Nanomaterials in macromolecular synthesis

Abstract

In nature, organic and inorganic structures always work successfully each other to form materials with unprecedented properties. Exoskeletons of crustaceans, mollusk shells and bones are the best examples of these structures in nature. Inspired by nature, scientists have developed synthesis strategies for next-generation materials with advanced optical, electronic, catalytic and magnetic properties that can fulfill several functions at the same time. In recent years, composite materials obtained as a result of the combination of an organic phase, especially macromolecules (polymers) with inorganic particles, have been attracted great attention in materials science due to their potential applications in optics, energy storage, catalysis, sensors, drug delivery and cancer therapy. Nanocomposite materials are structures that are formed by placing one of the phases smaller than 100 nanometers together in one, two or three dimensions. Metal, ceramic or polymer can be used as a matrix. With the emergence of exciting properties of nanocomposite materials, this interest has continued to grow in two directions, especially: i) synthesis and development of new nano-sized inorganic structures to be embedded in the polymer, and ii) synthesis of polymers with better properties. The idea behind creating nanocomposites is to use building blocks with dimensions in the nanometer range to design and develop new materials with unique physical properties. Nanoparticles, nanorods, nanofibers and carbon nanotubes are examples of the individual inorganic units that constitute the nanocomposite. These materials show synergistic properties that are not found in the individual components. The polymer gains new properties with the nanostructures placed in the polymer matrix which are not present the polymer itself, such as conductivity, heat resistance, high refractive index, etc. Nanocomposite materials can be obtained in two ways: i) by synthesizing inorganic particles in situ synthesis or ii) by dispersing inorganic filler building blocks in a polymeric matrix. There are also two methodologies in the synthesis of polymeric nanocomposite materials: i) bottom-up and ii) top-down. In the bottom-up approach, precursors are used to build and grow well-organized structures from the nanometric level. In the top-down approach, layered nanostructures are often dispersed in the polymer matrix using physical methods. By using the photocatalytic properties of semiconductor materials, nanocomposite materials can be obtained by in-situ polymerization. Semiconductor materials used as fillers of composite materials have unique properties in many respects. Inorganic semiconductor nanoparticles are highly sensitive to light and can release charge carriers under light exposure. Many photocatalytic applications are implemented by utilizing the photocatalytic properties of semiconductor materials. Examples include hydrogen evolution from water, biosensing, sensitizers in solar cells, treatment of pollutants and water purification, and many other photocatalytic processes. The catalytic activity of semiconductors under light exposure is based on the excitation of nanostructures and the consequent formation of electron-hole pairs. Solar energy is recognized as one of the most promising renewable energy sources. In this context, the use of semiconductor photocatalysts in a wide variety of applications has attracted great interest since the discovery of the catalytic effect of TiO2 under ultraviolet (UV) light. Important strides have been made in the field of materials science towards discovering new semiconductor materials and expanding their potential applications. UV light is required for the activity of many conventional semiconductors such as TiO2. However, 90% of the rays from the sun are not sufficient for the activation of conventional semiconductors. In the last decade, numerous strategies have been proposed for the design of new photocatalysts that operate in the broad spectrum of sunlight, particularly in the near-infrared (NIR) region. Many NIR light-operated photocatalysts used show low catalytic efficiency due to the narrow absorption band and some synthetic difficulties. Therefore, the development of new photocatalyst systems that absorb sunlight at much longer wavelengths, particularly in the visible and NIR, has attracted widespread interest. Polymer synthesis by light (photopolymerization) has receieved a growing importance. This methodology finds application in many different fields such as medical, electronics, automotive, pharmaceutical, lithography, coating, three dimensional (3D) printing and imaging, as it offers economical and more environmentally friendly conditions. The monomer units can turn into polymers when exposed to light with the appropriate frequency. However, this process often results in low yield and it is almost always necessary to add a light-sensitive additive to the system to escalate the efficiency. This additional substance is called as photoinitiator. It decomposes by absorbing light and provides the formation of reactive species necessary for the polymerization to start. Although the use of photoinitiator increases the polymerization efficiency, it also brings many disadvantages such as cost, odor, solubility, and yellowing of the final polymer. Because it is both reusable and environmentally friendly, the use of semiconductor materials in polymerization initiator systems offers additional advantages than chemical initiators. In addition to providing the necessary electron for polymerization, it also provides hybrid material to be obtained when it is not removed after the reaction. In this thesis, nanocomposites were obtained by including nanomaterials in various photopolymerization processes and presented in three different studies. In the first chapter of thesis, the synthesis, characterization and initiator activity of black phosphorus, which has 2D semiconductor properties, are reported. Black phosphorus, an important material in the 2D family, can initiate polymerization with the electron it produces when irradiated in the near infrared region, without an additional photoinitiator. The dispersed form of black phosphorus in a solvent can initiate the polymerization not only by radicals, but also indirectly formed cations. Therefore, it allows the polymerization of both free radical polymerizable acrylate type monomers and cationically polymerizable vinyl ethers and epoxides. In the second part of the study "copper-catalyzed azide-alkyne addition" (CuAAC) reactions by utilizing the photocatalytic property of black phosphorus in the NIR region is presented. It has been successfully shown that when black phosphorus is irradiated in the NIR region, it reduces CuII salts in the reaction medium to CuI salts and provides triazole ring formation between small molecules. The concept was further extended to the telechelic and block copolymer synthesis confirming the use of black phosphorus in the synthesis of low molecular weight as well as macromolecular structures. Obtained polymers were examined spectroscopically and chromatographically, and the efficient operation of the system was supported by various characterization methods. Finally, a successful adaptation of Curtius rearrangement to macromolecular structures has been achieved by the ligation process. It is known that when acyl azide compounds are exposed to UV light, they undergo intramolecular conversion and form isocyanate compounds. From this point of view, acyl azide functional compounds can produce urethane linkage with the compounds which have hydroxyl functional group under UV light. This ligation process has been successfully used for the modification of chain ends of macromolecules, the production of cross-linked polymers, and the modification of silica and graphene oxide surfaces with –OH functional groups.