Graphene Oxide: A Versatile Agent for Polyimide Foams with Improved Foaming Capability and Enhanced Flexibility

Close-celled aromatic polyimide (PI)/graphene foams with low density and improved flexibility were fabricated by thermal foaming of poly(amic ester)/graphene oxide (PAE/GO) precursor powders. The PAE/GO precursor powders were prepared by grafting GO nanosheets with PAE chains, which led to efficient dispersion of the GO nanosheets in PAE matrix. Incorporation of GO resulted in an enhanced foaming capability of the precursor, i.e., enlarged cell size and decreased foam density. Notably, a decrease of 50% in the foam density was obtained via the addition of only 2 wt % GO in the precursor. In the foaming process, the GO nanosheets functioned as a versatile agent that not only provided heterogeneous nucleation sites but also produced gaseous molecules. By analyzing the foaming mechanism, the excellent features of GO in heat transfer, gas barrier, and strength reinforcement also facilitated to obtain large and uniform cells in the foams. In addition, the PI/graphene foams exhibited a prominent flexibility and enhanced flexural strength, as an elastic-to-nonelastic conversion of the initial stage of the compressive stress-strain curves was observed by increasing the content of graphene in the PI matrix and an increase of 22.5% in flexural strength was obtained by addition of 0.5 wt % GO in the precursor.