Fabrication of Polymer Brushes via Light-Mediated Polymerization and Polymerization of Acrylic Acid

The development of polymer synthesis is of great significance towards the preparation of functional materials. Polymers have been used in surface modifications ranging from antifouling, antifogging to chemical sensing. Surfaces are often modified by a variety of techniques such as spin-coating, painting or dipping. However, these physisorption techniques lack stability against erosion. Covalent chemical attachment overcomes this disadvantage and offers chemical robustness with synthetic flexibility. Surface-initiated polymerizations are one of the most efficient strategies to modify surface properties via covalent attachment while providing high grafting density of a variety of functional groups as well as the ability to pattern the surfaces. This dissertation will highlight the development of three synthetic strategies. First, a novel strategy to synthesize branched polymer brushes by sequential light-mediated polymerization is demonstrated. Stepwise synthesis including linear copolymerization, deactivation of active chain ends and secondary graft polymerization is described affording branched polymer brush architectures. Secondly, the fabrication of well-defined, multifunctional polymer brushes using microliter volumes under ambient conditions is reported. This simple synthetic strategy uses photoinduced polymerization with metal-free photoredox catalyst which acts as both an oxygen scavenger and polymerization catalyst. Finally, the rapid copolymerization of acrylic acid and sodium acrylate is demonstrated. This polymerization uses an alkyl iodide/sodium iodide as a mediator in water affording copolymers with a moderate control over molecular weight and dispersity

Additive Manufacturing of Bovine Serum Albumin-Based Hydrogels and Bioplastics

Bio-sourced and biodegradable polymers for additive manufacturing could enable the rapid fabrication of parts for a broad spectrum of applications ranging from healthcare to aerospace. However, a limited number of these materials are suitable for vat photopolymerization processes. Herein, we report a two-step additive manufacturing process to fabricate robust protein-based constructs using a commercially available laser-based SLA printer. Methacrylated bovine serum albumin (MA-BSA) was synthesized and formulated into aqueous resins that were used to print complex 3D objects with a resolution comparable to a commercially available resin. The MA-BSA resins were characterized by rheometry to determine the viscosity and the cure rate, as both of these parameters can ultimately be used to predict the printability of the resin. In the first step of patterning these materials, the MA-BSA resin was 3D printed, and in the second step, the printed construct was thermally cured to denature the globular protein and increase the intermolecular noncovalent interactions. Thus, the final 3D printed part was comprised of both chemical and physical cross-links. Compression studies of hydrated and dehydrated constructs demonstrated a broad range of compressive strengths and Young’s moduli that could be further modulated by adjusting the type and amount of co-monomer. The printed hydrogel constructs demonstrated good cell viability (> 95%) after a 21-day culture period. These MA-BSA resins are expected to be compatible with other vat photopolymerization techniques including digital light projection (DLP) and continuous liquid interface production (CLIP)

Patterned Poly(acrylic acid) Brushes Containing Gold Nanoparticles for Peptide Detection by Surface-Assisted Laser Desorption/Ionization Mass Spectrometry

Patterned poly­(acrylic acid) (PAA) brushes was successfully generated via photolithography and surface-initiated reversible addition–fragmentation chain transfer (RAFT) polymerization of acrylic acid as verified by water contact angle measurements and FT-IR analysis. The carboxyl groups of PAA brushes can act as reducing moieties for in situ synthesis of gold nanoparticles (AuNPs), without the use of additional reducing agent. The formation of AuNPs was confirmed by transmission electron microscopy and X-ray photoelectron spectroscopy. The glass surface-modified by PAA brushes and immobilized with AuNPs (AuNPs-PAA) can be used as a substrate for SALDI-MS analysis, which is capable of detecting both small peptides having <i>m</i>/<i>z</i> ≤ 600 (glutathione) and large peptides having <i>m</i>/<i>z</i> ≥ 1000 (bradykinin, ICNK­QDCP­ILE) without the interference from matrix signal suggesting that AuNPs were stably trapped within the PAA brushes and the carboxyl groups of PAA can serve as internal proton source. By employing AuNPs as the capture probe, the AuNPs-PAA substrate can selectively identify thiol-containing peptides from the peptide mixtures with LOD as low as 0.1 and 0.05 nM for glutathione and ICNK­QDCP­ILE, respectively. An ability to selectively detect ICNK­QDCP­ILE in a diluted human serum is also demonstrated. The patterned format together with its high sensitivity and selectivity render this newly developed substrate a potential platform for high-throughput analysis of other biomarkers, especially those with low molecular weight in complex biological samples

