Effect of graphene nano-platelet morphology on the elastic modulus of soft and hard biopolymers

Free-standing biocomposites were fabricated by solvent casting and hot-pressing employing two bio-polyesters having diverse elastic (Young's) moduli (soft and hard), reinforced with different graphene nanoplatelets (GnPs). Systematic mechanical measurements were conducted to investigate the effect of GnP thickness and lateral size on the elastic moduli. Comparisons were made with other reinforcing nanostructured filers such as organoclay, MoS2, Fe2O3, carbon black and silica nanoparticles. Upon solvent casting, GnPs did not perform better than the other model fillers in increasing the elastic modulus of the soft bio-polyester. Upon hot-pressing however, large (>300 nm) multi-layer GnPs (≥8 layers) more than doubled the elastic modulus of the soft bio-polyester matrix compared to other GnPs and fillers. This effect was attributed to the optimized alignment of the large 2D GnP flakes within the amorphous soft polymer. In contrast, hot-pressing did not yield superior elastic modulus enhancement for the hard bio-polyester when hot-pressed. GnPs only induced 30% enhancement for both processes. Moreover, multi-layer large GnPs were shown to suppress the thermally-induced stiffness reduction of the soft bio-polyester near its melting temperature. A theoretical analysis based on the spring network model is deployed to describe the impact of the GnP alignment on the elastic moduli enhancement.