Correlation with mechanical and other properties

3 Correlation with mechanical and other properties 10.3.3.1 Gas permeation 

The diffusion of gases through a polymer matrix is determined by the mobility of gas molecules through the matrix. The diffusion coefficient is therefore, at least partially determined by the free volume size of the polymer. It has been shown, for example, that there is a correlation between the free volume measured by PAL and the diffusivity of carbon dioxide in a seriers of polycarbonates [58], In a study of poly (trimethylsilyl propyne) (PTMSP), which has an extremely high gas permeability and diffusion coefficients, it was found that the lifetime data could be resolved into four components [59]. The longest lifetime component (T4) had a lifetime of 

A number of PAL studies on polymers have attempted to correlate PAL parameters with macroscopic mechanical properties. The value of PAL in providing microscopic structural information for the observed macroscopic properties in polymer systems is illustrated by the examples bellow. 

Hsieh et al [62] found that for a range of thermotropic liquid crystalline polymers, the greater the free volume measured by PAL the greater the chain mobility at Tg and the higher the value of tan S (damping strength) measured by dynamic mechanical analysis. 

The effects of plasticizers has also been studied by PAL [64, 65]. The addition of a plasticizer to polymers generally has the effect of lowering the Tg, however in some cases an anti-plasticization can occur. Borek et al [65] have shown that the fraction of free volume in PVC polymers could be fit with a fourth order polynomial as a function of plasticizer concentration. The decrease in th Tg with increasing amount of plasticizer is attributed to this increase in the free volume of the polymers. 

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