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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 correctingdoi:10.1016/s0032-3861(04)00371-4 fatcat:anpmwpu7ive5rhj2vftketr6lu