Amorphous solids modelling atomistic structures

In this chapter we show how atomistic computer simulation methods can yield unique insights into the structural properties of amorphous solids. The range and scope of the materials that can currently be studied by modelling techniques is illustrated by recent results, predominantly on glassy materials (defined below), although a brief discussion of other classes of amorphous materials is given towards the end of the chapter. 

In another paper in this issue [1], the molecular motions involved in secondary transitions of many amorphous polymers of quite different chemical structures have been analysed in detail by using a large set of experimental techniques (dynamic mechanical measurements, dielectric relaxation, H, 2H and 13C solid state NMR), as well as atomistic modelling. 

The applications we now discuss relate to the modelling and prediction of crystal structures, to the development of atomistic models for amorphous materials, to the modelling of surfaces of inorganic solids, to the simulation of the dynamical and defect properties of solids and to the explicit calculation of the electronic structure of crystals. They will foreshadow the much more detailed accounts that follow in later chapters. 

Molecular dynamics (MD) is an invaluable tool to study structural and dynamical details of polymer processes at the atomic or molecular level and to link these observations to experimentally accessible macroscopic properties of polymeric materials. For example, in their pioneering studies of MD simulations of polymers, Rigby and Roe in 1987 introduced detailed atomistic modeling of polymers and developed a fundamental understanding of the relationship between macroscopic mechanical properties and molecular dynamic events [183-186]. Over the past 15 years, molecular dynamics have been applied to a number of different polymers to study behavior and mechanical properties [187-193], polymer crystallization [194-196], diffusion of a small-molecule penetrant in an amorphous polymer [197-199], viscoelastic properties [200], blend [201,202] and polymer surface analysis[203-210]. In this article, we discuss MD studies on polyethylene (PE) with up to 120,000 atoms, polyethylproplyene (PEP), atactic polypropylene (aPP) and polyisobutylene (PIB) with up to 12,000 backbone atoms. The purpose of our work has been to interpret the structure and properties of a fine polymer particle stage distinguished from the bulk solid phase by the size and surface to volume ratio. 

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