Oligodimethylsiloxanes with Charged Chain Ends: Synthesis, Characterization, and Properties

Charged polymers have been intensively studied because of their scientific and industrial importance. In certain cases, a single charged group in the chain end can alter the structure and properties of a polymer dramatically. On the other hand, oligomers, which are different from both discrete small molecules and higher molecular weight polymers, have become an emergent class of materials. There are several charged chain-end functionalized oligomers reported, but this area has been largely unexplored. The work described in this dissertation is aimed at using well-characterized end-group functionalized oligodimethylsiloxane (oDMS) as model materials to understand the effects of charged end groups on the structural organization of amphiphilic oligomers in bulk and across the interface, which aides in designing novel amphiphilic materials with desirable properties for certain applications, such as the mimicked lipidic memcapacitor for neuromorphic computing.

The first part of this work focuses on the synthesis of chain-end functionalized oDMS. Mono-ionic/zwitterionic chain ends were introduced from the hydroxyl terminal groups of oDMS. Cations and anions influenced their thermal degradation temperatures (T_d) of the resultant products with most chain ends increasing the T_ds, except morpholinium bromide, which decreased.

The second part includes the phase behavior of amphiphilic oDMS. These oligomers self-assembled into well-defined morphologies including lamella and hexagonally packed cylinders as revealed by small-angle x-ray scattering measurements. The domain spacings ranged from 6.04 to 8.34 nm with different order-disorder transition temperatures. To further understand the phase behaviors during the heating/cooling processes, ion conductivities were measured by dielectric spectroscopy and the data shows that increased conductivity suggests a more strongly connected domain.

The third part studies the molecular organization of amphiphilic oDMS on oil/aqueous interfaces using sum frequency generation (SFG) spectroscopy. SFG revealed that charged chain ends significantly impact the orientation of the hydrophobic oDMS tails in hexadecane/aqueous interfaces. Moreover, by spectral fitting, the changes of the oDMS conformations at the interface can be elucidated.

Finally, we also offer a summary and an outlook in this exciting area.