Activation volume in superpressed glass-formers

In pressurized glass-forming systems, the apparent (changeable) activation volume Va(P) is the key property governing the previtreous behavior of the structural relaxation time (τ) or viscosity (η), following the Super-Barus behavior: \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\boldsymbol{\tau }}{\boldsymbol{(}}{\boldsymbol{P}}{\boldsymbol{)}}{\boldsymbol{,}}{\boldsymbol{\eta }}{\boldsymbol{(}}{\boldsymbol{P}}{\boldsymbol{)}}{\boldsymbol{\propto }}{\bf{\exp }}{\boldsymbol{(}}{{\boldsymbol{V}}}_{{\boldsymbol{a}}}{\boldsymbol{(}}{\boldsymbol{P}}{\boldsymbol{)}}{\boldsymbol{/}}{\boldsymbol{R}}{\boldsymbol{T}}{\boldsymbol{)}}$$\end{document}τ(P),η(P)∝exp(Va(P)/RT), T = const. It is usually assumed that Va(P) = V#(P), where \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${{\boldsymbol{V}}}^{{\boldsymbol{\#}}}{\boldsymbol{(}}{\boldsymbol{P}}{\boldsymbol{)}}={\boldsymbol{R}}{\boldsymbol{T}}{\boldsymbol{d}}\,{\boldsymbol{ln}}\,{\boldsymbol{\tau }}{\boldsymbol{(}}{\boldsymbol{P}}{\boldsymbol{)}}{\boldsymbol{/}}{\boldsymbol{d}}{\boldsymbol{P}}$$\end{document}V#(P)=RTdlnτ(P)/dP or \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${{\boldsymbol{V}}}^{{\boldsymbol{\#}}}{\boldsymbol{(}}{\boldsymbol{P}}{\boldsymbol{)}}{\boldsymbol{=}}{\boldsymbol{R}}{\boldsymbol{T}}{\boldsymbol{d}}\,{\boldsymbol{ln}}\,{\boldsymbol{\eta }}{\boldsymbol{(}}{\boldsymbol{P}}{\boldsymbol{)}}{\boldsymbol{/}}{\boldsymbol{d}}{\boldsymbol{P}}$$\end{document}V#(P)=RTdlnη(P)/dP. This report shows that Va(P) ≪ V#(P) for P → Pg, where Pg denotes the glass pressure, and the magnitude V#(P) is coupled to the pressure steepness index (the apparent fragility). V#(P) and Va(P) coincides only for the basic Barus dynamics, where Va(P) = Va = const in the given pressure domain, or for P → 0. The simple and non-biased way of determining Va(P) and the relation for its parameterization are proposed. The derived relation resembles Murnaghan - O’Connel equation, applied in deep Earth studies. It also offers a possibility of estimating the pressure and volume at the absolute stability limit. The application of the methodology is shown for diisobutyl phthalate (DIIP, low-molecular-weight liquid), isooctyloxycyanobiphenyl (8*OCB, liquid crystal) and bisphenol A/epichlorohydrin (EPON 828, epoxy resin), respectively.


Activation volume in superpressed glass-formers
Aleksandra Drozd-Rzoska in pressurized glass-forming systems, the apparent (changeable) activation volume V a (P) is the key property governing the previtreous behavior of the structural relaxation time (τ) or viscosity (η), following the Super-Barus behavior: ( ), ( ) ( ( )/ ) ∝ τ P η P V P RT exp a , T = const. it is usually assumed that V a (P) = V # (P), where V P RTd ln τ P dP = ( ) ( )/ # or ( ) ( )/ # V P RTd ln η P dP = . this report shows that V a (P) ≪ V # (P) for P → P g , where P g denotes the glass pressure, and the magnitude V # (P) is coupled to the pressure steepness index (the apparent fragility). V # (P) and V a (P) coincides only for the basic Barus dynamics, where V a (P) = V a = const in the given pressure domain, or for P → 0. The simple and nonbiased way of determining V a (P) and the relation for its parameterization are proposed. the derived relation resembles Murnaghan - O'Connel equation, applied in deep Earth studies. It also offers a possibility of estimating the pressure and volume at the absolute stability limit. the application of the methodology is shown for diisobutyl phthalate (Diip, low-molecular-weight liquid), isooctyloxycyanobiphenyl (8*OCB, liquid crystal) and bisphenol A/epichlorohydrin (EPON 828, epoxy resin), respectively.
