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Dissertation zugänglich unter
Functionalization of Multi-walled Carbon Nanotubes and Localization of Functionalized Multi-walled Carbon Nanotubes in an SAN/PPE Blend
Funktionalisierung von Multi-Walled Carbon Nanotubes und Lokalisierung von funktionalisierten Multi-Walled Carbon Nanotubes in einer SAN / PPE-Mischung
Dokument 1.pdf (5.378 KB)
Freie Schlagwörter (Deutsch):
Carbon Nanotubes , chemische Funktionalisierung , Lokalisierung , SAN / PPE-Mischung
Freie Schlagwörter (Englisch):
Carbon Nanotubes , Chemical Functionalization , Localization , SAN/PPE Blend
Abetz, Volker (Prof. Dr.)
Tag der mündlichen Prüfung:
Kurzfassung auf Englisch:
The main objective of this work was to confine multi-walled carbon nanotubes (MWCNT) at the interface of binary blends by functionalizing MWCNT with desired polymers and study the effects of localization of MWCNT on the properties of the composites. In order to achieve this purpose, commercial MWCNT with amino groups were grafted with polystyrene (PS), poly(methyl methacrylate) (PMMA) and random copolymers of styrene and methyl methacrylate (P(MMA-co-S)). Since PS is miscible with poly(2,6-dimethyl-1,4-phenylene ether) (PPE) and PMMA has good miscibility with poly(styrene-co-acrylonitrile) copolymers (SAN) with specific acrylonitrile content, the grafted polymers on the surface of MWCNT are expected to provide a driving force to confine MWCNT at the interface of immiscible SAN/PPE phases.
The first part of this dissertation studied the functionalization of MWCNT by “grafting-from” and “grafting-to” methods. In both approach, the polymerization of selected polymers was realized by atom transfer radical polymerization (ATRP). In order to obtain sufficient functionalized MWCNT in one batch for preparing composites, the reactions of functionalization via “grafting-from” were significantly up-scaled. FT-IR spectra clearly indicated that the selected polymer chains were successfully grafted on MWCNT step by step from anchoring initiator to synthesize polymer via ATRP. From thermal gravimetric analysis (TGA) results, the content of grafted PS was found to proportionally increase with the molecular weight of polymer, which implied good controllability of ATRP reaction in the presence of MWCNT. Similar results and phenomena were also observed in the case of functionalized MWCNT with PMMA under optimum reaction conditions. The formation of grafted P(MMA-co-S) random copolymers was determined by nuclear magnetic resonance spectroscopy (1H-NMR), where signals of the main chains and the pendant groups were broad in all spectra and pronouncedly different from the ones of homopolymers. The controllability of the reaction was characterized by the fact that both the content and the molecular weight of grafted polymer increased linearly with the conversion of total amount of monomers. In present work, functionalizing MWCNT via “grafting-to” was also attempted by means of a Diels-Alder reaction between furfuryl-capped polymer and MWCNT. However, although both of PMMA and PS chains were successfully grafted on individual nanotubes, a much lower content of grafted polymer was achieved by “grafting-to” compared to the one in case of “grafting-from” because of the limited reactivity of MWCNT.
Then, the nanocomposites of SAN/PPE 40/60 blends filled with MWCNT were prepared by solution casting and melt processing. The localization of MWCNT in composites was characterized by transmission electron micrographs (TEM). The functionalized MWCNT with PS was found to migrate from the pre-mixed SAN phase to the PPE phase which had a better miscibility with grafted PS. The degree of migration was considerably influenced by the molecular weight of grafted PS. Nevertheless, as the dispersion of MWCNT in the pre-mixed phase was random, it was difficult to uniformly locate MWCNT at the interface through migration. Hence, introducing copolymer which contained both MMA and styrene units was considered as an alternative method. Through functionalizing with P(MMA-co-S) copolymers, many nanotubes were preferentially located at the interface of the blends. In contrast, pristine MWCNT were observed to concentrate selectively in the SAN phase, which was in agreement with theoretical prediction according to the calculated wetting efficiency. However, when molecular weight of grafted copolymers increased, pronounced agglomeration was observed nearby the boundary which should be contributed to a micellization of the high molecular weight P(MMA-co-S) copolymers covering MWCNT and the formation of SAN/PPE/P(MMA-co-S) ternary blend with a specific composition of copolymer. For the rheological properties, the dynamic moduli and the complex viscosity of SAN/PPE blends were enhanced after filling pristine MWCNT. The values further increased when MWCNT were functionalized with low molecular weight P(MMA-co-S) copolymers, as MWCNT locating at the interface formed a CNT-CNT network with good contacting with each other even if the boundary of blend was not entirely occupied by MWCNT. However, when molecular weight increase, the composites showed a similar behavior to the neat blends with low dynamic moduli, which resulted from the softening effect of the grafted copolymers and the agglomerations of MWCNT. Besides, since a well conductive network of MWCNT was not formed and negative effect of grafted polymer, little improvement of electrical conductivity was achieved in the composites with functionalized MWCNT.
In summary, grafting selected polymer chains on the surface of MWCNT presented a possible approach to tune the localization of MWCNT in binary phase separated blends. The molecular weight of grafted polymer shows a significant influence on the localization of MWCNT and the properties of composites. When the grafted polymers had specific relationship (either miscible or immiscible) with both components in the blends, MWCNT might be more easily to be located at the interface, such as functionalized MWCNT with random copolymers. The functionalizing approach used in this thesis is not only valid in binary blends but also helpful to control the localization of MWCNT in block copolymers. Moreover, although the influence of composition of grafted copolymer on the localization of MWCNT was not discussed in this work, it is possible to achieve more uniform localization of MWCNT at the interface by grafting random copolymer on MWCNT with specific composition which leads to good miscibility with both SAN and PPE. . It could be very interesting and promising for future investigation. The work studied in this dissertation also shows a potential of further investigation on the localization of other kinds of nanofillers, such as silica nanoparticles, metallic nanoparticles, and cellulose nanofibers, etc.