Rotational dynamics of anthracene and 9,10-dimethylanthracene in polyisoprene

Abstract

Singlet and triplet transient grating experiments have been used to measure rotational correlation functions for anthracene and 9,10-dimethylanthracene in cis-polyisoprene from Tg+120 °C to Tg+20 °C. Over this temperature range, the rotational correlation time varies by seven decades and follows the temperature dependence of the bulk viscosity. The observed correlation functions are not consistent with predictions of the anisotropic rotational diffusion model. The observed correlation functions for anthracene are consistent with a change in the predominant axis for rotational diffusion as a function of temperature. At high temperatures, in-plane rotation occurs most rapidly, while at low temperatures rotation about the long axis predominates.

Two-order-parameter model of the liquid–glass transition. II. Structural relaxation and dynamic heterogeneity

“…These points cause marked differences in the physical meaning and the behavior of n between glass transition and critical phenomena. This picture is consistent with the experimental indications of the existence of dynamic heterogeneity [8][9][10][11] (see Ref. [12] for review) and the absence of the characteristic length in scattering experiments [13].…”

“…It is known that below T Ã m (in our terminology) the translational self-diffusion in supercooled liquids has a significantly weaker temperature dependence than that of the viscosity or rotational diffusion [8,11]. The most intuitive explanation for the rotational-translational decoupling based on the dynamic heterogeneity is as follows [55]: The characteristic time scale of rotational motion is governed by the slower fraction of the distribution of relaxation times, D r / hs a i À1 (D r : rotational diffusion constant), while the average translational time is governed by the faster fraction, D t / hs À1 a i (D t : translational diffusion constant).…”

Two-order-parameter model of the liquid–glass transition. III. Universal patterns of relaxations in glass-forming liquids

“…This small-amplitude orientational fluctuation mode may be the slow b mode. This assigned type of motion is consistent with the assignment of this mode to small-amplitude rapid librational motion by Fujara et al [87], Spiess et al [88,89], and Ediger and coworkers [90] based on their experiments.…”

“…The experimental results mentioned above [87][88][89][90]104] also indicate the existence of spatial dynamic heterogeneity of the slow b mode. This is consistent with a wide distribution of the local density of metastable islands ranging continuously from the more liquid-like to more solid-like regions, which should exist in the frustrated metastable-liquid state between T Ã m and T 0 according to our model.…”

Two-order-parameter model of the liquid–glass transition. I. Relation between glass transition and crystallization

“…Another problem is related to the question whether there exists spatial or dynamic heterogeneity associated with the liquid-glass transition [69][70][71][72][73][74][75]. The MCT that deals with a two-body density correlator does not deal with any heterogeneity.…”