Investigating dependence of elastomeric fracture on temperature and rate

Decades of elastomeric fracture phenomenology due to Thomas and Smith demonstrated a remarkable fact that rubbers are stronger and tougher at lower temperatures. The prevailing explanation relates the fracture behavior to polymer viscoelasticity. Given the recent insight and evidence that toughness is determined by strength, we examine elastomeric fracture with a different perspective and conclude that chain scission dictates fracture characteristics including its temperature dependence. Working within a selected temperature range, stretching is shown to be entirely elastic at stretching rate below 0.17 s-1, we demonstrate that the same temperature and rate dependencies of strength and toughness, observed by Thomas and Smith, still occur in our crosslinked polybutadiene (BR) and styrene-butadiene rubber (SBR). The temperature effects on rate dependence of strength and toughness are found to be much stronger than that prescribed by the WLF shift factor aT. Moreover, crack propagates, upon either stepwise stretching or during creep, at a much lower speed at lower temperature that cannot be rationalized with polymer relaxation dynamics. Our new interpretation is that a carbon-carbon bond is stronger at a lower temperature, equivalently, it takes a longer time for a covalent bond to undergo dissociation in tension because available thermal energy is proportional to temperature T.