This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Glasses are (mostly) brittle, metals can be ductile (plastic), and liquids flow easily. Polymers are interesting because they exhibit all three types of behavior, depending on their chemical nature, temperature, and deformation rate.
If polymers are not able to crystallize, their low-temperature state is glassy, and they are hard and transparent. One example is poly(methylmethacrylate) [PMMA], which has been used as an artificial glass since the early 20th century. Surprisingly, polymer glasses can be ductile at temperatures significantly below their glass transition temperature, Tg.
For a typical laboratory strain rate (~10⁻³ s⁻¹), the brittle-ductile transition temperature (BDT or TBD) can be 50–100 K below Tg. This result is surprising—naively, one would expect a "glass" to be brittle—yet polymers can deform plastically well below Tg. In collaboration with Profs.
Oleg Gendelman (Technion, Israel) and Alessio Zaccone (University of Milan, Italy), I started by developing a simple model that captures the main features of the three types of stress-strain behavior: brittle (Type I), ductile with stress overshoot (Type II), and ductile with no stress overshoot (Type III). For more than a century, scientists have studied viscoelastic materials using simplified representations like the Maxwell element (spring and dashpot connected in series). Our model—now published in Soft Matter—is based on that Maxwell element, but with two big modifications.
First, the elastic component is not a pure linear spring, but a sinusoidal function of the elastic strain, reflecting the fact that the potential energy landscape has multiple minima. Second, the dashpot viscosity decreases at large strains, as predicted by the so-called Eyring-Long model. With our model, we can calculate the yield strain corresponding to each strain rate.
For small strain rates, the yield strain is small, and the stress-strain curve is of Type III. For intermediate strain rates, the yield strain is larger, and the stress-strain curve is of Type II. Finally, for large strain rates, there is no solution, the material does not flow, and the stress-strain curve is of Type I.
Extract — continue reading at the source.
