Hydrogels, a class of materials composed of polymer networks swollen with large amounts of water, have gained increasing attention in a range of fields, from tissue engineering to food science. However, synthetic hydrogels are known to be brittle and have poor mechanical properties, including low extension ratios and fracture toughnesses. It has been a great challenge to develop synthetic hydrogels with improved mechanical properties and correlate with their network composition. In this dissertation we developed two robust well-defined synthetic hydrogel systems by two simple strategies—thiol-norbornene chemistry and ring opening metathesis polymerization (ROMP). The swelling and mechanical properties were systematically characterized and correlated with their network structures. The PEG/PDMS hydrogels synthesized by the simple, efficient, photo-initiated thiol-norbornene chemistry have improved mechanical properties. By manipulating the volume fractions of the PEG and PDMS, a large range of water content (54%–97%) was achieved. The Young's modulus (E) was significantly improved by increasing the volume fraction of PDMS in the hydrogel, and the Voigt and Reuss models were used to quantify the relationship between the volume fraction and E. In addition, increasing the volume fraction and molecular weight of the PDMS led to tougher hydrogels with Gc more than 100 J/m2. Furthermore, a high resilience (more than 97%) was maintained across the entire range of strains, regardless of the composition of the PEG/PDMS hydrogels. Controlled polymerization provides another method to synthesize gels with improved mechanical performance. The properties of the ROMP-based gels were tuned by varying the initial molar ratio of the monomer to cross-linker from 7.5% to 20%. The mechanical properties of the gels were characterized via the cavitation rheology technique (CRT), which demonstrated a transition from reversible to irreversible deformation with the increase of the molar ratio. By combining CRT and contact mechanics, E and Gc for these gels were quantified. As the amount of the cross-linker increased, E increased while G c slightly decreased. Soft tissues with hierarchical structures share similar properties as synthetic hydrogels. The study of their mechanical properties would provide useful information in designing synthetic hydrogels with novel network structures. Eye lens was chosen in this study. The anisotropic mechanical properties of bovine eye lenses were measured using CRT over a range of length scales. E of the nucleus and cortex of the lens were determined, as approximately 11.8 and 0.8 kPa, respectively, on macroscopic length scales. We also measured the mechanical properties of the lens on a length scale of a single cell, suggesting that the stiffness significantly decreased from that in the bulk measurements for both the nucleus and cortex. In addition, during the growth of the cavity anisotropic propagation in the cortex was observed, while in the nucleus, the propagation was isotropic. We further explored the elasticity of the cavity deformation, showing both elastic and inelastic deformation occurred in the nucleus with equal contributions while deformation in the cortex was elastic and reversible. Lastly, we investigated adhesive properties of polymer networks with the thiol-norbornene chemistry to explore their possible applications. The mechanical and adhesive properties of these PDMS networks were quantified by the contact adhesion test (CAT), as well as DMA and compressive measurements. E of these end-linked PDMS measured by CAT was comparable with that measured by the compressive test. In terms of the adhesion properties, the energy release rate was described as a function of crack velocity. A higher molecular weight between cross-links led to a higher adhesion energy over a range of crack velocity for the PDMS with the same end-linking chemistry. Sylgard PDMS with 18 to 1 ratio was chosen to compare the mechanical and adhesive properties to the end-linked PDMS. With the similar Young's modulus and resilience, the adhesion energy of the Sylgard PDMS was comparable to that of the end-linked PDMS with 5 kDa molecular weight, which was likely to result from the comparable molecular weight between cross-links.

