Explaining the difference between brittle and ductile behavior in polymer glasses
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],…
The article explores the various types of behavior exhibited by polymers: brittle, ductile with stress overshoot, and ductile without stress overshoot. Glassy polymers, such as poly(methylmethacrylate) (PMMA), are hard and transparent at low temperatures, but can be ductile at temperatures well below their glass transition temperature, Tg.
For small strain rates, the stress-strain curve of a polymer glass is Type III, meaning it yields uniformly. As the strain rate increases, the yield strain grows, and the stress-strain curve transitions from Type III to Type II, indicating ductile behavior with stress overshoot. At high strain rates, the material does not flow and exhibits brittle behavior, represented by the Type I stress-strain curve.
The brittle-ductile transition temperature (BDT) is a temperature-dependent threshold that separates these three types of behavior. This threshold is inversely proportional to the Johari-Goldstein beta-relaxation time, which describes how the relaxation time decreases as the material is deformed. The alpha-relaxation time, which is large (on the order of the aging time) in undeformed glassy polymers, decreases to the beta-relaxation time as the material is deformed.
The BDT can be computed by considering the alpha- and beta-relaxation times as functions of temperature, and knowing their temperature dependence for different amorphous polymers allows for the calculation of polymer-specific BDT values.
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