Models amorphization

Kotelyanskii M 1997 Simulation methods for modelling amorphous polymers Trends Polym. Sol. 5 192-8... 

Product/ Temperature Amor- Amorphous form phous form Arrhenius stat. model Amorphous form observed Crystalline form Arrhenius Crys- talline form stat. model Crys- talline form observ- ed... 

The degree of crystallinity, indicates the proportion of solid that is crystalline. This magnitude has great practical importance, as the mechanical properties of the material, and most of its properties in general, depend on the crystallinity. The simplest method to determine the degree of crystallinity of a polymer is based on the measurement of the specific volume, v. Crystallization implies a compaction of polymer chains, i.e., a decrease in the volume (increase in density) with respect to the amorphous material. If the biphasic model (amorphous phase + erystalhne phase) is accepted for the semicrystalline sample, the specific volume can be written as... 

The largest contribution to this gas-solid interaction for oxide surfaces comes from the oxide anions (the BS neglects all other contributions). Thus, the parameters of the LJ T and e for, e.g., the Ar-0 interaction of some model amorphous oxide may be as a first approximation transferred from those for a different oxide with a well known regular atomic structure at the surface. An example is the (100) surface of MgO where the energy was evaluated and compared with reliable experimental data for a number of simple gases (for a discussion of such calculations see, e.g.. Refs. [26, 27]). Such transferability seems reasonable if one studies physical adsorption on a heterogeneous surface of MgO which (cf. Introduction) may be modelled as an amorphous MgO surface. Unfortunately, the transferability of LJ parameters is not generally possible. For example, e for Ar-0 interaction determined from the temperature dependence of the Henry s Law constants... 

Figure 2. The density of the Henry s Law constant (k(x,y)) for Ar on a model amorphous oxide (BS) at 150 K.

Approximation of the isotherm of axlsorption of Ar on the model amorphous oxide by the BET equation in the BET region... 

Feldman JL, Kluge MD, Allen PB, Wooten F. 1993. Thermal conductivity and localization in glasses Numerical study of a model amorphous silicon. Phys. Rev. B 48 12589-12602. 

In MD, too, demands for the future should be obvious from the discussion in Sections 3, 6, and 7.1. To obtain realistic simulations of the dynamics, not only is it essential to simulate considerably longer real-time relaxations but also to include an adequate number of particles in the simulation box. This is particularly important for modeling amorphous states of polymer systems for which the demands of computer storage and speed are critical and at present inadequately met. 

Isoharic (constant pressure) and isochoric (constant volume) glass transitions in polymers were first observed for bisphenol A polycarbonate (62). A molecular dynamics study of such transitions in a model amorphous polymer has also been reported (63). This study shows that the glass transition is primarily associated with the freezing of the torsional degrees of freedom of polymer chains (related to chain stiffness), which are strongly coupled to the degree of freedom associated with the nonbonded Lennard-Jones potential (related to interchain cohesive forces). 

Antonijevi c MD, Craig DQM, Barker SA (2008) The role of space charge formation in the generation of thermally stimulated current (TSC) spectroscopy data for a model amorphous drug system. Int J Pharm 353 8-14... 

A Monte Carlo method employed by Vacatello et al. upon a liquid of 31 C,o chains to model amorphous polyethylene found no tendency towards the formation of bundles of chains as judged by the reproduction of experimental JT-ray scattering curves and other measures of chtiin direction correlations. ... 

Kotelyanskii, M. Simulation methods for modeling amorphous pol5nners. Trends Polymer Sci. 1997, 5, 192-198. 

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