Spin probe dynamics in relation to free volume in crystalline organics by means of ESR and PALS: n-Hexadecane

We report on a combined study of the guest dynamics and free volume in n-hexadecane (n-HXD) using two external microscopic probes: stable free radical 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) and ortho-positronium (o-Ps) by means of ESR or PALS, respectively. Dynamic behavior of the molecular TEMPO probe and annihilation properties of the atomistic o-Ps one as a function of temperature are compared. Two coincidencies between the crossover effects at the characteristic ESR and PALS temperatures T50 G and Tb1cr below and TX2fast and TmPALS at the melting point TmDSC of n-HXD from macroscopic DSC scan were found. First, the slow to fast regime transition is ascribed to the spin probe TEMPO localization in the expanding interlamellar gap due to the induced enhanced end-chain mobility. Second, the high-temperature crossover within the fast regime is directly connected to the collective chain movements at the melting which takes place under the similar free volume fluctuation condition as revealed recently for a series of amorphous small molecular and polymer glass-formers.