Mixtures of m-fluoroaniline with apolar aromatic molecules: Phase behaviour, suppression of H-bonded clusters, and local H-bond relaxation dynamics

We employ differential scanning calorimetry, IR spectroscopy, X-ray powder diffraction and dielectric spectroscopy to characterize binary mixtures of m-fluoroaniline (mFA), a rigid-molecule glass former, with two non-self-associating molecules containing benzene rings, namely m-xylene (mX) and a distyrene derivative (DS). The addition of mX suppresses the formation of H-bonded mFA clusters. While both mX and DS dilute the H-bond density of mFA, they have opposite effects on the glass transition temperature (Tg) and structural relaxation time of mFA, with mX acting as a plasticiser and DS as an antiplasticiser. While the mFA-DS mixtures can be studied in the supercooled liquid regime at DS molar mass fraction as high as 0.9 without undergoing phase separation, the mFA-mX mixtures remain homogeneous at low temperatures only up to a mX molar fraction of 0.5. All homogeneous mixtures display a structural relaxation which shifts according to the Tg of the mixture, and a secondary relaxation which remains virtually invariant with respect to that of pure mFA. For mX molar fraction higher than 0.5, the mixtures phase-separate into almost pure crystalline mX domains and a mFA-rich amorphous phase. Upon partial crystallization the structural relaxation time and Tg of the remaining amorphous fraction shift to larger values, consistent with the increased effective mFA concentration. At the same time, the secondary relaxation time undergoes a dramatic shift of more than two decades. These findings allow identifying the secondary relaxation in the rigid-molecule mFA glass former as a local reorientational motion of a m-fluoroaniline moiety around one of its H-bonds, a rotation that is strongly affected if the mobility of the H-bond sharing species is quenched by crystallization.