Previtreous changes of the structural (primary, alpha) relaxation time (τ), viscosity (η), electric conductivity (σ), heat conductivity (κ), diffusion (d) or chemical reactions rates (k) in systems ranging from low-molecular-weight liquids and polymers to liquid crystals and plastic crystals are the key manifestation of the hypothetical universal dynamics emerging on approaching the glass transition (T g , P g ) [1][2][3][4][5] . Similar patterns are observed both for the temperature and pressure paths [6][7][8] . The temperature path is associated with the Super-Arrhenius (SA) dynamics, and it is governed by changes of the apparent activation energy E a (T), which strongly increases on approaching the glass transition temperature T g [9][10][11] . The non-biased way of determining E a (T) and its properties are discussed in refs [12][13][14] and recalled in Supplementary Info. This report focuses on the still puzzling case of the (high) pressure-induced glass transition. For compressed glass-formers, general features of the previtreous dynamics are described by the Super-Barus (SB) equation [7][8][9][10] : where T = const and P < P g ; V a (P) denotes the apparent activation volume, which changes on compressing. Generally, the name 'activation volume' is reserved for the basic Barus 15 equation with V a (P) = V a = const in the given domain of pressures. Prefactors τ 0 P and η 0 P in Eq. (1) refer to P = 0, but within the experimental error they can be approximated by atmospheric pressure values, i.e., τ 0 P = τ(P = 0) ≈ τ(P = 0.1 MPa) for the tested isotherm T. For high-pressure studies the experimental errors are ΔP ≈ ±0.2 MPa (moderate pressures) and ΔP > ±1 MPa (GPa domain). The shift of pressure by 0.1 MPa does not yield detectable changes of dielectric relaxation time [6][7][8] .
Similar SB dependences describe pressure changes of all physical properties recalled above: pressure dependences of τ(P) and η(P) are parallel (Eq. (1)) 6-8 , but for the remaining dynamic properties the translational -orientational decoupling have to be taken into account 8,10 . For instance, for DC electric conductivity 16 : , where E a stands for the activation energy and k is the reaction rate coefficient 20 . Generally, for ultraviscous/ultraslow glass-forming systems one should expect the Super-Barus (SB) behavior (Eqs (1) and (2)), where dynamic properties are governed by changes of the pressure-dependent activation volume: the apparent activation volume.
According to the above discussion one concludes that V a (P) governs the dynamics of ultraviscous/ultraslow systems, and its determination and understanding is the key to the ultimate insight into the glass transition problem [6][7][8][9] , the behavior of soft matter under pressure [7][8][9]21 , high-pressure chemistry and biochemistry 22 , innovative material engineering 23 , high-pressure biotechnology 24 , geophysics, and deep Earth science 25 .
Usually, for the previtreous domain the apparent activation volume V a (P) is calculated from τ(P) or η(P) experimental data via  : a # under the assumption that V a (P) = V # (P) and for 0.1 MPa < P ≤ P g . The analysis exploring the (apparent) activation volume determined via Eq. (3) is the key point of numerous research reports. In the framework of the transition state theory, the activation volume describes the difference between volumes occupied by a molecule in activated and non-activated states 22 . It is the essential parameter characterizing the sensitivity of the structural relaxation time or other dynamic properties to pressure changes 7,8,21 . It estimates the local volume required for a given dynamical process (in the case of τ denotes molecular rearrangements) 8,26-35 . Hong et al. 36,37 indicated that the activation volume correlates with the length scale of dynamical heterogeneities ξ 3 , which are considered as one of the essential sources of the previtreous 'universality' of dynamic previtreous properties. Tests in hydrogen-bonded molecular liquids showed the case-sensitivity of the activation volume, determined via Eq. (3) to subtle features of molecular structures [38][39][40] . Worth stressing is also the broadly used link between the activation volume at the glass transition, and the fragility: g is one of key 'universality' metrics for the glass transition phenomenon [2][3][4][5]9,10,43,44 . The analysis via Eq. (3) was also used for showing different activation volumes determined by dielectric and light scattering spectroscopies 8,[45][46][47][48][49][50][51][52][53] . Reasonings based on such analysis can yield important checkpoints for glass transition models 8,[45][46][47][48][49][50][51][52][53][54][55] . The activation volume is also significant for the thermodynamic scaling linking τ(T.P, V) experimental data 8,54,55 . There are also reports where V # (P) is recalled as the (apparent) activation volume, but the link to the steepness index m T (P) is indicated 56,57 .
