Thermooxidative stabilization of polymers using natural and synthetic melanins

Melanin is a biopolymer well-known for its intriguing chemical structure and physiological functions including photoprotection, radical scavenging, and metal-ion chelation. Although it has a suite of properties not common to many known organic materials, efforts to exploit those properties in technologically relevant materials have been few compared to other biopolymers such as cellulose, chitin, or collagen. Besides its natural presence in many animals including humans, melanin is also commonly consumed by humans in soups, sauces, and pastas and is widely available in large quantities from a variety of natural sources, suggesting it could serve as a nontoxic additive for enhancing the properties of common polymers. To this end, we report for the first time the potential of natural and synthetic melanins as thermal stabilizers for common polymers by evaluating the addition of melanin to several model polymers with well-known degradation pathways. When added to poly(methyl methacrylate) (PMMA) in very low amounts (0.5-5 wt %), synthetic melanin-like polymers significantly altered the radical initiated chain scission behavior of PMMA and caused a dramatic increase (by about 50-90 °C) in its onset decomposition temperature in both inert and air atmospheres. Moreover, PMMA samples with up to 1 wt % melanin achieved nearly the maximum enhancement level yet retained more than 80% light transmission from 350 to 800 nm in 100 μm thick films. Natural melanin extracted from the ink sac of Sepia officinalis (commonly known as cuttlefish) also displayed significant thermal stabilization effects on PMMA and polypropylene at similar loadings. From molecular weight characterization studies, the associated delay in the molecular weight decrease of PMMA and other polymers at elevated temperature could be potentially beneficial for high-temperature processing or increasing their upper use temperature in demanding applications. It is likely that the thermal stabilization benefits of melanin could be realized in many polymers due to the diversity of its known radical scavenging capabilities in both living systems and the polymers presented in this article. Since natural and synthetic melanin additives are macromolecules, they are also less likely to leach from the base polymer in the same way that small molecule additives often do.