Development of high performance carbon nanotube/polymer composites
thesisposted on 11.09.2012, 11:33 by Dongyu Cai
This project mainly concerned the development of novel engineering approaches to optimise the physical properties of the polymer composites with a low loading of carbon nanotubes (CNTs). It was additionally discovered that graphite oxide nanoplatelets (GONPs) can be a strong and affordable substitute for the CNTs in the polymer composites. Colloidal physics and coating methods were applied to fabricate semi-conductive CNT/polymer composites with low percolation threshold. Polyurethane (PU) latex and ultra high molecular weight polyethylene (UHWMPE) powder were used as hosting matrix in the colloidal physics method and coating method, respectively. In the colloidal physics method, the percolation threshold was found to be around O.5wt% MWCNTs and the electrical conductivity of the composites was improved by more than four orders of magnitude with the addition of I wt % multi-walled carbon nanotubes (MWCNTs). The study of rheological behaviour revealed that the addition of the MWCNTs led to the increase in the viscosity of the PU dispersion. In the coating method, the scanning electron microscopy (SEM) images confirmed the strong adhesion of the nanotubes on the surface of the powders. Sheet samples were prepared using compression moulding for electrical test. The percolation threshold for the powders with the size of 60)lm was around I wt% MWCNTs and the percolation threshold for the powders with the size of 100)lm was around 0.5wt% MWCNTs. A novel route was revealed to reduce the interfacial phonon scattering that is considered as the bottleneck for CNTs to highly improve the thermal conductivity of CNT/polymer composites. Semicrystalline PU dispersions were used as latex host to accommodate the MWCNTs following the colloidal physics method. The thermal conductivity increased from 0.15 Wm-'K-' to 0.47 Wm-'K", by -210%, as the addition of the MWCNTs increased to 3wt%. The morphology of the composites suggested that the continuous nanotube-rich phase existing in the interstitial space among the latex particles and the crystaIIites nucleated at the nanotube-polymer interface were the main factors for the effective reduction of interfacial phonon scattering. The optimisation of the crystalline layer around CNTs was studied based on the MWCNT/polycapro!actone (PCL) composites using differential scanning calorimetry (DSC). The study of the non-isothermal crystaIlisation showed that crystaIIisation temperature (Tc) increased with increasing incorporation of the nanotubes, and melting temperature (T m) and heat of fusion (ilHm) was almost unchanged. The incorporation of 2wt% nanotubes resulted in the biggest increase of the T c to be -11 QC. The study of the isothermal crystaIIisation showed the temperature, 14 DC higher than the Tc. was appropriate one to optimise the crystaIIine layer in the composite melts. It was revealed that the incorporation of 0.1 wt% nanotubes significantly affected the rate of crystal growth and crystalline morphology. For more incorporation of the nanotubes, the rate of crystal growth and crystaIIine morphology was less affected. The improvement in the Young's modulus of the composite with the thermal treatment confirmed the contribution of the crystalline layer to the load transfer across the non-covalent interface between the nanotube and polymer matrix. The preparation of the exfoliated GONPs in DMF was revealed. With this method in hand, two kinds of polymers including semi-crystaIline PCL and amorphous PU were selected to be incorporated with the GONPs using the solution method. It was found that the GONPs showed strong nucleating ability in the PCL matrix. The thermal treatment under the "14QC" rule could create an optimised crystalline layer on the surface of the GONPs from the composite melts. The bigger increase in the Young's modulus of the treated GONPIPCL composites confirmed that the crystaIIine layer nucleated on the surface of the GONPs could act as a non-covalent interface between the GONPs and PCL matrix. The significant reinforcement of the PU using GONPs was also disclosed. Morphologic studies showed thai, due to the formation of chemical bonding, strong interaction occurred between the GONPs and the hard segment ofthe PU, which allowed effective load transfer. The GONPs can prevent the formation of crystalline hard segments due to their two-dimensional structure. With the incorporation of 4.4wt% graphite oxide nanoplatelets, the Young's modulus and hardness of the PU were significantly increased by -900% and -327%, respectively. The resultant high anti-scratch property pointed to the promising application of these composite materials in surface coating.
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