Anomalous Diffusion of Probe Molecules in Polystyrene: Evidence for Spatially Heterogeneous Segmental Dynamics

Cicerone, Marcus T.; Blackburn, F. R.; Ediger, M. D.

doi:10.1021/ma00128a036

Cited by 261 publications

(315 citation statements)

References 16 publications

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“…However, it is well-known that near T g , translations are often enhanced relative to reorientational motions (such as associated with the dielectric α-process) 82,83,84 . The result is that a double logarithmic plot of σ versus τ α will have a slope deviating from unity 85 .…”

Section: Resultsmentioning

confidence: 99%

Temperature and Density Effects on the Local Segmental and Global Chain Dynamics of Poly(oxybutylene)

Casalini

Roland

2005

Macromolecules

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Dielectric spectroscopy measurements over a broad range of temperature and pressure were carried out on poly(oxybutylene) (POB), a type-A polymer (dielectrically-active normal mode). There are three dynamic processes appearing at lower frequency, the normal and segmental relaxation modes, and a conductivity arising from ionic impurities. In combination with pressure-volume-temperature measurements, the dielectric data were used to assess the respective roles of thermal energy and density in controlling the relaxation times and their variation with T and P. We find that the local segmental and the global relaxation times are both a single function of the product of the temperature times the specific volume, with the latter raised to the power of 2.65. The fact that this scaling exponent is the same for both modes indicates they are governed by the same local friction coefficient, an idea common to most models of polymer dynamics. Nevertheless, near T g , their temperature dependences diverge. The magnitude of the scaling exponent reflects the relatively weak effect of density on the relaxation times. This is usual for polymers, as the intramolecular bonding, and thus interactions between directly bonded segments, are only weakly sensitive to pressure. This insensitivity also means that the chain end-to-end distance is invariant to P, conferring a near pressure-independence of the (density-normalized) normal mode dielectric strength.The ionic conductivity dominates the low frequency portion of the spectra. At lower temperatures and higher pressures, this conductivity becomes decoupled from the relaxation modes (different T-dependence), and exhibits a significantly weaker density effect. At frequencies higher than the structural relaxation, both an excess wing on the flank of the α-peak and a secondary relaxation are observed. From their relative sensitivities to pressure, we ascribe the former to an unresolved Johari-Goldstein (JG) relaxation, while the higher frequency peak is unrelated to the glass transition. These designations are consistent with the relaxation time calculated for the JG process. 2The dynamic properties of the POB are essentially the same as those of polypropylene glycol, in accord with their similar chemical structures. However, POB is less fragile (weaker T gnormalized temperature dependence), its relaxation times less sensitive to density changes, and facilitating the measurements herein, its normal mode has a substantially larger dielectric strength

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Section: Resultsmentioning

confidence: 99%

Temperature and Density Effects on the Local Segmental and Global Chain Dynamics of Poly(oxybutylene)

Casalini

Roland

2005

Macromolecules

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“…We refer to a system as dynamically heterogeneous if it is possible to select a dynamically distinguishable subset of particles by experiment or computer simulation [20]. Simulations [21,22,23,24,25,26,27] and experiments [28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43] have demonstrated the cooperative and spatially heterogeneous nature of the liquid dynamics (for reviews of the experimental evidence for spatially heterogeneous dynamics, see, e. g., Refs. [44,45,46]).…”

