Influence of molecular weight of polymer matrix on the structure and rheological properties of graphene oxide/polydimethylsiloxane composites

The structure and rheological properties of graphene oxide (GO)/polydimethylsiloxane (PDMS) composites are examined as the molecular weight of PDMS and concentration of GO are varied. Clusters formed by GO sheets get smaller and disperse better with increasing molecular weight of PDMS, which results in the higher critical concentration to form network (Ccr). Moreover, at GO concentration just above Ccr, the plateau modulus of samples decreases with the molecular weight of PDMS. During shear experiments, negative normal stress differences (ΔN) are observed in composites with PDMS molecular weight lower than critical entanglement molecular weight (Mc). However, positive ΔN is found in samples with PDMS molecular weight above Mc. It can be concluded that the vorticity alignment of GO clusters induces the negative ΔN based on the optical shear experiments. The possible mechanism for the positive ΔN is also proposed.