is sharp only for N → ∞) is characterized by a flat region of f in the f vs. H isotherm, i.e. a jump in the height occurs at the transition from Hesc,t to Himp,t, with (Himp,t − Hesc,t)/Hesc,t ≈ 0.26.
Mechanical And Thermophysical Properties Of Polymers And Chemical Structure: From Computer Simulations To Experimental Data
1988
Chemical structure is related to properties on the basis of results of computer modelling of mechanical behavior. An equation for the shift factor aT,c dependent on temperature T and concentration c is provided, connection to the chain relaxation capability noted, and potential application range of the approach discussed. Constituents of free volume are defined, and effects of individual constituents on mechanical properties.