4 F. Gardea et al. Fig. 1.2 Photoreactive benzophenone (BP) molecule bonded to a CNT via (i) diazonium reaction and subsequent (ii) photoreactive bonding to a polymer chain forming a covalent link between the CNT and polymer chain Fig. 1.3 Normalized Raman spectra of (a) pristine and functionalized HiPco SWCNTs and (b) pristine and functionalized MWCNTs The approach was to chemically bond a BP molecule to a CNT such that when exposed to UV light, the BP would react and bond to the C-H bonds on the side groups or backbone of the polymer matrix, as schematically shown in Fig. 1.2. This photoreactive molecule would provide a controlled covalent link between the CNT and polymer chain. The orientation of the filler is considered extremely important as the effect of this chemical bonding enhancement on the interfacial mechanics might not be observable if additional mechanisms are introduced as a result of misorientation. Studying the effect of CNT alignment provided a more theoretically grounded understanding of the interfacial load transfer mechanics. 1.3.1 Characterization of CNT Functionalization In the Raman spectrum of a CNT, the peak ratio of the defect-derived D-band and the graphitic structure-derived G-band is a good indicator of the defect concentration in a CNT sample [27, 28, 32]. In Fig. 1.3a, the spectra of dispersed pristine HiPco SWCNTs is shown. It is observed that a high-intensity G peak (1600 cm−1) and low-intensity D peak (1350 cm−1) are present in the pristine sample, which indicates a low number of defect sites. After the reaction, a significantly enhanced D peak (at around 1350 cm−1) is observed, resulting in a D/G peak ratio > 1, indicative of a high quantity of defects caused by grafting of the BP molecule to the CNTs. The same chemistry was effective for funtionalizing both SWCNT and MWCNT films. The Raman spectra (Fig. 1.3b) shows the enhanced D/G ratio after the reaction for aligned MWCNT films. It is worth mentioning that a D peak with considerable intensity was observed for the pristine MWCNT sample, which is due to the defects generated in the synthesis process [33]. Therefore, only a slight enhancement of D/G ratio can be observed for the functionalized sample, however the result does verify the attachment of the BP molecule to the surface of the aligned MWCNTs.
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