Conjugated polymers are of interest in organic photovoltaics (OPVs) for the benefits of their low cost, ease of processing, and flexible design. OPV device performance greatly depends on the morphology of the donor (conjugated polymer) and acceptor (fullerene, CdSe, etc.) materials, which should ideally promote efficient exciton formation, dissociation, and charge transport to the respective electrodes. One potentially ideal active layer morphology would consist of an interpenetrating, bicontinuous network of donor and acceptor materials, having domain sizes of ∼10 nm (i.e., on the order of the exciton diffusion length). Such morphologies can be achieved through annealing processes (thermal or solvent) of the donor/acceptor blend, or the use of pre-formed, highly crystalline fibril nanowires of the conjugated nanowires. This thesis outlines the design of functional polythiophene copolymers with the ability to form novel assemblies through tailored functionalities. Amphiphilic P3HT-b-poly(3-(triethyleneglycol)thiophene) (P3TEGT) diblock copolymers were synthesized that were capable of microphase separating based on the difference in polarity between the blocks. We targeted P3HT-b-P3TEGT thin-film morphologies, where we could orient [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) (electron acceptor) in the polar P3TEGT domain, to allow P3HT to form pristine crystal domains. Thermal annealing P3HT-b-P3TEGT diblock copolymers led to microphase separation, as characterized by atomic force microscopy (AFM) and small-angle x-ray scattering (SAXS), and OPV devices were fabricated and characterized using P3HT-b-P3TEGT/PCBM blends. P3HT-based diblock copolymers that had hydroxyl- and amine- functionalities were synthesized in an effort to utilize their nucleophilic nature for further functionalization. These copolymers underwent a solvent-induced crystallization that provided a P3HT nanowire decorated with the hydroxyl/amine functionalities on the fibril exterior. Previously, cross-linking mechanisms for polythiophenes had occurred in thin films upon thermal annealing or exposure to radiation, with little control over the crystallinity of the polymer. We were able to utilize fibrils formed from our functional diblock copolymers to covalently cross-link the crystalline structures by reacting with diisocyanates. This led to the formation of robust fibrils that had not previously been reported, that maintained photophysical and electronic properties to the unmodified nanowires. These nucleophilic fibrils were reacted with a bis-diisocyanate functionalized fullerene derivative to yield stabilized p-type/n-type nanowires. This process led to robust p-type/n-type fibrils that displayed photoluminescence quenching and high charge transfer characteristics that were not observed for p-type/n-type blends. In an effort to simplify the fibril cross-linking, we designed a P3HT-based diblock copolymer that had thioacetate functionalities, which were deprotected and underwent an oxidative cross-linking process. These polymers formed nanowires by solvent induced precipitation, and upon deprotection of the thioacetate groups by reaction with dimethyl amine, experienced oxidative cross-linking to achieve robust fibrils. This provided a novel system in that the polymer was able to cross-links itself, and the cross-linking process was found to be reversible by reduction chemistry.

This dissertation aims to both deepen and broaden our understanding of copolymers with pendent benzophenone (BP) in relation to both established applications and novel directions in materials science. Photo-reaction of these BP copolymers is explored in attempts to achieve three distinct goals: (1) robust and efficiently photo-crosslinkable solid polymer films, (2) photo-reacted polymer blends with disordered bicontinuous nanostructures, and (3) photo-patterned hydrogel materials with environmental UV stability. We begin by investigating the fundamental gelation behavior of solid polymer films, finding BP copolymers to be particularly effective crosslinkable materials. Gelation efficiency can be tuned according to comonomer chemistry, as BP hydrogen abstraction on the main polymer chain increases chain scission, reducing crosslinking efficiency. This knowledge is then applied in Chapter 3, wherein we discuss two potential methods for preparing nanostructured polymer blends from these copolymers, namely spinodal decomposition of a photo-crosslinked polymer blend and solution-state photografting to create interfacially active species. While each technique shows promise, the ultimate goal of a disordered bicontinuous morphology will require further tuning of materials systems and protocols. Finally, chemical deactivation of BP photo-crosslinker in copolymers for use as photo-patternable and environmentally stable hydrogel materials is investigated. Reduction of BP by sodium borohydride proves a feasible route toward deactivating residual photo-crosslinker in patterned hydrogel films. These results confirm the utility of copolymers with pendent benzophenone photo-crosslinkers as useful tools for complex material systems.

