Influence of Cooperative α Dynamics on Local β Relaxation during the Development of the Dynamic Glass Transition in Poly(<i>n</i>-alkyl methacrylate)s

Garwe, F.; Schönhals, Andreas; Lockwenz, H.; Beiner, Mario; Schröter, Klaus; Donth, E.

doi:10.1021/ma9506142

Cited by 295 publications

(417 citation statements)

References 40 publications

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“…This behavior has been described previously for poly(alkyl methacrylate)s due to temperature changes alone. 35 The results in Figure 2 show that the relaxation strengths are connected to the dynamics; that is, the relative dielectric strength for the two processes depends on the magnitudes of τ α and τ JG . According to the Kirkwood− Frolich relation, 36 the dielectric strength should increase with increasing density (higher dipole concentration) and decreasing temperature (stronger dipole correlations).…”

Section: ■ Experimental Sectionmentioning

confidence: 97%

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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Section: ■ Experimental Sectionmentioning

confidence: 97%

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

Casalini

Roland

2013

Macromolecules

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“…Williams [45,46] defined this process as a new relaxation process where the motions associated to the and relaxations influence each other and not just a mere superposition of two relaxation mechanisms. However the process is not totally clarified, namely it is not so straightforward what happens in the region of frequencies and temperatures where occurs the separation of the two processes (splitting region) [47]. This problem is related to the glass transition dynamics because almost all…”

Section: Dma and Creep Datamentioning

confidence: 99%

“…was studied some years ago and several scenarios for the splitting region were proposed not only for these materials but for any material [47]. From this work it is inferred that the behaviour in the splitting region is much more complex than what may be expected, and the way the splitting occurs and the character of the local process could vary considerably in the splitting zone (two examples: the relaxation can be seen as a precursor process of the relaxation, or instead the relaxation process is continuous and the motions associated to the relaxation can only occur at temperatures much lower than T g ).…”

Section: Dma and Creep Datamentioning

confidence: 99%

Departure from the Vogel behaviour in the glass transition—thermally stimulated recovery, creep and dynamic mechanical analysis studies

Alves

Mano

Ribelles

et al. 2004

Polymer

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Elsevier Alves, N.; Mano, J.; Gómez Ribelles, JL.; Gómez-Tejedor, JA. (2004). Departure from the Vogel behaviour in the glass transition -thermally stimulated recovery, creep and dynamic mechanical analysis studies. Polymer. 45(3): 1007-1017. doi:10.1016/j.polymer.2003.04.002. N.M. Alves et al. / Polymer 45 (2004 AbstractIn this work the study of the dynamics of the segmental motions close to T g of a poly(methyl methacrylate), PMMA, network was analysed by distinct mechanical spectroscopy techniques. Three techniques were employed: dynamic mechanical analysis (DMA), creep and thermally stimulated recovery (TSR). The timetemperature superposition principle was applied to the DMA and creep results, and master curves were successfully constructed. A change from a Vogel to an Arrhenius behaviour was observed in these results.Above T g it was found a distinct temperature dependence for the retardation times calculated from creep and the relaxation times calculated from DMA. This unexpected behaviour was attributed to the merging of the and the relaxations that occurs in PMMA systems. The activation energies (E a ) were also calculated from DMA, creep and TSR experiments. Above T g the E a values obtained agreed very well for all the techniques. In addition, the fragility exhibited by this material was investigated by the mechanical spectroscopy techniques referred above and by differential scanning calorimetry (DSC). The obtained values of the fragility index m indicated that the PMMA network is a kinetically fragile system. The thermodynamic manifestation of the fragility was also analysed.

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“…Not only window glass, but also advanced materials for holographic data storage or nonlinear optical applications based on PMMA films are discussed [1]. To improve relevant physical properties, and also for fundamental reasons, a better understanding of the glass transition, molecular dynamics and the underlying microscopic relaxation processes in liquid and glassy PMMA films is necessary.…”

Section: Introductionmentioning

confidence: 99%

“…To improve relevant physical properties, and also for fundamental reasons, a better understanding of the glass transition, molecular dynamics and the underlying microscopic relaxation processes in liquid and glassy PMMA films is necessary. Experimental data about the dynamic processes in bulk materials in most cases are gained from dielectric spectroscopy that allows a broad range of frequencies and temperatures [1]- [3]. In addition, there have been various other experimental approaches such as photon correlation spectroscopy [4,5], multi-dimensional nuclear magnetic resonance [6,7] and mechanical spectroscopy [8]- [10], which are usually applied at constant temperature below and above the thermodynamic glass transition T g .…”

Section: Introductionmentioning

confidence: 99%

Comparison of mechanical and dielectric relaxation processes in laser-deposited poly(methyl methacrylate) films

et al. 2006

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Influence of Cooperative α Dynamics on Local β Relaxation during the Development of the Dynamic Glass Transition in Poly(n-alkyl methacrylate)s

Cited by 295 publications

References 40 publications

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

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

Departure from the Vogel behaviour in the glass transition—thermally stimulated recovery, creep and dynamic mechanical analysis studies

Comparison of mechanical and dielectric relaxation processes in laser-deposited poly(methyl methacrylate) films

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