# Beyond Visual Observations: Establishing the Mechanical Stability Threshold of Nanothin Polyethylene Layers

## TL;DR

This study uses simulations to determine the mechanical stability threshold of ultrathin polyethylene films and finds that visual observations underestimate their critical thickness.

## Contribution

The paper introduces a new mechanical isotropy-based stability criterion for ultrathin polyethylene films.

## Key findings

- Ultrathin polyethylene films transition to fibrillar structures at a critical thickness.
- Anisotropic pressure profiles indicate positive disjoining pressure and film instability.
- Critical thickness values are higher than previously estimated from visual observations.

## Abstract

This paper investigates the mechanical stability and critical thickness of free-standing, ultrathin molten polyethylene films using Molecular Dynamics simulations. By comparing the “interfacial drying” and “film stretching” methodologies, this research establishes that both methods consistently identify a stability threshold where continuous films transition into fibrillar and void structures known as “crazes”. A key finding is that films at extremely reduced thicknesses exhibit an anisotropic pressure profile in their core—characterized by a positive normal pressure—which serves as a manifestation of positive disjoining pressure and a precursor to film transformation. Consequently, the study proposes a more rigorous stability criterion based on mechanical isotropy, which yields higher critical thickness values (approximately 6.5 nm at 373.15 K and 9.3 nm at 673.15 K) than those previously estimated from short-term (100 ns) visual observations. Ultimately, the work concludes that maintaining a negative disjoining pressure is fundamental to the structural integrity of these polymeric nanomaterials.