Ionic Liquid Modified Polymer/graphite Composites

Owing to retaining excellent processing characteristics of polymer, light weight, adjustable resistance and low cost, conductive polymer based composites have attractive prospects in such applications as optoelectronic devices, information, sensors, electromagnetic shielding and so on. Consequently, the research of conductive polymer based composites has important practical value. Graphene has a unique electrical conductivity and thermal conductivity. Composites filled with graphene have extraordinary conductivity and mechanical properties, so the study on graphene-polymer composites is of scientific importance. Both delaminating graphite into single graphene and dispersing uniformly the graphene into polymer matrix are crucial to the realization the potential performance of the conductive composites. In this study, in order to obtain composites with excellent conductive properties, the following two aspects were researched.(1) Due to the presence of potential interaction between ionic liquids and graphene and the attractive characteristics of the ionic liquids such as conductity and stable chemical properties, it is expected that the combination of graphene and ionic liquid will render better conductive performance to the polymer composites. In this part, we chose epoxy resin as the matrix of the composite, the effects of the ionic liquids and graphite on the curing behavior of epoxy resin were discolosed. These results provided technical basis for optimizing the performance of such materials. Mainly conclusions were illustrated as follows.At lower concertration of eapanded graphite (EG), compared with the curing activation energy (Eα) of the neat epoxy resin, the composite with EG has a lower Eαbefore the gelation, but a higher Eαafter the gelation. However, at higher concertrations of EG, in the whole conversion range, the composite with EG shows a higher Eαcompared with the neat epoxy resin. Besides, as the curing proceeded, a peculiar increase of Eαis found in systems containing [BMIm]PF6. Due to the formation of hydrogen bonding between [BMIm]PF6 and the harder (Jeffamine),the reactivity of Jeffamine is considerably decreased, leading to a much higher Eαin [BMIm]PF6-containing systems, especially at higher conversion. In systems containing a combination of [BMIm]PF6 and EG, due to the unique interactions between EG and [BMIm]PF6, the shielding effect provided by the well-dispersed EG sheets constrains the formation of hydeogen bonding between [BMIm]PF6 and Jeffamine, leading to lowered Eαcompared with that for the system containing [BMIm]PF6 only.(2) Conductive polymer/graphene composites could be prepared by chemical oxidation and reduction process. However, during the reduction process, graphene is easy to stack together because ofπ-πinteraction. In order to overcome this problem, we chose hydropropyl cellouse (HPC) as the matrix for its low critical solution temperature (LCST) behavior. The reduction of oxidized graphene above LCST could effectively prevent the reuniting of the graphene into stacked structure. Meanwhile, based on the confirmed interaction between graphene and ionic liquids in the first part, incorporation of ionic liquids into the HPC/graphene systems further improved the conductivity of composites. The main results were concluded as follows.Oxidized graphene can be delaminated into only 1~2 layers via ultrasound treatment, which can form a stable and uniform dispersion in the HPC. [BMIm]Cl can significantly improve the electrical conductivity of composites. Under the same content of reduced graphite oxide (RGO), the conductivity of composite can be increased by 2~3 orders of magnitude with the help of 20 phr [BMIm]Cl. When the volume fraction of graphene was only 0.57%, the conductivity of composite reached 5.66×10-4 S/ m, while the content of RGO increased to 10.45 vol%, its electrical conductivity reached up to as high as 10.5 S/m. Besides, it was showed that graphene can effectively restrain the crystallization of HPC, and the corresponding diffraction intensity of crystalline phase (2θ= 8.2o) gradually decreased along with the increasing of the content of RGO. The crystallization of HPC was further suppressed when 20 phr of [BMIm] PF6 was included.