Simulations of the Structure and Properties of the Polyethylene Crystal Surface
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"Fold" Surface
Lateral Surface
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Figure 1: Schematic of a tightly-folded lamellar crystal. known as lateral and fold surfaces. Since the prevalent kinetic theories of polymer crystallization postulate that crystal growth occurs through the addition of chain segments to the lateral surface, it is expected that the structure and properties of the lateral surface will have a strong influence on polymer crystal growth behavior.[ I] The properties of the lateral crystal surface 469 Mat. Res. Soc. Symp. Proc. Vol. 408 ©1996 Materials Research Society
are also of interest in understanding surface properties of crystals grown under pressure, epitaxially grown crystals, crystals of rigid polymers, or any other system in which the lateral surface may be macroscopically observed. However, to this date, few studies have been conducted to quantify the structure and properties of the lateral surface in crystalline polymers.[2-4] The present study offers the first rigorous calculation of the temperature dependence of the structure and properties of the polyethylene lateral crystal surface, determined from first principles. This study uses consistent quasi-harmonic lattice dynamics simulations to find the free energy minimum structures of the (100), (010), and (110) lateral surfaces of extended chain polyethylene crystals, up to temperatures of 300 K. MODEL The present modeling approach is similar to that used in a previous study of bulk crystalline polyethylene. [5] In this work, the method is adapted to simulate two-dimensional crystal slabs. The surfaces were modelled by considering the thermal motions in thin crystalline slabs of extended chain polyethylene, such as that for the simulation cell used to represent the (100) surface shown in Figure 2. In this case, the crystalline slab with (100) surfaces was generated with a simulation cell (shaded in the diagram) containing 14 chains. In the diagram, diagonal
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Figure 2: Representation of thin molecular slab used to model the (100) surface: diagonal lines are projections of chain backbones as viewed down the chain axis; the shaded region indicates the simulation cell; bold chains are those which lie on the (100) surface. lines are the projections of the chain backbones as viewed down the c-axis. Bold-faced chains are those which lie on the (100) surface, and all chains outside ofthe shaded region are generated by periodic boundary conditions in the direction parallel to the surface. The (010) and (110) surfaces were similarly represented with appropriate changes in the simulation cell and the directions of periodicity considered. Slab thicknesses of 50 A were found to be sufficient to ensure non-interacting surfaces and attainment of bulk structure in the center of the slab. Simulations of thicker slabs did not change surface properties significantly. In all analyses, the unit cell was constrained to be orthorhom
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