Carbon Nanotube Polymer Composites: Mechanical, Electrical and Photorefractive Properties.

Doctoral thesis, 2007

Incorporation of carbon nanotubes (CNT) into polymers is attractive due to the generally good processability of polymers and unprecedented properties of nanotubes. In this work, preparation and matrix known earlier but not employed so-far for nanocomposites are studied towards enhanced mechanical and electrical performances. For another CNT composite, the photorefractive behaviour is studied. Commercial single-wall (SWNT) and multi-wall nanotubes (MWNT) grown by arc discharge and Chemical Vapour Deposition (CVD) of diameter 1.4 nm (average value) and 10-30 nm, respectively, were used. They were purified and functionalised by acid reflux, and characterised by X-ray Photoelectron Spectroscopy (XPS). From the C1s spectrum, the peak corresponding to carboxylic group increased from 7% (for as–received) to 16%, and similarly from 0% to 9%, for oxidised MWNT and SWNT, respectively. The presence of carboxylic moieties plays a role in the composite preparation and properties. Films containing 0-1.5 wt% CNT were synthesized by hot pressing from melamine-formaldehyde (MF) with -cellulose, using a film stacking route with layered cellulose, and using milled cellulose. Sonication, mechanical mixing and surfactant assistance were used in the low viscosity (~30 mPa·s) aqueous MF solution to de-agglomerate and disperse the nanotubes. Electron and optical microscopy was performed to observe the morphology. Also, composites containing 0.2-0.3 wt% untreated SWNT in aromatic polyimide (API) were prepared by coating. Obtained results for both, SWNT and MWNT MF composites demonstrate that improvements in stiffness and strength, practically without embrittlement (MWNT), can be achieved; for example improvements by around 40-50% at 0.1 wt% CNT. Continuum mechanics calculations of the modulus gave underestimated values. Superior stiffening of the structure is explained in terms of constituents linking (immobilisation). Conductive behaviour was found for SWNT/MF at filler content >1 wt% and >1.5 wt% (MWNT), and statically dissipative behaviour for CNT/cellulose/MF at filler content >0.05 wt%. From the percolation theory, a threshold of 0.13 wt% (SWNT) and 0.2 wt% (MWNT) were calculated. For SWNT/API, improvement in the photorefractive performance was observed at 1062 nm wavelength with 0.25 wt% CNT. The gain coefficient of 90 cm-1 and net gain coefficient of 65 cm-1 at 80 V/μm were obtained.