Characterization of free volume and density gradients of polystyrene surfaces by low-energy positron lifetime measurements

2004 Polymer  
Free volume and density gradient widths of the free surface of high molecular mass polystyrene was characterized by use of a pulsed lowenergy positron lifetime beam. A density gradient in terms of mass density (g/cm 3 ) versus depth (nm) was obtained from the experimental beam data using a novel approach, yielding a width of the density gradient of approximately 3.5 nm. The procedure accounted for the broadening of the positron implantation probe as a function of energy as well as correcting
more » ... the increase in positron implantation depths due to the presence of a lower density at the surface region. Moreover, the spectra in the low implantation energy range were found to contain a long-lived lifetime that yielded very large estimates of the free volume effects at the surface. Accounting for this lifetime in the evaluation procedure, resulted in a much improved agreement with the results of other experimental and theoretical investigations in the literature. q
doi:10.1016/s0032-3861(04)00371-4 fatcat:anpmwpu7ive5rhj2vftketr6lu