New model explains brittle and ductile transitions in polymer glasses

Polymer glasses, such as poly(methylmethacrylate) (PMMA), can exhibit brittle, ductile, or liquid-like behaviors depending on their chemical makeup, temperature, and the rate of deformation. While glasses are typically expected to be brittle, they can actually become ductile at temperatures significantly lower than their glass transition temperature (Tg). For instance, at standard laboratory strain rates, the transition from brittle to ductile behavior can occur 50 to 100 K below the Tg. (source 69)

Researchers have developed a micromechanical model to capture three specific types of stress-strain behavior: brittle fracture, ductile behavior with stress overshoot, and ductile behavior without stress overshoot. This model builds upon the traditional Maxwell element but introduces two primary changes: the elastic component uses a sinusoidal function of elastic strain to reflect a potential energy landscape with multiple minima, and the dashpot viscosity decreases at large strains as predicted by the Eyring-Long model. According to the study published in Soft Matter, this model allows for the calculation of yield strain based on specific strain rates. (source 69)

The researchers propose that a polymer glass yields uniformly if the strain rate remains below a certain temperature-dependent threshold, whereas it breaks brittly if the rate exceeds it. This threshold is hypothesized to be inversely proportional to the Johari-Goldstein beta-relaxation time. The model also accounts for the alpha-relaxation time, which is affected by the aging of the polymer. By using the dependence of these relaxation times on temperature, the researchers created a "universal" phase map to separate brittle and ductile regions. Consequently, the analysis of the brittle-ductile transition requires knowledge of the age of the material sample. (source 69)

When applied to PMMA, poly(vinylchloride) (PVC), and polystyrene (PS), the model's results were within 5 K of experimental observations. However, the model remains semiempirical because it relies on an adjustable parameter to describe the relaxation time floor. Future research aims to use molecular simulations and experiments to better predict full stress-strain curves and fracture stress. (source 69)