Structure and Function of Bottlebrush Polymers
Pesek, Stacy Lee
Doctor of Philosophy
Bottlebrush polymers, also known as molecular brushes, are a special class of polymers characterized by having a “side-chain” polymer grafted to every repeating unit along the main chain polymer “backbone”. This high grafting density is the source of unique and interesting physical properties, many of which are still actively being studied. The physical size and structure of bottlebrush polymers is governed by the length (degree of polymerization, DP, or molecular weight, Mn) of the side-chain and backbone polymers. The chemical properties (such as solubility, responsive behavior, etc) are governed primarily by side-chain polymer type or monomer selection. Together, the side-chain and backbone polymers determine bottlebrush molecular structure properties such as size, shape and stiffness. The type of side-chain polymer dictates bottlebrush functional properties, such as interaction with environment, hydrophobicity, and self-assembled structures. The main objective of the work described in this thesis is to develop an improved understanding of the relationship between bottlebrush structures and resulting material properties such that bottlebrush materials can be tailored to meet functional needs for target applications. These applications include drug delivery, where the size and shape of the bottlebrush in solution are important and as thin film coatings, where the side-chain structure and flexibility determines surface properties. The first chapter introduces bottlebrush polymers and discusses their synthesis and physical properties. The second chapter describes a study of the solution conformation of a series of bottlebrush polymers with polystyrene side-chains. The structure of the bottlebrushes was changed by varying the side-chain and backbone polymer lengths (Mn), which was quantified by nuclear magnetic resonance (NMR), gel permeation chromatography (GPC) and light scattering analysis. Solution conformation of bottlebrush polymers in toluene (a good solvent for polystyrene side-chains) was determined by fitting theoretical models to experimental small-angle neutron scattering (SANS) data. In this first study we measured a conformation change from globular molecules to highly elongated cylinders when bottlebrush backbones exceed 100 repeat units. Chapter three details a similar SANS study on bottlebrush polymers with biocompatible poly(lactic acid) (PLA) side-chains. Well-defined PLA bottlebrushes with two side-chain lengths and systematic variations in backbone length were made, resulting in a total of nine bottlebrush samples. The samples display a similar change in conformation from globular to elongated after the backbone length exceeds 50 units for both side-chain lengths. Bottlebrush radii were dependent primarily on side-chain length, and bottlebrush length increases with backbone size, as expected. By fitting the results for bottlebrush radii with a power-law model, we found side-chain polymer conformation scales with length as ν = 0.56 ± 0.013, consistent with a 3-dimentional self-avoiding walk. Next, we studied bottlebrush polymers as additives to thin films that spontaneous migrate to the top film surface, allowing small amounts of polymer to significantly change the surface properties in polymer coatings. Novel poly(dimethyl siloxane) (PDMS)- based bottlebrush polymers and PDMS-PLA bottlebrush copolymers were synthesized and cast in films as additives. Surface properties of bulk polymer and polymer with 1 or 5 wt % bottlebrush copolymer additives were characterized by contact angle, X-ray photoelectron spectroscopy (XPS) and microscopy. Mixed-arm PDMS/PLA bottlebrush copolymers spontaneously segregate to the top interface with large surface excess, which result in changes in water contact angle of over 30 ° in some cases. In most cases, these bottlebrush polymers did not induce phase separation in thin films. Bottlebrush polymer segregation in thin films is attributed to both entropic demixing of branched species from a linear matrix and enthalpic migration to the polymer-air interface with a low-surface energy polymer (PDMS). In the fifth chapter we present bottlebrush copolymers with poly(ethylene gycol) (PEG) and PLA components with hydrophobic cores for drug encapsulation and release applications. Bottlebrush copolymers were synthesized with “core-shell” architecture from PLA-block-PEG linear side-chains resulting in a hydrophobic core and hydrophilic, water soluble shell. A second type of bottlebrush copolymers, “blocky” bottlebrushes, was synthesized with a block copolymer backbone from PLA and PEG homopolymer side-chains. These “blocky” bottlebrushes self-assemble in water to form large, spherical micelle structures. Bottlebrush copolymers were characterized by SANS, dynamic light scattering (DLS), and electron microscopy to determine size and shape of bottlebrush materials. An outlook on future applications of bottlebrush polymers and preliminary data is presented in chapter six. Small amounts of nanoparticle additives have been shown to have significant changes in composite thermal properties. Using a hypothesis presented in recent literature, we calculate the surface free energies of bottlebrush polymers and linear host polymers (PS and poly(methyl methacrylate) (PMMA)) and predict a directional change in host glass transition temperature. Bottlebrush polymer additives in bulk polymers are prepared from 1 – 40 wt % additives and composite glass transition temperature is measured by differential scanning calorimetry (DSC). Bottlebrush polymers could also be beneficial as surface coatings for anti-biofouling applications. Brush polymers and thermoresponsive materials, such as poly(N-isopropyl acrylamide) (PNIPAAm) have demonstrated decreased adhesion from fouling biomass. We discuss the synthesis of crosslinkable PNIPAAm bottlebrush polymers and the preparation of thin film coatings to be studied for anti-biofouling.