Development of bionanocomposites based on PCL/PBS double crystalline blends and carbon nanotubes
Gumede, Thandi Patricia
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The main purpose of this research was to use commercially available crystalline biobased polymers, namely poly(ε-caprolactone) (PCL) and poly(butylene succinate) (PBS), and introduce polycarbonate (PC)/multi-walled carbon nanotubes (MWCNTs) masterbatch into these polymers in order to provide additional functionalities, especially those associated with electronic applications. All the samples were prepared through melt- mixing in a twin-screw extruder. According to our results, the PCL/PBS blends showed a sea-island morphology with discrete droplets of the minor phase within the matrix of the major phase, indicating immiscibility. The introduction of the PC/MWCNTs masterbatch to PCL, PBS and the PCL/PBS blends showed partial miscibility, where PC-rich, PCL-rich and PBS-rich phases were formed. A number of MWCNTs diffused from the PC-rich phase to the PCL- rich and the PBS-rich phases, although the MWCNTs were mostly agglomerated in the PC-rich phases. However, the extent of partial miscibility was different for each system. The polar component surface energy, interfacial tension and isothermal crystallization results suggested that the MWCNTs would preferably diffuse into the PBS-rich phase, rather than the PCL-rich phase. Standard DSC measurements for the PCL/PBS blends, PCL/(PC/MWCNTs) and PBS/(PC/MWCNTs) nanocomposites demonstrated nucleation effects. In the PCL/PBS blends, nucleation was ascribed to (1) transference of the impurities from the PCL phase to the PBS phase, and (2) since the PBS crystallizes first, the PCL droplets may have crystallized by surface induced nucleation on the interface with the PBS crystallized matrix and nucleate at the interphase. In the case of the nanocomposites, the nucleation effect was attributed to the MWCNTs that diffused from the PC-rich to the PCL-rich and PBS-rich phases, even though the nucleating efficiency was lower than reported in the literature, which probably was due to the limited phase mixing between the PC-rich, the PCL-rich and PBS-rich phases. For the PCL/PBS/(PC/MWCNTs) nanocomposites, there was a decrease in the Tc values. This was due to the competition between two effects: (1) the partial miscibility of the PC-rich with the PCL-rich and PBS-rich phases, and (2) the nucleation effect of the MWCNTs. The decrease in the Tc values indicated that miscibility was the dominating effect. Isothermal crystallization experiments performed by DSC showed an increase in the overall crystallization rate of PCL with increases in MWCNTs contents, which was the result of their nucleating effect. However, for the PBS/(PC/MWCNTs) nanocomposites, the crystallization rate increased up to 0.5 wt% MWCNTs, while further increases in MWCNTs loading (and also in PC content) resulted in progressive decreases in crystallization rate. The results were explained through increased MWCNTs aggregation and reduced diffusion rates of PBS chains, as the masterbatch content in the blends increased. In the case of the PCL/PBS/(PC/MWCNTs) nanocomposites, the overall crystallization rates decreased as a result of the competition between the nucleating effect and miscibility. Since the PC-rich phase is partially miscible with the PCL-rich and PBS-rich phases, the PC probably immobilized the PCL and PBS chains and inhibited the rate of crystallization. The thermal conductivities and mechanical strengths of the nanocomposites were generally enhanced compared to those of the neat material. The nanocomposites prepared in this work could be used in applications where electrical conductivity, as well as low weight and tailored mechanical properties, are required.