Do Theories of the Glass Transition, in which the Structural Relaxation Time Does Not Define the Dispersion of the Structural Relaxation, Need Revision?

Ngai, K. L.; Casalini, R.; Capaccioli, S.; Paluch, Marian; Roland, C. M.

doi:10.1021/jp053439s

Cited by 220 publications

(305 citation statements)

References 74 publications

Supporting

Mentioning

277

Contrasting

Order By: Relevance

“…Since the (normalized) Φ(t) depends only on τ α [1,2] (which itself implies N c is constant at fixed τ α if the nonexponentiality of Φ(t) is attributed to dynamic heterogeneity), it follows from eq. (6) that     The interest in dynamic correlation arises from the idea that a growing spatial extent of intermolecular cooperativity underlies the supercooled dynamics, perhaps leading ultimately to vitrification [17,18,19,41].…”

Section: Resultsmentioning

confidence: 99%

“…Kohlrausch exponent, β, describing the breadth of the relaxation function, is a unique function of τ α [1,2], as is the thermodynamic quantity TV γ , where V is the specific volume and γ a material constant [3,4,5]. Thus, the distribution of relaxation times (reflected in β) and the set of state points associated with a given distribution are both determined entirely by the magnitude of τ α .…”

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Density Scaling and Dynamic Correlations in Viscous Liquids

2009

Self Cite

View full text Add to dashboard Cite

show abstract

Section: Resultsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Density Scaling and Dynamic Correlations in Viscous Liquids

2009

Self Cite

View full text Add to dashboard Cite

show abstract

“…94 For non-associated materials at elevated pressure, m usually decreases, 32 while β KWW is unchanged for conditions of T and P such that τ α is constant. 95,96 In figure 8 we show spectra for the trimer and tetramer measured at different T and P chosen such that the τ α ∼ 10s. Good superpositioning of the peaks is found, in accord with the behavior of other glass-formers, both molecular and polymeric.…”

Section: Resultsmentioning

confidence: 99%

Effect of chain length on fragility and thermodynamic scaling of the local segmental dynamics in poly(methylmethacrylate)

Casalini

Roland

Capaccioli

2007

The Journal of Chemical Physics

Self Cite

View full text Add to dashboard Cite

Local segmental relaxation properties of poly(methylmethacrylate) (PMMA) of varying molecular weight are measured by dielectric spectroscopy, and analyzed in combination the equation of state obtained from PVT measurements. The usual variation of glass transition temperature and fragility with molecular weight are observed. We also find, in accord with the general properties of glass-forming materials, that a single molecular weight dependent scaling exponent, γ, is sufficient to define the mean segmental relaxation time, τ α . and its distribution. This exponent can be connected to the Grüneisen parameter and related thermodynamic quantities, thus demonstrating the interrelationship between dynamics and thermodynamics in PMMA. Changes in the relaxation properties ("dynamic crossover") are observed as a function of both temperature and pressure, with τ α serving as the control parameter for the crossover. At longer τ α another change in the dynamics is apparent, associated with a decoupling of the local segmental process from ionic conductivity.2

show abstract

“…28 According to eq 2, if β KWW is constant or a function of τ α , 29,30 both relaxation times should conform to density scaling with equivalent values of γ; otherwise stated, the ratio τ JG /τ α is a constant for any value of τ α for a material conforming to density scaling. In this paper we describe dielectric measurements on a low molecular weight poly(methyl methacrylate) (PMMA) obtained over a range of temperatures and pressures.…”

Section: ■ Introductionmentioning

confidence: 99%

Density Scaling of the Structural and Johari–Goldstein Secondary Relaxations in Poly(methyl methacrylate)

Casalini

Roland

2013

Macromolecules

Self Cite

View full text Add to dashboard Cite

Dielectric spectra were obtained on a low molecular weight poly(methyl methacrylate) (PMMA) over a range of temperatures (331 < T (K) < 386) at pressures approaching 0.8 GPa. The α relaxation times, τ α , superpose when plotted versus T/ρ γ , where ρ is density and γ a material constant, in accord with results for many other van der Waals liquids and polymers. However, the Johari−Goldstein (JG) relaxation times, τ JG , do not conform to this density scaling for the same value of the exponent γ. Likewise, the frequency separation of the α and JG loss peaks in the spectrum increases with pressure for constant τ α ; that is, state points having the same α relaxation time and same peak breadth have different τ JG . Similar results were obtained on a lower molecular weight PMMA, for which there was less overlap of the two peaks. The implication is that density scaling of the segmental relaxation times originates in the glass transition dynamics, not, as recently suggested, in higher frequency secondary processes. ■ INTRODUCTIONDensity scaling refers to the superpositioning of structural relaxation times, τ α , when expressed as a function of the ratio of temperature to density, where the latter is raised to the power γEquation 1 has been shown to apply to experimental measurements on virtually all nonassociated liquids and polymers. 1−6 Equivalent relations describe the viscosity, diffusion constant, and other dynamical properties, although when data extend to high temperatures, as common for viscosities 7,8 and molecular dynamics simulations (mds), 9,10 the use of scaled quantities improves the superpositioning. 11 Beyond a means to categorize and organize experimental data spanning broad ranges of temperature and pressure, interest arises in density scaling because of the physical meaning that may be ascribed to the material constant γ. For example, from mds it has been shown that γ (i) is approximately one-third the effective slope of the intermolecular repulsive potential, 9,12 (ii) equals the proportionality constant between isochoric equilibrium fluctuations of the virial and potential energy, 10,13 and (iii) from the latter can be connected to linear thermoviscoelastic constants. 14 Density scaling is invariably applied to τ α or other dynamical quantities that are coupled to the α-relaxation time such as the viscosity and diffusion constant. However, there are two relaxation processes that, while not directly related to structural relaxation, are considered to bear a relationship to τ α : the terminal relaxation giving rise to a low-frequency dispersion in the mechanical loss of polymers and the Johari−Goldstein (JG) secondary relaxation found in all molecular liquids and polymers. The former is responsible for the normal mode peak in the dielectric loss of polymers having a dipole moment parallel to the chain backbone, such as 1,4-polyisoprene, 15 polyoxybutylene, 16 and poly(propylene glycol). 17 Since theories of polymer dynamics such as the Rouse and reptation models 18,19 posit that chain moti...

show abstract

Do Theories of the Glass Transition, in which the Structural Relaxation Time Does Not Define the Dispersion of the Structural Relaxation, Need Revision?

Cited by 220 publications

References 74 publications

Density Scaling and Dynamic Correlations in Viscous Liquids

Density Scaling and Dynamic Correlations in Viscous Liquids

Effect of chain length on fragility and thermodynamic scaling of the local segmental dynamics in poly(methylmethacrylate)

Density Scaling of the Structural and Johari–Goldstein Secondary Relaxations in Poly(methyl methacrylate)

Contact Info

Product

Resources

About