Detailed Modeling of Structure and Deformation of Glassy Polymers
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DETAILED MODELING OF STRUCTURE AND DEFORMATION OF GLASSY POLYMERS DOROS N.THEODOROU* **, PETER J.LUDOVICE*, AND ULRICH W.SUTER* * Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139 Current address: Department of Chemical Engineering, University of California, Berkeley, CA 94720-9989
INTRODUCTION The molecular structure of macromolecules determines to a very large extent the macroscopic properties of the polymers they make up, and delimits what variations in properties can potentially be effected by processing. It is, therefore, important to understand and to be able to reliably predict the relevant properties of polymers and the limits of these properties upon treatment from the knowledge of the molecular structure of the constituent chains. For many properties of amorphous polymeric glasses correlations have been introduced in the last decades; some of them have been very useful [1-3], but they lack basic understanding of the processes and mechanisms operative in the generation of the properties in question. Their application is, by definition, limited to interpolation in that part of "chemical structure space" used as basis for the correlation. Prediction implies the power of extrapolation and, consequently, some use of first-principle methods. Remarkably little such work [1, 4-6] has been done to date on amorphous polymeric glasses. We have recently begun to investigate the atomistic-level modeling of structure and properties of amorphous, "fully relaxed" polymeric glasses [7-9]. MODELING THE
DETAILED MOLECULAR
STRUCTURE OF A GLASS
The systems under consideration are pictured as small sections of a fully aged bulk amorphous material, initially of cube-shape and after deformation of the shape of a general parallelepiped. For simplicity spatially periodic boundary conditions are imposed (a common device in the detailed modeling of condensed phase materials), and the contents of the cube are obtained from a single "parent chain." The entire bulk of the material, therefore, is thought to be made up of the translated images of a single parent chain, typically "randomly" coiled and extending beyond the confines of the cube. In the studies reported here, this parent chain was of the type CH 3 CHR(CH 2 CHR)Z_ where R is either methyl (-CH 3 ) or chlo1 CH 3 , rine, and methyl is regarded as a simple "pseudo-atom." The systems are static embodiments of Cohen and Turnbull's [10) concept of glasses being in a state of frozen-in liquid disorder, and employ rigid bond lengths and bond angles. The polymer is regarded as an ensemble of such microscopic structures. The assumption of full aging implies that all parts of the microstructures, i.e., of the cubes, satisfy the requirement of detailed mechanical equilibrium (the sum of all forces and torques, acting through space or through bonds, on each
Mat. Res- Soc. Symp. Proc. Vol. 79
1987 Materials Research Society
388
and, simultaneously, that the potential atom must be zero), energy of the microstructures is at a minimum with
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