Cooling-Rate versus Compression-Rate Dependence of the

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Cooling-Rate versus Compression-Rate Dependence of the Crystallization in the Glass-forming Liquid, Propylene Carbonate Grzegorz Szklarz, Karolina Adrjanowicz, and Marian Paluch Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00123 • Publication Date (Web): 26 Feb 2018 Downloaded from http://pubs.acs.org on March 1, 2018

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Crystal Growth & Design

Cooling-Rate Versus Compression-Rate Dependence of the Crystallization in the Glass-forming Liquid, Propylene Carbonate Grzegorz Szklarz,1,2* Karolina Adrjanowicz,1,2* Marian Paluch1,2 1

2

Institute of Physics, University of Silesia, 75 Pulku Piechoty 1, 41-500 Chorzow, Poland

Silesian Center for Education and Interdisciplinary Research (SMCEBI), 75 Pulku Piechoty 1a, 41-500 Chorzow, Poland * email: [email protected], [email protected]

ABSTRACT The effect of cooling and compression rates on the tendency to crystallize/vitrify of the canonical glass-forming liquid, propylene carbonate (PC), was studied by using dielectric spectroscopy. Based on constructed Time-Temperature-Transformation (TTT) and Continuous-HeatingTransformation (CHT) diagrams, we have determined the critical scanning rates that allow avoiding crystallization on cooling from the liquid state and reheating of the glassy sample, respectively. In a similar way to isobaric temperature-dependent studies, we have also carried out isothermal high-pressure measurements upon which the crystallization tendency of PC was examined as a function of varying compression and decompression rates (pressures up to 1.3 GPa). We propose Time-Pressure-Transformation (TPT) and Continuous-Decompression Transformation (CDT) diagrams as the pressure analogs of the TTT and CHT diagrams. Obtained results demonstrate that, qualitatively, one gets the same picture when pressure (instead of the temperature) is used as the principal adjustable thermodynamic parameter. In agreement with this finding, a careful comparison of the time-dependent crystallization results collected under isobaric and isothermal conditions has revealed that within the considered T-p range the 1 ACS Paragon Plus Environment

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maximal crystallization rate and dimensionality of the growing crystals do not depend significantly on whether we vary with the temperature at a fixed pressure or pressure at a constant temperature.

INTRODUCTION Glass formation plays an important role in several research disciplines (including physics, chemistry, biology or material science) as well as numerous technological applications (e.g., amorphous semiconductors, solid-state electrolytes, optical fibers, energy and waste storage).1,2,3,4,5,6Because of the above reasons, understanding the conditions which may favor vitrification on cooling, rather than crystallization, have great scientific and practical meaning. This, however, remains a long-standing scientific problem. Over the past decades, numerous attempts to provide universal criteria enabling to predict/quantify the glass-forming tendency of various materials were proposed, e.g. based on the molecular architecture7,8,9 the strength of the intermolecular attractions10,11values of Tb/Tm or Tg/Tm ratios (where Tg is the glass transition temperature,

Tm

is

the

melting

temperature,

and

Tb

is

the

absolute

boiling

temperature).12,13,14,15,16Unfortunately, none of the proposed measures provide a convincing prediction of the glass-forming tendency that applies to a broad spectrum of different systems. Vitrification takes place when the rate of cooling is fast enough so that there is no time for nuclei to form and grow. The two essential constituents of the crystallization process, i.e., nucleation and crystal growth, are determined by the interplay between thermodynamic and kinetic factors. With increasing undercooling, the free energy barrier between liquid and crystalline states decreases driving the system towards crystallization. On the other hand, the concurrent slowing down of the molecular movement needed at the liquid-crystal interface impede the crystallization process. This results in a characteristic bell-shaped curves of the 2 ACS Paragon Plus Environment

