Effect of reduced graphene oxide on nucleation, crystallisation, self-nucleation and thermal fractionation of in-situ polymerised cyclic butylene terephthalate

Reduced graphene oxide (rGO) drastically affected the crystallisation of poly (butylene terephthalate) (pCBT) prepared by in-situ ring-opening polymerization of cyclic butylene terephthalate (CBT). The effect on the crystallisation was investigated by means of differential scanning calorimetry (DSC) and Wide Angle X-ray scattering (WAXS). Furthermore, part of the rGO was thermally treated at 1700°C in vacuum, resulting in a reduction of defectiveness, and its effect on the crystallization of pCBT was compared to that of pristine rGO. Standard DSC experiments evidenced the complete conversion of CBT into pCBT, with no detectable residual traces of CBT melting/crystallization. The addition of rGO strongly affected melting and crystallization behavior of pCBT. In the presence of rGO, the crystallization peak temperature was shifted to higher temperature values and the melting/recrystallization of imperfect crystals, typically observed during heating scans for neat pCBT, was suppressed, thus indicating the formation of more stable crystals during cooling. Furthermore, a new peak at higher temperature was observed in both DSC heating and cooling scans, especially in nanocomposites containing annealed rGO. This peak could be related to the formation of a stack of extended chain crystals at the pCBT/rGO interface, promoted by the change in the surface chemistry in annealed rGO. Isothermal crystallization tests revealed faster crystallization in the presence of rGO, without altering the crystallization mechanism of pCBT. In fact, an Avrami index of about 2 was calculated for both neat pCBT and nanocomposites, thus resulting in the nucleation of instantaneous axialites. Self-nucleation experiments were performed to determine a nucleation efficiency (NE) of 164% and 270% in the presence of pristine and annealed nanoparticles, respectively, thus indicating a super-nucleating effect. Self-nucleation and successive annealing tests revealed that the high melting/crystallization “phase” can be fractionated with the higher melting fraction centered at ~ 253 °C, which is close to the values of the equilibrium melting temperature reported in literature. These are a further confirmation of the polymeric nature of this phase and the possible formation of extended chain crystals. Finally, WAXS experiments showed that the polymer chains and rGO oriented simultaneously, the higher orientation obtained with annealed rGO. Tests at different temperatures showed the presence of crystalline reflections at temperatures higher than the melting temperature of neat pCBT: these traces were related to the same crystalline structure of neat pCBT (α-form). The orientation and the crystalline form observed for the new phase gave a further proof to the chain conformation as thick stack of extended chain crystals, especially in presence of rGO with low defectiveness and oxidation.