The process of molecular self-assembly has attracted tremendous attention due to its novel structural organizations and applications. The self-organization of amphiphilic molecules provides unique opportunities for designing novel materials for advanced nanotechnology. The development of molecular self-assembly involves the precise tailoring of chemical structures and the effective use of non-covalent forces. We are interested in the molecular design and synthesis of amphiphilic macromolecules that exhibit various self-assembled nanostructures. We modify the amphiphilic building blocks during their synthesis, investigate the structure-property relations of these amphiphilic molecules and explore their applications. Amphiphilic dendrimers are obtained when these building blocks are grown in a perfectly branched fashion, whereas amphiphlic homopolymers are produced from the linearly grown building blocks. We show that facially amphiphilic dendrimers exhibit significant difference in surface wettability due to subtle changes in structure. These amphiphilic dendrimers respond to the surface polarity and modify the polar surfaces from hydrophilic to hydrophobic. The monodendrons are capable of providing hydrophobic surfaces, while the didendrons provide superhydrophobic surfaces. This provides an example of how a molecular level change could result in dramatic changes in surface property. Amphiphilic homopolymer films have been immobilized onto substrates and shown to reduce protein adsorption, despite the high affinity of the hydrophobic or hydrophilic groups by themselves toward proteins. This protein-resistant property seems to arise from the unique molecular-scale alternation of incompatible functionalities. The combination of incompatible functionalities with a predefined alternating pattern within a monomer could provide a potential design for nonfouling materials. We also designed and synthesized proton conducting systems that derived from facially amphiphilic polymers. We show that our novel molecular design leads to organized supramolecular assemblies that dramatically enhance the anhydrous conductivity. We describe the design, synthesis, and characterization of these materials, which suggest that nanoscopic organization of proton conducting functionalities should be a key consideration in obtaining efficient anhydrous proton transport.

Nanoimprinting with anodized aluminum membranes was performed to produce nanoposts with controlled diameter and aspect ratio. The polymer nanoposts were found to remain upright and preserve the post-packing structure at low aspect ratios, but succumbed to elastocapillary coalescence at higher aspect ratios, with the morphology of the aggregates directly related to the aspect ratio of the polymer nanoposts. Replication of the replicated nanoposts at low aspect ratio was achieved to reproduce the pore packing structure of the original alumina membrane. Teflon microparticles were found to be effective stabilizers for inverse foams, producing dry water with excellent flow properties and contact stability and consisting of non-spherical liquid marbles 90-500 microns in diameter. The chemical inertness of the PTFE particles allowed for use of not only water, but also aqueous solutions of acids and bases and organic molecules including ionic liquids and water-soluble polymers. The teflon particle shell stabilized the liquid drop such that two particles containing two solutions which would ordinarily mix and/or react would remain separate. The wettability studies focused on demonstrating that entrapped gases are not responsible for Cassie superhydrophobic wetting behavior, that the removal of the pockets of air would not lead to Wenzel wetting behavior with an increase in contact angle hysteresis. The measurement of advancing and receding contact angles on surfaces with controlled topography consisting of square posts holes showed that the contact angles remained unchanged despite removal of over 90% of the air. It showed that water was not intruding into the hydrophobic topography because the Laplace pressure was thermodynamically preventing water from increasing its interaction with the topographically-patterned surface. Wettability studies were also aimed at extending our understanding of wettability as a one-dimensional phenomenon from the three-phase contact line perspective, by investigating the ability of hydrophilic arcs, short and long wedges, and the outlines of the wedges, to pin water drops on hydrophobic, low hysteresis surfaces. They were additionally aimed at studying the ability of hydrophilic lines to deform the three-phase contact line of a water drop and kinetically trap a water drop in a distorted shape on a hydrophobic surface.

Fabrication and characterization of different nanomaterials by using porous anodic aluminum oxide (AAO) templates were studied. Amorphous carbon nanotubes were prepared by casting thin films of polyacrylonitrile (PAN) and polystyrene-block-polyacrylonitrile (PS-b-PAN) within a AAO membrane followed by pyrolysis. Raman and wide angle X-ray diffraction (WAXD) measurements indicate that the carbon nanotubes are of low crystallinity. When diblock copolymers of PS-b-PAN were used, it was found that, nanopores were created within the nanotube walls after pyrolysis. Nanotubes of the cylinder-forming polystyrene-block-poly(ethylene oxide) (PS-b-PEO) copolymer nanotubes were generated. Because of the water solubility of the cylindrical PEO microdomains and the orientation of the cylindrical PEO microdomains with respect to the nanotube walls, the nanotubes were permeable to aqueous media. Rayleigh instabilities in thin polymer films confined within AAO membranes were studied. Thin films of PMMA were prepared by filling cylindrical nanopores in an AAO membrane with a PMMA solution in chloroform followed by solvent evaporation. When the PMMA nanotubes were annealed above the glass transition temperature (Tg), undulations in the film thickness were observed that were induced by a Rayleigh instability. The amplitude of the undulations increased with time and eventually bridged across the cylindrical nanopore in the AAO membrane, resulting in the formation of polymer nanorods with periodically encapsulated holes. A facile route to prepare hierarchical structures by wetting polymer microspheres into the nanopores of AAO templates was presented. In this approach, polystyrene (PS) microspheres were first spread and self-assembled into well-ordered monolayers on a silicon wafer. By contacting the porous AAO template, polymer chains wet the porous template and form short nanorods on top of the micrsopheres after thermal annealing. These hierarchical structures show ordering at two length scales which can be controlled by the size of the polystyrene microspheres and the pore sizes of the template. The generation of one-dimensional mesoporous silica and titania nanomaterials by using poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) as precursors was described. The porous structures were fabricated by evaporation induced self-assembly followed by pyrolysis. The orientation of the mesopores is parallel to the channels of the AAO membrane.

