Linking Structure and Properties: Multiscale Simulations of Macromolecules

Kurt Kremer
Max Planck Institute for Polymer Research

The huge variety of the macroscopic properties of polymeric materials is based on the many different possible chemical molecular building units as well as on various molecular architectures and huge differences in molecular weights. They can be stiff or flexible along the backbone, meaning that they have a huge or a rather small conformational freedom. Their local interactions, which results e.g. in a characteristic bead friction or determines the miscibility of the components of a blend are determined by the details of the chemistry of the monomers. In all cases, both the generic (essentially entropy driven) properties as well as the chemistry specific (essentially energy driven) aspects have to be taken into account in order to provide a quantitative understanding of a certain systems. It is this combination, which results and the rather delicate interplay of local chemical with more global architectural and size properties, which makes macromolecules so versatile and interesting. This means that rather different length and time scales are relevant, and that understanding the properties on one scale is not at all sufficient to understand material properties. The relation between atomistic structure, architecture, molecular weight and material properties is a basic concern of modern polymer material science. The longstanding aim by now goes far beyond standard properties of bulk materials. A typical additional focus is on surface interface aspects or the relation between structure and function in nanoscopic molecular assemblies. This all implies a thorough understanding on many length and correspondingly time scales ranging from (sub)atomic to macroscopic. At this point also computer simulations are playing an increasingly important role. Traditionally simulations have been separated in two main groups, namely simplified models to deal with generic or universal aspects, i.e. critical exponents, of polymers and those employing classical force field simulations with (almost) all atomistic detail, i.e. for the diffusion of small additives in small "sample". Both approaches in their own right are and have been invaluable tools for research over the last 20 years. They significantly improved over that time because of ever improved models and algorithms and the dramatic increase of computer power. Still characteristic problems, which require huge systems and or long times in combination with a chemistry specific model, cannot be tackled by these methods alone.

More recently with the development of scale bridging or multi scale simulation techniques, these different approaches have been combined into an emerging rather powerful tool. It is the purpose of this talk to give a few examples of how such an approach, which combines ab initio quantum level calculations, force field simulations as well as coarse grained molecular dynamics simulations in a systematic manner, can be used to understand specific material properties. Questions considered range from surface morphologies of polymer melts close to a metal surface experiencing specific interactions to the classical problem of entanglements. By using a mapping scheme, which allows for a scale bridging in both the coarsening as well as the detailing direction, new classes of problems can be tackled by simulations. Some recent attempts to construct a small molecular tool set for such simulations as well as first steps towards an adaptive scheme will be mentioned shortly.

J. Baschnagel et al, ''Bridging the Gap Between Atomistic and Coarse-Grained Models of Polymers: Status and Perspectives'',
Advances in Polymer Science, Springer Verlag, 152, 41 (2000)
C.F. Abrams, L. Delle Site, K. Kremer in "Bridging Time Scales: Molecular Simulations for the Next Decade",
P. Nielaba, M. Mareschal, G Ciccotti (Eds)) Proceedings Simu Conference, August 2002, Springer, Berlin - Heidelberg, (2002)
N. Attig, K. Binder, H. Grubmüller, K. Kremer (Eds) "Computational Soft Matter: From Synthetic Polymers to Proteins"
NIC Lecture Notes, 23, FZ Jülich (2004)
L. Delle Site, C. F. Abrams, A. Alavi, K. Kremer ''Polymers Near Metal Surfaces: Selective Adsorption and Global Conformation'' Phys. Rev. Lett. 89, 156103 (2002)
L. Delle Site, S. Leon, K. Kremer ''BPA-PC on a Ni(111) surface: The interplay between adsorption
energy and conformational entropy for different chain end modifications'' JACS 126, 2944 (2004)
R. Everaers, S. K. Sukumaran, G. S. Grest, C. Svaneborg, A. Sivasubramanian, and K. Kremer
''Rheology and Microscopic Topology of Entangled Polymeric Liquids'' Science 303, 823 (2004)

Presentation (PowerPoint File)

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