For the validation of Eq. (3) reports by Whalley 17,18 and Williams 19 are most often cited 8,30-34,38-42,48-54 . However, these reports did not consider the Super-Barus dynamics with the pressure depending apparent activation volume but the basic Barus behavior with the constant activation volume. This issue is worth stressing, since the SB Eq. (1) directly yields the differential equation: Comparing Eqs (3) and (4) one obtains that generally: V a (P) ≠ V # (P). The second term in Eq. (4) disappears only for two 'special' cases: (i) for P = 0, or (ii) for V a (P) = V a = const, i.e., for the basic Barus behavior in the given pressure domain. Worth recalling is the difference between the free volume (V f ) as the volume not occupied by molecules and the activation volume (V a ) as the volume required for the given process, for instance, the molecular rearrangement or reorientation. Then, one can expect V f > V a 8,9 . Consequently, the question arises for the (proper) estimation of the (apparent) activation volume in the previtreous domain. This report proposes the solution to this problem and discusses the meaning and behavior of both V # (P) and V a (P) for P < P g . The discussion is supported by the analysis of the τ(P) experimental data for glass-forming low molecular weight liquid diisobutyl phthalate (DIIB, T g (0.1 MPa) = 196.8K), epoxy resin bisphenol A/epichlorohydrin (EPON 828, T g (0.1 MPa) = 253.9K) and liquid crystalline isooctyloxycyanobiphenyl (8*OCB, T g (0.1 MPa) = 220.7K). The latter vitrifies in the isotropic liquid phase, and the possible nematic phase is hidden below the glass transition. In given studies, pressures up to P ≈ 1.2 GPa were reached, i.e. for the domain hardly available in high resolutions tests carried out so far 8,10 . Experimental details are described in the Methods section. Figure 1 shows the pressure evolution of the structural relaxation time for selected isotherms for three qualitatively different glass formers 8*OCB, DIIP and EPON 828, in the pressure range 0.1 MPa < P < P g . They served for estimating both V # (P) and V a (P).

Results and Discussion
When discussing the physical meaning of V # (P) one can recall the definition of the pressure-related steepness index (the normalized rate of changes of the relaxation time, viscosity, …) in the previtreous domain 8 , which leads to the relation: T g g 10 It terminates at the pressure-related fragility 11 : , which is the key metric for glass-forming ultraviscous/ultraslow systems. Then, the pressure-dependent (isothermic) coefficient m T (P) for P < P g can be called the apparent fragility. Linking Eqs (3) and (5) one obtains the relation showing that Considering further the ratio of fragilities along T g (P) curve/line: and linking this with Eq. (6) the following relations are obtained: Equation (7b), originally derived in ref. 40 , is broadly used for calculating isobaric fragilities m = m P (T g ) for different isobars, based on the knowledge of the '~activation volume V # (P)' calculated via Eq. (3) and the pressure shift of T g 8,26-34,38-42,47-54 . Notwithstanding, Eq. (7a) is fundamentally more correct than Eq. (7b), since V # (P) should not be recalled as the apparent activation volume.
Recently, it was shown experimentally that changes of the pressure-related apparent fragility can exhibit a 'universal' previtreous behavior 5,7 : where T = const, the amplitude A = const, and P * is for the extrapolated singular pressure. Regarding pressures: P < P g and P * > P g . Following Eqs (6) and (8)   1/2 vs. P plot 8,59 , which is parallel to (m T (P)) −1/2 vs. P presentation 5,7,56 . Such analysis assumes a priori the validity of the pressure counterpart of the Vogel-Fulcher-Tammann (VFT) relation for describing τ(P) experimental data 5,7,56 . Recently, an alternative and a model-free way for detecting the dynamic crossover via 1/m T (P) vs. P analysis was indicated 7 . This report shows that the pressure evolution of 1/V # (P) follows the pattern noted for 1/m T (P): this is shown in insets in Figs 2 and 3. The lack of P B for 8*OCB in the inset in Fig. 4 results from the limited tested pressure range, between P g (T) = 0.56 GPa and P = 0.1 MPa. Consequently, in 8*OCB measurements were carried out only in the high-pressure dynamical domain (P g > P > P B ).
The above discussion shows the direct link between the magnitude considered so far as the apparent activation volume V # (P) and the apparent fragility m T (P). To determine the 'real' apparent activation volume V a (P) the  . It is visible that both quantities follow the same pattern, in agreement with Eq. (8) and ref. 7 . One can also see that V # (P) ~ V a (P) only for P → 0. Some distortion between V a (P → 0) and V # (P → 0) visible in Figs 2-4 can be associated with the distortions sensitive nature of Eq. (3) containing derivatives and used for calculating V # (P). For higher pressures, i.e. P → P g : V # (P) ≫ V a (P). Interestingly, the pressure evolution of the apparent activation volume V a (P) seems to be poorly, if at all, sensitive to the dynamical crossover phenomenon (see main parts of Figs 2 and 3).
