Explanation of the difference in temperature and pressure dependences of the Debye relaxation and the structural α-relaxation near Tg of monohydroxy alcohols

doi:10.1016/j.chemphys.2019.110617

Chemical Physics

Volume 530, 1 February 2020, 110617

https://doi.org/10.1016/j.chemphys.2019.110617 Get rights and content

Abstract

Advance was made recently in identifying the nature of the Debye relaxation observed by dielectric spectroscopy in monohydroxy alcohols by other experimental techniques. Consistent with the experimental findings, the transient chain model give a good description of the origin of the Debye relaxation. Notwithstanding, a general property of the Debye relaxation remains not explained, and that is the weaker temperature and pressure dependences of its relaxation time τ_D(T,P) compared to τ_α(T,P) of the faster structural α-relaxation at temperatures approaching the glass transition temperature T_g. Based on the transient chain model for the Debye relaxation, and the Coupling Model for the α-relaxation, we have explained the difference in the temperature and pressure dependences of τ_D(T,P) compared to τ_α(T,P) quantitatively.

Introduction

The molecular dynamics of monohydroxy alcohols are distinctively different from most glass-forming substances by the presence of a very intense dielectric loss peak at lower frequencies than the ubiquitous structural α-relaxation. Some have narrow frequency dispersion corresponding to an exponential time dependence for the relaxation function. Found by Debye from electrical absorption experiment in alcohols and for which he proposed a relaxation model for application to the dielectric properties of water and alcohols [1], [2], it is commonly referred to as the Debye relaxation or Debye-like relaxation if the frequency dispersion is broader. The study and detection of the Debye relaxation in the past was mainly by dielectric relaxation techniques, and the results were used exclusively to deduce its molecular origin. Although the suggestions offered differ in detail, its origin was considered in the past as a direct consequence of H bond switching in the intermolecularly H-bonded structure. Notwithstanding, recent studies using advanced experimental techniques have yielded valuable information for understanding the origin of the Debye relaxation and its characteristics that differ from the structural α-relaxation. The techniques used are broadband dielectric relaxation at ambient pressure [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], and elevated pressures [15], [16], [17], [18], [19], [20] over wide frequency ranges, NMR [21], [22], calorimetric [23], [24], viscoelastic [25], light scattering ³, depolarized light scattering (DLS) [13], [14], [26], shear rheology [27], near infrared [28], neutron scattering [9], [29], [30], and simulation [31]. Performed on several monohydroxy alcohols, the Debye relaxation has also been revealed in the non-dielectric experimental studies although with a significantly lower intensity than in dielectric measurements. Details can be found in a comprehensive review of the dynamics and thermodynamics of monohydroxy alcohols by Böhmer, Gainaru, and Richert [32].

The results from studies using the near infrared and NMR spectroscopies in monohydroxy alcohols show the equilibration of the H bond population proceeds much faster than the time scale associated with the Debye peak [28]. This is evidence that the origin of the Debye relaxation cannot come from H bond switching as was previously assumed. This finding can also be used to critically examine the validity of other models of the Debye relaxation, one of which is the transient chain model by Gainaru et al. [21], [32] According to this model, the Debye relaxation originates from the fluctuations of the end-to-end dipole moment of the transient hydrogen-bonded chains, corresponding to a ‘‘snakelike motion induced by a successive loss (or gain) of segments at its one end and a gain (or loss) of segments at its other end’’. The transient chain model has support from recent experiments [21], [32] and high-field dielectric measurements by Singh and Richert [33] from which they found that a rearrangement of the chain structure was observed to occur on the time-scale of the Debye relaxation.

Despite the advance made in identifying the nature of the Debye relaxation, the weaker temperature T dependence [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14] and pressure P dependence [15], [16], [17] of its relaxation time τ_D(T,P) compared to τ_α(T,P) of the faster structural α-relaxation have not been addressed on a theoretical basis and explained quantitatively. Over some temperatures not far above T_g at ambient pressure P₀, the temperature dependence of τ_D(T,P₀) is weaker than τ_α(T,P₀), and consequently the slower Debye relaxation appears to merge with the faster α-relaxation on decreasing temperature and approaching T_g. The purpose of this paper is to provide a quantitative explanation of this generally observed property of monohydroxy alcohols. Based on the transient chain model of the Debye relaxation, the Coupling Model [34], [35] is used to account for the weaker temperature dependence of τ_D(T,P₀) than τ_α(T,P₀) near T_g at ambient pressure P₀ quantitatively, and also the weaker pressure dependence of τ_D(T,P) than τ_α(T,P) qualitatively.

Section snippets

Theoretical explanation

In this section we give an explanation of the weaker temperature dependence of τ_D(T) compared to τ_α(T) found generally in monohydroxy alcohols based on the Coupling Model (CM) applied before to polymer dynamics and viscoelasticity [34], [35]. The current explanation in the case of monohydroxy alcohols is analogous to the explanation of the weaker temperature [36], [37] and pressure dependence [38], [39], [40], [41], [42] of the chain relaxation time τ_c(T) than the structural α-relaxation time τ_α

Results and discussion

Before testing the prediction of the temperature dependence of τ_D(T) is the same as τ₀(T), we are mindful that this should be performed over a limited temperature region neighboring T_g. This is because the prediction is only valid over the temperature range in which the structure of the transient chain remains unchanged or changes slightly, and that is near T_g where all processes slow down. The ideal monohydroxy alcohols to perform the test are those having the values of τ_α(T) and (1-n)

Conclusion

The recent and renewed interest in the nature of the Debye relaxation in monohydroxy alcohols is driven by the studies using experimental techniques other than dielectric spectroscopy. The results particularly the microscopic ones are instrumental to critically test different models proposed to explain the origin of the Debye relaxation. The transient hydrogen-bonded chain model for the Debye relaxation stands out to be most consistent with all the experimental findings. Despite this success,

Acknowledgment

The current work was realized due to the ﬁnancial support of the project No. UMO-2015/17/B/ST3/01221 by the National Science Centre, Poland.

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Explanation of the difference in temperature and pressure dependences of the Debye relaxation and the structural α-relaxation near Tg of monohydroxy alcohols

Abstract

Introduction

Section snippets

Theoretical explanation

Results and discussion

Conclusion

Acknowledgment

Solids

Phys. Rep.

Prog. Polym. Sci.

Polar Molecules

Verh. Dtsch. Phys. Ges.

J. Chem. Phys.

Europhys. Lett.

J. Chem. Phys.

J. Chem. Phys.

Phys. Chem. Chem. Phys.

J. Chem. Phys.

J. Chem. Phys.

J. Chem. Phys.

J. Chem. Phys.

J. Phys. Chem. B

Phys. Rev. Lett.

J. Chem. Phys.

J. Chem. Phys.

J. Chem. Phys.

Phys. Rev. Lett.

J. Phys. Chem. B

Colloid Polym. Sci.

Phys. Rev. Lett.

J. Chem. Phys.

J. Chem. Phys.

Explanation of the difference in temperature and pressure dependences of the Debye relaxation and the structural α-relaxation near T_g of monohydroxy alcohols