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nucleation and crystal growth rates maxima,17,18,19 as demonstrated in Scheme 1a. Depending on the intensity and the extent of overlap, avoiding crystallization on cooling requires a bit different efforts. For example, when the maxima of nucleation and crystal growth rates are located near to each other, a liquid would be prone to crystallize rather than vitrify on cooling. In such case, the smaller the liquid volume and higher the cooling rate, the lower probability of crystallization.7 On the other hand, when the optimal temperature regions for nucleation and crystal growth do not superimpose, a liquid can be quenched and forms a glass. However, on subsequent heating of such sample, it is susceptible to crystallization after passing through the nucleation and then the crystal growth zones in the right and non-interchangeable order.20 For practical reasons, understanding the conditions which must be fulfilled to bypass crystallization on cooling a liquid from the melt are expressed in terms of the time-temperaturetransformation (TTT) diagrams. TTT diagram demonstrates changes in the crystallization rate below Tm as a characteristic ‘C-shaped’ curve in the time-temperature coordinates. The ‘nose’ in TTT diagram defines, via temperature at which the crystallization rate reaches its maximum value, the critical cooling rate needed to avoid crystallization when cooling a liquid from Tm. When the transformation kinetics takes place under non-isothermal conditions, just like in the industrial processing, continuous-cooling-transformation (CCT) diagrams are more useful. CCT diagrams provide information on the initial and the final transformation temperatures as well as the final product obtained during continuous cooling at various constant cooling rates. On the other hand, the heating analog of the TTT diagram is continuous-heating-transformation (CHT) diagram which reports on time and temperature required to reach a certain phase transformation during the course of continuous heating with various constant heating rates. TTT, CCT and CHT

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diagrams allow developing the time-temperature conditions essential for the glassformation/crystallization of organic and inorganic-based substances.21,22,23,24 Without any doubts, the temperature is a fundamental thermodynamic variable controlling vitrification. However, the results collected over the past years have demonstrated that only in combination with high-pressure studies, one gets a complete description of the crystallization and glass-formation phenomena.25,26,27,28,

29,30

Pressurization of liquid at a fixed

temperature can be also used to produce glasses.31Scheme 1b demonstrates a very schematic overview on the pressure profile of the nucleation and the crystal growth rates. Upon compression at fixed temperature and above the melting point, a liquid overpass first via the optimal zone for the crystal growth (located at lower pressures), and afterward the region where the nucleation process is favorable. However, it is completely unknown if both thermodynamic parameters, i.e., temperature and pressure, affect location/separation of the nucleation and the crystal growth rates maxima with respect to each other in exactly the same way. Herein, one should remember that temperature and pressure are not equivalent thermodynamic variables. While temperature affects the kinetic energy of the molecules, pressure acts mostly on the intermolecular distances between them. Thus, understanding the response of the glass-forming liquids to both variables, temperature and pressure, is essential to take full advantage of the twodimensional T-p phase space and produce materials with desired properties and structure. In this paper, we wish to compare the effect of temperature and pressure on the glassforming tendency of the van der Waals liquid, propylene carbonate (PC). We show that crystallization behavior of the investigation sample can be controlled not only by changing the rate of cooling/heating but also compression/decompression rate. Based on obtained results we have constructed Time-Pressure-Transformation (TPT) and Continuous-Decompression-


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Transformation (CDT) diagrams as the pressure analogs of the TTT and CHT diagrams. In both cases, avoiding crystallization on re-heating/decompression from the glassy state was found to be far more challenging that cooling/pressurizing a liquid from Tm. Surprisingly, we found that within the studied T-p range the crystallization rate and dimensionality of the growing crystals, do not depend significantly on whether temperature or pressure is used as the controlling thermodynamic variable. This means that to some of the extent the increasing pressure mimics the crystallization behavior of the investigated sample as on lowering the temperature, and thus, can be used interchangeably. (a)

(b)

Scheme 1. Schematic evolution of the temperature (a) and pressure (b) dependences of the nucleation N and crystal growth G rates. The overlap zone of these two processes and their absolute magnitudes determine the overall crystallization behavior of a material upon upward and downward T/p scans.

