A Combined Atomic and Molecular Probe Characterization of Aromatic Hydrocarbons via PALS and ESR: Methylbenzene

Abstract

A combined study of one of the simplest aromatic hydrocarbons, i.e., methylbenzene (toluene) (TOL), via the annihilation of an ortho-positronium (o-Ps) probe via positron annihilation lifetime spectroscopy (PALS) and the rotation dynamics of nitroxide spin probe 2,2,6,6-tetramethyl-piperidinyl-1-oxy (TEMPO) using electron spin resonance (ESR) over a wide temperature range, 10-300 K, is reported. The o-Ps lifetime, τ₃, and the relative o-Ps intensity, I₃, as a function of temperature exhibit changes defining several characteristic PALS temperatures in the slowly and rapidly cooled samples. Similarly, the spectral parameter of TEMPO mobility in TOL, 2A_zz', and its correlation time, τ_c, reveal several effects at a set of the characteristic ESR temperatures, which were determined and compared with the PALS results. Finally, the physical origins of the changes in free volume expansion and spin probe mobility are revealed. They are reflected in a series of the mutual coincidences between the characteristic PALS and ESR temperatures and appropriate complementary thermodynamic and dynamic techniques.

Keywords: ESR; PALS; free volume; methylbenzene or toluene; positronium; relaxation dynamics; spin probe dynamics; thermodynamics.

Figure 1

Basic DSC thermogram of TOL consisting of cooling scan followed by heating one with cooling/heating rates of −5/+10 K/min. All the exothermic effects from hot and cold crystallizations and the endothermic effects from devitrification and melting are marked by the following four onset characteristic DSC temperatures T_hc,on^DSC = 143 K, T_g,on^DSC = 117.5 K, T_cc,on^DSC = 135 K and T_m,on^DSC = 178 K as described in the text.

Figure 2

DSC thermograms for the TOL samples prepared using two distinct “boundary” formation methods: (a) under very slow cooling with cooling rate of −1 K/min and (b) under relative rapid cooling with cooling rate of −20 K/min followed by heating with heating rate of +10 K/min together with the observed thermal events.

Figure 3

o-Ps annihilation parameters of TOL during various heating runs: (i) heating from 10 K up to 170 K after cooling from RT down to 10 K in 2 h consisting of rapid cooling down to 180 K, i.e., slightly above T_m^DSC = 178 K, followed by slower cooling in the non-degassed state (black) down to 10 K, (ii) heating from 110 K up to 190 K with 2 and 1 K step after slow cooling (sc) (magenta) with ΔT = 1 K and (iii) heating from 140 K up to RT K with 5 K and then with 10 K step in the degassed state (red). The characteristic PALS temperatures of various effects are mentioned in the text and the characteristic DSC temperatures T_g,on^DSC and T_m,on^DSC from Figure 1 together with the characteristic dynamic temperatures T_X, T_m^c are also included.

Figure 4

o-Ps annihilation parameters of TOL obtained during heating runs: heating from 110 K to 195 K after slow cooling (sc) in the degassed state (pink); heating from 128 K to195 K after rapid cooling (rc) in the non-degassed state (olive). The characteristic PALS temperatures are mentioned and discussed in the text. The basic thermodynamic temperatures from cold crystallization at T_cc,on^DSC and melting at T_m,on^DSC together with the characteristic dynamic temperatures of the two structural relaxation models (T_X,T_m^c)—see Section 4—are also included.

Figure 5

o-Ps annihilation parameters for TOL during various cooling runs on the degassed TOL sample: (i) cooling from RT with ΔT = 10K down to 180 K close to T_m,on^DSC = 178 K and then with ΔT = 5 K down to 145 K (light blue squares); (ii) cooling from 200 K with ΔT = 5 K down to125 K (blue squares); (iii) cooling from 190 K with ΔT =1 K down to 135 K (violet squares) on the non-degassed TOL; iv) cooling from RT, first with ΔT = 10K down to 130 K (light blue crosses). The characteristic PALS temperature of the change in the liquid state T_b1^liq,τ3, of solidification T_sol(ΔT) and the melting point at T_m,on^DSC from DSC and finally of the change in the solid state T_b2^sol,I3 ≅ 158 K are marked in the upper or lower T scales, respectively.

Figure 6

Spectral evolution of the spin TOL/TEMPO system as a function of temperature. Three main regions of the two monomodal (A,C) and the one bimodal (B) type of spectra are evident.

Figure 7

Spectral parameter of mobility 2A_zz‘ as a function of temperature for the spin system TOL/TEMPO. Insets show the details of the slow and fast motion regime regions. Error bars are of the point size.

Figure 8

Experimental (black) and simulated (green) ESR spectra of the spin system TOL/TEMPO from three main regions of the two monomodal (A,C) and the one bimodal (B) type of ESR spectra.

Figure 9

Arrhenius pot of the correlation time for the spin system TOL/TEMPO (a) and relative fraction of slow and fast spectral component as a function of temperature (b) as obtained via the NLSL program. Arrhenius equation fitting is as follows: slow region 1: τ₀₁^slow = (3.5 ± 0.1) × 10⁻⁷ s, E₁^slow = 0.35 ± 0.01 kJ/mol, r = 0.938; slow region 2: τ₀₂^slow = (3.6 ± 0.05) × 10⁻¹⁹ s, E₂^slow = 18.5 ± 0.01 kJ/mol, r = 0.995; fast region 1: τ₀₁^fast = (1.6 ± 0.06) × 10⁻¹⁰ s, E₁^fast = 1.6 ± 0.04 kJ/mol, r = 0.929; fast region 2: τ₀₂^fast = (7.2 ± 0.02) × 10⁻¹⁴ s, E₂^fast = 5.7 ± 0.13 kJ/mol, r = 0.998; fast region 3: τ₀₃^fast = (2.9 ± 0.1) × 10⁻¹² s, E₃^fast = 2.3 ± 0.06 kJ/mol, r = 0.999.

Figure 10

Relaxation map of the primary α− and secondary β−processes in amorphous TOL from DS [17], NMR [15], LS [18] and VISC [37,38,39] data.

Figure 11

Fits of the structural α relaxation time in amorphous phase of TOL using the PL equation or the idealized MCT model and the TOP model. The PL or MCT parameters are: τ_∞,α = 5.6·10⁻¹² s, T_X^{PL or MCT} = 153 K and µ = 1.89. The TOP model parameters are: τ_∞,α = 1.5·10⁻¹³ s, E_∞ = 4.6 kJ/mol, T₀^TOP = 130 K, T_m^c = 156 K and T_A = 220 K, taken from [55].

Figure 12

Comparison of the correlation times of TEMPO in the partially crystalline (below T_m^DSC) and amorphous (above T_m^DSC) spin probe TEMPO/TOL system with the time scales of the primary α– and secondary β relaxations in TOL from NMR [17,18,19,20] and VISC [51,52,53] data in the amorphous TOL medium.