Search results for "Glass transition"

On the equation of state for thermal polymer solutions and melts with attractive interaction

1996

We perform Monte Carlo simulations of a lattice model for polymer melts, i. e., the bond fluctuation model in three dimensions. By using an energy parameter that prefers relatively long bonds, the model exhibits a glass transition at low temperatures, in close qualitative similarity to experiment. We modify this model by adding an attractive interaction of variable strength. We demonstrate that a small interaction strength has only a very small effect on the static properties of the melt. For a fixed strength of the potential, the chemical potential is measured by a modified particle-insertion method over a large range of temperatures and densities. The osmotic pressure is obtained by therm…

Synthesis and structural characterization of ethylene copolymers containing double-decker silsesquioxane as pendant groups and cross-linkage sites by…

2018

Abstract The copolymers of ethylene with the double-decker silsesquioxane (DDSQ) were synthesized by copolymerization with the use of metallocene and bis(phenoxy-imine) catalysts. The influence of the kind of the catalyst and polyreaction conditions on the performance of copolymerization as well as on the properties of the copolymers was studied. Depending on polyreaction parameters, the DDSQ contents in the copolymer varied in the range of 0.93–11.53 wt% which determined the compositions and the structural properties of copolymers. DDSQ incorporated into the polymer chain could constitute pendant groups in the main chain or it could act as a cross-linking agent. The ethylene/DDSQ copolymer…

Sugar reduction in extruded cereal based products : impact of water content on the structure and molecular dynamics in such material

2019

Low-moisture biopolymer-based systems are commonly encountered in food. Obviously, understanding the physical basis of their quality: e.g texture, or performance over time or as a function of their composition is of primary importance. The objectives of this work were to evaluate how the presence of sucrose and water content affects physico-chemical properties. The physical stability of these materials were monitored through an insight at different molecular scales. Then the relations between the multi-scale studies were investigated. The effect of sucrose (0–20%) and feed water (10 and 15%) on extrusion blends was studied using a twin screw extruder under the same processing settings. The …

Density Fluctuation in Amorphous Polymers by Small Angle X-Ray Scattering

1984

Density fluctuations in an infinite volume can be obtained by extrapolating the scattering intensity to the zero scattering angle, while those in a finite volume having a radius of several tens of Angstrom can be obtained from intensities at non-zero scattering angles. Small angle X-ray scattering intensities for condensed phases were approximated by I(s)=I(0)exp(As2). This s dependence of the intensity arises from the repulsive interaction between two particles. Since the density fluctuations in the above two types of volume (infinite and finite) were nearly the same in magnitude and temperature dependence for an amorphous polymer (PMMA), it was concluded that no density fluctuations due t…

Aging and the fluctuation dissipation ratio in a Lennard-Jones fluid

1999

We discuss numerically the relaxation dynamics of a simple structural glass which has been quenched below its (computer) glass transition temperature. We demonstrate that time correlation functions show strong aging effects and compute the fluctuation dissipation ratio of this non-equilibrium system.

Glass Transition and Glass Dynamics

2014

The transition from an undercooled liquid towards a glass (glass transition) is introduced and discussed in terms of mode-coupling theory. It is demonstrated that mode-coupling theory leads to a two-step relaxation scenario near the transition with time-critical exponents, which characterize the two relaxation steps (beta and alpha relaxation). The anomalous vibrational properties of a disordered solid (glass) is explained in terms of a model with spatially fluctuating harmonic force constants.

Simulation of the dynamics of hard ellipsoids

2008

We study a system of uniaxial hard ellipsoids by molecular dynamics simulations, changing both the aspect-ratio X-0 (X-0 = a/b, where a is the length of the revolution axis and b is the length of the two other axes) and the packing fraction phi. We calculate the translational and rotational mean squared displacements, the translational D-trans and the rotational D-rot diffusion coefficients and the associated isodiffusivity lines in the phi - X-0 plane. For the first time, we characterize the cage effect through the logarithmic time derivative of log and log . These quantities exhibit a minimum if the system is supercooled and we show that, consistently with our previous findings, for large…

Positronium as a probe in natural polymers: decomposition in starch

2009

Ortho-positronium (o-Ps) is used as a probe in positron annihilation lifetime spectroscopy (PALS) experiments, to characterise the behaviour of free volumes in natural starch samples, as a function of temperature (T). Up to about 540 K, the o-Ps intensity, I(3), remains constant at 26.2% while its lifetime, tau(3), is found to increase linearly. Both parameters undergo a decrease above this T, due to the onset of decomposition, which results in a shrinking of the sample pellets. The results indicate that the glass transition temperature should be above 501 K. Data from thermal gravimetry analysis (TGA) measurements are well described by supposing a first order process for the survival proba…

1986

Investigation of the Na2O/Ag2O ratio on the synthesis conditions and properties of the 80TeO2–10ZnO–[(10−x)Na2O–xAg2O] glasses

2019

International audience; Properties of the tellurite glasses 80TeO2–10ZnO–[(10−x)Na2O–xAg2O] are investigated as a function of the substitution ratio x between Na2O and Ag2O. One observe that the variation of glass transition temperature decreases monotonously with x and that surface crystallization mechanism is favored. The assignment of the Raman bands and their relation with the underlying glass structure is discussed. While both Na2O and Ag2O oxides act as glass network modifiers, their progressive equimolar substitution does not lead to a meaningful evolution in the structure of the TZ[Na10-xAgx] glass. The refractive index and the cut-off wavelength are found to increase with x. The co…