Efficient synthesis of branched poly(acrylic acid) derivatives via postpolymerization modification

The utility of pentafluorophenyl esters for the selective introduction of functional units and branch points in well-defined poly(acrylic acid) (PAA) derivatives is demonstrated using a combination of controlled radical polymerization and postpolymerization modification. Reversible addition-fragmentation chain transfer enables the synthesis of well-defined copolymers-poly(pentafluorophenyl acrylate-co-tert-butyl acrylate)-with the active ester repeat units serving as attachment points for reaction with primary amines, specifically tris(2-(t-butoxycarbonyl)ethyl)methyl amine (Behera's amine). Deprotection using trifluoroacetic acid removes both the backbone and side chaint-butyl esters to give a series of branched PAA derivatives containing novel tricarboxylic acid side chains that are well suited to complexation and multidentate interactions. Surprisingly, the active ester homopolymer is shown to have the highest reactivity with Behera's amine when compared to copolymers with lower incorporation of pentafluorophenyl esters, suggesting an intriguing interplay of neighboring group effects and steric interactions. The ability to tune the efficiency of postpolymerization modification gives a library of PAA derivatives

Investigating temporal control in photoinduced atom transfer radical polymerization

External regulation of controlled polymerizations allows for controlling the kinetics of the polymerization and gaining spatial or temporal control over polymer growth. In photoinduced atom transfer radical polymerization (ATRP), light irradiation (re)generates the copper catalyst to switch the polymerization on. However, removing the light does not immediately inactivate the catalyst, nor does the rate of polymerization become zero as chains may grow in the dark because of continued activation by the residual activator catalyst or regeneration of the Cu catalyst in the dark. In this paper, the effect of polymerization components on photoinduced ATRP was investigated to understand the interplay of temporal control and light switching. Kinetics of polymerization were monitored using in situ NMR as well as under conventional batch conditions. The extent of the polymerization in the dark depended on the activity of the Cu catalyst, which was regulated by the nature of the ligand and reaction medium. For highly active catalysts, the equilibrium concentration of the L/CuI activator is very low, and it was rapidly depleted by radical termination reactions, yielding temporal control which closely matched the switching of light to on or off. Decreasing the activity of the Cu catalyst increased the equilibrium concentration of the activator, leading to significant chain growth in the dark

Novel Strategy for Photopatterning Emissive Polymer Brushes for Organic Light Emitting Diode Applications.

A light-mediated methodology to grow patterned, emissive polymer brushes with micron feature resolution is reported and applied to organic light emitting diode (OLED) displays. Light is used for both initiator functionalization of indium tin oxide and subsequent atom transfer radical polymerization of methacrylate-based fluorescent and phosphorescent iridium monomers. The iridium centers play key roles in photocatalyzing and mediating polymer growth while also emitting light in the final OLED structure. The scope of the presented procedure enables the synthesis of a library of polymers with emissive colors spanning the visible spectrum where the dopant incorporation, position of brush growth, and brush thickness are readily controlled. The chain-ends of the polymer brushes remain intact, affording subsequent chain extension and formation of well-defined diblock architectures. This high level of structure and function control allows for the facile preparation of random ternary copolymers and red-green-blue arrays to yield white emission

Simultaneous preparation of multiple polymer brushes under ambient conditions using microliter volumes

The fabrication of well-defined, multifunctional polymer brushes under ambient conditions is described. This facile method uses light-mediated, metal-free atom-transfer radical polymerization (ATRP) to grow polymer brushes with only microliter volumes required. Key to the success of this strategy is the dual action of N-phenylphenothiazine (PTH) as both an oxygen scavenger and polymerization catalyst. Use of simple glass cover slips results in a high degree of spatial and temporal control and allows for multiple polymer brushes to be grown simultaneously. The preparation of arbitrary 3D patterns and functional/emissive polymer brushes demonstrates the practicality and versatility of this novel strategy

Simple Benchtop Approach to Polymer Brush Nanostructures Using Visible-Light-Mediated Metal-Free Atom Transfer Radical Polymerization.

The development of an operationally simple, metal-free surface-initiated atom transfer radical polymerization (SI-ATRP) based on visible-light mediation is reported. The facile nature of this process enables the fabrication of well-defined polymer brushes from flat and curved surfaces using a "benchtop" setup that can be easily scaled to four-inch wafers. This circumvents the requirement of stringent air-free environments (i.e., glovebox), and mediation by visible light allows for spatial control on the micron scale, with complex three-dimensional patterns achieved in a single step. This robust approach leads to unprecedented access to brush architectures for nonexperts