Toughenability of polymers

Abstract

We demonstrate that all solid polymers are intrinsically brittle and will undergo a ductile to brittle fracture transition based on the nature of their bonding alone. The most effective way of avoiding a ductile to brittle transition is to reduce the plastic resistance to delay reaching the brittle strength which in unoriented polymers is governed by intrinsic cavitation. While a number of possibilities for this exist, the most widely used techniques involve incorporation of rubbery particles that can cavitate or rigid particles that can debond prior to plastic flow. In both approaches the continuous homo-polymer is transformed into a quasi-regular cellular solid that is much more capable of undergoing large local plastic flow by ligament stretching between cavitated particles and is less susceptible to the propagation of brittle cracks under the usual conditions of tensile straining. Under impact conditions, however, in a notched sample which concentrates the strain rate at the notch root, the plastic resistance of the stretching ligaments rises sharply due to two separate but related effects. First, by an increase in the shear modulus due to the high frequency nature of the Izod impact test to fracture, viewed as a quarter cycle oscillation, which directly elevates the flow resistance and second, by the further effect of increase due to the much increased plastic strain rate. At the notch root then, the plastically stretching and strain hardening ligaments are left with a much reduced capacity to strain further before the cavitation stress is reached. While rubber particle-modified polymers can still exhibit considerable toughening, rigid-particle-modified polymers suffer severely from clustering of rigid particles into super critical flaws that trigger brittle response, much like what is encountered in structural steels.Based on their known mechanical response in neat form six, semi-crystalline polymers have been analyzed in detail to evaluate their potential for toughening under impact conditions. The results correlate very well with the experimental findings.