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We study the formation of novel bicontinuous phases in binary mixtures of AB diblock copolymers (DBCP) using the polymeric self-consistent field theory. We predict that the bicontinuous double-diamond (DD) and plumber’s nightmare (P) phases, which are metastable phases of neat diblock copolymers, could be stablized in gyroid-forming A-minority DBCPs via the blending of specifically designed A-majority DBCPs. The mechanisms of stabilizing different bicontinuous phases are revealed by analyzing the spatial distribution of the different DBCPs. It is found that the A-majority DBCPs residing mainly in the nodes of the structure, thus alleviating the packing frustration of the A-blocks. Furthermore, a local segregation of the two DBCPs occurs at the AB interface, thus regulating the local curvature of the interfaces. A synergetic interplay of these two mechanisms results in a larger stable region of the DD and P phases via the addition of tailored A-majority DBCPs. The theoretical study provides an efficient route to obtain novel bicontinuous phases. / Thesis / Master of Science (MSc)

Microgels, colloidal networks of crosslinked water-soluble polymers with dimensions < 1 μm, have been demonstrated to be useful materials in a wide range of biomedical and environmental applications. In particular, temperature-responsive microgels based on poly(N- isopropylacrylamide) (PNIPAM) have attracted significant research interest in drug delivery applications. However, conventional precipitation-based PNIPAM microgels are functionally non-degradable, problematic for biomedical applications. To resolve this issue, a thermally- driven self-assembly approach based on hydrazide and aldehyde functionalized PNIPAM oligomers to form an acid-labile hydrazone bond was developed in the Hoare Lab to produce thermoresponsive, colloidally stable, monodisperse and degradable microgels. In this thesis, the internal structure of these self-assembled microgels was investigated using small and ultra-small angle neutron scattering and surface force experiments. Contrary to expectations based on the assembly technique, all these characterization strategies suggested that self-assembled microgels have a homogeneously cross-linked internal structure. It is anticipated that these well-defined degradable and homogeneous nanoscale gel networks offer opportunities for addressing challenges in drug delivery, biosensing, and optics by exploiting the predictable diffusive and refractive properties of the homogeneous microgel networks. In addition, the co-self-assembly of a moderately hydrophobic anti-inflammatory drug (dexamethasone) during the microgel self-assembly process was demonstrated to enable five-fold higher drug encapsulation (75-80%) relative to the conventional partition/diffusion- based drug loading processes. This result addresses a key challenge in delivering hydrophobic drugs using conventional precipitation-based microgel systems due to the inherent hydrophilicity of the crosslinked network. The potential of the self-assembly approach to fabricate multi-responsive smart microgels was demonstrated by incorporating pH-ionizable functional groups (via the copolymerization of acrylic acid and 2-dimethylaminoethylmethacrylate to introduce anionic and cationic charges respectively) into the hydrazide and aldehyde-functionalized precursor polymers prior to self-assembly. The self-assembled charged microgels showed the same pH- responsive swelling behaviours of conventional microgels, including amphoteric microgels that can be formed at any desired cationic:anionic charge density by simply mixing different ratios of cationic and anionic precursor polymers. Such microgels offer significant potential to improve the performance of microgels in applications demanding dual pH/temperature specific drug delivery. / Thesis / Master of Applied Science (MASc) / Medications can exist in many different forms. From pills to injections, existing drug delivery systems require a high frequency of drug administration and often result in low efficacy of drug once administered to the human body. Polymer-based drug delivery systems have the potential to improve this delivery. In particular, microgels, water-filled crosslinked polymer networks with a size less than one micron, offer promise as a drug delivery vehicle. The size and chemical composition of microgels can be tailored to enable their use in a wide array of drug delivery applications. In addition, microgels can be loaded with a therapeutic agent and transported in the blood stream to deliver drug at a rate and/or location tunable based on the internal structure of the microgel. “Smart” microgels have the particularly attractive ability to change their properties in response to certain environmental stimuli (i.e. temperature or pH). However, current smart microgel systems are non-degradable and would accumulate in the body, causing undesired side-effects. In this thesis, a new self-assembly approach has been used to produce degradable microgels with the potential to switch properties in response to both temperature and pH. Water-insoluble drugs can be encapsulated more efficiently with this method, and the dual-responsive behaviour is expected to improve our capacity to deliver drug at the rate and location desired in the body.

