Structure, Dimensions, and Entanglement Statistics of Long Linear Polyethylene Chains
K Foteinopoulou and NC Karayiannis and M Laso and M Kroger, JOURNAL OF PHYSICAL CHEMISTRY B, 113, 442-455 (2009).
This work elucidates the effect of both temperature and molecular length on the conformational and structural properties as well as on the entanglement statistics of long amorphous, polydisperse, and molten linear polyethylene (PE). A large number of PE samples are modeled in atomistic detail, with average molecular lengths ranging from C(24) up to C(1000) over a wide range of temperatures in the interval of 300 <= T <= 600 K under constant pressure (P = 1 atm). By employing enhanced chain-connectivity-altering moves, full-scale equilibration is achieved within modest computational time even for the longest molecules at ambient conditions. At a second stage, direct geometrical analysis is applied on all equilibrated polymer configurations providing the corresponding primitive paths and intermolecular entanglements. Simulation findings on the characteristic ratio, density, and atomic packing are in excellent agreement with available experimental data. The same holds for the calculated plateau modulus; simulation predicts 1.8 +/- 0.1 MPa. Regarding the primitive path statistics, the average contour length and the number of entanglements are found to exhibit a simple exponential-type dependency on temperature. For the polydisperse samples studied here, a superposition of Poissonians (often represented by a negative binomial) describes best the distribution of entanglements of the primitive paths.
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