Role of Crystalline Structure in the Marine Biodegradation Process of Polyamide 6

Abstract Biodegradable plastics capable of fully decomposing in natural environments offer a promising solution to global plastic pollution. Polymer biodegradability is governed by multiple parameters, including chemical composition, crystalline morphology, and sample geometry. Although the amorphous phase is hypothesized to undergo preferential degradation over its crystalline counterpart, direct evidence remains elusive. Herein, we investigated the marine biodegradability of polyamide 6 (PA6) with varying crystallinities to elucidate the contribution of crystalline structure to its biodegradation behavior. This study also addresses the ongoing controversy in literature, where PA6 is interchangeably reported as either biodegradable or nonbiodegradable. We demonstrated that PA6 biodegradability is strongly associated with its crystalline structure and sample morphology, including crystallinity, crystallite size, and surface accessibility. Cryo-milling enhanced the biodegradation of PA6 particles by simultaneously altering their particle size, crystalline structure, and molecular weight, whereas subsequent reannealing markedly suppressed their biodegradation despite their small particle sizes. These results further support a strong contribution of crystalline structure to PA6 biodegradation while also demonstrating that the effects of crystalline structure and sample geometry are coupled. Furthermore, the preferential degradation of less-ordered regions may leave crystalline remnants that are relatively resistant to further biodegradation and could potentially contribute to the formation of persistent nanoscale particles. However, direct characterization of such particles is required to verify this possibility and evaluate their environmental implications.