Functionalized carbon nanotubes for molecular recognition and mechanical properties modulation of polymers

Abstract

Carbon Nanotubes (CNTs) are well known to exhibit a unique combination of optical, electrical, chemical and mechanical properties. In addition, the high surface area to volume ratio of CNTs makes them an ideal candidate for their use in various areas of science and engineering. This work primarily addresses the application of CNTs in two broad areas-molecular recognition and mechanical property enhancement of polymer matrix. To this end, CNTs are grown using chemical vapor deposition (CVD) process in a three zone tubular reactor. Argon, hydrogen and acetylene are used as precursor gases. Since, the as grown CNTs contain amorphous carbon as well as catalytic and other carbonaceous impurities. Therefore, CVD grown CNTs are subjected to various oxidative and chemical treatments before using them in recognition and nanocomposites studies carried out in this work. In the first part of this work, metal nanoparticles - CNT based nano-hybrids are developed for the detection of several environmentally important analytes. Metal nanoparticles of Ag, Ni and Cu have been prepared, characterized and decorated on the CNT surface to form CNT@Ag, CNT@Cu and CNT@Ni nano-hybrid. The developed nano-hybrids have successfully captured CO2, CN-, and HSO4- ions in aqueous medium. The detection limit of ions captured is found to be in nanomolar range with no interference from competing ions which states that prepared nano-hybrids are highly sensitive and selective. It is observed that the chemical approaches for purification of CNTs cause fewer defects in its structure which disturbs the optical and chemical properties of CNTs; therefore, to prevent its intrinsic properties non-covalent functionalization is performed. Pyrene based organic receptors have been prepared and functionalized with CNTs by employing  interactions amongst their aromatic rings. These CNT-pyrene based hybrid structures form a cavity for entrapment of analytes. Fabricated CNT@Pyrene hybrid assemblies have been employed to detect NO3- and CH3COO- ions in nano-molar concentration range. It is very important to detect these ions to monitor environmental activities and for the well-being of humans and aquatic life. Owing to the unique surface properties of organic nanoparticles (ONPs), which offer 100% solubility in aqueous medium, these have been fabricated in present work using re-precipitation method. These ONPs exhibit size in range of 10-100 nm, which can be tailor-made during sonication process. The interesting fact about these ONPs is their easy solubility in any medium, which aided in recognition processes in biological systems. In designing of sensors, interactions of the ions with the receptor depends upon the size of cavity, electrostatic interactions and hydrogen bonding. Several urea, thiourea, amide and imine linked receptors has been synthesized and processed to its organic nanoparticles. These dipodal receptors provide sufficient cavity size for binding of PO43-, Br- and HSO4- ions through hydrogen bonding. To confirm the recognition and to examine the efficacy of proposed sensors in real life applications, its real sample analysis shows accuracy as high as 90%. These ONPs meet all the basic requirements of a sensor such as varied pH stability, salt concentration effect and time stability. In the second part of this work, an experimental study has been carried out to establish the relationship between CNT purification and functionalization routes with the average mechanical response of CNT/epoxy nanocomposites under static and dynamic loading. A better dispersion of CNTs has been recorded for the functionalized CNTs as compared to the oxidized and CVD grown CNTs. Tensile, 3-point bending and nanoDMA analysis has been performed on nanocomposites processed with CVD-grown, oxidized and functionalized CNTs. The relative improvement in the mechanical properties of epoxy is found to be a function of purification and functionalization treatment imparted to as grown CNTs. Moreover, it is shown that COOH functionalized CNTs which are initially oxidized at 350 oC provides maximum relative improvement in mechanical properties of epoxy matrix in comparison to all other purification routes studied in this work. It is further extended to analyze the effect of various types of CNT incorporation on the interfacial properties of carbon fiber (CF)/epoxy multiscale composites. To this end, epoxy matrix is modified with as grown, oxidized and functionalized CNTs and later on this modified matrix is used to process carbon fiber/epoxy single fiber composites. It is observed that epoxy matrix modified with COOH functionalized CNTs shows better wettability and hence higher interfacial shear strength in comparison to epoxy matrix modified with other types of CNTs. The present work provides a tool to harness the excellent properties of CNTs in improving average mechanical as well as interfacial properties of CNT based composites for structural applications.