Crystallization behavior of functional polypropylene grafted graphene oxide nanocomposite

Abstract

With an aim to understand the role of polymer grafted graphene oxide (GO) in crystallization processes, a functional polypropylene (PP) grafted GO nanocomposite, prepared by isocyanate group-contained polypropylene (PP-g-TMI) reacting with hydroxyl and carboxyl groups on GO, was investigated in terms of isothermal and non-isothermal crystallization by differential scanning calorimetry. Comparing with the PP-g-TMI/natural graphite (NG) nanocomposite, a fully exfoliated and uniformly dispersed GO in the PP-g-TMI matrix for the PP-g-TMI/GO nanocomposite was affirmed by transmission electron microscopy and a large enhancement of its storage modulus. For isothermal crystallization at 140 °C, the addition of GO into PP-g-TMI accelerates the crystallization rate dramatically more than that of NG, indicating the grafted GO can act as a very efficient heterogeneous nucleation agent to increase nucleation dramatically. However, for isothermal crystallization at 126 °C, the crystallization rate of PP-g-TMI accelerated by grafted GO is inferior to that by NG, which can deduce that the grafted GO can restrict migration and diffusion of polymer molecular chains to the surface of the nucleus for spherulitic growth due to strong covalent binding with PP-g-TMI, the formed highly viscous and dense GO layers. These two converse effects of the grafted GO on crystallization can also explain the interesting non-isothermal crystallization behavior that the grafted GO increases the crystallization rate of PP-g-TMI at a low cooling rate, while it decreases the crystallization rate at a high cooling rate.