The chemistry of the non-covalent bond has developed rapidly over the last few decades. In particular, the successful construction of nanoscale assemblies by non-covalent forces has been described more frequently in the recent literature. This significant progress is largely due to transferring of concepts found in the biological systems (e.g., the tobacco mosaic virus and the DNA double helix) to the area of synthetic chemistry. As an example, the architecture of the double helix, perhaps the most well-known biological self-assembling structure, remarkably demonstrates the ability of biological systems to construct large supramolecules by multiple aggregations of relatively simple building blocks by means of hydrogen bonding. Scientists have begun to employ such synthetic strategy adopted in Nature to construct nanoscale systems. The use of pseudorotaxane assemblies formed between the suitably sized crown ethers and dipyridinium salts (paraquats) or dibenzylammonium ions is a viable synthetic strategy to construct non-covalent systems because of their selectivity and strong hydrogen bonding ability. We describe the syntheses and characterization of non-covalent assemblies of different sizes and shapes via the pseudorotaxane approach. A series of dendritic pseudorotaxanes were efficiently prepared from self-assembling complimentary building blocks, namely a triply charged ammonium ion and the 1st, 2nd, and 3rd generations of benzyl ether dendrons bearing dibenzo-24-crown-8 moiety. The wholly complexed self-assembling dendrimers were evidenced by 1H NMR spectroscopy and mass spectrometry. Linear supramolecular pseudorotaxane polymers were formed with reversible chain extension in solution by self-assembly of two complimentary homoditopic molecules with secondary ammonium ion and dibenzo-24-crown-8 moieties. The fraction of the cyclic dimer and the size of the linear suprastructures were determined in solution by 1H NMR spectroscopy as a function of concentration. Viscosity measurements corroborated the presence of aggregates of large hydrodynamic volume at high concentrations. The solid state samples of the supramolecular polymers, prepared by freeze-drying, were analyzed by DSC and optical microscopy and shown to be distinct from the starting materials and the cyclic dimer. Fibers and films were formed from high concentration solutions, corroborating the polymeric nature of the aggregates. Similarly, polymolecular arrays were formed in solution from a heteroditopic self-complimentary molecule, comprising bis-m-phenylene-32-crown-10 and a paraquat unit. Side-chain polypseudorotaxanes were prepared from spontaneous association of polymethacrylates bearing dibenzo-24-crown-8 and secondary ammonium ions. The complexation behavior in solution was investigated using 1H NMR spectroscopy. The solid state samples of side-chain polypseudorotaxanes, prepared by freeze-drying, showed noticeable changes in thermal behavior and morphology from the individual components. / Ph. D.

Rotaxanes can be synthesized using a thermodynamically driven approach of self-assembly. The thermodynamically driven approach is an efficient method to provide a better controlled synthesis of specific structures. This synthetic approach takes advantage of a labile bond between the guest molecule and the end stopper group. The reversibility of this bond allows for threading by the host molecule via chemical equilibrium. Intramolecular interactions such as hydrogen bonding and π-π stacking facilitate threading to form the pseudorotaxane which is endcapped to form the thermodynamically stable rotaxane. In this work, the synthesis and characterization of rotaxanes using a thermodynamically driven approach is reported. New OH-functionalized secondary dibenzyl ammonium hexafluorophosphate and tetrafluoroborate salts were synthesized and complexed with dibenzo-24-crown-8. The complexation between the salts and dibenzo-24-crown-8 was observed using 1D and 2D ¹H NMR spectroscopy. An association constant of 110 M⁻¹ was determined by integration for the pseudorotaxane from the ammonium hexafluorophospate salt and dibenzo-24-crown-8. The new guest species were endcapped in situ as trityl ethers to form new thermodynamically stable rotaxanes. Further work to pursue would include synthesis of rotaxanes using functionalized crown ethers for polymerization to make polyrotaxanes and synthesis of self-assembled polymers using this synthetic method. / Master of Science