Cavitation in polymer melts and rubbers is of great technological significance. It can be described as a phase separation phenomenon, wherein a cavity develops within a condensed phase that is being subjected to an isotropic stress. Below a certain negative pressure, the dense polymer phase becomes unstable. From thermodynamic considerations, the stress at the limit of stability is higher than experimental observations. On the other hand, a continuum mechanics analysis of heterogeneous nucleation predicts that a preexisting cavity in a neo-Hookean material will grow without limit when the hydrostatic pressure is more tensile than the Young's modulus, in approximate agreement with experimental observations.
A molecular-level understanding of cavitation in amorphous polymers upon imposition of mechanical stress is still lacking. Molecular Dynamics simulations of crosslinked amorphous Polyethylene (PE) and of linear PE melts were conducted in order to study cavitation as a function of prevailing stress and molecular characteristics and to understand its dependence on cohesive interactions, entropy elasticity of the chains, network defects, and particle inclusions.
Triaxial loading of homogeneous crosslinked and linear PE melts, in the absence of flaws, leads to an estimated limit of stability at a stress (negative pressure) of 65 MPa. Our systems cavitate at a smaller stress ~ 45 MPa, since they become metastable as they approach the limit of stability. At these stress levels, which are much larger than the Young's modulus of the materials, cohesive interactions in the polymer are overcome. The stress at the limit of stability depends slightly on factors such as, crosslink density, density of chain ends, network defects, and interfacial interactions of particle inclusions with the host matrix. Factors which decrease cohesion lower the stress, and vice versa.
Starting from a cavitated initial state and by unloading the systems, we find that a preexisting cavity cannot survive in a rubber sample, below a certain stress which is comparable to the Young's modulus of the systems. In the P-V diagram of the samples, this stress appears as a lower limit of stability. We further show, clearly, that that this lower stress is of entropic origin, in agreement with the continuum mechanics analysis of heterogeneous nucleation.