Viscoelasticity and diffusion in miscible blends of saturated hydrocarbon polymers

Linear viscoelasticity and tracer diffusion were investigated as functions of temperature, component molecular weight and blend composition for entangled, single-phase blends of nearly monodisperse poly(ethylene-alt-propylene) (PEP) and head-to-head polypropylene (HHPP). Both components are non-polar and, despite evidence for slight differences of component glass temperatures in their blends, the viscoelastic data obey time-temperature superposition rather well. The properties of the blends were compared at constant T-T _g (blend) with predictions of the tube-model theories. The composition dependence of viscosity agrees best with the double-reptation prediction, as had been found earlier for molecular weight blends. The variation in plateau modulus with composition is consistent with reptation, but the changes are too small to provide a definitive test. The tracer diffusion coefficients, D ^* _PEP and D ^* _HHPP are nearly independent of composition, consistent with the reptation prediction and in sharp contrast with tracer diffusion for blends with specific associations. Results for the recoverable compliance depart from this pattern, varying differently and much less strongly with composition than the predictions of either single or double reptation. It thus seems that microstructural blends may behave in significantly more complex ways than molecular weight blends even for components with only weak dispersive interactions and rather modest differences in glass temperature and plateau modulus.Dedicated to Prof. John D. Ferry on the occasion of his 85th birthday.     

Glass transition of polymer/single-walled carbon nanotube composite films

The glass-transition temperatures (T_g's) of nanocomposites of polystyrene (PS) and single-walled carbon nanotubes were measured in the bulk and in thin films with differential scanning calorimetry and spectroscopic ellipsometry, respectively. The bulk T_g of the nanocomposites increased by approximately 3 °C and became much broader than that of PS. For the nanocomposite films thinner than 45 nm, T_g decreased with decreasing film thickness [i.e., ΔT_g(nano) < 0]. This phenomenon also occurred in thin PS films, the magnitude of the depression in PS [ΔT_g(PS)] being somewhat larger. The film thickness dependence and the differences in the magnitude of ΔT_g in the two systems were examined in light of current theory, and a quantitative comparison was made. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 3339–3345, 2003     

Constraining the low-energy pion electromagnetic form factor with space-like and phase of time-like data

The Taylor coefficients c and d of the EM form factor of the pion are constrained using analyticity, knowledge of the phase of the form factor in the time-like region, 4m _π ²≤t≤t _in and its value at one space-like point, using as input the (g−2) of the muon. This is achieved using the technique of Lagrange multipliers, which gives a transparent expression for the corresponding bounds. We present a detailed study of the sensitivity of the bounds to the choice of time-like phase and errors present in the space-like data, taken from recent experiments. We find that our results constrain c stringently. We compare our results with those in the literature and find agreement with the chiral perturbation-theory results for c. We obtain when c is set to the chiral perturbation-theory values.     

Theory of unitarity bounds and low-energy form factors

We present a general formalism for deriving bounds on the shape parameters of the weak and electromagnetic form factors using as input correlators calculated from perturbative QCD, and exploiting analyticity and unitarity. The values resulting from the symmetries of QCD at low energies or from lattice calculations at special points inside the analyticity domain can be included in an exact way. We write down the general solution of the corresponding Meiman problem for an arbitrary number of interior constraints and the integral equations that allow one to include the phase of the form factor along a part of the unitarity cut. A formalism that includes the phase and some information on the modulus along a part of the cut is also given. For illustration we present constraints on the slope and curvature of the K_l3 scalar form factor and discuss our findings in some detail. The techniques are useful for checking the consistency of various inputs and for controlling the parameterizations of the form factors entering precision predictions in flavor physics.     

Patterned removal of molecular organic films by diffusion

We demonstrate that "contact patterning" subtractively patterns a wide range of molecular organic films of nanoscale thickness with nanometer-scale accuracy. In "contact patterning", an elastomeric stamp with raised features is brought into contact with the organic film and subsequently removed, generating patterns by the diffusion of the film molecules into the stamp. The mechanism of material removal via diffusion is documented over spans of minutes, hours, and days and is shown to be consistently repeatable. Contact patterning provides a photolithography-free, potentially scalable approach to subtractive patterning of a wide range of molecular organic films.