Solid–solid phase transitions and molecular motions in long-chain paraffins studied by positron annihilation techniques

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Abstract
Positron annihilation lifetime and Doppler-broadening measurements were performed on n-dotriacontane (n-C32H66) and n-tritriacontane (n-C33H68) between 80 K and their melting points. The annihilation parameters plotted as a function of temperature show irregularities above the room temperature, which reflect solid–solid phase transitions in these materials. The transition temperatures derived from the positron annihilation measurements are in excellent agreement with literature values determined by other techniques. In the case of n-C32H66 additional phase transitions not reported previously in the literature were observed. The lifetime vs temperature curves could be reasonably well interpreted within the framework of the free volume model taking into account the effect of special types of molecular motions and structural defects. On the basis of our measurements, linear dependence of ortho-positronium (o-Ps) lifetime on the size of the cavity around o-Ps could be expected. The proposed empirical relationship proved to fit well to lifetime data for molecular crystals as a function of their molecular volume. The o-Ps yield showed an abrupt decrease with increasing temperature at 190 and 165 K in the case of n-C32H66 and n-C33H68, respectively. This effect was attributed to the onset of rotation of the methyl end groups of the paraffin chains.