The extraordinary electrical, mechanical, and thermal properties of carbon nanotubes (CNTs) make them the strongest candidates for their application in composite materials [1–3]. Good dispersion of CNTs is required for their application in many composites. However, CNTs are produced in the form of bundles where they are attracted together by van der Waals interactions. The aggregation of CNTs in the form of bundles influences the properties of the resulting composite materials e.g., ineffective stress transfer and higher percolation thresholds for electrical conductivity. Moreover, the agglomerates of carbon nanotubes may act as conventional carbon black and hence, to obtain improved material properties, the disaggregation of CNTs agglomerates is necessary [4–6].
Surface modification of CNTs is a tool to improve their dispersion in various solvents and matrices and it can be grouped into two different categories: (a) binding the functional groups on the π-conjugated skeleton of carbon nanotube via covalent bonding [7–9] and (b) physical adsorption or wrapping of a variety of functional molecules via non-covalent interactions [10–14]. Non-covalent functionalization has an advantage over covalent functionalization because no major side wall defects occur thus preserving the electronic properties of π-conjugated tubular structure of CNTs. Such functionalization involves wrapping of the CNTs’ surface by various polymers, polynuclear aromatic compounds, surfactants, or biomolecules . Ionic and biological surfactants improve the CNTs’ dispersion in aqueous solutions where CNTs are entrapped into micelles leading to a stable dispersion . Conjugated polymers interact with CNTs by π−π stacking, resulting in better dispersion of CNTs in specific organic solvents . Block copolymers, having at least one block exhibiting conjugation and other having high affinity toward solvent, lead to a better dispersion of CNTs in solvents of different polarities . Polycyclic aromatic compounds e.g., pyrene, are also well known for their π−π stacking on CNTs surface, and the attachment of pyrene to molecular species which are soluble in organic solvents or aqueous media creates the possibility to effectively disperse CNTs in these solvents .
Polyhedral oligomeric silsesquioxanes (POSS) possess an inorganic cage structure, with the possibility for a variety of functional groups of different nature to be attached to the Si8O12 core [18, 19]. The reactive functional groups on POSS have been subjected to functionalize CNTs, allowing a good dispersion of CNTs in solvents such as chloroform and tetrahydrofuran (THF) [20–22]. POSS-functionalized CNTs resulted in nanocomposites with improved CNTs dispersion and mechanical properties . In this study, 1-pyrenebutyric acid was attached covalently to POSS, and multi-walled carbon nanotubes (MWCNTs) were non-covalently modified with the resultant pyrene-POSS. The obtained hybrid material can be efficiently dispersed in various organic solvents such as n-hexane, toluene, and THF to form stable dispersions. The dispersibility of the hybrid material is provided by the presence of the POSS with aliphatic moieties having high affinity toward organic solvents.
To demonstrate the dispersing effect of pyrene-POSS also in a polymer matrix, pyrene-POSS-modified MWCNTs were used for the fabrication of conductive polydimethylsiloxane (PDMS) nanocomposite membranes. Such conductive membranes, when applied in spiral wound membrane modules with plastic housing, can provide the opportunity to neutralize electrostatic charges and hence may provide, for example, a safer separation of hydrocarbons. In order to avoid electrostatic charging, a surface resistivity between 106 Ω/□ and 109 Ω/□ or bulk electrical conductivity above 10−6 Sm−1 are required [23, 24].
It has been a challenge to uniformly disperse non-functionalized MWCNTs in a thin PDMS selective layer because of the indispersibility of unmodified MWCNTs in nonpolar solvents like n-hexane and toluene. However, pyrene-POSS provided a way to effectively disperse MWCNTs in PDMS nanocomposite membranes with the advantage of a well-preserved tubular conjugated MWCNTs structure which, in turn, conserves their electrical properties. The PDMS nanocomposite membranes were characterized by optical microscopy, scanning electron microscopy, gas permeation, and sheet resistance measurements.