Rheology of supercooled P–Se glass-forming liquids: From networks to molecules and the emergence of power-law relaxation behavior

Abstract

The effect of the network-to-molecular structural transformation with increasing phosphorus content in P xSe 100-x (30 {less than or equal to} x {less than or equal to} 67) supercooled liquids on their shear-mechanical response is investigated using oscillatory shear rheometry. While network liquids with 30 {less than or equal to} x {less than or equal to} 40 are characterized by shear relaxation via a network bond scission/renewal process, a Maxwell scaling of the storage (G') and loss (G') shear moduli and a frequency-independent viscosity at low frequencies, a new relaxation process emerges in liquids with intermediate compositions (45 {less than or equal to} x {less than or equal to} 50). This process is attributed to an interconversion between network and molecular structural moieties. Predominantly molecular liquids with x {greater than or equal to} 63, on the other hand, are characterized by a departure from the Maxwell behavior as the storage modulus shows a linear frequency scaling G'(ω) ~ ω over nearly the entire frequency range below the G'-G' crossover and a nearly constant ratio of G˝/G´ in the terminal region. Moreover, the dynamic viscosity of these rather fragile molecular liquids shows significant enhancement over that of network liquids at frequencies below the dynamical onset and does not reach a frequency-independent regime even at frequencies that are four orders of magnitude lower than that of the onset. Such power-law relaxation behavior of the molecular liquids is ascribed to an extremely broad distribution of relaxation timescales with the coexistence of rapid rotational motion of individual molecules and cooperative dynamics of transient molecular clusters, with the latter being significantly slower than the shear relaxation timescale.