Functionalization of polylactide with multibranched poly(ethyleneimine) by in situ reactive extrusion

Abstract

Polylactide (PLA) was functionalized with multibranched polyethyleneimine (PEI) for the addition of active functional groups along the molecular chain by in situ reactive extrusion. The synthesis of PLA-PEI was performed in two steps through a PLA-maleate (PLA-MA) precursor varying the proportion of maleic anhydride (MA) at 1, 2, 3, or 4%(w/w) and the free radical initiator, 2,5-dimethyl-2,5-di-(tert-butylperoxy)hexane, at 0.1, 0.25, or 0.5% (w/w). The effects of the amount of initiator and MA on the degree of maleate grafting was investigated in terms of the acid value by titration. The %MA was found to depend on the amount of initiator, but not on the amount of MA. Next, two different molecular weights of PEI at different proportions [PEI800 at 1–4% (w/w) and PEI25k at 1–2% (w/w)] were modified onto the PLA-MA by the ring opening reaction of the anhydride group and the amino groups of PEI during extrusion. The amount of amino groups was quantitatively determined using the ninhydrin method. The structure of the obtained PLA-MA and PLA-PEI were characterized by Fourier transformed infrared spectroscopy and nuclear magnetic resonance analyses. Thermal properties were analyzed by differential scanning colorimetry. The molecular weight determination and melt flow index results implied the degradation of PLA during synthesis of the PLA-MA precursor, or substitution of PEI for PLA-PEI. Antibacterial properties of PLA-PEI were investigated against Escherichia coli and Staphylococcus aureus. The multibranched PLA-PEI800 showed a higher crystallization than neat PLA. The PLA modified with a PEI800 content of more than 1% (w/w) and PEI25k showed antibacterial activities against S. aureus in 24 h, but not against E. coli. The % reduction in the number of viable cells (colonies) was proportional to the amino group contents.