Interfacial Forces in Free-Standing Layers of Melted Polyethylene, from Critical to Nanoscopic Thicknesses

Molecular dynamics simulations of ultrathin free-standing layers made of melted (373.15-673.15 K) polyethylene chains, which exhibit a lower melting temperature (compared to the bulk value), were carried out to investigate the dominant pressure forces that shape the conformation of chains at the interfacial and bulk liquid regions. We investigated layer thicknesses, t(L), from the critical limit of mechanical stability up to lengths of tens of nm and found a normal distribution of bonds dominated by slightly stretched chains across the entire layer, even at large temperatures. In the bulk region, the contribution of bond vibrations to pressure was one order of magnitude larger than the contributions from interchain interactions, which changed from cohesive to noncohesive at larger temperatures just at a transition temperature that was found to be close to the experimentally derived onset temperature for thermal stability. The interchain interactions produced noncohesive interfacial regions at all temperatures in both directions (normal and lateral to the surface layer). Predictions for the value of the surface tension, gamma, were consistent with experimental results and were independent of t(L). However, the real interfacial thickness-measured from the outermost part of the interface up to the point where gamma reached its maximum value-was found to be dependent on t(L), located at a distance of 62 angstrom from the Gibbs dividing surface in the largest layer studied (1568 chains or 313,600 bins); this was similar to 4 times the length of the interfacial thickness measured in the density profiles.

Automated summary

Molecular dynamics simulations were used to investigate the pressure forces shaping the conformation of polyethylene chains in ultra-thin free-standing layers. It was found that the chains were slightly stretched and the contributions of bond vibrations to pressure were greater than interchain interactions. The interchain interactions resulted in noncohesive interfacial regions that were independent of temperature. The predicted surface tension values were consistent with experimental results and were not dependent on layer thickness, while the real interfacial thickness was dependent on layer thickness.