116
S. Pakdel et al.
rGO aggregates could be easily redispersed in suitable organic media as individual
nanosheets by using NaCl salts as flocculants with the help of sonication, which is an
economical and facile technique for the large-scale production of graphene/GO/rGO.
The homogeneous dispersion of graphene/GO/rGO in organic solvents facilitates
solution-processing method for new composites’ development. The covalent [61–63]
and non-covalent [64–67] modifications are the strategies that have been incorporated toward the homogeneous dispersion of graphene in various solvents. However,
the reaction condition for the grafting modification is usually complex, while for the
non-covalent methods, the stabilizing agents are difficult to be removed completely
[60]. It should be considered that the organic solvents are generally adsorbed between
the graphene layers that adversely impact the performance of the prepared nanocomposites [68]. Solution blending of polymer and GO is often carried out in laboratory
preparation [69], because large amounts of organic solvents are required to be used
in the preparation especially when the nanocomposite is used for industrial production [68]. Also, it is difficult to completly remove the large quantities of the utilized
solvent that makes it not eco-friendly [70].
2.2 In Situ Polymerization
In situ polymerization method is often used to prepare graphene-filled polymer
composites, such as those with epoxy [71–75]. In this method, graphene/GO/rGO is
mixed with pre-polymers or monomers [50]. First, graphene or modified graphene
is swollen inside the liquid monomer. Then an appropriate initiator is diffused, and
polymerization is begun either by radiation or heat [74, 76]. In epoxy–graphene
composite, at first, the graphene-based filler is mixed with epoxy resins by applying
high-shear forces and then a curing agent is added to initiate the polymerization [53,
77, 78]. In situ polymerization of monomer in the presence of GO fillers can efficiently improve the interfacial bonding strength between GO and polymers. Besides,
it improves the dispersion status of GO nanosheets in the polymer matrix [79], but
this technique could be carried out only on the laboratory scale [69].
Wang and coworkers [80] introduced an efficient approach of in situ polymerization in order to fabricate GO/polyimide (PI) composites with greater performances. NH 2 -functionalized GO (ODA-GO) was a versatile starting platform for
the in situ preparation of composite films via the embedding of polyamic acid onto
the GO surface. The graphene sheets, bearing grafted polymer layers, exhibit great
dispersibility and compatibility with the polymer matrix and also create strong bonds
with the PI polymer chains. The functionalized graphene nanosheets strengthen the PI
composites greatly by incorporating the ultrahigh contact area and strong interfacial
interactions of the PI matrix. The PI–ODA-GO films, prepared by in situ polymerization, exhibit around a 6.4-times increase in tensile modulus, and 240% improvement
in tensile strength, for a very low GO content (0.3 wt%). Furthermore, the bulk dielectric constant is decreased from 3.3, for the neat PI film, to just 2.0 for the PI–ODA-GO
composite film. Consequently, in situ polymerization to produce PI–ODA-GO films
S. Pakdel et al.
rGO aggregates could be easily redispersed in suitable organic media as individual
nanosheets by using NaCl salts as flocculants with the help of sonication, which is an
economical and facile technique for the large-scale production of graphene/GO/rGO.
The homogeneous dispersion of graphene/GO/rGO in organic solvents facilitates
solution-processing method for new composites’ development. The covalent [61–63]
and non-covalent [64–67] modifications are the strategies that have been incorporated toward the homogeneous dispersion of graphene in various solvents. However,
the reaction condition for the grafting modification is usually complex, while for the
non-covalent methods, the stabilizing agents are difficult to be removed completely
[60]. It should be considered that the organic solvents are generally adsorbed between
the graphene layers that adversely impact the performance of the prepared nanocomposites [68]. Solution blending of polymer and GO is often carried out in laboratory
preparation [69], because large amounts of organic solvents are required to be used
in the preparation especially when the nanocomposite is used for industrial production [68]. Also, it is difficult to completly remove the large quantities of the utilized
solvent that makes it not eco-friendly [70].
2.2 In Situ Polymerization
In situ polymerization method is often used to prepare graphene-filled polymer
composites, such as those with epoxy [71–75]. In this method, graphene/GO/rGO is
mixed with pre-polymers or monomers [50]. First, graphene or modified graphene
is swollen inside the liquid monomer. Then an appropriate initiator is diffused, and
polymerization is begun either by radiation or heat [74, 76]. In epoxy–graphene
composite, at first, the graphene-based filler is mixed with epoxy resins by applying
high-shear forces and then a curing agent is added to initiate the polymerization [53,
77, 78]. In situ polymerization of monomer in the presence of GO fillers can efficiently improve the interfacial bonding strength between GO and polymers. Besides,
it improves the dispersion status of GO nanosheets in the polymer matrix [79], but
this technique could be carried out only on the laboratory scale [69].
Wang and coworkers [80] introduced an efficient approach of in situ polymerization in order to fabricate GO/polyimide (PI) composites with greater performances. NH 2 -functionalized GO (ODA-GO) was a versatile starting platform for
the in situ preparation of composite films via the embedding of polyamic acid onto
the GO surface. The graphene sheets, bearing grafted polymer layers, exhibit great
dispersibility and compatibility with the polymer matrix and also create strong bonds
with the PI polymer chains. The functionalized graphene nanosheets strengthen the PI
composites greatly by incorporating the ultrahigh contact area and strong interfacial
interactions of the PI matrix. The PI–ODA-GO films, prepared by in situ polymerization, exhibit around a 6.4-times increase in tensile modulus, and 240% improvement
in tensile strength, for a very low GO content (0.3 wt%). Furthermore, the bulk dielectric constant is decreased from 3.3, for the neat PI film, to just 2.0 for the PI–ODA-GO
composite film. Consequently, in situ polymerization to produce PI–ODA-GO films