Structural composition and mechanical performances of poly (vinyl alcohol) materials: experimental and molecular dynamics simulation studies

Structural composition and mechanical performances of poly (vinyl alcohol) materials: experimental and molecular dynamics simulation studies

The addition of small molecules can significantly influence the macroscopic mechanical property and microscopic chain dynamics of poly(vinyl alcohol) (PVA) materials. However, to date the mechanical mechanism behind remains not well understood experimentally and theoretically. The goal of this work is to gain an in-depth understanding of the effects of structures and content of small molecules on physical structures and dynamics properties of PVA composites by a combined use of molecular dynamics (MD) simulation and experimental methods. Three kinds of small molecules are assessed: water, 2,4,5,6-tetraaminopyrimidine (4N-2456), and phytic acid (PA).

First, we systematically examine the governing effect of water content on the mechanical property, glass transition, free volume, and intermolecular interactions by combining experimental and MD simulation. Our results show that the presence of water significantly reduces the mechanical strength of PVA, and only 1.8 wt% of water reduces the tensile strength by ~32% but notably increases the plasticity of PVA by ~2.5 times. Meanwhile, the inclusion of water remarkably lowers the glass transition temperature, increases free volume, and promotes the relaxation and mobility of PVA chains. This is mainly because the presence of water gives rise to both the plasticization and even lubricating effect on the PVA chains. This work unravels how water governs the mechanical performances of the PVA.

Then, we study the hydrogen-bond cross-linking effect of 4N-2456 molecules on the structure, chain dynamics and mechanical properties of the PVA matrix. It is found that the addition of 4N-2456 leads to a nonlinear decrease of the free volume of PVA. A critical concentration about 5.0 wt% is identified, resulting in the formation of 4N-2456 clusters. At this concentration, the PVA chains show a relatively slow mobility, a higher glass transition temperature and higher elastic modulus. A further increase in the 4N-2456 concentration leads to aggregation, and conversely weakens the hydrogen-bond interactions between PVA chains. Our work offers an understanding of how the 4N-2456 molecules influence the PVA chain dynamics and mechanical properties of the PVA matrix on molecular level.

Finally, we explore the structure, chain dynamics and mechanical properties of PVA/PA composite films showing a certain antibacterial capability by combining MD simulation and experimental investigations. We show that the addition of 10 wt% PA endows PVA with a good antibacterial capability. The number of PVA-PA H-bonds per PA molecule and that of total H-bonds show different PA content dependence. The PVA composite film containing 1.9 wt% of PA shows a lower free volume and a smaller diffusion coefficient, and the highest strength and ductility. Meanwhile, the glass transition temperature (Tg) of PVA reaches the maximum value at ca. 1.25 wt% of PA. This work reveals how small molecules affect structure and mechanical properties of polymers and contributes to expanding practical applications of PVA/PA in many industrial sectors (e.g., packaging).

This work offers an in-depth understanding of how small molecules affect physical structure and mechanical properties of polymers through intermolecular interactions between small molecules and the polymer matrix. This work is expected to contribute to the creation of mechanically strong, tough and ductile polymeric materials, ultimately expanding the real-world applications of polymers in industries.