Fracture Behavior of Polymers in Plastic and Elastomeric States

Our recent studies departed from the conventional description of polymer fracture behavior while maintaining consistency with the principles of material mechanics including linear elastic fracture mechanics (LEFM). In traditional fracture mechanics of brittle materials, crack resistance is quantified in terms of toughness Gc, which represents the critical energy release per unit fracture surface area. This perspective suggests that high Gc involved high energy dissipation. A new perspective has surfaced, proposing a fundamental yet underexplored, seemingly universal fracture mechanism and explaining the origin of high Gc within an alternative framework. According to this view, for unfilled plastics and elastomers, fracture initiates when local tensile stress surpasses the polymer's fracture strength σF(inh), and high toughness is a consequence of high fracture strength. Remarkably, for polymers in both plastic and elastomeric states, their inherent strength σF(inh) appears to be of a comparable magnitude to the nominal tensile strength σb. Spatial–temporal resolved polarized optical microscopic (str-POM) measurements have started to provide insight into the fracture mechanism and unveil a concealed length scale (P) representing the size of a stress saturation zone at the crack tip. The two-parameter theoretical framework shows (a) toughness of brittle plastics increases quadratically with its strength σF(inh) and linearly with P and (b) elastomers exhibit significantly greater toughness and higher tensile strength at lower temperatures due to the increased stability of covalent bonds given the lower thermal energy and plausible frictional effect on bond vibration frequency. Embracing this emerging paradigm where we recognize polymer strength to be time-dependent, we anticipate new advancements in polymer design and a clearer understanding of the fracture behavior of polymeric materials.