The degree of polymerization (DP n ) of poly(p-phenylene) (PPP) 3 with appendant third-generation, Fréchet-type dendrons was determined as 110 after a chemical modification. This DP n is extraordinarily high and shows that the mechanistically complex Suzuki polycondensation can thus be employed even in the case of sterically highly loaded (dendronized) AA-type monomers, which paves the way to obtain extremely rigid nanorods. A molecular model of 3 was obtained by molecular dynamics simulations which show that the diameter of the rodlike dendrimer fluctuates between approximately 2 and 4 nm in vacuo. Scanning force microscopy (SFM) on 3 adsorbed on graphite indicates the formation of multilayer films made of densely packed nanorods. SFM with molecular resolution reveals highly ordered domains in which the molecules are packed parallel to each other, separated by a lateral periodicity of 4.8 ± 0.5 nm. Lateral and vertical spacings indicate that the (110) plane of the densely packed nanorods is exposed at the film surface. SFM further indicates that different domains exhibit three molecular orientations, reflecting the 3-fold symmetry of the graphite substrate. Chain ends can be resolved at grain boundaries. Time-dependent SFM experiments show reorientation of small domains on a time scale of minutes.

No abstract

Scanning tunneling microscopy (STM) studies at the interface between the basal plane of graphite and organic solutions or melts of long chain alkanes and alkyl derivatives reveal that the molecules order in lamellae with the main molecular axes oriented parallel to the substrate. Here we employ molecular dynamics (MD) simulations to obtain more details on the molecular order and dynamics within the alkane lamellae as a function of density. We find that the orientation of the molecular carbon zigzag planes relative to the graphite is governed by a subtle interplay of packing and entropic effects. In addition, we consider multiple layer adsorption and investigate the rapid loss of order with increasing distance from the interface. Finally, we study the diffusive behavior of an isolated long chain alkane, C350H702, on graphite, which is of interest in the context of STM imaging of isolated macromolecules at interfaces. The sensitive dependence on atomic parameters renders MD simulations a valuable complementary tool for the detailed interpretation of STM data.

indicated by the topography of the films shown in Figure 6 (1c and 2c). Studies by Raman spectroscopy and X-ray diffraction are in progress in our laboratory to investigate the effect of canalization and a possible one-pot intercalation on these systems. Work is in progress to assemble an electrochromic device using this material and to study its photoelectrochemical properties.

High‐density polyethylene (HDPE) and isotactic polypropylene (PP) were mixed either with a stabilizer or with a stabilizer and a compatibilizer in different mixing ratios. The structure and properties of these blends were analyzed by methods such as torsion pendulum measurements, mechanical short time experiments, electron microscopy, and fracture mechanical toughness tests. The results display a strongly increased impact strength in the HDPE/PP blend with compatibilizer within a specific mixing region. The deformation behavior and the mechanism leading to the increased impact strength of the blends were investigated in tensile tests by acoustic emission analysis and scanning electron microscopy: Increased fibrilation and strong strain was registered in the blend with compatibilizer. The impact strength was modeled, using experimentally measured properties such as energy release rate, matrix and inclusion volumes, the impact strength of each component. The inclusion volume that causes plastic deformation was chosen as an additional parameter. The calculated results are in good agreement with the experimental ones.

No abstract

Molecular dynamics simulations of pure benzene and a poly(oxyethy1ene) chain in benzene are performed. The simulation of pure benzene is found to agree excellently with previous simulations despite using a different force field. A comparison is made between the results of simulations of the poly(oxyethy1ene) chain in benzene and in water and of stochastic simulations with respect to mean torsional angles, translgauche fractions, and transition rates. Characteristic deviations are found for the simulation in water and explained by specific atomic interactions, while there is satisfactory agreement with a stochastic simulation based upon the simple Langevin equation using a friction coefficient of 1 ps-I. The characteristic ratio of poly(oxyethy1ene) in benzene is calculated on the basis of the rotational isomeric state model. 0 1992 by John Wiley 8, Sons, Inc.

No abstract