Direct observation of dynamic crossover in fragile molecular glass formers with 2D IR vibrational echo spectroscopy

Hoffman, David J.; Sokolowsky, Kathleen P.; Fayer, M. D.

doi:10.1063/1.4978852

Cited by 15 publications

(29 citation statements)

References 93 publications

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“…The isotropic signal, P(t), shows that the vibrational population of the PhSeCN decays exponentially with a lifetime of 446 ± 8 ps across the entire absorption band. This lifetime is very similar to the lifetime of PhSeCN measured in other non-H-bonding chemical systems (22). The vibrational lifetime determines the maximum timescale that is measurable in the pump-probe experiment: Typically, signals out to several factors of the lifetime are measurable.…”

Section: Methodssupporting

confidence: 73%

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Amorphous polymer dynamics and free volume element size distributions from ultrafast IR spectroscopy

Hoffman

Fica-Contreras

Fayer

2020

Proc. Natl. Acad. Sci. U.S.A.

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A method for measuring the size and size probability distribution of free volume regions in polymeric materials using ultrafast infrared (IR) polarization-selective pump–probe experiments is presented. Measurements of the ultrafast dynamics of a vibrational probe (the CN stretch of phenyl selenocyanate) in poly(methyl methacrylate) show that the probe dynamics are highly confined. The degree of confinement was found to be both time-dependent and dependent on the vibrational frequency of the probe molecule. The experiments demonstrate that different vibrational frequencies correspond to distinct subensembles of probe molecules that have different dynamic properties determined by their local structural environments. By combining the degree of dynamical confinement with the molecular size of the probe molecule, the free volume element size probability distribution was determined and found to be in good agreement with the best established experimental measure of free volume. The relative probability of a free volume element size is determined by the amplitude of the nitrile absorption spectrum at the frequency of the measurement. The inhomogeneous broadening of the spectrum was linked to the vibrational Stark effect, which permits site selectivity. The observed dynamics at each frequency were then associated with a different size free volume element and distinct local electric field. The multiple timescales observed in the pump–probe experiments were connected to local structural fluctuations of the free volume elements.

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Section: Methodssupporting

confidence: 73%

“…Previous 2D IR experiments on small-molecule glass-forming liquids show that the dynamics that randomize the vibrational frequency can often be associated with the viscosity of a liquid system (22). Raising the temperature of a glass former well above the glass transition then leads to rapid decorrelation.…”

Section: Methodsmentioning

confidence: 99%

Amorphous polymer dynamics and free volume element size distributions from ultrafast IR spectroscopy

Hoffman

Fica-Contreras

Fayer

2020

Proc. Natl. Acad. Sci. U.S.A.

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“…The dynamical slowing in supercooled liquids near glass transition temperatures ( T g ) has often been discussed in a similar manner, i.e., the growing dynamical correlation lengths of cooperatively rearranging regions as the liquids are cooled toward T g . − In this sense, the dynamical behavior of ionic liquid molecules at the interfaces may resemble that of a supercooled liquid near T g . The observations here might provide insights into the behavior of supercooled liquids under confinement. , …”

Section: Resultsmentioning

confidence: 99%

Extraordinary Slowing of Structural Dynamics in Thin Films of a Room Temperature Ionic Liquid

Nishida

Breen

et al. 2018

ACS Cent. Sci.

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The role that interfaces play in the dynamics of liquids is a fundamental scientific problem with vast importance in technological applications. From material science to biology, e.g., batteries to cell membranes, liquid properties at interfaces are frequently determinant in the nature of chemical processes. For most liquids, like water, the influence of an interface falls off on a ∼1 nm distance scale. Room temperature ionic liquids (RTILs) are a vast class of unusual liquids composed of complex cations and anions that are liquid salts at room temperature. They are unusual liquids with properties that can be finely tuned by selecting the structure of the cation and anion. RTILs are being used or developed in applications such as batteries, CO2 capture, and liquids for biological processes. Here, it is demonstrated quantitatively that the influence of an interface on RTIL properties is profoundly different from that observed in other classes of liquids. The dynamics of planar thin films of the room temperature ionic liquid, 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (BmimNTf2), were investigated using two-dimensional infrared spectroscopy (2D IR) with the CN stretch of SeCN– as the vibrational probe. The structural dynamics (spectral diffusion) of the thin films with controlled nanometer thicknesses were measured and compared to the dynamics of the bulk liquid. The samples were prepared by spin coating the RTIL, together with the vibrational probe, onto a surface functionalized with an ionic monolayer that mimics the structure of the BmimNTf2. Near-Brewster’s angle reflection pump–probe geometry 2D IR facilitated the detection of the exceedingly small signals from the films, some of which were only 14 nm thick. Even in quarter micron (250 nm) thick films, the observed dynamics were much slower than those of the bulk liquid. Using a new theoretical description, the correlation length (exponential falloff of the influence of the interfaces) was found to be 28 ± 5 nm. This very long correlation length, ∼30 times greater than that of water, has major implications for the use of RTILs in devices and other applications.

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“…One should stress that the critical-like behavior mentioned in issue (iii) above was associated with the low-temperature domain (close to T g ) and has no links to the MCT critical-like behavior associated with the high-temperature domain for T >> T g . Notable, that the existence of two dynamical domains was also confirmed beyond BDS studies (Wu et al, 2009;Hoffman et al, 2017;Bartoš et al, 2018). Generally, the dynamical crossover phenomenon is associated with the crossover from the ergodic to the nonergodic (ultraviscous, ultraslow) domain on approaching the glass transition (Roland et al, 2005;Floudas et al, 2011;Ngai, 2011), Stickel et al (1995), and Casalini et al (2003).…”

Section: Introductionmentioning

confidence: 77%

Pressure-Related Universal Previtreous Behavior of the Structural Relaxation Time and Apparent Fragility

Drozd-Rzoska

2019

Front. Mater.

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The report presents the evidence for the hypothetically "universal" previtreous behavior of the pressure related apparent fragility: m T (P) ∞1/ (P * − P), where P * is the extrapolated singular pressure. This finding was the basis for deriving the "model-free" dependence for the pressure evolution of the structural relaxation time: τ (P) = τ P o (P * − P) −. All these led to the new way of testing the dynamic crossover phenomenon via 1/m T (P) vs. P plot. Finally, the behavior of electric conductivity in the previtreous domain is discussed. The experimental evidence covers 8 * OCB (liquid crystal), EPON 828 (resin), diisobutyl phthalate, and propylene carbonate (low molecular weight liquids). Experiments were carried up to extreme P∼ 2.2 GPa, in the "bulk" measurement capacitor. This report also contains a summary of existing descriptions of the pressure evolution of structural relaxation time or viscosity on approaching the glass transition.

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Direct observation of dynamic crossover in fragile molecular glass formers with 2D IR vibrational echo spectroscopy

Cited by 15 publications

References 93 publications

Amorphous polymer dynamics and free volume element size distributions from ultrafast IR spectroscopy

Amorphous polymer dynamics and free volume element size distributions from ultrafast IR spectroscopy

Extraordinary Slowing of Structural Dynamics in Thin Films of a Room Temperature Ionic Liquid

Pressure-Related Universal Previtreous Behavior of the Structural Relaxation Time and Apparent Fragility

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