POLYMER-FILLER INTERACTIONS IN POLYETHER BASED THERMOPLASTIC POLYUREATHANE/SILICA NANOCOMPOSITES
Materials Science and Engineering, PhD Dissertation, 2013
Prof. Dr. Yusuf Z. Menceloğlu (Thesis Supervisor), Prof. Dr. İskender Yılgör, Assoc. Prof. Melih Papila, Asst. Prof. Burç Mısırlıoğlu, Prof. Dr. Yaşar Gürbüz
Date &Time: June 07th, 2013 - 09:00
Place: FENS G032
Keywords: Nanocomposites, Polyether Based Thermoplastic Polyurethaneurea, Colloidal Silica, Solution Blending, Polymer-Filler Interfacial Interaction.
Thermoplastic polyurethane ureas (TPU) are a unique class of materials that are used in a broad range of applications due to their tailorable chemistry and morphology that enable engineering materials with targeted properties. The central theme of this dissertation is to develop an understanding on polymer-filler interfacial interactions and related reinforcing mechanism of silica nanoparticles in polyether based TPU/silica nanocomposites. First, the growth of silica nanoparticles in different solvents was studied. In-situ liquid-state 29Si nuclear magnetic resonance (NMR) was used to investigate the temporal concentration changes during ammonia-catalyzed initial hydrolysis of tetraethyl orthosilicate (TEOS) in different solvents (methanol, ethanol, n-propanol, iso-propanol and n-butanol). Dynamic light scattering (DLS) was employed to monitor simultaneous changes in the average diameter of silica particles and atomic force microscopy (AFM) was used to image the particles within this time frame. Solvent effects on initial hydrolysis kinetics, size and polydispersity of silica particles were discussed and explained on the basis of polarity and hydrogen-bonding ability. Stable silica sols in isopropanol, which is a common solvent for studied TPU copolymers, were prepared in a controlled manner. Second, nanocomposites consisting of TPU and silica nanoparticles of various size (20-250 nm) and filler loadings (1-40 weight%) were prepared by solution blending and characterized. TPU copolymer was based on a cycloaliphatic diisocyanate and hydroxyl terminated poly(tetramethylene oxide) or poly(ethylene oxide) with number average molecular weight of 2000 g/mol and 2-methyl-1,5-diaminopentane as a chain extender. Solution blending using isopropyl alcohol resulted in even distribution of silica nanoparticles in the polyurethane-urea matrices. FTIR spectroscopy indicated strong interactions between silica particles and polyether segments. Incorporation of silica nanoparticles of smaller size led to higher modulus and tensile strength of the nanocomposites, and elastomeric properties were retained. Increased filler content of up to about 20 weight% resulted in materials with higher elastic moduli and tensile strength while the glass transition temperature remained the same. Improvements in tensile properties of the nanocomposites, particularly at intermediate silica loading levels and smaller particle size, are attributed to the hydrogen-bonding interactions between silanol groups on the surface of silica nanoparticles and ether linkages of the polyether segments of the copolymers. In summary, we have introduced a simple method to prepare nanocomposites of polyether based TPU/silica nanocomposites with a range of mechanical properties and homogeneous distribution of silica particles using solution blending of a copolymer solution and silica sols “as synthesized and aged” in common solvents at room temperature.