Interpenetrating network formation in isotactic polypropylene/graphene composites

Morphological structure development of the in-situ polymerized isotactic polypropylene/graphene nanocomposites was investigated during the annealing process. The material was prepared via an in-situ slurry polymerization method with graphene-supported Ziegler-Natta catalyst. An interpenetrating network was observed at a relatively low graphene concentration after the sample was annealed for a long period (one to several hours). Alternative Conductivity impedance spectroscopic measurements showed a remarkable electrical conductivity increase of several orders of magnitude after the network was formed. At very low graphene content, although macroscopic network cannot be formed, the electrical conductivity can be enhanced through annealing as well. It implied that the rearrangement of the clusters or aggregates was necessary and such rearrangement favored the network formation. The network forming process was investigated through optical and electron microscopy methods and discussed based on the viscoelastic phase separation model. The effect of network on the crystallization behaviors was also investigated. It showed that the overall crystallization was improved by the presence of graphene platelets, but depressed after the network was formed, which was due to the spatial confinement and the decrease of total surface area available for heterogeneous nucleation. (C) 2013 Elsevier Ltd. All rights reserved.