Dielectric Study of Molecular Mobility in Poly(propylene-graft-maleic anhydride)/Clay Nanocomposites
(2005)
Polymer/clay nanocomposite materials based on poly(propylene-graft-maleic anhydride) (PPgMAH) and two different organophilic modified clays were investigated by dielectric relaxation spectroscopy (DRS). In contrast to ungrafted polypropylene (PP), PPgMAH shows a dielectrically active relaxation process which can be assigned to localized fluctuations of the polar maleic anhydride groups. Its relaxation rate exhibits an unusual temperature dependence, which could be attributed to a redistribution of water molecules in the polymeric matrix. This is confirmed by a combination of Raman spectroscopy and thermogravimetric experiments (TGA) with real-time dielectric measurements under controlled atmospheres. In the nanocomposites this relaxation process is shifted to higher frequencies up to 3 orders of magnitude compared to the unfilled polymer. This indicates a significantly enhanced molecular mobility in the interfacial regions. In the nanocomposite materials a separate high-temperature process due to Maxwell-Wagner-Sillars (MWS) polarization was observed. The time constant of this MWS process can be correlated with characteristic length scales in nanocomposites and therefore provides additional information on dispersion and delamination/exfoliation of clay platelets in these materials. These properties also influence the diffusivity of the water molecules as revealed by real-time dielectric investigations.
Nanocomposites of bisphenol A polycarbonate with 2, 4, 6, and 15 wt% multiwall carbon nanotubes (MWNT) and their use in fire retardancy are investigated. Their thermal behavior and pyrolysis are characterized using thermogravimetry, differential scanning calorimeter, oscillatory shear rheology, and dynamic mechanical analysis. The flammability is addressed using LOI and UL 94; the fire behavior, with a cone calorimeter using different irradiation. With increasing MWNT content the storage modulus is increased (10-20%) and melt viscosity increases by several orders of magnitude, particularly for low shear rates. The melt flow, dripping, and deformation during fire are hindered, which influences UL 94 and cone calorimeter results. The peak heat release rate is reduced up to 40-50% due to an improved barrier for small amounts (2 wt%) of MWNT and for low irradiation, whereas the effect is reduced for increasing irradiation and nearly vanishes for increasing filling. Adjuvant but also deleterious mechanisms result in the complex dependency on the MWNT content. Significant flame retardancy effects are specific and limited to only some fire properties. This study allows the materials' potential for implementation in different fire scenarios and tests to be assessed and provides insight into active mechanisms.