O-terphenyl Research Articles

Interfacial effects on vitrification of confined glass-forming liquids

Mesoporous silica phases, with uniform pores of dimensions in the 2-30 nm range, offer a uniquely well-defined environment for the study of the effects of two-dimensional spatial confinement on the properties of glass-forming liquids. We report observations by differential scanning calorimetry of the vitrification of o-terphenyl (OTP), salol, and glycerol in hexagonal mesoporous silica (MCM-41 and SBA-15) in a wide range of pore sizes from 2.6 to 26.4 nm. In agreement with previous studies, where a controlled porous glass is used as a solid matrix, the glass transition temperature for o-terphenyl diminishes with decreasing pore size. In contrast to OTP, glycerol shows a gradual increase in glass transition temperature, while in salol a slight reduction of glass transition temperature is observed, followed by an increase, which results in glass transition temperature indistinguishable from that of the bulk for the smallest pores. These results are discussed in terms of liquid-surface interactions in an interfacial layer, monitored by Fourier-transformed infrared spectroscopy in the study. The hydrogen bonding with silica surface silanols dominates the glass transition trends observed in salol and glycerol.

Sub -T g relaxations due to dipolar solutes in nonpolar glass-forming solvents

It is well known that rigid dipolar solutes (in smaller quantity) dispersed in a nonpolar glassy matrix exhibit a sub-T(g) (or beta(s)) relaxation due to the solute often designated as Johari-Goldstein (JG) relaxation, which is intermolecular in nature. In this article, we report the results of our study of such a sub-T(g) process in a wide variety of dipolar solutes in different glassy systems using dielectric spectroscopy over a frequency range of 20-10(6) Hz down to a temperature of 77 K. The T(g) of these solutions are determined using differential scanning calorimetry. The solvents used in this study are o-terphenyl (OTP), isopropylbenzene (IPB), and methylcyclohexane. In the case of rigid molecular solutes, like mono-halogen benzenes, the activation energy (DeltaE(beta)) of the beta(s) process is found to increase with decreasing T(g) of the solvent, with a corresponding decrease in the magnitude of the beta(s) process. In the case of more symmetrical molecular solute, for example, tert-butylchloride, the change in DeltaE(beta) is not very appreciable. These results emphasize the importance of the size of the cage of the host matrix in the relaxation of the solute molecules. We have also studied the sub-T(g) relaxation(s) due to some flexible molecular solutes, viz., 1butylbromide, 1hexylbromide, 1butylacetate, and benzylacetate. These solutes in IPB matrix exhibit only one relaxation, whereas in OTP matrix they exhibit an additional sub-T(g) process, which may be identified with a JG type of relaxation. These observations lead us to the conclusion that the beta process observed in the glassy states of these pure solutes is predominantly intramolecular in nature.

Role of the density in the crossover region of o-terphenyl and poly(vinyl acetate).

The coupling between the reorientation of molecular probes and the density in one low-molar mass glass former [ o -terphenyl (OTP)] and one polymer [poly(vinyl acetate) (PVAc)] is studied in the Goldstein's crossover region where the structural (alpha) and the secondary (beta) relaxations bifurcate. The coupling is found to be strong in OTP and virtually absent in PVAc. The probes sense both the alpha and beta relaxations, and locate their splitting accurately. It is concluded that the density affects the relaxation occurring in the crossover region of OTP but not of PVAc at subnanometer length scales. The findings are compared with recent assessments of the role of the molecular packing close and above the glass transition temperature T(g).

Functional Electrolyte: Additives for Improving the Cyclability of Cathode Materials

A novel type of additives for improving cathode cyclability has been developed. Three electrolyte additives, biphenyl (BP), o-terphenyl (OT), and m-terphenyl (MT) have been investigated for improving the cyclability of cathode materials like LiCoO 2 and LiMn 2 O 4 . The effect of these additives is correlated to the properties of the surface film on the cathode electrode, which comes from the decomposition of the additives at high potentials.

Heterogeneous Dynamics and Domains in Supercooledo-Terphenyl: A Single Molecule Study

Single molecule spectroscopy was used to characterize the rotation of fluorescent probe molecules in supercooled o-terphenyl (OTP) just above the glass transition. Rotational motions of spatially i...

O-Terphenyl: flips of the end rings in the crystal phase

We demonstrate by selective saturation deuteron NMR experiments on a crystal of selectively deuterated o-terphenyl (OTP) that both end rings I and II attached to the central ring undergo thermally activated flip motions. In crystals of OTP, the end rings I and II are not symmetry related, their dynamics can be different and, in fact, is different, although mutual steric hindrance of the rings suggests a strong correlation. We measured the rates k I and k II of the flips of both end rings I and II as a function of temperature. We find that, independent of the temperature, k I exceeds k II by roughly a factor of 100. This result excludes correlated flips of rings I and II in the sense that a flip of one ring necessarily entails a flip of the other. The activation energies EaI and EaII of the two flip processes turn out to be equal (80.5 kJ mol−1) within experimental errors. This is taken as a hint that the flips are, after all, related to each other. A mechanism is proposed of how this is possible under the constraint k I » k II.

