Modification of multiwall carbon nanotubes by grafting from controlled polymerization and their dispersion in a block copolymer matrix
One of the aims of this work was the identification of the factors that affect the grafting of polymer from the surface of commercially available MWCNTs. The grafting of polystyrene from the surface of two commercially available MWCNTs was performed under atom transfer radical polymerization conditions after successive functionalization reactions. The purity, the state of aggregation and the size of the MWCNTs were found to influence the content of grafted polystyrene. The multiwall carbon nanotube with lower purity, relative shorter length and higher state of aggregation (MWCNTBMS95) showed higher grafting efficiencies along all the surface functionalization reactions than the MWCNTFC99, i.e., the one with higher carbon purity and relative longer lengths. The presence of compact aggregates was found to decrease the viscosity during the reactions, which increases the yield of the polymerization reaction but the grafting at the surface of aggregates is favored. As a consequence, high polymer grafting contents are relatively easy to obtain with the MWCNTBMS95, but probably there is a high amount of aggregates coated with polymer. In the case of the MWCNTFC99, the nanoparticle length and the looseness of the bundles promote the disaggregation of the nanoparticles during the functionalization reaction. This leads to the increase of the viscosity of the media and decreases the yield of the reaction. However, higher polymer content can be achieved with the MWCNTFC99 when the right combination of length of glycol spacer, concentration of the anchored initiator and monomer to carbon nanotube weight ratio are employed. These studies demonstrate that there is a correlation between the characteristics of the carbon nanotubes and the polymer grafting. Parameters like the carbon purity, the length and the state of aggregation of the carbon nanotubes are important to determine the extent of the polymer grafting. Though, the functionalization could be adjusted in order to obtain the desired degree of functionalization and molecular weight of the grafted polymer. Nanocomposites based on pristine MWCNTs, pre-functionalized and polymer grafted MWCNTs and the block copolymer AS-SB26 were prepared by film casting from solution. The dispersion of the nanoparticles improves with the increase in molecular weight and grafting content of the polystyrene from the carbon nanotube. In the case of the system prepared with the polymer grafted MWCNTFC99, the nanoparticle dispersion was also enhanced by the increase of the casting temperature. For the nanocomposite based on the MWCNTBMS95, the mechanical properties of the block copolymer do not appreciably improve by compounding with polymer grafted carbon nanotubes at ambient temperature. The storage modulus at 90 °C slightly increases in the case of the polystyrene grafted MWCNTBMS95 based nanocomposites due to a better load transfer between the block copolymer and the functionalized nanotubes. In the case of the MWCNTFC99, the enhancement of the dispersion of the carbon nanotubes does not improve the mechanical properties of the nanocomposites at ambient temperature. It seems that the load transfer from the polymer matrix to the dispersed carbon nanotubes in this thermoplastic elastomer is not efficient under the compounding conditions employed in the range of composition evaluated. The increase in the grafting density and molecular weight of the polystyrene grafted from the MWCNTBMS95 and the MWCNTFC99 influences the morphology of the AS-SB26. In the case of the nanocomposite based on PS9120MWCNTBMS95 and AS-SB26, the PS9120MWCNTBMS95 “locally” templates the lamellae morphology of the AS-SB26, so that the lamellae resemble the contour of the PS9120MWCNTBMS95. A similar observation was obtained for the nanocomposite containing PS8524MWCNTFC99, however, comparatively a lower quantity of lamellae adapts to the shape of the PS8524MWCNTFC99. The difference could be related with the lower grafting density of the polystyrene and with the difference in length and the curvature of the nanotubes, the longer the nanotube the more irregular is the contour length. The orientation of the nanocomposite under a linear deformation was observed for the samples containing MWCNTBMS95. The PS9120MWCNTBMS95 increases the orientation factor of the AS-SB26. Due to the polymer coating, the PS9120MWCNTBMS95 align perpendicular to the strain direction. With this study it could be demonstrated that the local orientation of nanofillers in an external field (here: mechanical force field) can be controlled by variation of the interaction between nanofiller and matrix. The studies on the electrical conductivity of the nanocomposites based on AS-SB26 and MWCNTFC99 demonstrate that the dispersion and polymer coating of the nanofiller influence the properties of the nanocomposite. The blend containing carbon nanotubes with a moderate grafting content and a low polystyrene molecular weight (PS142MWCNTFC99) showed an increase in the electrical conductivity at lower weight percentage than the oxidMWCNTFC99. The improvement on the dispersion of the carbon nanotubes due to the polymer coating improves the electrical conductivity of the nanocomposite compared with the neat AS-SB26, at lower loadings than the oxidized nanofillers. However, the polymer coating characteristics have to be controlled in order to avoid the insulation of the carbon nanotube as in the case of the PS8524MWCNTFC99. The synthetic strategies presented in this work open the gate to further developments in the functionalization of various kinds of nanofillers, besides carbon nanotubes also inorganic materials like silica, or organic ones like the upcoming cellulose nanofibers. Besides functionalization with homopolymers, also the attachment of block copolymers to appears possible, which should allow for the adjustment of transfer of properties (e.g. electrical, mechanical or thermal properties) between matrix and filler. To some extent, the systems presented in this thesis contained already a short elastomeric oligo(ethylene oxide) block between carbon nanotube and the polystyrene, Also the attachment of different types of polymers to the same nanofiller for the adjustment of interfacial properties (useful in multiphase polymer blends), or the attachment of polymers with functional groups for further reactions (useful, for example, in melt blending) are possible future developments which will potentially lead to new nanocomposites with tailored properties.
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