Volume 111, Issue 13, 01 October 1999
Index of content:
111(1999); http://dx.doi.org/10.1063/1.479863View Description Hide Description
A variable temperature scanning tunneling microscope was used to directly observe the thermally induced rotation of a single acetylene molecule between two equivalent orientations on Cu(001) above 68 K. Measurements of the rotation rate as a function of temperature yielded an energy barrier of meV and a preexponential factor of Thermal diffusion of individual acetylene molecules was monitored above 178 K by single molecule tracking. A diffusion barrier of eV and a preexponential factor of were determined by measuring the hopping rate as a function of temperature.
Glass transition of one-dimensional molecular chains of p-nitroaniline confined in nanopores revealed by dielectric spectroscopy111(1999); http://dx.doi.org/10.1063/1.479864View Description Hide Description
The dynamics of molecular chains of hydrogen-bonded p-nitroaniline (pNA), confined in the one-dimensional pore system of was studied by broadband dielectricspectroscopy Hz). Two relaxation processes were observed. A fast process (the process) shows above a Vogel–Fulcher–Tammann behavior typical of glass-forming materials, indicating cooperative dynamics of the “condensed” pNA chains. Below the glass temperature the -relaxation peak broadens significantly and obeys an Arrhenius law. Both observations imply a change in length scale of cooperativity from a temperature-controlled to a chain-length-dependent one. The second relaxation process is tentatively assigned to structuralrelaxations of the host lattice itself, since it is also present in calcined (empty) crystals.
111(1999); http://dx.doi.org/10.1063/1.479865View Description Hide Description
We analyze how the addition of a small number of polymer molecules influences the diffusion constant of a spherical tracer, whose radius is small compared to the size of the polymer. We show that the polymer chain can be regarded as a two-dimensional object which is an impenetrable obstacle for the tracer. It is also shown that the diffusion constant of the tracer, in contrast to the solutionviscosity, is independent of chain length, depending only on the monomer concentration.