Bioinspired catecholic polymers for functional materials design
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Melanins and Polydopamine (PDA) are bioinspired catecholic polymers that are well known for their intriguing chemical structure and physiological functions. Indeed, PDA has a suite of properties that are uncommon to many known organic materials and over the last decade researchers have endeavored to exploit those properties in technologically relevant materials. Those efforts notwithstanding, PDAs’ chemical structures have yet to be revealed in their entirety and their useful properties yet to be fully explored. In addition, given the natural presence of melanins and dopamine in many animals (including humans), along with their versatile functional features, these materials can serve as non-toxic additives to common polymers and thereby enhance their properties. Or they could, on their own, serve as eco-friendly functional organic films to be used in a variety of useful applications. To this end, we suggest first the potential of melanins as thermal stabilizers for common polymers by evaluating the addition of melanin to several model polymers with well-known degradation pathways. Small loadings of natural and synthetic melanins significantly alter the radical-initiated chain scission behavior of conventional polymers. Such loadings cause a dramatic increase in its onset decomposition temperature, indicating potential benefits for high temperature processing or increasing their upper use temperature in demanding applications. Second, we suggest thin films of synthetic melanin and poly(allylamine hydrochloride) be deposited layer-by-layer from dilute aqueous solutions in ambient conditions. The multilayer films show superior UV-protection performance and substantially extend the useful life of a conductive polymer film under UV light, demonstrating the utility of melanin films in high-tech applications. Third, we establish a synthetic approach to prepare block copolymers of poly(methyl methacrylate) (PMMA) and PDA using a modified atom transfer radical polymerization (ATRP) technique. These copolymers display very good solubility in a range of organic solvents and the spin cast thin films of the copolymers show a sharp reduction (by up to 50%) in protein adsorption compared to those of neat PMMA. The enhanced solvent processability, thermal stability, and low protein adsorption characteristics of this copolymer make it an attractive choice for antifouling coatings on large surfaces. Fourth, we exploit PDA to achieve block copolymer (BCP) lithography on a variety of soft-material surfaces. This biomimetic film serves as a reactive platform for subsequently grafting a surface neutral layer to chemically guide the perpendicular orientation of BCP lamellae. BCP nanopatterning may now be achieved over a large area on cheap, rough, and commercially available roll-to-roll flexible polymer substrates having a wide range of surface energies, surfaces that are of interest to be adapted for patterning. Fifth, we develop an efficient, environmentally friendly, and water-based flame-retardant surface nanocoating for highly flammable foamed materials such as flexible polyurethane (PU) foams. Upon exposure to flame, a PDA coating remains intact on the surface, completely stopping the melting and interrupting foam collapse. In addition, given the reported radical-scavenging capability of catechols, the PDA layer is hypothesized to remove flammable radicals which further retards flame spread during a fire. From cone calorimeter data, peak heat release rate of PDA-coated foams shows a sharp reduction, of up to 67%, relative to a control foam. This represents much better performance than many conventional additives for flexible PU foams that have been reported in the literature. We additionally investigated the effect of catechol functionality on the flammability of PU foams through the comparison of cone calorimetric analysis between PDA-coated flexible PU foams with pristine catechol functionality and LD-containing rigid PU foams with mostly depleted catechols. This new knowledge will be potentially useful in the design of flame-resistant foams and surface coatings.