Complexity of crystallization amazes physics
On the one hand, gently organized and ordered, on the other hand, flowing like water or honey – this is the dual nature of liquid crystals. Scientists from Krakow have looked in detail at one aspect of liquid crystals: crystallization processes. Their research shows that in the case of certain types of liquid crystals, these processes not only occur when they are cooled and heated, but are also surprisingly complex in nature.
Liquid crystals are versatile. They are found in liquid crystal displays (LCDs), which use not single liquid crystal compounds, but mixtures thereof. It turns out that glass-forming materials are more useful when designing next-generation LCDs because they are better suited as components of mixtures than easily crystallizable compounds. Substances that create chiral smectic phases are especially promising for LCD display technology, since thanks to them it is possible to build devices with short switching time, high contrast and wide color scale.
“First: recognition. In order to determine whether a given liquid crystal is suitable for specific applications, fundamental research must be carried out using complementary experimental methods to know the physicochemical properties, structure and dynamics of the compound,” says Dr. Eng. Anna Drzewicz of the Institute of Nuclear Physics of the Polish Academy of Sciences (IFJ PAN) in Krakow, and Dr Eng. Małgorzata Jasiurkowska-Delaporte (FIJ PAN) adds: “…and also to check if it crystallizes, or if it is able to form a glassy state.”
Crystallization is a complex process comprising nucleation (i.e. formation of nuclei) and crystal growth which is controlled by both kinetics (i.e. molecular mobility) and forces thermodynamic drivers. The phenomenon of crystallization is generally associated with the cooling of a given material. However, if the compound under study is cooled fast enough, it has a chance of vitrifying. The assumed cooling rate depends on the nucleation rate, which is an individual characteristic of the substance. The glassy state is related to the slowing (or even freezing) of stochastic motions.
“The game of ‘Statues’ comes to mind here (it has a different name in different regions). The participants move freely, but on the command of the leader, they must immediately remain still in the pose they have just performed,” says Dr. Drzewicz.
The liquid crystal 3F5HPhH7, which is the subject of the research, was obtained at the Chemical Institute of the Military University of Technology in Warsaw. IFJ PAN scientists found that it forms several chiral smectic phases (SmC*, SmCA* and SmXA*). Slow cooling of the sample resulted in its crystallization, and on rapid cooling it showed a greater tendency to vitrify into the SmXA* phase than into a crystalline form. In addition, as a result of heating the sample (after its rapid cooling), the phenomenon of so-called cold crystallization occurred. This type of crystallization appears during the heating of a previously vitrified disordered thermodynamic state. The detailed characteristics of the 3F5HPhH7 liquid crystal can be found in the article published in the prestigious physico-chemical journal Physical Chemistry Chemical Physics.
In the next step, the IFJ PAN scientists took a closer look at the two crystallization processes of the compound 3F5HPhH7.
“We wanted to check the kinetics of crystallization during both cooling and heating of the compound and determine if we are able to control or modify these processes by the rate of temperature change,” says Dr. Jasiurkowska-Delaporte.
Differential Scanning Calorimetry (DSC), Broadband Dielectric Spectroscopy (BDS) and Infrared Spectroscopy (FTIR) methods were used to follow crystallization kinetics. These techniques very precisely measure the response of a sample to the action of various external factors, providing additional information on its state and the processes in progress, necessary for understanding the phenomena. The results, recently published in the journal CrystEngComm, shed new light on the mechanisms underlying 3F5HPhH7 crystallization processes.
Avrami’s model was used to analyze crystallization kinetics. The results obtained showed that the crystallization during the cooling of the sample is controlled by the nucleation and is carried out by the three-dimensional growth of the crystallites (the dimensions of the growing crystallites are indicated by the value of the so-called Avrami parameter).
