UC Santa Barbara

Polysiloxanes, commonly known as silicones, are ubiquitous in daily life, serving as key components in numerous commercial products. From lubricants to flexible electronics, polysiloxanes are used in a vast range of applications due to their desirable and unique combination of material properties that arise from the distinctive character of the siloxane (Si–O) bonds that comprise the silicone polymer backbone. The development of next-generation polysiloxanes is crucial for improving both the efficacy and the sustainability of silicone-based materials.Chapter 2 and Appendix A present our work developing a method to prepare polyborosiloxane networks via hydrosilylation. Polyborosiloxane networks synthesized this way from readily available building blocks cure in ∼2 min at convenient temperatures (e.g., 90 °C) and exhibit enhanced viscoelastic behavior when compared to traditional polyborosiloxane networks fabricated via the conventional condensation route. By virtue of using efficient hydrosilylation chemistry, another key advantage of this synthetic platform is the ability to synthesize dynamic polyborosiloxanes with different network connectivity by simply using silicones with Si–H moieties placed at the chain ends or distributed throughout the repeat-unit structure. The availability of other alkenes amenable to hydrosilylation provides an additional formulation handle to synthesize mixed dynamic–static networks with tunable control over stress relaxation and solvent resistance. In summary, this synthetic approach is a simple and accessible platform for preparing dynamic polyborosiloxanes. The work detailed in Chapter 3 and Appendix B builds upon that project by seeking to manipulate the rate of stress relaxation in polyborosiloxane networks through the design of a series of second-generation crosslinkers that feature a tunable chemical handle. Next-generation polyborosiloxane networks are fabricated from a backbone functionalized polydimethylsiloxane derivative and each of these crosslinkers via a hydrosilylation cure. Crucially, it is this hydrosilylation chemistry that enables the use of these ditopic designer crosslinkers. Rheological experiments demonstrate that the rate of stress relaxation of these networks could be readily tuned by varying the structure of the borosiloxane crosslinker. In Chapter 4 and Appendix C, we report the preparation and characterization of carbosiloxane bottlebrush networks with enhanced performance and recyclability compared to their traditional siloxane analogues. The preparation of these materials is enabled by the synthesis of well-defined heterotelechelic macromonomers with Si–H and norbornene chain ends via anionic ring-opening polymerization of the hybrid carbosiloxane monomer 2,2,5,5-tetramethyl-2,5-disila-1-oxacyclopentane. These novel heterotelechelic α-Si–H/ω-norbornene macromonomers undergo efficient ring-opening metathesis copolymerization to yield functional bottlebrush polymers with accurate control over molecular weight and functional-group density. Si–H groups retained at the ends of side-chains after ring-opening metathesis copolymerization allow for the preparation of super-soft networks via hydrosilylation. In contrast to traditional PDMS systems, the incorporation of poly(carbosiloxane) side chains allows the resulting networks to be recycled back to the original monomer (>85% recovery) via depolymerization at elevated temperatures in the presence of base catalysts. Recovered monomer was successfully repolymerized through anionic ring-opening polymerization with no decrease in structural fidelity or activity. In summary, this combination of unique (macro)monomer design and bottlebrush architecture creates new opportunities in sustainable practices by offering a robust, recyclable alternative to commercial silicone-based materials. In Chapter 5 and Appendix D, we examine the fundamental interactions of a novel siloxane-containing methacrylate monomer (MD′M-ALMA) with poly(dimethylsiloxane) and poly(methyl methacrylate). Well-defined block, random, and block-random copolymer libraries of these three building blocks were synthesized and characterized. Simply by changing the feed ratio of the methacrylate monomers, we observed dramatically different physical properties and segregation strengths in the final block-random copolymers ranging from viscous disordered liquids at high MD′M-ALMA loadings to well-ordered lamellar structures at high methyl methacrylate loadings. Given these striking changes, MD′M-ALMA shows promise as a potential compatibilizing agent capable of bridging the gap between silicones and traditional organic polymers. Chapter 1 offers an introduction to polysiloxanes. Chapter 6 presents an outlook on the work presented in this dissertation. Appendix E details a collaborative effort exploring the tribological properties of pH-responsive hydrogels in extreme environments.