Nanoparticles were functionalized in order to incorporate their unique properties into functional materials. Gold nanoparticles were functionalized to direct their assembly at the oil-water interface, and further modified to achieve cross-linking at the interface, incorporation of charged groups or targeting groups, and extrusion to resize the capsules for potential delivery applications. Capsules were characterized by fluorescence microscopy by encapsulation of a fluorescent dye, and after drying on substrates by scanning force microscopy (SFM) and transmission electron microscopy (TEM). Gold nanoparticles were functionalized for their assembly into a microphase separated block copolymer, polystyrene-b-poly(2-vinyl pyridine) (PS-PVP) and the nanoparticles were directed within the domains by modification of the ligand periphery. Varying the ratio of hydrophobic to hydrophilic ligands allowed for the controlled assembly of the nanoparticles within the PVP domain of the diblock copolymer or at the interface between the two blocks. Thermal annealing resulted in ripening of the particles and migration of all particles to the center of the PVP domain. Location of the nanoparticles was determined by TEM and SFM. Gold nanoparticles were modified with acid-labile groups for potential use in photolithography applications, and with amine groups for incorporation in water purification membranes. Silica particles were modified with a dithiocarbonate chain transfer agent to achieve controlled polymerization by reversible addition fragmentation chain transfer polymerization (RAFT) of vinyl acetate from the particle surface. The poly(vinyl acetate) was hydrolyzed to poly(vinyl alcohol) to achieve particles dispersible in water with potential gas barrier properties. Functionalized silica particles were characterized by thermogravimetric analysis, TEM, and polymer was characterized by size exclusion chromatography.

The autonomous organization of components into patterns or structures without human intervention is known as self-assembly. This process is common throughout nature and technology and may involve many different kinds of interactions. This thesis treats the self assembly of block copolymers (BCPs) confined within cylindrical nanopores and generation of novel structures resulting from the constraints of size and forced curvature. Lamella-, cylinder-, and sphere-forming BCPs were drawn into the pores of anodized aluminum oxide (AAO) membranes in the melt phase by capillary forces. After thermal annealing, BCP nanorods were isolated by dissolution of the AAO with a weak acid and transmission electron microscopy (TEM) was used to investigate the resultant morphologies of the confined BCPs. The diameter and surface chemistry of AAO nanopores and molecular weight of BCP were varied to investigate the effect of confinement on the microphase separation of BCP. Concentric cylinders were observed for the lamella-forming BCPs under 2D confinement and deviations of the lamella repeat period were measured as a function of AAO pore diameter. For the bulk cylinder-forming BCP, a rich variety of morphologies, not seen in bulk, were observed that included stacked discs, toruses, single, double and triple helices and helices with a cylinder in the core. The specific morphology observed depended on D/Lo, where D is the pore diameter and Lo is the period of the BCP in the bulk. Electron tomography was performed on the cylinder-forming BCP to obtain 3D image of the confined morphology. For bulk sphere-forming BCP, novel structures, such as core-shell cylinders and spiraling rows of single, double and triple paired spherical microdomains were observed. The bulk cylinder-, and sphere-forming BCP were also placed in silane modified AAO and a rich variety of novel structures were observed. Inside silane modified AAOs, the preference of the blocks towards the pore wall was also altered. The results of cylinder-forming BCP were consistent with the results from the simulations. This method offers and exciting opportunity to manipulate the phase separation of BCPs and discover the novel periodic structures that are significantly different from those observed in bulk.

