Dynamical Heterogeneity in Glassy o-Terphenyl. 2. Measurement of Environment Structure Lifetime Using Reversible Reactions

Abstract

A new method is proposed for measuring the lifetime of heterogeneities in a glassy matrix. UV-vis absorption spectroscopy has been used to monitor the kinetics of photoinduced cis-->trans isomerization of 1-naphthyl-azomethoxybenzene (NAMB) in o-terphenyl (OTP) below T(g). The dependence of isomerization rate on the duration of dark interval after generation of cis molecules was established; an increase in the dark interval causes a decrease in isomerization rate. This dependence is shown to be due to the change in the local environment of NAMB molecules. The time required for the environment to change was estimated over a temperature range of 234 (T(g) - 9 K) to 241.5 K (T(g) - 1.5 K). The change in the environment of the guest molecules has been interpreted in terms of the exchange processes. The values obtained for the exchange time coincide with the rotation times of the NAMB molecule. Also, it is shown that the time of structural rearrangement of the environment near the fast reacting molecules is of the same order of magnitude as the time of structural rearrangement of the environment near the slow reacting ones.

Supercooled Liquid Dynamics: Advances and Challenges

“…In this case, Q was observed to decrease strongly with temperature such that no contradiction with the NMR results appeared [155]. In a more recent study, the long exchange times for o-terphenyl at T g could not be confirmed, as measurements using photochromic probes were indicative of Q ≈ 2 [190]. Solvation dynamics experiments covering the time window of 50 s-3 ms regarding τ α resulted in Q > 9 [191], but the timescale of collective solvent responses as detected by the Stokes shift correlation function turns out a factor of 6 faster than other common metrics of structural relaxation.…”

Evolution of the Environment of Guest Molecules in Dynamically Heterogeneous Matrices of Poly(ethyl methacrylate) and Poly(n-butyl methacrylate) Far below Tg

“…The simulation procedure is described in more detail elsewhere. 41 Numerical simulations of the isomerization kinetics using the model of random changes fail to provide adequate description of the experimental curves (the simulation results are not presented). The reason for the failure can be understood from the qualitative analysis of the kinetic curves shown in the bottom panel of Figure 4: the slowest molecules do not manifest themselves in the kinetics of isomerization at low τ dark values (5À15 min).…”

“…The equilibrium values of C i ( t ) and k i were found from the kinetic curves obtained at the maximum value of τ dark . The simulation procedure is described in more detail elsewhere …”

“…Each step leads to only a moderate change in the environment. Note that, in contrast to the studied polymers, the exchange in low-molecular-weight glass-formers , can be described with the model of random changes in environments.…”

“…The goal of present study was to investigate the exchange dynamics in two polymeric matrices, PEMA and P n BMA, far below T g using cis – trans photoisomerization of guest molecules as a probe process. Earlier, utilizing this approach, it has been found that, in glassy o -terphenyl, the time required for the environment to change is close to the rotational time of the probe molecules and follows the temperature dependence of α-relaxation time. Because of the chain molecular structure, the relaxation behavior of polymers differs noticeably from that of small molecule glass-formers. − Therefore, we can expect that the exchange dynamics in polymers also differs from that in low molecular weight glasses.…”