Poly-butylene terephthalate/graphene nanoplates nanocomposites via ring-opening polymerization during melt mixing: effects of nanoparticles structure and defectiveness on crystallinity and thermal conductivity

In this work, poly (butylene terephthalate) nanocomposites were prepared by in-situ ring-opening polymerization of cyclic butylene terephthalate (CBT) in the presence of carbon nanoplates. Different types of graphene-related material were used, with variable size and structural defectiveness. Selected nanoparticles were annealed at 1700°C in vacuum to reduce defectiveness of the carbon structure, evidencing that thermal annealing can considerably reduce the amount of defects, as consistently proven by Raman measurements, X-ray photoelectron spectroscopy, X-ray diffraction and thermogravimetry. Dramatic thermal conductivity improvement of nanoparticles upon annealing was confirmed both by direct measure of thermal dissipation on individual nanoflakes and by indirect measures on nanopapers prepared with pristine or thermally annealed nanoparticles. For the preparation of nanocomposites, pristine or thermally annealed nanoflakes were pre-dispersed in cyclic butylene terephthalate oligomers (CBT) followed by catalyzed ring-opening polymerization of CBT in extrusion. This technique allowed to obtain significantly improved dispersion of nanoflakes compared to conventional melt processing methods, thanks to facile distribution in CBT and further dispersion as a result of high shear applied once viscosity increased during polymerization. Crystallization of nanocomposites was found to be strongly affected by the presence of nanoflakes, evidencing significant nucleation effects. However, differences were clearly observed as a function of nanoparticles grade, leading to a higher nucleation efficiency for less defective nanoplates. Thermal conductivity results showed significant variability as a function of nanoparticles properties, particularly in terms of defectiveness, surface area and lateral dimensions. Furthermore, a dramatic two- to three-fold increase in the thermal conductivity of the nanocomposite was observed in the presence of annealed nanoflakes compared to pristine ones, evidencing the importance of using low defectiveness. The study of nanocomposite thermal conductivity before and after polymerization of cyclic butylene terephthalate into poly (butylene terephthalate) was also addressed, to gain insight in the modification of nanocomposites morphology upon polymerization. Furthermore, the effect of different processing parameters (time, temperature, shear rate) during the preparation nanocomposites was addressed. Thermal conductivity up to 1.7 W/mK, i.e., one order of magnitude higher than for pristine polymer, was obtained with 10%wt loading, which is in line with state of the art nanocomposites prepared by more complex and less upscalable in-situ polymerization processes.