Dynamic Heterogeneity and Density Scaling in 1,4-Polyisoprene

Fragiadakis, D.; Casalini, R.; Bogoslovov, R. B.; Robertson, Christopher G.; Roland, C. M.

doi:10.1021/ma102795w

Cited by 39 publications

(41 citation statements)

References 60 publications

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“…lines of equal relaxation time). Contrary to previous reports [16,17] and to theoretical predictions [26,27], the extent of the spatial correlations in the dynamics is therefore not uniquely determined by the relaxation time. …”

Section: ∂F(t)contrasting

confidence: 97%

Effect of pressure on the number of dynamically correlated molecules when approaching the glass transition

Alba‐Simionesco¹,

Dalle-Ferrier²,

Tarjus³

2013

AIP Conference Proceedings

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We characterize the heterogeneous character of the dynamics of liquids approaching the glass transition through an experimental determination of the number of dynamically correlated molecules N corr as obtained from dynamical susceptibilities. To do so, we have obtained a new set of dielectric spectroscopy data for liquid dibutyl-phtalate on a fine and extended temperature and pressure grid, and we have used it in conjunction with high-pressure data from the literature. We have been able to evaluate the contributions to N corr that are due to fluctuations associated with density and with temperature separately, thereby improving the estimate of N corr . We find that N corr increases along the glass transition line, and more generally along any isochronic line, as pressure increases (up to 1 GPa), a result which is at odds with recent reports and theoretical predictions.

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Section: ∂F(t)contrasting

confidence: 97%

Effect of pressure on the number of dynamically correlated molecules when approaching the glass transition

Alba‐Simionesco¹,

Dalle-Ferrier²,

Tarjus³

2013

AIP Conference Proceedings

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“…The coincidence of the scaling exponent for the segment relaxation and the chain reorientation has been noted in poly(propylene glycol), 1,4-polyisoprene as well as in poly(oxybutylene) [114,115]. Nonetheless, Fragiadakis et al, investigating very carefully the Table II. density scaling in 1,4-Polyisoprene (PI) of different molecular weight by dielectric relaxation, noted that there is a small difference in the exponent γ ts for the segmental and the chain modes of the lowest molecular weight PI with degree of polymerization 18 [116].…”

Section: Thermodynamic Scaling Of Relaxationmentioning

confidence: 99%

Thermodynamic scaling of vibrational dynamics and relaxation

Puosi

Chulkin

Bernini

et al. 2016

The Journal of Chemical Physics

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We investigate by thorough Molecular Dynamics simulations the thermodynamic scaling (TS) of a polymer melt. Two distinct models, with strong and weak virial-energy correlations, are considered. Both evidence the joint TS with the same characteristic exponent γts of the fast mobility -the mean square amplitude of the picosecond rattling motion inside the cage -, and the much slower structural relaxation and chain reorientation. If the cage effect is appreciable, the TS master curves of the fast mobility are nearly linear, grouping in a bundle of approximately concurrent lines for different fragilities. An expression of the TS master curve of the structural relaxation with one adjustable parameter less than the available three-parameters alternatives is derived. The novel expression fits well with the experimental TS master curves of thirty-four glassformers and, in particular, their slope at the glass transition, i.e. the isochoric fragility. For the glassformer OTP the isochoric fragility allows to satisfactorily predict the TS master curve of the fast mobility with no adjustments.

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“…To a good approximation, this correspondence is borne out by experimental data. [15][16][17]20 The JG relaxation refers to local, noncooperative dynamics not requiring intramolecular degrees of freedom. The JG peak in the relaxation spectrum occurs at the lowest frequency of any secondary processes, and the fact that it precedes in time the α-relaxation has been taken to imply that the JG process plays a role in structural relaxation, even serving as its precursor.…”

Section: ■ Introductionmentioning

confidence: 99%

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

Casalini

Roland

2013

Macromolecules

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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...

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Dynamic Heterogeneity and Density Scaling in 1,4-Polyisoprene

Cited by 39 publications

References 60 publications

Effect of pressure on the number of dynamically correlated molecules when approaching the glass transition

Effect of pressure on the number of dynamically correlated molecules when approaching the glass transition

Thermodynamic scaling of vibrational dynamics and relaxation

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

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