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

Casalini, R.; Roland, C. M.; Capaccioli, S.

doi:10.1063/1.2728898

Cited by 53 publications

(59 citation statements)

References 108 publications

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“…tinct features. In stiffer polymers, like polymethylmethacrylate, the exponent decreases abruptly as the length of the chain increases [109]. It must be also pointed out that high molecular weight polymers are characterized by small values of the exponent, γ ts < 3 [7].…”

Section: Thermodynamic Scaling In the Cage Regimementioning

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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Section: Thermodynamic Scaling In the Cage Regimementioning

confidence: 99%

Thermodynamic scaling of vibrational dynamics and relaxation

Puosi

Chulkin

Bernini

et al. 2016

The Journal of Chemical Physics

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“…Equation 8 was derived by expressing the entropy change as a function of density and temperature, with γ identified as the Gruneisen constant. 37,38 This equation satisfies the density scaling (eq 1). From the fit of eq 8 to the τ α (T,ρ) (solid lines in Figure 3), the scaling parameter is determined, γ = 1.94 ± 0.02.…”

Section: ■ Experimental Sectionmentioning

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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“…[40,46] These results indicate that the chain length also serves as an important molecular parameter to control the properties of glassy polymers. Recently, Casalini et al [35] have experimentally detected the impact of varying chain length on thermodynamic scaling by analyzing data for the relaxation times of PMMA samples with four molecular weights. The scaling exponent decreases from 3.7 to 1.8 when the chain length grows from N = 3 to N = 1500, while the isochoric fragility varies in the opposite direction.…”

Section: Influence Of the Chain Lengthmentioning

confidence: 99%

“…Although the thermodynamic scaling is a universal property that is common to both small molecules and polymers, a better understanding of its origins benefits from exploring the special features of polymer systems, such as the large changes in properties that may be produced by small alterations in molecular parameters. While dielectric spectroscopy experiments [35] and molecular dynamics simulations [36] have probed the dependence of thermodynamic scaling on chain length for poly(methylmethacrylate) (PMMA) and Lennard-Jones chains, respectively, it is in general difficult experimentally to finely tune a single molecular parameter, and simulations are restricted to systems far above the glass transition temperature. Moreover, some striking empirical correlations between the scaling exponent and the fragility parameter [8,12,37,38] also invite theoretical considerations.…”

Section: Introductionmentioning

confidence: 99%

Thermodynamic scaling of dynamics in polymer melts: Predictions from the generalized entropy theory

Freed

2013

The Journal of Chemical Physics

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Many glass-forming fluids exhibit a remarkable thermodynamic scaling in which dynamic properties, such as the viscosity, the relaxation time, and the diffusion constant, can be described under different thermodynamic conditions in terms of a unique scaling function of the ratio ρ γ /T , where ρ is the density, T is the temperature, and γ is a material dependent constant. Interest in the scaling is also heightened because the exponent γ enters prominently into considerations of the relative contributions to the dynamics from pressure effects (e.g., activation barriers) vs. volume effects (e.g., free volume). Although this scaling is clearly of great practical use, a molecular understanding of the scaling remains elusive. Providing this molecular understanding would greatly enhance the utility of the empirically observed scaling in assisting the rational design of materials by describing how controllable molecular factors, such as monomer structures, interactions, flexibility, etc., influence the scaling exponent γ and, hence, the dynamics. Given the successes of the generalized entropy theory in elucidating the influence of molecular details on the universal properties of glass-forming polymers, this theory is extended here to investigate the thermodynamic scaling in polymer melts. The predictions of theory are in accord with the appearance of thermodynamic scaling for pressures not in excess of ∼ 50 MPa. (The failure at higher pressures arises due to inherent limitations of a lattice model.) In line with arguments relating the magnitude of γ to the steepness of the repulsive part of the intermolecular potential, the abrupt, square-well nature of the lattice model interactions lead, as expected, to much larger values of the scaling exponent. Nevertheless, the theory is employed to study how individual molecular parameters affect the scaling exponent in order to extract a molecular understanding of the information content contained in the exponent. The chain rigidity, cohesive energy, chain length, and the side group length are all found to significantly affect the magnitude of the scaling exponent, and the computed trends agree well with available experiments. The variations of γ with these molecular parameters are explained by establishing a correlation between the computed molecular dependence of the scaling exponent and the fragility. Thus, the efficiency of packing the polymers is established as the universal physical mechanism determining both the fragility and the scaling exponent γ.

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

Cited by 53 publications

References 108 publications

Thermodynamic scaling of vibrational dynamics and relaxation

Thermodynamic scaling of vibrational dynamics and relaxation

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

Thermodynamic scaling of dynamics in polymer melts: Predictions from the generalized entropy theory

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