Study on damping mechanism based on the free volume for CIIR by PALS.

The influences of the free volume and the termperature on the damping property of chlorinated isoprene-isobutylene rubber (CIIR) were first investigated by positron annihilation lifetime spectroscopy (PALS), dynamic mechanical analysis (DMA), and differential scanning calorimetry (DSC), respectively. From the variations of ortho-positronium (o-Ps) lifetimes as a function of temperature, two transition temperatures can be observed, i.e., Tg and TB. We found that there exists a pre-rubbery state between the glass transition temperature Tg and TB. The DMA results indicate that the tan delta peak of CIIR has a very broad temperature distribution because of the existence of a liquid-liquid transition (Tll). The temperature dependence of the average free volume size can be linearly fitted below Tg and above TB, respectively. It is very interesting that the difference spectrum of the free volume size between the results fitted by two lines (below Tg and above TB) and experimental data is very similar to the dynamic mechanical spectrum of CIIR. In order to clarify the damping mechanics, the Williams-Landel-Ferry (WLF) equation based on free volume theory has been successfully used to establish a direct quantificational relationship between the free volume and the damping property, which indicates that the free volume plays an important role in determining the damping property.