Polymers are interesting macromolecules that have gained great interest in a wide variety of applications. In this thesis, polymers have been utilized for gene and drug delivery, and to modulate enzyme activity. Simply, the idea of gene therapy is to deliver a therapeutic gene into defective cells. This field of medicine has enormous potential to cure many genetic diseases such as cancer, Alzheimer’s, and Huntington’s disease. A major obstacle in gene therapy is in safely and efficiently delivering the exogenous DNA into the nucleus of a cell. After the tragic failures in virus based gene therapy in clinical trials, there is a necessary search for a non-viral material. Polymers have been shown to interact and facilitate the entry of therapeutic DNA into the nucleus. Several types of polymers, including polyethyleneimine and poly-lysine, have had initial success in gene delivery. The first part of this thesis discusses about interesting approaches to further enhance the level of transfection and decrease toxicity. In one approach, an inspiration is taken from HIV-TAT protein that translocate exogenous materials into the nucleus. The basic domain of HIV-TAT protein, TAT peptide has been utilized in the polymeric design. A method has been developed to effectively display the TAT peptide on the polyplex surface. This significantly enhances the transfection efficiency of polyethylenimine polymers. In the search for a successful polymeric design for gene delivery, several naturally occurring amino acids are placed on a biocompatible polymer backbone. These polymers have high transfection and low toxicity when compared to the polymeric gold standards for gene delivery PEI and PLL. The design of these polymers also provided an opportunity to understand the structure-property relationships of different amino acid based polymers. Polymeric materials are also an interesting candidate for drug delivery. Polymeric micelles are designed to incorporate the cytotoxic anti-cancer drug, doxorubicin, and target it to cancer cells. These amphiphilic polymers have redox sensitive disulfide bonds and their cleavage causes the micelle to fall apart. This helps the polymeric micelles to selectively release the drug in high redox environment of cancer cells. Proteins regulate most of the cellular functions in our bodies and the alteration of their activities can lead to life threatening diseases. In the final part of this thesis, polymeric scaffolds are designed and developed to tune enzyme activity. The cationic polymers have the ability to electrostatically interact with proteins, namely serine protease chymotrypsin. At the physiological pH, these polymers can regulate chymotrypsin activity from 20% to 200% at nanomolar concentration without denaturing the protein.

Modern polymer research is uncovering new materials for biomedical applications. This thesis centers on novel polymer syntheses towards anti-cancer therapeutics termed polymer “pro-drugs”. In particular, aliphatic polyesters are potentially useful for drug delivery due to their biocompatibility and biodegradability. However, conventional aliphatic polyesters lack functionality. Incorporating functionality into aliphatic polyesters carries the potential to tailor their properties, and provides a method to attach drugs covalently. There are significant challenges associated with aliphatic polyester functionalization due to side-reactions including degradation of the polymer backbone. To overcome this challenge, alkyne functionalized lactones were synthesized, and click cycloaddition chemistry was employed to covalently attach novel azides to these alkyne containing aliphatic polyesters. A novel trimethylsilane protected alkyne d-valerolactone was synthesized and used in the preparation of block copolymers allowing for orthogonal functionalization strategies. Tin (II) mediated ring-opening polymerization of the lactones led to aliphatic polyesters with a narrow molecular weight distribution that had pendent alkynes available for post-polymerization chemistry. Click cycloaddition chemistry afforded water-soluble aliphatic polyesters by attaching PEGylated and zwitterionic azides to the polymer backbone. Novel phosphorylcholine and phosphobetaine-azides were prepared and grafted to the polyester chain. Camptothecin (CPT), an anti-cancer drug, was covalently attached to the aliphatic polyesters through a variety of covalent linkers. High pressure liquid chromatography (HPLC) was used to examine the release of CPT from the pro-drugs. With some of the linkers, CPT was seen to release from the aliphatic polyester with half-lives of 2–4 hours in human plasma. Physicochemical characterization techniques, including light scattering and atomic force microscopy (AFM), were used to investigate the properties of the polymeric pro-drug micelles. These micelles were approximately 60-120 nm in hydrodynamic radius. In vitro assays with MCF7 (breast cancer) and COLO205 (colorectal cancer) cells were used to evaluate the cytotoxicity of the polymers pro-drugs. IC50 values as low as 4 µM in COLO205 cells indicated the release of CPT in its cytotoxic form, and the potential of these aliphatic polyesters to function as a drug delivery platform.