Time Domain Optical Studies of Dynamics in Supercooled o-Terphenyl: Comparison to Mode Coupling Theory on Fast and Slow Time Scales

Orientational relaxation of supercooled o-terphenyl (OTP) is studied over a wide range of temperatures and time scales using optical heterodyne detected optical Kerr effect techniques. A combination of experimental setups made it possible to study the dynamics for greater than six decades of time (100 fs to hundreds of nanoseconds). The complex shape of the data is analyzed in terms of power laws at intermediate times and an approximately exponential decay at long times. Most features of the relaxation curves are in accord with predictions of mode-coupling theory (MCT) for supercooled liquids. In particular, at long times the relaxation data show a temperature independent shape in the temperature regime 361−290 K. When plotted vs temperature, the long time decay constants scaled by a relationship given by MCT fall on a line as predicted and give a value of the MCT critical temperature, Tc = 285 K, in accord with literature values. On a faster time scale, the slowest of the power law decays has a temperatu...

How Long Do Regions of Different Dynamics Persist in Supercooled o-Terphenyl?

In supercooled o-terphenyl (OTP), subsets of probe molecules in more mobile environments can be selectively photobleached. The time required for the remaining slower-than-average probes to be redis...

Comment on “A 250 GHz ESR study of o-terphenyl: Dynamic cage effects above Tc” [J. Chem. Phys 106, 9996 (1997)

In a recent ESR study Earle, Moscicki, Polimeno and Freed investigated the rotational dynamics of tracers in supercooled o-terphenyl (OTP). Their claims were (1) the correct analysis of the ESR data must account for the possible nonexponential decay of the the rotational correlation function; (2) the average rotational correlation time, 〈τ〉 follows the Stokes–Einstein–Debye law (SED); (3) there are substantial voids in OTP. We show that (i) claims #1 and #2 become very questionable after having corrected inconsistencies between the data and the related plot; (ii) the claim #3 is not supported by additional experimental data.

Response to “Comment on ‘A 250 GHz ESR study of o-terphenyl dynamic cage effects above TC’ ” [J. Chem. Phys. 109, 10523 (1998)

We address the points raised by Giordano and Leporini (GL) and show that accounting properly for the nonexponential decay of the rotational correlation function leads to improved agreement with the Stokes–Einstein–Debye (SED) relation above the crossover temperature TC for those probes 3,3′-dimethyloxazolidinyl-N-oxy-2′,3-5α-cholestane (CSL), and perdeuterated 2,2′,6,6′-tetramethyl-4-methyl aminopiperidinyl-N-oxide) (MOTA) that are well-coupled to the viscous modes of o-terphenyl (OTP) when the average relaxation rate 16〈τ〉 is plotted versus 1/T. On the other hand, 2,2′,6,6′-tetramethyl-4-piperidine-N-oxide (PDT) shows simple Arrhenius behavior in this regime, because of weak coupling to the solvent cage, inconsistent with SED, which was clearly shown in our paper. We also suggest that the difference in chemical structure of the PDT probe, studied by us, compared to 2,2′,6,6′-tetramethylpiperidine-N-oxyl (TEMPO), studied by GL, accounts for the difference in the low-temperature relaxation behavior of the two probes.

Observation of metastable ordered structures and the kinetics of crystallization of o-terphenyl

We have investigated the crystallization behavior of the organic glass former o-terphenyl (OTP) as a function of time and temperature by X-ray diffraction. Unstable crystalline structures, which appear and disappear alternately in time, were observed in the amorphous phase at the temperature T=258 K. These unstable crystalline structures differ from the thermodynamically stable structure of crystalline OTP and may be correlated to density fluctuations. At temperatures above 258 K a normal crystal growth was found. This nucleation shows an Arrhenius-like behavior with an activation energy of 160 kJ/mol.

Dynamic structure factor of glassy o-terphenyl: a Brillouin light scattering study

A Brillouin light scattering study of the fragile glass-former o-terphenyl (OTP) has been performed in the glassy state, between 186 and 245 K. The spectra of density fluctuations have been obtained from direct subtraction of depolarized spectra from polarized ones. A full spectrum analysis has been performed giving evidence of a vibrational relaxation process at about 6 GHz still active in the glassy state.

Linear and non-linear electron spin resonance study of the rotational diffusion of a molecular tracer in supercooled o-terphenyl

The long-time and the short-time rotational relaxation of a molecular tracer in supercooled o-terphenyl (OTP) have been studied by linear and non linear electron spin resonance (ESR) spectroscopy, respectively. At higher temperatures the related correlation times, τ l and τ s, are equal within the experimental errors, i.e. the relaxation is exponential, and well coupled to the viscosity, η. On cooling, τ s, and, at lower temperatures, τ l, are partially coupled to the viscosity in agreement with the fractional Debye–Stokes–Einstein (DSE) law τ i ∝( η/ T) ξ i , i=l,s with ξ i constant. For T < 330 K the rotational correlation function is not exponential.