However, analysis of cold crystallization kinetics yielded surprising results. Under non-isothermal conditions, the process depends on the heating rate of the sample. When the temperature increases slowly, cold crystallization depends mainly on molecular diffusion and, under conditions of rapid heating, on nucleation. The speed of temperature changes was found to be significant also in the case of cold crystallization tested under isothermal conditions. When the sample is rapidly heated to the cold crystallization temperature, a two-step process emerges, with each step having a different dimensionality of the resulting crystallites. Slow heating of the compound changes the nature of cold crystallization from a two-step process to a one-step process.
Interestingly, the two-step and one-step isothermal cold crystallization processes are mainly controlled by the diffusion rate. This contrasts with the faster crystallization in the molten state which is solely governed by thermodynamics.
“We are satisfied that all the experimental methods used provided consistent information, which allowed us to recreate the course of crystallization processes in detail and proved the correctness of using complementary methods to study liquid crystals” , summarizes Dr. Drzewicz.
“The compound 3F5HPhH7 shows how diverse the crystallization process can be and how it depends on the rate of temperature change. This knowledge is perfectly in line with the current challenges of condensed-phase physics and in particular of soft matter,” emphasizes Dr. Jasiurkowska-Delaporte.
The results obtained are of great value but they do not exhaust the subject. The in-depth understanding of the possibilities of controlling the crystallization process of vitrifying materials is a stimulus for continued research, also in the context of potential applications.
The Henryk Niewodniczański Institute of Nuclear Physics (IFJ PAN) is currently one of the largest research institutes of the Polish Academy of Sciences. A wide range of research carried out at IFJ PAN covers fundamental and applied studies, from particle physics and astrophysics, to hadron physics, high, medium and low energy nuclear physics, from condensed matter (including materials engineering), to various applications of nuclear physics in interdisciplinary research, covering medical physics, dosimetry, radiology and environmental biology, environmental protection and other related disciplines. IFJ PAN’s average annual publication output includes more than 600 scientific articles in high-impact international journals. Each year, the Institute hosts around twenty international and national scientific conferences. One of the most important facilities of the Institute is the Cyclotron Center Bronowice (CCB), which is a unique infrastructure in Central Europe, serving as a clinical and research center in the field of medical and nuclear physics. In addition, IFJ PAN operates four accredited research and measurement laboratories. IFJ PAN is a member of the Marian Smoluchowski Kraków: “Matter-Energy-Future” research consortium, which in the years 2012-2017 enjoyed the status of Leading National Research Center (KNOW) in physics. In 2017, the European Commission awarded the Institute the HR Excellence in Research award. The Institute holds category A+ (the highest scientific category in Poland) in the field of science and engineering.
Dr. Eng. Anna Drzewicz
Institute of Nuclear Physics, Polish Academy of Sciences
Phone. : +48 12 662 8063
Email: [email protected]
Dr. Eng. Małgorzata Jasiurkowska-Delaporte
Institute of Nuclear Physics, Polish Academy of Sciences
Phone. : +48 12 662 8481
email: [email protected]
SCIENTIFIC PUBLICATIONS :
“On relaxation and vibrational dynamics in the thermodynamic states of a chiral smectogenic glass former”
A. Drzewicz, M. Jasiurkowska-Delaporte, E. Juszyńska-Gałązka, A. Deptuch, M. Gałązka, W. Zając, W. Drzewiński
Physico-chemistry Physics-chemistry, 24, 4595-4612, 2022
“Overview of cold and melt crystallization phenomena of a smectogenic liquid crystal”
A. Drzewicz, E. Juszyńska-Gałązka, M. Jasiurkowska-Delaporte, P. Kula
CristEngComm, 24, 3074-3087, 2022
The website of the Institute of Nuclear Physics of the Polish Academy of Sciences.
Press releases from the Institute of Nuclear Physics, Polish Academy of Sciences.
A better understanding of the complex crystallization processes occurring in liquid crystals brings us closer to a new generation of liquid crystal displays. (Source: IJF PAN)
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Overview of cold and melt crystallization phenomena of a smectogenic liquid crystal
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