Molecular dynamics studies on local ordering in amorphous polyethylene

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Abstract
A molecular dynamics (MD) simulation was performed on a linear poly(ethylene) model under bulk amorphous conditions. A molecule was composed of united atoms ( CH 2 and CH 3 unit), and the pressure of the MD cell was controlled to be 1 atm. The system was stepwise cooled from 600 to 120 K. At each temperature 2 ns simulation was performed. Local structural ordering proceeded as the temperature was lowered. The mean length of the trans sequences increased with decreasing temperature according to the Arrhenius law above 300 K. The correlation volume abruptly increased between 300 and 330 K, which is the measure of parallel ordered structure of the model chain used by Rigby and Roe. Fluctuations of the density, the length of the trans sequences, and the correlation volume developed toward the maximum values between 300 and 330 K. Then the fluctuations decreased with further cooling and the glass transition occurred at 230 K (≡T g ). A cluster was observed below T g , which consisted of many planar-zigzag segments oriented in the same direction.

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