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Abstract
Positron annihilation lifetime spectroscopy (PALS) has been applied in natural rubber to determine the properties of nano free volumes (size, concentration, distribution) as a function of temperature.
A fast fast coincidence system with a time resolution (FWHM) of 300 Ps with 60Co has been used. A positron source of 22Nacl covered with kapton foil is used. The source is sandwiched between two identical samples and completely enclosed in a copper sample holder at the end of the cold finger of the liquid-helium cryostat with temperature controlled to better than ±0.5K.
The lifetime spectra at each temperature (ranging from 173K to 313K) consisted of 106 counts at a rate of 40 counts/s.
The analysis of the PAL spectra have been performed by finite as well continuous methods using the routine L.T 9.0 in its discrete and distribution modes respectively.
The results have proven, that the PALS is sensitive to probe the glass transition temperature and for the first time the melting transition temperature in natural rubber. In addition, the free positrons are found to be sensitive to the low density domains in the crystalline matrix as well as in the interphase between crystalline and amorphous phase. The ortho-positronium (o-ps) is found to probe the variation of free volumes in the amorphous domains with temperature ranging from 42Å3 to 252Å3 at -100oC and 40oC respectively. The deduced expansion coefficient indicating that o-ps is probing selectively free volume sites in amorphous domains.