Merging of the α and β relaxations in polybutadiene: A neutron spin echo and dielectric study

Arbe, Arantxa; Richter, D.; Colmenero, Juan; Faragó, B.

doi:10.1103/physreve.54.3853

Cited by 270 publications

(306 citation statements)

References 21 publications

Supporting

Mentioning

263

Contrasting

Unclassified

Order By: Relevance

“…Also one may doubt if a simple additive superposition of α and β relaxation is valid in this region [33]. However, an alternative, non additive superposition [33] led to results consistent with those shown in Fig. 2.…”

supporting

confidence: 77%

Dielectric spectroscopy in benzophenone: Theβrelaxation and its relation to the mode-coupling Cole-Cole peak

2007

View full text Add to dashboard Cite

We report a thorough characterization of the glassy dynamics of benzophenone by broadband dielectric spectroscopy. We detect a well pronounced β relaxation peak developing into an excess wing with increasing temperature. A previous analysis of results from Optical-Kerr-effect measurements on this material within the mode coupling theory revealed a highfrequency Cole-Cole peak. We address the question if this phenomenon also may explain the Johari-Goldstein β relaxation, a so far unexplained spectral feature inherent to glass-forming matter, mainly observed in dielectric spectra. Our results demonstrate that according to the present status of theory, both spectral features seem not to be directly related.PACS numbers: 64.70. Pf, 77.22.Gm Aside of the α relaxation, characterizing the slowing down dynamics during the glass transition, glassy matter reveals a rich variety of further, so far only poorly understood dynamic processes [1,2,3]. Among the most prominent ones is the socalled Johari-Goldstein (JG) β relaxation [4]. In spectra of the imaginary part of the susceptibility χ" (the "loss") it shows up as second peak at a frequency beyond the α relaxation. It is inherent to the glassy state of matter but its microscopic origin still is controversially discussed. Even if superimposed by a dominating α relaxation, it still may show up in the loss as a socalled excess wing [5]. This long known feature [2,3,6,7] only recently was discovered to be due to a second relaxation peak [8].The most prominent theory of the glass transition is the mode coupling theory (MCT) [9]. It predicts a so-called fast β relaxation, showing up in the GHz -THz range, which was experimentally verified in numerous investigations [10]. It is envisioned by the rattling motion of a particle in the cage formed by the surrounding particles. It should be noted that this fast β process usually is presumed not to be identical with the JG β relaxation, sometimes also termed "slow β process". In the basic versions of MCT, a combination of two asymptotic power laws is predicted to describe the data in the regime of the fast process, with both exponents determined by a system parameter λ. They form a shallow minimum in the frequency-dependent loss, followed by the so-called microscopic peak located in the THz range [ Fig. 1(a)]. Some consensus seems to develop in the glass community that MCT is the correct description for high temperatures, T > T c . The critical temperature T c can be regarded as idealized glass-transition temperature often roughly located at 1.2 T g with T g the experimental glass temperature. The original MCT only revealed three characteristic spectral features, the α relaxation, the minimum, and the microscopic peak. The high-frequency flank of the α relaxation peak directly crosses over into the low-frequency wing of the minimum, the so-called von-Schweidler law ν -b [ Fig. 1(a)]. At high temperatures this is in accord with experimental observations. However, excess wing or JG β peak developing at lower temperatures seemed not t...

show abstract

supporting

confidence: 77%

Dielectric spectroscopy in benzophenone: Theβrelaxation and its relation to the mode-coupling Cole-Cole peak

2007

View full text Add to dashboard Cite

show abstract

“…This unusual behavior, namely a β-process with an amplitude becoming comparable or even exceeding that of the α-process at high temperatures, to our knowledge so far was never observed in any plastic crystal and is realized only in few canonical glass formers, e.g. toluene or polybutadiene [21,38]. In general, the occurrence of β-relaxations seems to be a common feature of canonical glass formers.…”

Section: Discussionmentioning

confidence: 94%

“…When considering these results, however, one should be aware that especially at high temperatures the determination of β CD of the α-process has a high uncertainty due to the strong overlap with the β-process. In addition, one should mention that it is not so clear if a simple additive superposition of different contributions to ε ′′ (ν) is really justified [37,38]. While alternative approaches may lead to somewhat different parameters, we believe that the overall trend of two broad relaxations exchanging the role of the dominating process depending on temperature will remain the same.…”

