Effects of oxidative functionalized and aminosilanized carbon nanotubes on the behaviours of polyamide-6 nanocomposites

Şankal, Seçil
The first aim of this dissertation was to modify carbon nanotubes to be used as nano-reinforcements in the polyamide-6 matrix. Surfaces were first oxidative functionalized by sulphuric acid/nitric acid mixture, then aminosilanized by γ-aminopropyltriethoxysilane. Chemical groups formed on carbon nanotubes due to these surface treatments were characterized by X-ray photoelectron spectroscopy, Fourier-transform infrared spectroscopy and also energy dispersive spectroscopy. Morphological changes and crystal structure of surface-treated carbon nanotubes were analyzed by scanning electron microscopy and X-ray diffraction, respectively. Thermogravimetric analysis was also used to observe thermal degradation of the chemical groups formed on the nanotube surfaces. Then, unmodified, oxidative functionalized and aminosilanized carbon nanotubes with three different loading levels (0.1, 0.5 and 1.0 wt%) were melt mixed with polyamide-6 matrix via laboratory size twin-screw extruder, followed by specimen shaping via injection molding. As the second aim of this dissertation, morphology and dispersion states of carbon nanotubes in polyamide-6 matrix was investigated by scanning and transmission electron microscopy; which revealed more homogeneous dispersion of functionalized and aminosilanized carbon nanotubes due to their increased chemical interactions with the matrix. The third aim of this dissertation was to investigate effects of oxidative functionalized and aminosilanized carbon nanotubes on the (i) isothermal and (ii) non-isothermal crystallization kinetics of polyamide-6 by DSC analyses, and (iii) crystal structure of injection molded specimens by XRD analyses. Due to basically very effective heterogeneous nucleation effect, both increasing amount and surface functionalization of carbon nanotubes by oxidation and aminosilanization resulted in higher relative crystallinity for all three cases. The increases were as much as 40% for the isothermal and non-isothermal crystallization, and it was up to more than two times in the injection molding. Crystallization parameters and Avrami constants indicated that crystallization rate increases in isothermal crystallization while it decreases in non-isothermal crystallization due to the delayed conformational mobility of polymer chains via physical hinderance of carbon nanotubes. Parameters also revealed that growth mechanism of crystallites might change during isothermal crystallization while there was no significant change during non-isothermal crystallization. XRD deconvolution analyses indicated that during injection molding, due to the constraints of carbon nanotubes only α-crystal structure was formed. Finally, as the fourth aim of this dissertation, effects of oxidative functionalized and aminosilanized carbon nanotubes on the mechanical and thermal properties of polyamide-6 nanocomposites were investigated. Flexural and tensile tests indicated that, increases in the flexural strength and tensile yield strength were 30% and 20%, while in the flexural modulus and Young’s modulus were 40% and 23%, respectively, with only 1 wt% aminosilanized carbon nanotubes; due to very efficient load transfer from the matrix to covalently bonded carbon nanotubes. Both dynamic mechanical analysis and thermogravimetric analysis showed that surface modified carbon nanotubes improve all thermal properties due to decreased matrix mobility and physical barrier formation. For example, increases in the storage modulus values were as much as 25%, while the increase in the thermal degradation temperatures were as much as by 5°C in the specimens with only 1wt% aminosilanized carbon nanotubes. Increases in the mechanical and thermal properties should be also due to the increased crystallinity of polyamide-6 matrix via carbon nanotubes acting as heterogeneous nucleation sites.