Sub-critical crack growth rate of soda-lime-silicate glass and less brittle glass as a function of fictive temperature

Sub-critical crack growth rates of soda-lime-silicate glass and less brittle glass with different fictive temperatures were compared using the DCDC method under both dry and humid atmospheres in order to investigate the origin of the unique mechanical features of the less brittle glass developed by Ito and his collaborators. In both dry and humid atmospheres, the crack velocity of the soda-lime-silicate glass was slower than that of the less brittle glass. For both glasses, the glass sample with higher fictive temperature showed a slower crack growth rate under both dry and humid atmospheres. These observations can be explained by the tendency for the plastic flow at the crack tip; the soda-lime-silicate glass is expected to show easier plastic flow under tension than the less brittle glass, and also the samples with higher fictive temperatures are expected to show easier plastic flow, leading to greater fracture toughness, K_IC, and slower crack growth rate.     

The factorization approximation for the Master Equation

We examine the validity of the application of the Factorization Approximation to derive the Master Equation for a microscopic system coupled to a reservoir. We developed a formal perturbation expansion for the time evolution of the system reduced density matrix. We employed a diagrammatic schemes to produce each term of the perturbation series. The diagrams in the time domain provide a distinct criteria to distinguish the diagrams which survive the Factorization Approximation. The Feynmann-like diagrams in the energy domain, originated from the Resolvent method, are used for execution of diagram summations to estimate their overall contributions. We demonstrated that for a two level atomic system, interacting with a thermal reservoir, the summation over the diagrams which survived the Factorization Approximation, yields the proper time evolution of the system, in agreement with the solution of the Master Equation. The summation of the diagrams which are excluded by applying the Factorization Approximation are characterized by a dimensionless parameter: Γ/ω₀, where ω₀ is the frequency of the transition line, and Γ is the line width. The Factorization Approximation is thus rigorously justified when this expansion parameter is very small.     

Optimal Kinoshita wave functions for heliumlike atoms

An extensive optimization has been performed for the composition ofN terms, as well as the exponent and the mixing coefficients, of Kinoshita wave functions for heliumlike atoms with atomic numberZ. The optimalN-term Kinoshita functions have been constructed forN=1–10, 20, 30, 40, 50, 100 andZ=1(H^–)–10(Ne⁸⁺). The present results demonstrate that the optimal term selection dramatically improves the accuracy of the Kinoshita function: In the case of He, for example, the optimal 100-term Kinoshita function gives – 2.903 724 376 95 hartrees, which is only 8×10^–11 hartrees higher than the most accurate literature value.     

Thermal conductivity of C60 and C70 crystals

We have performed thermal conductivity measurements on C₆₀ and C₇₀ crystals grown by sublimation. For single crystal C₆₀, the thermal conductivity k is 0.4 W/m K at room temperature and is nearly temperature independent down to 260K. We observed a sharp orientational phase transition at 260K, indicated by a 25% jump in k. Below 90K, k is time dependent, which manifests itself as a shoulder-like structure at 85K. The temperature and time dependence of k below 260K can be described by a simple model which accounts for the thermally activated hopping of C₆₀ molecules between two nearly degenerate orientations, separated by an energy barrier of 240 meV. It is found that solvents have a strong influence on the physical properties of C₇₀ crystals. For solvent-free C₇₀ crystal, k is about constant above 300K. There is a broad first-order phase transition in k at 300K with a 25% jump. We associate this transition with the aligning of the fivefold axes of the C₇₀ molecules along the c-axis of the hexagonal lattice. Upon further cooling, k increases and is time independent.     

Miscibility in poly(L-lactide)- b -poly( $ \upvarepsilon$ -caprolactone) double crystalline diblock copolymers

Thermally stimulated depolarization currents, TSDC, wide-angle X-ray scattering, WAXS, differential scanning calorimetry, DSC, and polarized light optical microscopy, PLOM, have been used to examine poly(L-lactide)-b -poly( -caprolactone) diblock copolymers in a wide composition range. Both components are crystallizable and the miscibility in the amorphous phase has been determined from the behavior of the primary relaxations which are the dielectric manifestation of the glass transition, and also from the superstructural morphology revealed by PLOM and the compositional dependence of the melting points as determined by DSC. Distinct segmental mobilities in the amorphous phase which can be well resolved by TSDC are present; the mode of the slower component shifts to lower temperatures as the PCL content increases while the glass transition of neat PCL is present for all compositions. A relaxation times bimodal distribution is apparent for PCL-rich copolymers. The composition dependence of the multiple glass transitions detected in these weakly segregated copolymers are predicted by the self-concentration model for a miscible blend made of components with a large T_g contrast.     

Dynamical precursor of nematic order in a dense fluid of hard ellipsoids of revolution

We investigate hard ellipsoids of revolution in a parameter regime where no long range nematic order is present but already finite-size domains are formed which show orientational order. Domain formation leads to a substantial slowing down of a collective rotational mode which separates well from the usual microscopic frequency regime. A dynamic coupling of this particular mode into all other modes provides a general mechanism which explains an excess peak in spectra of molecular fluids. Using molecular dynamics simulation on up to 4096 particles and on solving the molecular mode coupling equation we investigate dynamic properties of the peak and prove its orientational origin. Received 19 September 2001 and Received in final form 19 March 2002