Antimicrobial polyurethane foams blown by CO2 adducts from polyethylenimines grafted with alkyl quaternary ammonium groups

Based on previous endeavors on antimicrobial polyurethane (PU) foams and CO2-releasing environment-friendly blowing agents, we explored a series of modified polyethylenimine (PEI)-CO2 adducts, serving as both blowing and antimicrobial agents for PUs. For this purpose, a quaternary ammonium chain with a C-12 alkyl (C-12(+)), either alone or together with neutral (C-8 and C-12) alkyls, was grafted onto branched PEI (M-n = 25,000 Da) backbones prior to saturation with CO2. The resultant four CO2 adducts, 5C(12)(+)5C(8)-, 5C(12)(+)5C(12)-, 10C(12)(+)- and 15C(12)(+)-PEI-CO2 (where the number indicates grafting rate in percentage of each side chain), all can well disperse into PU foaming mixtures, generating flexible PU foams with antimicrobial activity. 15C(12)(+)-PEI-CO2 displays the best dispersibility and highest foaming efficiency, while 10C(12)(+)-PEI-CO2 is the most antimicrobial toward both Gram positive and negative bacteria. The relatively high grafting rate in 15C(12)(+)-PEI-CO2 drives the formation of micelle-like particles in the foaming system; the outermost hydrophobic chains can somewhat prevent the internal quaternary ammoniums from accessing to the negatively charged cell membrane of bacteria, causing a reduction in antimicrobial efficacy compared with that of 10C(12)(+)-PEI-CO2. The developed PEI-CO2 adducts might replace the traditional ozone depletion and/or global warming blowing agents, and would show prolonged antimicrobial activity over rapidly leachable small molecule antimicrobial agents.