polymerization [114]. Park et al. [115] investigated π–π interactions between
MWCNT and the Cp ligand of Cp 2 ZrCl 2 , which was immobilized on MWCNT.
They succeeded in preparing HDPE/MWCNT and UHMWPE/MWCNT nanocomposites. Tong et al. prepared HDPE/SWCNT nanocomposites by in situ
Ziegler–Natta polymerization using MgCl 2 /TiCl 4 . Prior to the catalyst preparation,
MgCl 2 in n-butanol was impregnated with SWCNT [116]. In spite of the progress
made, the marginal improvement of the polyolefin/CNT property balance does not
yet justify the high cost of CNT additives. In contrast to CNT, graphene offers
unique opportunities both with respect to lower costs and high performance of
polyolefin/carbon composites.
In 2004 Geim and Novoselov prepared defect-free graphene by peeling off
individual graphene layers from highly orientated graphite. Their discovery
triggered a boom in graphene research. As listed in Table 1, single layer graphene
and CNT (SWCNT and MWCNT) exhibit similarly high performance with respect
to their excellent mechanical and electrical properties [89, 117–127]. In contrast to
CNT, however, graphene, as a two-dimensional polycyclic aromatic carbon polymer, forms electrically conductive, ultralight and optically transparent films that
consist of only one carbon atom layer. Owing to thermal fluctuation, graphene
does not form planar two-dimensional crystals but has a wavy structure
[128, 129]. Graphene is highly impermeable, thus improving barrier resistance
against permeation of gas and liquids. Hence, among carbon allotropes, only
graphene is capable of simultaneously improving matrix reinforcement, electrical
and thermal conductivity, as well as abrasion and barrier resistance. The envisioned
applications of defect-free graphene include displays, carbon-based electronics,
ultracapacitors. and organic solar cells [117, 119, 122, 130–133]. As illustrated in
Fig. 7, the graphene family ranges from sub-micron graphite and multilayer
graphene, also referred to as graphene nanoplatelets (GNP), to single-layer
graphene. Whereas defect-free graphene additives, produced by chemical vapor
deposition (CVD) and other “bottom-up” processes, are not cost-competitive with
polyolefins, several “top-down” strategies are at hand, solving this cost/performance problem encountered for MWCNT/polyolefin composites. In principle, as
illustrated in Fig. 8, there exist two routes toward producing graphene from
graphite: (1) solution exfoliation and mechanical delamination of graphite, and
(2) graphite intercalation and subsequent exfoliation.
Intercalation of graphite with sulfuric acid has been commercially used for many
years to produce expandable graphite salts as flame retardants [134–136]. During
combustion, the salt decomposes to expand graphite, accounting for intumescence
[137–140]. The process for making expanded graphite can be modified to prepare
functionalized graphene. Typically, graphite is swollen in sulfuric acid and
oxidized to produce graphite oxide (GO), containing polar groups such as epoxy,
hydroxyl, phenolic, and carboxylic acid groups [141]. As illustrated in Fig. 8, in the
subsequent step, GO is chemically or thermally reduced. During thermal reduction,
the functional groups decompose, evolving gases such as carbon monoxide and
carbon dioxide, while de novo graphitization of the GO layers occurs. In this
thermal reduction, rapid heating up to temperatures well above 400
C is required
290
M. Stu ¨rzel et al.
MWCNT and the Cp ligand of Cp 2 ZrCl 2 , which was immobilized on MWCNT.
They succeeded in preparing HDPE/MWCNT and UHMWPE/MWCNT nanocomposites. Tong et al. prepared HDPE/SWCNT nanocomposites by in situ
Ziegler–Natta polymerization using MgCl 2 /TiCl 4 . Prior to the catalyst preparation,
MgCl 2 in n-butanol was impregnated with SWCNT [116]. In spite of the progress
made, the marginal improvement of the polyolefin/CNT property balance does not
yet justify the high cost of CNT additives. In contrast to CNT, graphene offers
unique opportunities both with respect to lower costs and high performance of
polyolefin/carbon composites.
In 2004 Geim and Novoselov prepared defect-free graphene by peeling off
individual graphene layers from highly orientated graphite. Their discovery
triggered a boom in graphene research. As listed in Table 1, single layer graphene
and CNT (SWCNT and MWCNT) exhibit similarly high performance with respect
to their excellent mechanical and electrical properties [89, 117–127]. In contrast to
CNT, however, graphene, as a two-dimensional polycyclic aromatic carbon polymer, forms electrically conductive, ultralight and optically transparent films that
consist of only one carbon atom layer. Owing to thermal fluctuation, graphene
does not form planar two-dimensional crystals but has a wavy structure
[128, 129]. Graphene is highly impermeable, thus improving barrier resistance
against permeation of gas and liquids. Hence, among carbon allotropes, only
graphene is capable of simultaneously improving matrix reinforcement, electrical
and thermal conductivity, as well as abrasion and barrier resistance. The envisioned
applications of defect-free graphene include displays, carbon-based electronics,
ultracapacitors. and organic solar cells [117, 119, 122, 130–133]. As illustrated in
Fig. 7, the graphene family ranges from sub-micron graphite and multilayer
graphene, also referred to as graphene nanoplatelets (GNP), to single-layer
graphene. Whereas defect-free graphene additives, produced by chemical vapor
deposition (CVD) and other “bottom-up” processes, are not cost-competitive with
polyolefins, several “top-down” strategies are at hand, solving this cost/performance problem encountered for MWCNT/polyolefin composites. In principle, as
illustrated in Fig. 8, there exist two routes toward producing graphene from
graphite: (1) solution exfoliation and mechanical delamination of graphite, and
(2) graphite intercalation and subsequent exfoliation.
Intercalation of graphite with sulfuric acid has been commercially used for many
years to produce expandable graphite salts as flame retardants [134–136]. During
combustion, the salt decomposes to expand graphite, accounting for intumescence
[137–140]. The process for making expanded graphite can be modified to prepare
functionalized graphene. Typically, graphite is swollen in sulfuric acid and
oxidized to produce graphite oxide (GO), containing polar groups such as epoxy,
hydroxyl, phenolic, and carboxylic acid groups [141]. As illustrated in Fig. 8, in the
subsequent step, GO is chemically or thermally reduced. During thermal reduction,
the functional groups decompose, evolving gases such as carbon monoxide and
carbon dioxide, while de novo graphitization of the GO layers occurs. In this
thermal reduction, rapid heating up to temperatures well above 400
C is required
290
M. Stu ¨rzel et al.