Atomistic Details of Disordering Processes in Superheated Polymethylene Crystals II. Effects of Surface Constraints
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II. EFFECTS OF SURFACE CONSTRAINTS B. WUNDERLICH, G. L. LIANG, B. G. SUMPTER, AND D. W. NOID Department of Chemistry, University of Tennessee, Knoxville, TN 37996-1600 and Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6197. ABSTRACT
Atomistic details of disordering in superheated polymethylene crystals have been studied using full molecular dynamics simulations of crystals containing 9600 CH 2-groups. The crystal size was about 227 nm . Simulations were carried out for up to 100 ps, starting at temperatures about 100 K above the melting temperature. Typically 1.5 h of CPU time on a Cray X-MP were necessary per ps simulation. Superheating causes a quick development of large-scale disorder throughout the crystal, including reorientation, translation, and the destruction of crystal symmetry. This is followed ultimately by surface melting. Crystallization centers with hexagonal packing are found in superheated, unconstrained crystals. On cooling during the simulation, recrystallization processes compete with the disordering, resulting in a reorientation of the molecular chains and reorganization of the crystal. Neither the fully amorphous phase nor the ordered crystal are reached during these shorttime simulations using an instantaneous temperature increase to above the melting temperature, followed by a slow cooling into the crystallization temperature region. INTRODUCTION
Molecular dynamics simulations of the disordering process in unconstrained, superheated polymethylene crystals with sizes close to experimentally observed small crystals have been carried out earlier.' In these unconstrained crystals, the disordering developed instantaneously throughout the whole crystal. Surface melting followed by reorganization and recrystallization were observed when the temperature was reduced. We have now extended these simulations to surface-constrained crystals of the same size. In this paper we report new results from such molecular dynamics simulations and compare them with the prior results. COMPUTATIONAL DETAIL
The simulations were carried out starting with (CH 2)50-chains in perfect orthorhombic (ORTH) or monoclinic (MONO) arrangement. The crystals are illustrated in x-y-projection (along the chain axes) in Fig. 1. The following polyethylene crystal-lattice parameters were used: ORTH: a = 0.74, b = 0.49, c = 0.25 nm with c being the chain axis; 2 MONO: a = 0.81, b = 0.25, c = 0.48 nm, and P = 1080 with b being the chain axis.3 The double crosses mark the zigzag positions of the carbon atoms of the all-transchains. In Fig. 1 each crystal contains 192 chains and each chain consists of 50 CH 2-groups, i.e. there are 9600 CH 2-groups 93 Mat. Res. Soc. Symp. Proc. Vol. 321. ©1994 Materials Research Society
in each crystal with an initial of the Initial Crystals along the Chain-axis dimension of about 6.0 x 6.0 Projection (Each cross represents the carbon position and eaoh chain x 6.3 z 227 nm3. Each of the oontains 50 oarbon atoms In the z-direotion) 4.0 1 CH 2-groups was treated as a Oflhorhomn
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