Energy Landscape and Isotropic Tensile Strength of n-Alkane Glasses

Abstract

We report results from a systematic simulational study of the ultimate mechanical strength of n-alkane glasses for carbon numbers n ) 1, 2, 3, 4, 6, 8, 16, 24, and 48. The ultimate isotropic tensile strength was determined by constructing the equation of state of energy landscape for this homologous series. The tensile strength depends nonmonotonically on carbon number, exhibiting a maximum at n ) 3. The mass density at which fracture occurs initially increases with chain length and then reaches a plateau value for n > 8. The predictions of the landscape equation of state are entirely consistent with results generated by a direct inherent structure deformation procedure. Although the ultimate isotropic tensile strength maximum at n ) 3 would seem to contradict physical intuition regarding chain entanglement, we present a simple mean-field theory that reveals the underlying physics responsible for the tensile strength maximum, namely the simple competition between intermolecular interactions and intramolecular packing effects.

The effect of nanoparticle shape on polymer‐nanocomposite rheology and tensile strength

“…A similar decrease of τ was observed in simulations of n-alkanes 43 and experimentally it has been found that ultimate tensile strength of certain polymeric systems, such as amorphous polyethylene, decreases with increasing molecular weight. 63 Such a decrease in τ is non-trivial, since one expects longer chains to exhibit more chain "entanglement".…”

“…Our results are based on molecular dynamics (MD) computer simulations, using non-equilibrium methods to evaluate η, 40,41 and exploit the "inherent structure" formalism to determine τ . 42,43 We find that the rod-like nanoparticles give the largest enhancement to η, which we correlate with the presence of chains that bridge between the nanoparticles. The sheet nanoparticles offer the weakest increase in η, and correspondingly have the smallest fraction of bridging chains.…”

“…The method of ref. 43 relates τ to the maximum tension the IS can sustain, determined by an appropriate mapping from equilibrium configurations to energy minimized configurations. In other words, τ is the upper bound on the tension at the breaking point of an ideal glass state in the T → 0 limit.…”

“…43 As chains grow longer, it becomes more difficult to pack effectively due to the constraints of chain connectivity. The poorer chain packing in turn can lead to an increase in the "fragility", in the context of fragile and strong glass formers.…”

Intrinsic vulnerabilities to mechanical failure in nanoscale films

“…3b and 3c, cohesive failure does show qualitatively different structural consequences in ultrathin and thicker films. While ultrathin films fail by forming planar voids between molecular layers parallel to the substrates, thicker films generally fail by forming voids that are 'local' and more isotropic (more balanced in the transverse and normal directions), not so different from the "bubble-like" voids that form from weak spots in bulk materials (Sastry et al, 1997;Shen et al, 2002). Note that this local cohesive failure mechanism produces a film with a much smaller tensile strength than the ultrathin film of Fig.…”