Insights into the π – π interaction driven non-covalent functionalization of carbon nanotubes of various diameters by conjugated fluorene and carbazole copolymers

Abstract

We investigate the interaction of polyfluorene and fluorene/carbazole copolymers bearing various functional groups and side chains with small to large diameter-from 1.7 to 9 nm-carbon nanotubes (CNTs) in vacuo. We use variable-charge molecular dynamics simulations based on the reactive force field ReaxFF. We show that non covalent functionalization of nanotubes, driven by π − π interactions, is effective for all the polymers studied, thanks to their conjugated backbone and regardless of the presence of specific functional groups. The geometry at equilibrium of these polymer/CNT hybrids is analyzed in detail at the scale of each fluorene or carbazole unit. The role of both the functional groups and the alkyl chains length is analyzed in detail. Adsorption of the polymers on the nanotube sidewalls is shown to be either complete-with the whole chain physisorbed-or partial-due to intrachain coiling or interchain repulsion-depending on the initial geometry, number of polymers and nanotube diameter. Energetic arguments supplement the described geometric features. Both energetic and geometric adsorption features are derived here for the first time for large diameter carbon nanotubes (up to 9 nm) and fluorene/carbazole copolymers having up to 30 monomers and bearing different functional groups. The force field ReaxFF and its available parametrization used for the simulations are validated thanks to a benchmark and review on higher level quantum calculations-for simple π − π interacting compounds made up of polycyclic aromatic molecules adsorbed on a graphene sheet, or bilayer graphene. Although it is shown that the influence of the nanotube chirality on the adsorption pattern and binding strength can not be discussed with our method, we highlight that an available force field such as ReaxFF and its parametrization can be transferable to simulate new systems without specific re-parametrization, provided that this model is validated against reference methods or data. This methodology proves to be a valuable tool for optimal polymer design for nanotube functionalization at no re-parametrization cost, and could be adapted to simulate and assist the design of other types of molecular systems.

Comparing Commercial Metal-Coated AFM Tips and Home-Made Bulk Gold Tips for Tip-Enhanced Raman Spectroscopy of Polymer Functionalized Multiwalled Carbon Nanotubes

“…TERS has already been employed in the study of the surface interactions between the polymer and the CNT [ 69 , 71 ] and has provided a new insight on the relative orientation between the polymer and the CNT [ 69 ], suggesting that the main bonding mechanism is based on the non-covalent interactions between the delocalized electrons of the aromatic rings present in the polymeric chain and the electrons of the sp 2 carbons of the CNT lattice [ 69 ]. The polymer used for the functionalization of our MWCNT sample is the FFUR-14 ( Figure 6 a), whose backbone is composed by fluorene groups bearing either dihexyl units or bis urea group (one every nine fluorene/dihexyl groups) [ 70 ]. Calculations of the electronic structure of FFUR-14 show that it can efficiently bind to the CNT surface through interactions, giving rise to a stable functionalized nanomaterial, with the fluorene groups either parallel or perpendicular to the CNT surface within few nanometers, with no preferential configuration [ 70 ].…”

“…The polymer used for the functionalization of our MWCNT sample is the FFUR-14 ( Figure 6 a), whose backbone is composed by fluorene groups bearing either dihexyl units or bis urea group (one every nine fluorene/dihexyl groups) [ 70 ]. Calculations of the electronic structure of FFUR-14 show that it can efficiently bind to the CNT surface through interactions, giving rise to a stable functionalized nanomaterial, with the fluorene groups either parallel or perpendicular to the CNT surface within few nanometers, with no preferential configuration [ 70 ]. The typical Raman spectrum of FFUR-14 is shown in Figure 6 b (blue line).…”

“…Finally, CNT ink was mixed with the polymer solution and sonicated for 5 min in an ultrasonic bath. The prepared nanohybrid ink was aged for 24 h before usage in order to optimize the formation of polymer-functionalized CNTs (f-CNTs) [ 70 , 73 ]. Finally, the mixed solution was centrifuged in order to separate the f-CNTs from free or loosely bound polymers.…”

“…By providing spectroscopic information at the nanoscale, TERS represents an ideal technique in fulfilling this task. In this context, Suzuki et al, exploited TERS for the characterization of styrene–butadiene rubber-MWCNT nanocomposite [ 69 ], highlighting that the local order of the polymer chains at the interface is dependent on the non-covalent interaction between the phenyl rings of the polymer and the sp 2 /sp 3 carbon atoms of the MWCNT walls [ 65 , 70 ]. The same group also carried out TERS measurements on SWCNTs/polystyrene nanocomposites, characterizing at the local level the mechanical stress introduced by the polymer compression on the SWCNTs [ 71 ].…”

Flexible sensors based on assembled carbon nanotubes

“…[ 2 ] Owing to the strong conjugation within the large‐sized conjugated structure, CNTs could generate π‐π stacking with other conjugated materials and thus enable an easy and compact combination. [ 16 ] The performances of these sensors mainly depend on the properties of matrix materials, where CNTs generally act as conductive and reinforced components. With the aim to fully extend the advantages of excellent properties of individual CNTs to macroscopic scale and to expand their application areas, CNTs have been extensively assembled into aligned structures with macroscopic architectures.…”