Isotropic tensile strength of molecular glasses.

The relationship between the bulk density and pressure of configurations corresponding to local minima on the potential energy surface of molecular models of ethane, n-pentane, and cyclopentane (the equation of state of their energy landscape) has been explored. Like simpler, atomic fluids, these systems exhibit a limiting bulk density below which minimum energy configurations are no longer spatially homogeneous, but consist instead of a locally dense fraction and large, system-spanning voids. In the case of n-pentane, the sampling of the minima on the energy landscape was found to depend strongly on temperature, due to changing Boltzmann factors associated with the different conformers in the liquid. The pressures of the minimum energy configurations, in contrast, were found to be essentially independent of the liquid temperature in all cases. The highest amount of isotropic tension (negative pressure) that minimum energy configurations can sustain is reached at the limiting densities, and is of similar magnitude (approximately 250 MPa) for all three model substances. Crystalline configurations of ethane and n-pentane, in contrast, were found to exhibit higher isotropic tensile strength than their amorphous counterparts. A pronounced segregation of end groups on the boundary of large voids was observed in the minimum energy configurations of low bulk density pentane. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]