Research Projects

Synthetic Materials

POSS - Siloxane Oligomers

One potentially interesting material is composed of POSS cages connected by a single oxygen atom. These systems, either isolated or combined with other polymer systems such as PMA, are expected to have unique physical and chemical properties. In an effort to understand the underlying basis for the characteristic properties of these materials, we have investigated small oligomers using ion mobility mass spectrometry (data obtained with MALDI-TOF instrument) and molecular modeling. An overview of some of our results is given below. Details can be found in "Structural Characterization of POSS Siloxane Dimer and Trimer," S. E. Anderson, C. Mitchell, T. S. Haddad, A. Vij, J. J. Schwab, M. T. Bowers, Chem. Mater. 2006, 18, 1490-1497

[(Cp7T8)2O]Na+We have investigated the dimer (Cp7T8)2O where the POSS cage capping agents are cyclopentyl groups (Cp). The sodiated dimer gives an ATD with a single narrow peak in our ion mobility experiment, indicating the presence of a single conformer family. The corresponding experimental cross section was determined to be 355 Å2. We also modeled this species with molecular mechanics, identifying two families of structures. Representative conformers from each family are shown in the figure at right as space-filling models (hydrogens omitted for clarity). In the lower-energy, more compact family, the Cp groups on the adjacent POSS cages are "staggered" with respect to each other due to a rotation of approximately 30° about the Si-O-Si bond connecting the two POSS cages. This family has an average cross section of 357 Å2, which is in good agreement with the experimental cross section. The higher-energy "eclipsed" form does not have this rotation, causing capping group repulsion and a small but measurable displacement of one cage relative to the other. The "eclipsed" conformers have an average cross section of 365 Å2, 3.1% larger than the experimental value. For relatively simple structures such as these, agreement between experiment and theory should be of the order of 1% and certainly no larger than 2%. Hence, exclusive formation of the "staggered" family of structures is indicated. This system gives us an idea of the lower limit for cage-cage separation in POSS-containing polymers. The "staggered" conformers have separations of 8.3 to 8.5 Å. This should be very near the minimum possible separation since the two cages are held together by a single oxygen atom and have rotated to best accommodate the capping groups. This distance will be used as a benchmark to characterize cage-cage interactions in oligomeric systems with large, more flexible backbones.

We have also investigated the POSS siloxane dimer with cyclohexyl capping groups (Cy) in place of the Cp groups. Again, we observed an ATD with a single narrow peak. The modeling, however, revealed that the (Cy7T8)2O·Na+ system is somewhat more complicated than the corresponding Cp system. The Cy groups can be attached to the POSS cage either through an axial or equatorial bond. When connected through an axial bond, the Cy group is folded into the POSS cage and the overall cross section of the dimer is reduced. Conformers with 2 to 4 folded Cy groups match the experimental cross section of 402 Å2. Dynamics calculations suggest that interconversion between folded and unfolded Cy groups is facile and we believe a rapid equilibrium is taking place in our experiment. Thus we observe a single peak in the ATD corresponding to an average cross section for 2 to 4 folded Cy groups.

[Cy7T8O-Cy8T6D2-OCy7T8]Na+In POSS siloxane oligomers larger than dimer, the central POSS units have an open-cage structure where one of the Si-O bonds of the T8 cage is broken. The dangling oxygen atom from the broken bond then binds another POSS cage (substituting for an R-group) and the dangling silicon atom adds an oxygen that bridges it to a second POSS cage. We have investigated the trimer with Cy capping groups, Cy7T8O-Cy8T6D2-OCy7T8, where D = SiO. The ATD of the sodiated trimer has only a single narrow peak, consistent with a single family of structures. Modeling suggests that a low-energy exo-exo family of structures (consistent with retention of configuration in the synthesis from the exo-diol precursor) fits the experimental cross-section (σexpt = 557 Å2) only if several Cy-groups are folded on average (σth = 557 Å2) compared to all Cy groups being equatorial (σth = 570 Å2). The figure above shows an example of a theoretical structure obtained in our molecular modeling. The Cy groups shown in green display the folded (axial) and extended (equatorial) configurations. The other Cy groups and all of the hydrogens are omitted for clarity.

We plan to investigate larger siloxane oligomers as soon as they can be synthesized by our collaborators. With larger oligomers, in addition to studying the effects of increased length, we will also be able to gauge the influence of branching.