Thickness-dependent crystallization and mechanical properties of thermoplastic nanofilms in nonsolvent environments

Background

: Nanofilms exhibit distinct behaviors compared to bulk materials, particularly concerning the effects of film thickness on melting temperature and Young’s modulus, which are not yet fully understood.

Methods

: In this study, dissipative particle dynamics simulations are used to explore the crystallization and stiffness of nanofilms immersed in a non-solvent bath.

Significant findings

: The solid state of the nanofilm, as confirmed by consistent melting temperatures determined from the polymer's radius of gyration, heat capacity, and crystallinity, indicates that thinner nanofilms have higher melting temperatures. Through uniaxial extension, the stress-strain curve of the nanofilm is obtained, and Young’s modulus generally increases toward a plateau with decreasing film thickness. Under strain, the decrease in crystallinity correlates with increased internal energy and positive entropy change, in contrast to typical rubber elasticity where entropy decreases upon stretching. It is found that local crystallinity near the interface is significantly higher than in the central region. Both regions show an increase in crystallinity as thickness diminishes, due to the surrounding non-solvent environment enhancing polymer alignment and crystallization. As the film thickness decreases to the nanoscale, the influence of the interfacial region becomes more pronounced, thereby increasing the film’s stiffness.