Carbon nanotube induced structure and phase evolution in polypropylene crystallised at elevated pressures: effect on physical properties

The dispersion of carbon nanotubes (CNTs) in polypropylene (PP) introduces a significant change in the structure and phase evolution during crystallisation at ambient and elevated pressures. The differences between the behaviours of PP and PP containing CNTs are attributed to CNT/polymer interface driven nucleation, which is also responsible for the significant reduction in spherulite size and is associated with the change in equilibrium melting point and percentage crystallinity. This opens the gateway for the use of polymer CNT composites for functional devices, including biomedical and electronic devices. A study of the impact of 1 wt-% CNT additions on the microstructure evolution of PP during pressure induced crystallisation is reported. It is demonstrated that the dispersion of CNTs produces a significant difference in the macromolecular structure and phase evolution during crystallisation at ambient and elevated pressures. The change in equilibrium melting point with pressure is higher for PP containing CNT, which is indicative of the chemical interaction between polymer and CNT. It is found that CNT additions promote the formation of gamma-phase at low crystallisation pressure of 0.1-25 MPa, in contrast to the neat PP where alpha-crystals nucleate at low to moderate pressures (about 0.1-60 MPa) and the gamma-phase nucleates at crystallisation pressures exceeding similar to 60 MPa. The dependence of equilibrium melting temperature on crystallisation pressure and spherulite size delineates two distinct regimes, characterised predominantly by a-and c-phases respectively, for both PP and PP-CNT systems.