Enumeration of Isobaric Inherent Structures for the Fragile Glass Former o-Terphenyl

The widely investigated organic substance o-terphenyl (OTP) displays thermal and kinetic behavior prototypical for “fragile” glass formers. The special relationship between its heat capacity in sup...

Enhanced translation of probe molecules in supercooled o-terphenyl: Signature of spatially heterogeneous dynamics?

A holographic fluorescence recovery after photobleaching technique has been used to measure translational diffusion coefficients DT for four probes in supercooled o-terphenyl (OTP). DT values from 10−6 to 10−14 cm2/s were observed in the temperature range from Tg+8 K to Tg+90 K (Tg=243 K). In agreement with previous reports, the translational diffusion of probe molecules which are the same size as OTP molecules has a significantly weaker temperature dependence than T/η. However, as the size of the probe molecule is increased the temperature dependence of DT tracks T/η increasingly well. The transition between a weak temperature dependence to that of T/η occurs over only a factor of 3 in probe size. Previous work established that the rotational correlation times τc of these four probes in OTP tracks η/T. The product DTτc is independent of temperature for the largest probe but increases almost 2 orders of magnitude for the smaller probes as Tg is approached. A strong correlation is observed between this enhancement of translational diffusion and the KWW β values obtained from rotation measurements; probes with small β values show enhanced translation. These observations are qualitatively explained by spatially heterogeneous dynamics. The probes studied are rubrene, 9,10-bis(phenylethynyl)anthracene, tetracene, and anthracene.

Relaxation of spatially heterogeneous dynamic domains in supercooled ortho-terphenyl

A photobleaching technique has been used to observe rotational dynamics of dilute probe molecules in supercooled o-terphenyl (OTP). The nonexponential rotational relaxation of the probe molecules is shown to be due, at least in part, to the presence of spatial heterogeneity in the host dynamics. Under appropriate photobleaching conditions, a nonequilibrium distribution of probe molecule mobilities can be created by preferentially bleaching the more mobile probe molecules in a sample. Near Tg this nonequilibrium distribution is observed to return to an equilibrium distribution of relaxation times over times on the order of 103 τc, where τc is the average rotational correlation time of a probe molecule. The time required to return to equilibrium is interpreted as a structural relaxation time for dynamic heterogeneities in OTP.

How do molecules move near T g? Molecular rotation of six probes in o-terphenyl across 14 decades in time

Time resolved optical spectroscopy was used to observe molecular rotation over more than 14 decades in time for six probes in o-terphenyl (OTP). In contrast to previous studies, probe rotation times are found to depend significantly upon probe size in the deeply supercooled regime. Systematic deviations from the temperature dependence of the Debye–Stokes–Einstein equation are observed, however, these deviations are relatively small. These observations are inconsistent with some models of cooperative molecular motion near Tg which invoke rigid aggregates or locally liquidlike regions. The width of the relaxation spectrum (characterized by the KWW β parameter) systematically decreases with increasing probe size. Near Tg, the largest probe (rubrene) rotates with nearly a single exponential correlation function. Based on the observed trend in β, it is estimated that OTP is homogeneous on length scales greater than 2.5 nm at Tg.

Solidification of acoustically levitated o-terphenyl crystals: a Raman study

Ultrasonic levitation of millimeter size o-terphenyl (OTP) crystals was carried out in a variable temperature enclosure. Levitated crystals were melted (MP = 331 K), undercooled and then seeded in order to initiate solidification. The incidence of an oargon laser beam on the sample, during containerless processing, allowed elastic and low frequency (< 200 cm−1) shifted inelastic Raman scattering to be monitored. The Raman spectra undergo remarkable changes during thermal processing, enabling the tracking of inter-molecular structural reconfigurations occurring in the solid and liquid states. Furthermore, the Raman scattered light proved to be a reliable tool for monitoring the kinetics of phase transformation. Thus tracking the peak intensity of a Raman feature allowed a measure of volume fraction transformation rates, during solidification initiated at different undercooling levels. Growth rates as a function of undercooling were inferred from these measurements and are compared to existing models.

Photobleaching technique for measuring ultraslow reorientation near and below the glass transition: tetracene in o-terphenyl

A photobleaching technique for measuring probe rotational diffusion in supercooled and glassy materials is presented. Rotational correlation times from 10 -2 to 10 +4.5 s have been obtained for tetracene in o-terphenyl (OTP). Relative to the conventional T g for OTP (-30 o C), this corresponds to a temperature range of T g +15 o C to T g -5 o C. These results, combined with nanosecond measurements of anthracene rotation in OTP, are compared to OTP viscosity data. We find that, in contrast to translational diffusion measurements of probes in OTP, rotational diffusion follows the temperature dependence of η/T over the 14 decades of time and viscosity explored