Electrical And Mechanical Properties Of Polymer Nanocomposites
Abstract
This thesis consists of three projects concerning the electrical and mechanical properties of polymer nanocomposites. We study the effect of nanoscale filler particles on the polymer dynamics at different length and time scales. In the first study, poly(ethylene oxide) (PEO)-multiwalled carbon nanotubes (MWCNT) nanocomposites with a MWCNT concentration ranging from 0 to 5 wt\% were prepared by both melt-mixing and twin-screw extrusion. Their electrical properties were studied over a wide range of frequency and temperature using a dielectric spectrometer. A percolation transition is observed at which the electrical conductivity of the nanocomposites increases by several orders of magnitude. The percolation threshold concentration p_c is very well-defined in the twin-screw extruded material, but less so in the melt-mixed nanocomposites. We identify two different dielectric relaxation processes in our PEO/MWCNT nanocomposites, which we attribute to polymer dynamics at different length scales. The second project is a study of the mechanical properties of PEO/MWCNT nanocomposites made by melt-mixing. We used a rotational shear rheometer to perform measurements during thermal cycling. Our results show that there are three main mechanical relaxation times in the nanocomposites, all of which are much slower than the relaxation times observed in dielectric data. One of these processes is due to the reptation of polymer chains. Another is due to the relaxation of PEO chains whose motions are restricted by MWCNT. The third one is related to the sample preparation process. In the last project, we used dielectric spectroscopy to investigate the electrical properties of polystyrene (PS)-MWCNT nanocomposites made using twin-screw extrusion. Our data suggest that the percolation threshold for these nanocomposites is between 4 and 5 wt\%, but the transition only occurs once the sample has been heated above 330 K. In most cases, the dielectric spectrum did not show any relaxation features. A dielectric relaxation was only observed for a MWCNT concentration of 5 wt\%, and the relaxation peak disappeared when the sample was heated above 330 K due to the high electrical conductivity of the sample. Our studies showed several examples of polymer dynamics influenced by the presence of MWCNT on time scales ranging from microseconds to hundreds of seconds.