Spin probe dynamics in relation to free volume in crystalline organics from ESR and PALS: N-tridecane

摘要

A joint external probe ESR and PALS study on n-tridecane (n-TRD) using stable free radical 2,2,6,6-tet-rarnethyl-1-piperidinyloxy (TEMPO) or ortho-positroniurn (o-Ps), respectively, is presented. In contrast to n-hexadecane (n-HXD) [Physica B 430, 99 (2013)], five crossovers in the spin probe TEMPO mobility coinciding with those in the o-Ps probe annihilation parameters at the characteristic ESR and PALS temperatures in the solid phase were revealed: 1) acceleration within the slow regime at T-xl(slow) in accordance with the changes in both o-Ps lifetime, tau(3), and relative o-Ps intensity, I-3, at T-bl(CT)* in the rigid crystalline solid are related to very local end methyl group dynamics, 2) slow to fast regime transition at T-50G < < T-m(DSC) is similarly as for n-HXD ascribed to the spin probe TEMPO dynamics in the expanding interlamellar gap due to the molecular probe deformation induced enhanced end-chain mobility close to the rigid to soft crystalline solid transition at T-bl(CT) being connected also with the starting dominance of electron detrapping over positron mobility, 3) the first change within the fast regime at T-X1(fast) coinciding with reduction of 13 at T-b2(cr**) is related to strong electron detrapping due to longitudinal disordering and 4) the next one at T-X2(fast*) is related to the soft to plastic crystalline solid phase transition at T-b2(cr)*=T-ss(DSC)=0.95T(m)(DSC). Finally, similarly as for n-HXD the highest T crossover at T(X2)(fas)t congruent to T-m(PALS) is related with the collective chain dynamics of the melting transition T-m(DSC). The underlying structural-dynamic processes behind the observed ESR and PALS crossover coincidencies are discussed in the light of the present thermodynamic DSC and reported dynamic NMR, QENS and IR data. (C) 2015 ELsevier B.V. All rights reserved.