Diisocyanate modified graphene oxide network structure: steric effect of diisocyanates on bimolecular cross-linking degree

Relative reactivity and the extent of functionalization of graphene oxide (GO) during its bimolecular reaction with three different classes of diisocyanates, i.e., aliphatic (hexamethylene diisocyanate, HMDI), alicyclic (isophorone diisocyanates, IPDI), and aromatic (toluene diisocyanates, TDI), are correlated on the basis of steric and electronic effect of diisocyanate functionalizing agents. The bimolecular reaction produces cross-linked network structure and the steric nature of diisocyanates dominates over electronic factor in regulating the relative reactivity, extent of functionalization and disorder in final structure. Formation of carbamate and amide bonds on participation of GO’s sp3-hydroxyl groups at top and bottom of basal plane and sp2-carboxyl groups at the edges are recognized from FTIR and XPS spectra. Extent of functionalization in different locations are predicted from relative ratio of hydroxyl and carboxyl groups participated in diisocyanate reaction and N1s XPS spectra show nearly 3:1, 2:1, 1.3:1 in HMDI, IPDI, and TDI respectively. The disorders in final structure are evaluated by comparing Raman, UV–Vis, and C1s XPS spectra of functionalized materials with GO. TDI, the most sterically hindered diisocyanate among the reported one, shows slowest reactivity and minimum disorder in the final structure compared to long chain aliphatic diisocyanate, HMDI. Depending on the relative participation of oxygenated functional groups at basal and/or edges of two adjacent GO nano sheets, the three possible reaction patterns, i.e., basal to basal, basal to edge, and edge to edge, are proposed. Different crystallographic orientations and out-of-plane arrangement of graphene layers arises from these reactions are correlated through XRD data.