UC Berkeley

In this thesis, challenges associated with the creation of anti-fouling surfaces and polymer-based electrolytes are addressed using a versatile polymer chemistry that affords control over material functionality through the manipulation of the polymer’s side chain composition. The polymers used in this thesis generally have a backbone comprised in-part or wholly of poly(ethylene oxide) (PEO) with pendant allyl side groups, to which various moieties can be attached using a straightforward thio-ene click. I describe the attachment of different side groups to design polymers with desirable properties, then characterize these polymers’ performance when used as anti-fouling surface coatings and electrolytic solutions.

The development of high lithium cation transference number electrolytes, those in which the ionic conductivity is carried predominantly by the lithium ion instead of its counteranion, could limit concentration polarization that plagues current battery electrolytes. The typical approach for achieving high transference number electrolytes is to immobilize the anion on a polymer to limit anion motion. We designed a lithium neutralized PEO backbone with pendent sulfonate groups to use as a dry single ion conductor. Interestingly, these sulfonate functionalized materials self-assembled into very well ordered, small (2.5nm) lamellae, which result from aggregation and phase segregation of the attached ions from the polymer backbone. Unfortunately, given that the ions are not solvated (i.e., dissociated) by the polymer backbone, no conductivity was observed in these materials, including at high (120°C) temperatures. The lack of ion dissociation suggests that a solvent with higher Lewis basicity or acidity than the PEO backbone is needed to disrupt the contact ion pairs and allow conductivity.

Polyelectrolyte solutions were therefore prepared from the polyanion described above and a strong Lewis basic solvent, dimethyl sulfoxide. These solutions exhibited an excellent combination of high ionic conductivity and Li+ transference number (e.g., 1.2 mS/cm and 0.99, respectively, at 25°C). In addition, this system is tunable (polyanion molecular weight and charge concentration can be manipulated), and provided a platform to explore the tradeoff between conductivity and transference number in liquid electrolytes with an unprecedented combination of both. Taking advantage of this tunability, I explored the effect of temperature, polymer molecular weight, solvent choice, and small molecule salt addition on the solution transport properties. Li+ transference number was found to be temperature independent, as expected for solutions that are fully dissociated at all temperatures measured. Scaling theories for molecular weight dependence of diffusion coefficients described the polymer behavior well but did not capture the behavior of the lithium cation, which was generally found to be independent of polymer molecular weight. Solutions made in a variety of solvents all displayed high cation transference number and high conductivity, but there was no simple relationship between solvent and electrolyte properties. Finally, when mixed into the polyelectrolyte solution, small molecule salts, such as LiTFSI, increase the solution conductivity, but at a substantial cost to Li+ transference number.

In another application, I designed polymer materials that prevent marine fouling, the attachment of organisms to human-made structures in marine environments. Amphiphilic polymers have been shown to be effective as antifouling materials against a variety of organisms and organic compounds. We therefore designed a polymer platform to present sequence-specific peptoids at the surface of thin film coatings. These amphiphilic coatings were used to study the effect of hydrophobic and hydrophilic residue sequence on antifouling performance. The polymer platform had a polystyrene (PS) anchoring block, and a PEO presentation block to which peptoids were attached. We showed that fluorinated moieties are very surface active and direct the surface composition of the polymer thin film. The position and number of fluorinated groups in the polypeptoid was shown to affect both the surface composition and antifouling properties of the film. Specifically, the position of the fluorinated units in the peptoid chain changes the surface chemistry and the antifouling behavior, while the number of fluorinated residues affects the fouling release properties.

In addition to the peptoid sequence, the specific composition of the polymer platform is an important consideration for film stability and surface presentation. The anchoring block molecular weight plays an important role in film stability, but also affects the overall film hydrophobicity, which adversely affects antifouling performance. It was found that the coatings using an entirely hydrophobic scaffold, polystyrene-b-poly(dimethyl siloxane), performed well against only one of the marine organisms studied (Ulva linza), but that hydrophilic coatings comprised of polystyrene-b-poly(ethylene oxide) performed well against all marine organisms studied, including U. linza and Navicula. Finally, it was shown that perfluorooctane thiol attached directly to the polymer chain can direct surface presentation of the entire polymer system, thereby allowing improved anti-fouling film formation.