| Abstract: | Positron annihilation lifetime spectroscopy (PALS) was used to study the microstructure of PTFE/silica composites. The positron lifetimes (τn) and intensities (In) of PTFE and the composites (30-62% silica) were measured at room temperature as a function of specimen thickness. Four lifetime components were found in PTFE and the composites. The longer lifetime components, τ3 = 1.4 ns and τ4 = 4.4 - 4.1 ns, were interpreted to be due to the presence of two different sized free volume cavity distributions within the PTFE/silica composites. A strong silica concentration dependence was found in the bulk intensities (I3b and I4b). The I3b value increased from 13.0% in PTFE to 28.2% in the 62% composite, while the I4b value decreased from 17.5% in PTFE to 4.5% in the 62% composite. The smaller-void size, free volume fraction (τ3I3b) values increased linearly between 0 and 100% silica concentration, while the larger void size, free volume fraction (τ4I4b) values decreased nonlinearly with silica concentration. Since silica has a long lifetime component (τ3 = 1.6 ns), this behavior is ascribed to silica particles occupying the large free volume cavities (370 Å3) in the PTFE/silica composites. © 1996 John Wiley & Sons, Inc. |
