Efficient mechanical toughening of polylactic acid without substantial decreases in stiffness and transparency by the reactive grafting of polyrotaxanes

A mechanical tough polylactic acid (PLA) that substantially retains its stiffness and transparency was created through reaction with a polyrotaxane derivative. A graft polyrotaxane (GPR), composed of a linear Polyethylene glycol backbone threaded with cyclodextrins bearing grafted poly(e-caprolactone), was bonded to the hydroxyl end-groups of PLA with hexamethylene diisocyanate (HDI), which then solidified to form a PLA-bonded GPR that was distributed throughout the PLA matrix. The addition of 5 wt% GPR resulted in a seven-fold increase in Izod impact strength and a three-fold increase in fracture elongation, with only an 11% loss of stiffness; the transparency was comparable to neat PLA. Electron microscopy revealed that HDI facilitated the dispersal of GPR in the PLA matrix and avoided the sea-island phase-separated structure observed in the PLA/GPR blend devoid of HDI, and that the homogenized structure led to ductile deformation. A GPR content of only 5 wt% efficiently increased PLA-matrix-chain flexibility.