Section: Freon112mentioning

confidence: 99%

α and β relaxation dynamics of a fragile plastic crystal

Pardo

Lunkenheimer

Loidl

2006

The Journal of Chemical Physics

View full text Add to dashboard Cite

We present a thorough dielectric investigation of the relaxation dynamics of plastic crystalline Freon112, which exhibits freezing of the orientational degrees of freedom into a glassy crystal below 90 K. Among other plastic crystals, Freon112 stands out by being relatively fragile within Angell's classification scheme and by showing an unusually strong β-relaxation. Comparing the results to those on Freon112a, having only a single molecular conformation, points to the importance of the presence of two molecular conformations in Freon112 for the explanation of its unusual properties. 1 INTRODUCTIONGlass phenomena occur when a dynamically disordered system (e. g. aliquid, plastic crystal or paramagnet) freezes as a function of external temperature or pressure devoid of long range order. In this freezing process, one or more degrees of freedom of atoms or molecules continuously slow down, reaching the so called glass transition when their dynamics has a characteristic time, generally chosen to be 10 2 s (for recent reviews on glass transition see, e.g., [1]). Since in the liquid phase there is translational and orientational disorder, the glass transition of canonical glass formers is associated with the freezing of these two degrees of freedom. But a mesophase can exist between the completely ordered crystalline phase and the translationally and orientationally disordered liquid phase, the so called plastic phase or orientationally disordered (OD) phase [2,3]. In the plastic phase, the centers of mass of the molecules have spatial long range order, forming a lattice which generally has high symmetry (such as cubic, quasi-cubic or rhombohedral [2, 3, 4]), but there is only short-range order with respect to the orientational degrees of freedom [5].As for glass-forming liquids, the typical phenomenology of a glass transition also can be realized in various plastic crystals when the temperature is decreased, but in this case only the orientational degrees of freedom are frozen, yielding the formation of a "glassy crystal"[6], also called OD glass.Concerning the dynamics of OD phases, dielectric spectroscopy has been revealed to be a useful tool to understand the complex dynamics of these phases [7,8,9,10,11,12,13].Based on these works there seem to be some general dynamic features of OD phases and its glasses: They are rather strong (following the definition of Angell [14]) and they follow the Böhmer relation between non-exponentiality of the α relaxation and fragility [15,16].On the contrary, the so called Nagel scaling [17] does not seem to work for these phases [7,11,18]. In addition there is only a weak or no β-relaxation at all and the excess wing, showing up as a second power law at the high frequency flank of the α peak in many canonical glass formers [17,19,20,21], is either absent in plastic crystals [11] or can be ascribed to a weak secondary relaxation [7,9].

show abstract

“…(iii) Johari-Goldstein type β-process (secondary process in the context of dielectric relaxation) [8,9,10,11,12,13] merges into the α-relaxation around T D , extrapolating the Arrhenius-type temperature dependence below T g of the β-relaxation times [6,14,15,16,17,18,19].…”

mentioning

confidence: 99%

Merging of α and slow β relaxation in supercooled liquids

2002

View full text Add to dashboard Cite

Dielectric relaxation spectroscopy (1 Hz -20 GHz) has been performed on supercooled glassformers from the temperature of glass transition (Tg) up to that of melting. Precise measurements particularly in the frequencies of MHz-order have revealed that the temperature dependences of secondary β-relaxation times deviate from the Arrhenius relation in well above Tg. Consequently, our results indicate that the β-process merges into the primary α-mode around the melting temperature, and not at the dynamical transition point T ≈ 1.2Tg.PACS numbers: 64.70. Pf, 77.22.Gm, In recent years, much of the focus on glassy dynamics has been shifting to a considerably higher temperature than T g , the glass-transition one [1,2,3,4]. The topical temperature is located around T D ≡ 1.2T g , where the dynamics of supercooled liquids has been found to change fairly. So far, there have been observed the following phenomena:(i) Rössler scaling reveals that, when cooled, the Debye-Stokes-Einstein relation becomes invalid around T D [5,6]. This indicates a change of diffusion mechanism there.(ii) Stickel analysis [7] clarifies that temperature dependence of viscosity changes around T D . Therefore, in order to fit the primary α-relaxation time τ α using the Vogel-Fulcher-Tamman (VFT) relation,the coefficients (τ 0 , C, T 0 ) have to vary at T B ≈ T D . This suggests that the mechanism of slow structural-relaxation makes some alternation there.(iii) Johari-Goldstein type β-process (secondary process in the context of dielectric relaxation) [8,9,10,11,12,13] merges into the α-relaxation around T D , extrapolating the Arrhenius-type temperature dependence below T g of the β-relaxation times [6,14,15,16,17,18,19].Theoretically, the characteristic temperature T D is thought to be comparable to T C where the idealized Mode Coupling Theory (MCT) predicts a dynamical phase transition [20]. Indeed, the above first two phenomena (i, ii) can be regarded as indicators of the dynamical transition at T C . However, the MCT is irrelevant to the third one (iii), the bifurcation of α, β-modes; even the existence of the Johari-Goldstein type β-process cannot be derived.Furthermore, from experimental aspects, while the dynamical transition phenomena (i, ii) have been confirmed from either Rössler or Stickel plot definitely, the bifurcation of (iii) is inferred from the extrapolation. Actually, however, it remains an open problem as to whether the Arrhenius behavior of the β-process persists in higher temperatures near T D : the T D -decoupling of α, β-relaxations is not conclusive.This letter thus aims to investigate the secondary β-mode in high temperatures well above T g , by carrying out precisely the broad-band measurements of dielectric relaxation. Our main result is the following: as will be seen in Figures 2 and 5, the β-relaxation times deviate from the Arrhenius relation, indicating that the β-relaxation merges into the α-mode not at T D but around the Arrhenius-VFT crossover temperature T A (generally close to the melting one) where the temperatu...

show abstract

Merging of the α and β relaxations in polybutadiene: A neutron spin echo and dielectric study

Cited by 270 publications

References 21 publications

Dielectric spectroscopy in benzophenone: Theβrelaxation and its relation to the mode-coupling Cole-Cole peak

Dielectric spectroscopy in benzophenone: Theβrelaxation and its relation to the mode-coupling Cole-Cole peak

α and β relaxation dynamics of a fragile plastic crystal

Merging of α and slow β relaxation in supercooled liquids

Contact Info

Product

Resources

About