Controlling molecular mobility and ductile–brittle transitions of polycarbonate copolymers

To control molecular mobility and study its effects on mechanical properties, we synthesized two series of poly(ester carbonate) and polycarbonate copolymers with different linkages: (B_xt)_n (x = 3, 5, 7, 9) and (B_xT)_n (x = 1, 3, 5, 7, 9), where t represents the terephthalate, T represents the tetramethyl bisphenol A carbonate linkages, and B is the conventional bisphenol‐A (BPA) carbonate. These two series of materials have distinct differences in their relaxation behaviors and chain mobility, as indicated by the π‐flip motion of the phenylene rings in the B_x blocks. Uniaxial tensile tests of the copolymers indicate that the brittle–ductile transition (BDT) temperatures of the copolymers are correlated to whether the γ‐relaxation peaks due to the B_x sequence is fully established. The materials possessing more fully established low‐temperature γ peaks give rise to a lower BDT. Also, the locations of the γ peaks are correlated to the ring flips of the B_x blocks of polymer chains. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 1730–1740, 2001     

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On Affine-Scaling Interior-Point Newton Methods for Nonlinear Minimization with Bound Constraints

A class of new affine-scaling interior-point Newton-type methods are considered for the solution of optimization problems with bound constraints. The methods are shown to be locally quadratically convergent under the strong second order sufficiency condition without assuming strict complementarity of the solution. The new methods differ from previous ones by Coleman and Li [Mathematical Programming, 67 (1994), pp. 189–224] and Heinkenschloss, Ulbrich, and Ulbrich [Mathematical Programming, 86 (1999), pp. 615–635] mainly in the choice of the scaling matrix. The scaling matrices used here have stronger smoothness properties and allow the application of standard results from non smooth analysis in order to obtain a relatively short and elegant local convergence result. An important tool for the definition of the new scaling matrices is the correct identification of the degenerate indices. Some illustrative numerical results with a comparison of the different scaling techniques are also included.     

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Adding a length scale to the polyamorphic ice debate

X-ray scattering and molecular dynamics simulations have been used to correlate the short range oxygen-oxygen structure with the intermediate range ordering (IRO) upon annealing very high density amorphous ice. While it is clear that the IRO that defines the network structure breaks down continuously to a minimum level, where there are weakened correlations extending beyond 7 Angstrom, at this point the local structure (O-O-O angles) is observed to change abruptly, allowing a continuous reemergence of a new IRO network. This is very different from a classic first order transition and helps reconcile previous data.     

Failure of viral shells

We report a combined theoretical and experimental study of the structural failure of viral shells under mechanical stress. We find that discontinuities in the force-indentation curve associated with failure should appear when the so-called F?ppl-von Kármán (FvK) number exceeds a critical value. A nanoindentation study of a viral shell subject to a soft-mode instability, where the stiffness of the shell decreases with increasing pH, confirms the predicted onset of failure as a function of the FvK number.