Polymer-assisted in-situ thermal reduction of silver precursors: A solventless route for silver nanoparticles-polymer composites

A simple one-step solventless in-situ reduction method was developed to prepare silver nanoparticle (AgNP) based polymer composites using different silver precursors and thermoplastic polymers. This method utilizes the mild reducing environment of thermoplastic polymer melts to convert silver precursors to AgNPs during the extrusion process. Complete reduction of Ag2O with broad AgNP size distribution (20 +/- 18 nm) in polyamide 6 (PA6) was obtained after 5 min processing. For same matrix, the uniformity of AgNPs improved significantly after 10 min processing time, associated with a slight increase in the particle diameter (26 +/- 9 nm). No significant impact of in-situ AgNP synthesis method on chemical and macromolecular characteristics of polyamide 6 was detected. When this method was used to synthesize AgNPs in polylactic acid and polypropylene, the influence of surface energy of polymer melt was evident. Reduction of Ag2O in polylactic acid (at 165 degrees C) was slower compared to PA6 and 95 +/- 3% Ag2O reduction was achieved after 10 min of processing. When polypropylene was used as the matrix, only 60% reduction of Ag2O was achieved, possibly due to poor wetting of the silver precursor, owing to the low surface energy of polypropylene. AgNP-PA6 composites were also successfully prepared using Ag2CO3 and Ag-palmitate as precursors. To demonstrate the potential application of such composites in food packaging, the silver release and antibacterial activity of AgNP-PA6 composite were also studied. Though the composites released 500 times less silver compared to the permitted limits of different safety standards, they exhibited an excellent antibacterial activity against a typical food pathogen (Listeria monocytogenes) already at a low level of AgNP loading i.e. 0.5 wt%.