Volume 110, Issue 22, 08 June 1999
Index of content:
- POLYMERS, BIOPOLYMERS, AND COMPLEX SYSTEMS
110(1999); http://dx.doi.org/10.1063/1.479046View Description Hide Description
In a recent paper DiMarzio and Yang [J. Res. Natl. Inst. Stand. Technol. 102, 135 (1997)] predicted that transport properties such as viscosity and diffusion coefficient do not follow the typical Williams, Landel, and Ferry (WLF) [J. Am. Chem. Soc. 77, 3701 (1955)] or Vogel–Fulcher-type of temperature dependence as the glass transition is approached. Rather, a transition to an Arrhenius-type of temperature dependence is predicted. Here we describe long term aging experiments that explore the temperature dependence of the viscoelastic response of polycarbonate in the vicinity of the glass transition.Aging the material for long times below the nominal glass transitiontemperature, assures that equilibrium is attained and we can directly test the DiMarzio–Yang prediction. In tests in which glassy samples of polycarbonate were aged into equilibrium at temperatures up to 17 °C below the conventionally measuredglass transitiontemperature, we find that the results are consistent with a transition from Vogel–Fulcher or WLF-type behavior to Arrhenius-type behavior. Our results are discussed within the context of other measurements on nonpolymeric glasses and other recent results on polymeric glass formers.
110(1999); http://dx.doi.org/10.1063/1.479042View Description Hide Description
Sorption of in both the glassy and the rubbery state of an amorphous polyethylenelike polymer was investigated using molecular dynamics simulations. The temperature was chosen such that the system was in its glassy state at low solute concentrations and its rubbery state at large solute concentrations. Both the pressure and the volume isotherms changed character at the transition concentration. The physical origin of these changes was clarified by investigation of the excess thermodynamic properties of the solute both below and above the transition concentration. Dynamical changes occuring at the glass transition were studied through the self-intermediate scattering function of the polymer atoms. This function was found to excellently reveal the difference between the dynamics of the glassy and rubbery state and therefore served as an independent tool monitoring the glass transition.
110(1999); http://dx.doi.org/10.1063/1.479043View Description Hide Description
We have carried out normal modeanalysis for four B-DNA molecules with different lengths and sequences. By focusing our attention on motions of the helical axes of the molecules and comparing them with those of the ideal rod obeying vibrational theory, we have shown that DNA molecules behave like isotropic and homogeneous elastic rods. Then, we have calculated dynamic constants representing their rigidities for bending and twisting. In the analysis of their twisting motions, we have shown that sliding and shifting motions between adjacent bases are important for the motions.
110(1999); http://dx.doi.org/10.1063/1.479006View Description Hide Description
The phase behavior near the order-disorder transition(ODT) in diblock copolymers was characterized by ultra-small-angle x-ray scattering (USAXS) method for two polystyrene-block-polyisoprene copolymers having about equal block volume fraction (f is the volume fraction of PS) but different molecular weights. By using USAXS method, the peak width and peak intensity were found to change by about 2 orders of magnitude across the ODT temperature for both Furthermore, in a narrow temperature region very close to the ODT, it is revealed that the coexistence of the ordered and the disordered phases occurs at thermal equilibrium for both The systematic results in the present study clearly show the thermal fluctuation effects on the phase diagram in the low molecular weight diblock copolymer systems.
The influence of intramolecular chain dynamics on the diffusion of small penetrants in semicrystalline aromatic polymers110(1999); http://dx.doi.org/10.1063/1.479044View Description Hide Description
A recently developed molecular model for diffusion in dense aromatic polymers, which attempts to explain penetrant jump frequencies in terms of phenyl ring partial flips, is investigated via molecular simulation. The model polymer system under consideration in this paper corresponds to the interfacial domains generated by the lateral chain invariant (LCI) grain boundaries between crystallites of a stiff chain polyamide, poly(p-phenylene terephthalamide) (PPTA), and the low molecular weight penetrant selected for study is water. Fully atomistic constraint molecular dynamics simulations are conducted with interatomic and intramolecular interactions described by the DREIDING potential [S. L. Mayo, B. D. Olafson, and W. A. Goddard III, J. Phys. Chem. 94, 8897 (1990).] The coupling of the diffusive motion of water with the local polymer chain dynamics is examined at two temperatures and over a range of grain boundary densities.