1995 Volume 44 Issue 9 Pages 608-615
Positron lifetime spectra for general purpose polystyrene (GPPS), polybutadiene rubber (PBR), and their copolymers, styrene-butadiene rubber (SBR) and high impact polystyrene (HIPS) have been measured. A fast-fast coincidence system with BaF2scintillation crystals coupled to photomultiplier tubes (Philips XP2020Q) and constant fraction differential discriminators (OR TEC 583) for selecting the energy of γ-rays was used.
It has been found that the free volume in the copolymers is smaller than the average over the individual polymers, probably due to the interfacial interaction between the styrene and rubber phases.
A long-lived component with a mean lifetime of about 123 ns exists in the spectrum for deformed HIPS. These results from the ortho-positronium in the deformation induced crazes. The long lifetime becomes shorter when the sample is placed in oxygen gas atmosphere. This indicates that the ortho-positronium in the crazes undergoes spin-conversion into para positronium on collision with an oxygen molecule. The oxygen pressure in the crazes has been analyzed to be the same as in the external space, showing that the crazes accessible for the positronium, i. e., those existing within about 50μm from the HIPS surface are connected to the external space.