To the best knowledge of the author, similar discussions of the apparent activation volume as introduced via Eq. (9) have appeared only in few reports so far. Beyeler and Lazarus 61 considered the diffusion processes during chemical reactions under high compression and introduced a similar apparent activation volume concept. Recently, Kornilov et al. 62 experimentally studied reaction rate constants for which the following relation was applied: k P ln ( ) = a′P + b′P 2 , for P < (220-300 MPa). It can be linked to the simple approximation of the SB Eq. (1): Consequently, recalling Eq. (9) the activation volume is given via: When considering the description of V a (P) in the broad range of pressures one can recall a recently derived relation for the previtreous pressure evolution of τ(P), originating directly from 'universal' changes of the apparent fragility described via Eq. (8) 7 :  www.nature.com/scientificreports www.nature.com/scientificreports/ V P V P RT C P P ln P P C P P P However, in Eq. (12) the apparent activation volume V a (P) → ∞ for P → 0, and then the anomalous increase occurs for P → 0.1 MPa. This problem can be avoided when taking into account that for solids, including liquids, the available range of pressures extends from the 'normal' positive domain (the isotropic compression) to the negative one, associated with the isotropic stretching 63 . This concept proved its fundamental significance for the general equation of state for water or critical mixtures and blends 63 . Angell and Quing experimentally showed the appearance of negative pressures and passing P = 0 without any hallmark, in glass-forming liquids using the centrifugal method 64 . Consequently, the 'positive' (isotropic compressing) and 'negative' (isotropic stretching) pressure regions can be considered as the common area, terminating at the absolute stability limit (SL) spinodal, hidden under negative pressures [63][64][65] . To describe experimental data in both pressure domains, one can consider the transformation P → ΔP = P − P SL 57 . Introducing the latter to Eq. (11) the modified dependence appears: The above relation describes pressure changes of the apparent activation volume in the whole tested pressure range, up to P ≈ 1.2 GPa, as shown in Figs 2-4. It can be also extended into the negative pressures domain. Fitted parameters are given in Table 1. Values P * were estimated from insets in Figs 2-4 using the condition 1/V # (P * ) = 0; this also includes parameters C and Ψ estimated following ref. 7 .
The main result of the given report is the simple and non-biased way of determining the apparent activation volume V a (P) via Eq. (9) and the proposal for the parameterization of V a (P) evolution given by Eq. 13. It is worth mentioning that problems with the estimation and meaning of the activation volume seem to be absent for geophysics/deep Earth science where Murnaghan -O'Connel relation is broadly applied 70,71 : where K 0 , ′ K 0 denotes 4/9 of the bulk modulus and its first derivative. Notable is the similarity of Eq. (14) to approximated Eqs (12) and (13)], as shown above. Results presented focused on the pressure evolution of the primary relaxation time, but they can also be applied for the viscosity, electric conductivity, diffusion, equilibrium, and reaction rates coefficients, in ultraviscous/ultraslow systems what indicates the broad range of fundamental and practical applications ranging from the glass transition physics and the solid state physics to 'extreme' chemistry, geophysics, petrology, innovative material engineering, high pressure preservation of food and biotechnology under pressure.

Methods
In the last decades, the broadband dielectric spectroscopy (BDS) has become the key tool for studying previtreous behavior, including challenging insights from high-pressure studies 8,72,73 . In this report, BDS is used to determine the pressure evolution of the primary (structural, alpha) relaxation time 72, 73 . Studies were carried out using the Novocontrol impedance analyzer, model 2015. BDS studies were carried out between the atmospheric pressure (P = 0.1 MPa) and the glass transition pressure, estimated via the empirical condition τ(T g , P g ) = 100 s 8 www.nature.com/scientificreports www.nature.com/scientificreports/ structural relaxation times were determined from the peak frequencies of primary relaxation loss curves ε"(f): τ = 1/2πf peak 8,9,72 . Tested samples were placed in the flat-parallel measurement capacitor made from Invar. The gap between plates d = 0.2 mm and their diameter 2r = 16 mm. Samples were entirely isolated from the pressurized medium (Plexol). They were in contact only with Invar, quartz (the spacer between plates) and Teflon. The pressure was transmitted to the sample via the deformation of 50 mm thick Teflon film. The process was supported by the computer-controlled pump, enabling pressure changes and programming with the precision ΔP = ±0.2 MPa, The pressure chamber was surrounded by a special jacket associated with the Julabo high-accuracy thermostat with the external circulation and the volume of the thermostated liquid V = 20L. These enabled temperature changes and control with accuracy ΔT = ±0.02 K. The temperature was monitored using the thermocouple within the pressure chamber and two platinum mini-resistors placed at the bottom and the top of the chamber. The high-pressure system was designed and produced by UnipresEquipment (Poland). Further experimental details are given in refs 6,7,74 . Notable, that the examined range of pressures was extended up to P ~ 1.2 GPa, the still hardly available range in high-resolution BDS pressure studies 8, . Experimental results cover time-scales from τ(P = 0.1 MPa) to τ(P g , T g ) = 100s. The latter is commonly applied as the practical empirical estimation of (T g , P g ) 8,10,26-54 .

Data Availability
The data supporting the findings of this study are available from the author upon reasonable requests.