EXPERIMENTAL Materials.



Propylene carbonate (MW=102.09 g·mol−1) of purity >99% was purchased from Sigma-Aldrich and used as received. The chemical structure of PC (Tg=159 K at 0.1 MPa) can be found in Figure 1a, as an inset. Methods. The real part of complex dielectric permittivity (ε’) was measured with the use of Novocontrol GMBH Alpha dielectric spectrometer. For studies carried out at atmospheric pressure, the temperature was controlled by Quattro Novocontrol system with a stability better than 0.1K. The liquid sample was placed between two stainless steel electrodes separated by a gap of 0.05 mm provided by a Teflon spacer. Measurements of the dielectric permittivity at 1MHz frequency were performed under nitrogen atmosphere within the temperature range from 250K to 150 K using various scanning rates. For high-pressure studies, we have used Unipress high-pressure equipment (Institute of High-Pressure Physics, Warsaw, Poland). The high-pressure setup consists of MP5 micro-pump with two-pulse step motor, control unit allowing to regulate the speed of compression by changing the frequency of the motor movements (and therefore also the piston position), stainless-steel vessel (shown elsewhere)32 and hydraulic ram for monostat LC20T which replaces a laboratory hydraulic press LCP20. The latter one is supplied with pressure generated by MP5 micro-pump. A ram piston via the pusher transfers the force to the monostat and generates the pressure in it. The ratio of the ram’s piston diameter to the monostat’s piston diameter provides appropriate multiplication factor and allows to obtain desired pressure in the LC20T monostat (high-pressure vessel).33 High-pressure plug with a dielectric cell (for details see the reference34) filled with the investigated sample (~3 ml volume) was placed into Teflon bellow mounted in the high-pressure vessel. The temperature in the pressure vessel was controlled by Tenney Junior environmental chamber. Routinely, we have


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also measured the temperature inside the high- pressure vessel by a Pt100 sensor located about 2.5 cm away from the sample. For thermally equilibrated systems, the sample and high-pressure cylinder temperature are almost the same (± 2 K). We have also carried out time-dependent isobaric (p=0.1 MPa) and isothermal (T=243 K and T=253 K) dielectric studies to determine the temperature and pressure evolution of the crystallization rate and Avrami parameter. By following changes in the dielectric response of the sample upon crystallization progress, it is possible to study the crystallization kinetics at the realtime of the measurements. To do that, obtained data were expressed in terms of the normalized permittivity ε ' N (t ) = (ε 'initial −ε ' (t )) /(ε 'initial −ε ' final ) and then fitted with the use of Avrami equation,35,36 ε 'N (t ) = 1 − exp(−kt n ) where n is Avrami parameter, k is crystallization rate. Changes in the static dielectric permittivity were recorded for every 300 seconds. Prior, the sample was cooled down (with the rate of ~5K/min) or either compressed from the liquid state (compression rate ~40 MPa/min) to the desired crystallization conditions (T, p), so without approaching the glassy state. Crystallization tendency of PC during cooling/heating with different scanning rates, as predicted by TTT and CHT diagrams, was verified with the use of dielectric and calorimetric techniques. RESULTS AND DISCUSSION Figure 1a demonstrates changes in the real part of complex dielectric permittivity (at 1 MHz frequency) recorded for PC upon cooling with different scanning rates at 0.1 MPa. A characteristic step of ε’ when lowering the temperature is due to the dielectric dispersion. For glass-forming liquids, it signifies that the dielectric α-relaxation systematically slows down and eventually moves out of the experimental window as the glass transition temperature, Tg, is



approached. Since ε’(T) dependences collected upon scanning with different cooling rates are almost identical, one can suppose that the glassy state can be reached easily, even if a very slow cooling rate is applied. On the other hand, from such a naive picture one cannot evaluate whether cooling of the investigated liquid down to T

Cooling-Rate versus Compression-Rate Dependence of the

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