Section: Introductionmentioning

confidence: 99%

Dynamical Heterogeneity and Jamming in Glass-Forming Liquids

Lacevic

Glotzer

2004

J. Phys. Chem. B

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The relationship between spatially heterogeneous dynamics (SHD) and jamming is studied in a glass-forming binary Lennard-Jones system via molecular dynamics simulations. It has been suggested by O'Hern et al. [1] that the probability distribution of interparticle forces P (F ) develops a peak at the glass transition temperature Tg, and that the large force inhomogeneities, responsible for structural arrest in granular materials, are related to dynamical heterogeneities in supercooled liquids that form glasses. It has been further suggested that "force chains" present in granular materials may exist in supercooled liquids, and may provide an order parameter for the glass transition. Our goal is to investigate the extent to which the forces experienced by particles in a glass-forming liquid are related to SHD, and compare these forces to those observed in granular materials and other glass-forming systems. Our results are summarized as follows. We find no peak in P (F ) at any temperature in our system, even below Tg. We also find that particles that have been localized for a long time are less likely to experience high relative force and that mobile particles experience higher relative forces at shorter time scales, indicating a correlation between pairwise forces and particle mobility. We construct force chains based on the magnitude of pairwise forces. We find that force chains constructed in this manner are composed of both localized and mobile particles, therefore there is no one-to-one correspondence between force chains as defined here and locally mobile or immobile regions of the liquid. We also find that force chains do not play the same role as force chains in granular materials, but may indicate a difference in the evolution of the local environment of particles with different mobility. We also discuss a possible relationship between force chains found here and the development of string-like motion found in other glass-forming liquids [2,3].

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“…28 A laser beam is split into two paths with a beam splitter, and the two beams are then recombined in the sample (overlapping at 180°) to create an interference pattern. This interference pattern, which has a d-spacing of 150 nm for these experiments, is used to create a grating pattern of bleached and unbleached probes in the sample, that is, in regions of constructive interference, probes will be bleached, and in regions of destructive interference, no probes will be bleached.…”

Section: Probe Translation Experimentsmentioning

confidence: 99%

“…Given the assumptions stated above, D T c is a very sensitive measure of the width of the distribution of segmental relaxation times in the sample. 28,34 Figure 4 displays equilibrium values of D T c for tetracene in PS as a function of temperature. In a homogeneous system, rotational and translational diffusion would have the same temperature dependence, and D T c would be a temperature-invariant quantity.…”

Section: T C In Equilibrium Systemsmentioning

confidence: 99%

“…As depicted by the curve for 95.6°C, upon 28 Upon lowering the temperature toward T g , the product becomes a strong function of temperature, and translational diffusion is enhanced by up to four orders of magnitude because of the presence of spatially heterogeneous dynamics. The inset portrays the correlation of enhanced translation with the KWW ␤ parameter observed in probe rotation experiments in equilibrium.…”

Section: T C In Nonequilibrium Systemsmentioning

confidence: 99%

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Spatially heterogeneous dynamics during physical aging far below the glass transition temperature

Thurau

Ediger

2002

J Polym Sci B Polym Phys

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ABSTRACT:Physical aging experiments after a deep quench were performed via an optical photobleaching technique to examine the evolution of the segmental dynamics of a polystyrene melt far below the glass transition temperature (T g ). Both rotational and translational diffusion measurements of dilute tetracene probes dispersed in the polymer matrix were monitored as a function of aging time. Following a temperature quench from T g ϩ2°C deep into the glass (with isothermal aging temperatures ranging from T g Ϫ15 to T g Ϫ67°C), either rotational or translational diffusion measurements were performed for up to 10 5 s of elapsed time. During this period, relaxation times for both observables were retarded by at least a factor of 3. Rotational and translational observables respond differently to the initial temperature quench and to the subsequent isothermal aging. These differences likely indicate that the dynamics responsible for physical aging in a quenched glass are spatially heterogeneous. Immediately following the quench, regions of faster mobility age toward equilibrium faster than regions of slower mobility. These findings compliment recent aging experiments close to T g and bolster the conclusion that heterogeneous dynamics are important for the accurate description of physical aging.

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Anomalous Diffusion of Probe Molecules in Polystyrene: Evidence for Spatially Heterogeneous Segmental Dynamics

Cited by 261 publications

References 16 publications

Temperature and Density Effects on the Local Segmental and Global Chain Dynamics of Poly(oxybutylene)

Temperature and Density Effects on the Local Segmental and Global Chain Dynamics of Poly(oxybutylene)

Dynamical Heterogeneity and Jamming in Glass-Forming Liquids

Spatially heterogeneous dynamics during physical aging far below the glass transition temperature

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