Oligomer-grafted graphene in a soft nanocomposite augments mechanical properties and biological activity

The present work aims to develop strong yet tough composites of polycaprolactone (PCL) reinforced with 5 wt% graphene oxide (GO) covalently modified with PCL chains by "graft-from" approach. Oligomers were grafted by in situ polymerization from GO and amine-functionalized GO (AGO) yielding GO-g-PCL and AGO-g-PCL, respectively. The elastic modulus of PCL increased by 48% with the addition of unmodified GO. With GO-g-PCL and AGO-g-PCL, the modulus was increased by 76% and 71%, respectively. Similarly, the yield strength increased by 21%, 50% and 14% for GO, GO-g-PCL and AGO-g-PCL, respectively. Whereas GO significantly compromised the elongation to break of PCL (970%) to 93%, GO-g-PCL and AGO-g-PCL based composites exhibited remarkably higher ductility of 560% and 367%, respectively. These observations may be attributed to the grafted oligomers affording enhanced interfacial interactions at the soft-hard interface. Cell studies in vitro revealed that composites containing oligomer grafted GO significantly enhanced the proliferation and osteogenesis of human mesenchymal stem cells compared to neat PCL. Thus, the grafting of polymeric chains on the filler is a promising strategy for engineering graphene composites with enhanced modulus and strength that exhibit excellent ductility and toughness in combination with enhanced biological activity for bone tissue regeneration.