Molecular Dynamics Simulation and Thermal Analysis of Macromolecular Crystals

Abstract

Thermal analysis as developed in the ATHAS, the Advanced THermal Analysis System, has led in the last 10 years to the description of about 150 polymers. The discussion of the entropy and the detailed study of heat capacities has resulted in understanding of disorder and motion in many crystalline polymers. The conformationally disordered state (condis crystalline state) was identified and shown to be different from liquid or plastic crystals. Example polymers range from the flexible polyethylene, polybutadienes, PTFE, and nylons to more rigid polyparaxylylene and aromatic polyesters and polyamides. The conformational disorder and motion could be shown to be introduced either gradually, or at a sharp transition. To get more insight, molecular dynamics simulation on supercomputers was used in cooperation with Dr. D. W. Noid and Dr. B. Sumpter of our institutions. At present crystals with over 6000 chain atoms can be simulated for times up to 30 ps. The crystal is normally 100 chain atoms long (crystallographic c-direction) and data were obtained for a variety of temperatures for a) single chains, b) chains included in one rigid ring nearest neighbors (7 chains), and a chain surrounded by c) one, d) two and e) three mobile rings of nearest neighbors held at constant volume by an additional rigid ring (19 and 37 and 61 chains, respectively) as well as f) without a rigid ring of chains (37 mobile chains). The computed heat capacity effect agrees with the simulation. Rotational isomers seem to be strongly altered by the intermolecular forces. Defects that were proposed based on molecular mechanics calculations seem to have extremely limited life-time.