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19 pages, 3017 KiB

Open AccessArticle

Molecular Dynamics and Near-T_g Phenomena of Cyclic Thioethers

by Hubert Hellwig, Andrzej Nowok, Paulina Peksa, Mateusz Dulski, Robert Musioł, Sebastian Pawlus and Piotr Kuś

Int. J. Mol. Sci. 2023, 24(24), 17166; https://doi.org/10.3390/ijms242417166 - 06 Dec 2023

Cited by 1 | Viewed by 589

Abstract

This article presents the synthesis and molecular dynamics investigation of three novel cyclic thioethers: 2,3-(4′-methylbenzo)-1,4,7,10-tetrathiacyclododeca-2-ene (compound 1), 2,3,14,15-bis(4′,4″(5″)-methylbenzo)-1,4,7,10,13,16,19,22,25-octathiacyclotetracosa-2,14-diene (compound 2), and 2,3,8,9-bis(4′,4″(5″)-methylbenzo)-1,4,7,10-tetrathiacyclododeca-2,8-diene (compound 3). The compounds exhibit relatively high glass transition temperatures (T_g), which range between 254 [...] Read more.

This article presents the synthesis and molecular dynamics investigation of three novel cyclic thioethers: 2,3-(4′-methylbenzo)-1,4,7,10-tetrathiacyclododeca-2-ene (compound 1), 2,3,14,15-bis(4′,4″(5″)-methylbenzo)-1,4,7,10,13,16,19,22,25-octathiacyclotetracosa-2,14-diene (compound 2), and 2,3,8,9-bis(4′,4″(5″)-methylbenzo)-1,4,7,10-tetrathiacyclododeca-2,8-diene (compound 3). The compounds exhibit relatively high glass transition temperatures (T_g), which range between 254 and 283 K. This characteristic positions them within the so-far limited category of crown-like glass-formers. We demonstrate that cyclic thioethers may span both the realms of ordinary and sizeable molecular glass-formers, each featuring distinct physical properties. Furthermore, we show that the T_g follows a sublinear power law as a function of the molar mass within this class of compounds. We also reveal multiple dielectric relaxation processes of the novel cyclic thioethers. Above the T_g, their dielectric loss spectra are dominated by a structural relaxation, which originates from the cooperative reorientation of entire molecules and exhibits an excess wing on its high-frequency slope. This feature has been attributed to the Johari–Goldstein (JG) process. Each investigated compound exhibits also at least one intramolecular secondary non-JG relaxation stemming from conformational changes. Their activation energies range from approximately 19 kJ/mol to roughly 40 kJ/mol. Finally, we analyze the high-pressure molecular dynamics of compound 1, revealing a pressure-induced increase in its T_g with a dT_g/dp coefficient equal to 197 ± 8 K/GPa. Full article

(This article belongs to the Special Issue Glass Transition and Related Phenomena 2.0)

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22 pages, 7495 KiB

Open AccessArticle

Effects of Segment Length and Crosslinking via POSS on the Calorimetric and Dynamic Glass Transition of Polyurethanes with Aliphatic Hard Segments

by Konstantinos N. Raftopoulos, Panagiotis A. Klonos, Patrycja Tworzydło, Jan Ozimek, Edyta Hebda, Apostolos Kyritsis and Krzysztof Pielichowski

Int. J. Mol. Sci. 2023, 24(22), 16540; https://doi.org/10.3390/ijms242216540 - 20 Nov 2023

Viewed by 899

Abstract

The glass transition in polyurethanes is a complicated phenomenon governed by a multitude of factors, including the microphase separation, which in turn depends strongly on the molar mass of the hard and soft segments, as well as the presence of additives. In this [...] Read more.

The glass transition in polyurethanes is a complicated phenomenon governed by a multitude of factors, including the microphase separation, which in turn depends strongly on the molar mass of the hard and soft segments, as well as the presence of additives. In this work, we study the effects of the segments’ length on the microphase separation and consequently on the calorimetric and dynamic glass transition of a polyurethane with aliphatic, “flexible” hard segments. It is found that the dependence of the calorimetric glass transition follows the same principles as those in systems with aromatic hard segments. Strikingly, however, the dynamic glass transition, as studied by dielectric spectroscopy, shows a slowing down of its dynamics despite a decrease in

T_{g}

. This discrepancy is discussed in terms of the strong dielectric response of the flexible segments, especially those close to the interface between the hard domains and soft phase, as opposed to a weak thermal one. In addition, polyhedral oligomeric silsesquioxanes (POSS) are introduced in the soft phase of the three matrices as crosslinking centres. This modification has no visible effect on the calorimetric glass transition; nevertheless, it affects the microphase separation and the dielectric response in a non-monotonic manner. Full article

(This article belongs to the Special Issue Glass Transition and Related Phenomena 2.0)

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19 pages, 4483 KiB

Open AccessArticle

How the Presence of Crystalline Phase Affects Structural Relaxation in Molecular Liquids: The Case of Amorphous Indomethacin

by Roman Svoboda, Marek Pakosta and Petr Doležel

Int. J. Mol. Sci. 2023, 24(22), 16275; https://doi.org/10.3390/ijms242216275 - 13 Nov 2023

Viewed by 684

Abstract

The influence of partial crystallinity on the structural relaxation behavior of low-molecular organic glasses is, contrary to, e.g., polymeric materials, a largely unexplored territory. In the present study, differential scanning calorimetry was used to prepare a series of amorphous indomethacin powders crystallized to [...] Read more.

The influence of partial crystallinity on the structural relaxation behavior of low-molecular organic glasses is, contrary to, e.g., polymeric materials, a largely unexplored territory. In the present study, differential scanning calorimetry was used to prepare a series of amorphous indomethacin powders crystallized to various extents. The preparations stemmed from the two distinct particle size fractions: 50–125 µm and 300–500 µm. The structural relaxation data from the cyclic calorimetric measurements were described in terms of the phenomenological Tool–Narayanaswamy–Moynihan model. For the 300–500 µm powder, the crystalline phase forming dominantly on the surface led to a monotonous decrease in the glass transition by ~6 °C in the 0–70% crystallinity range. The activation energy of the relaxation motions and the degree of heterogeneity within the relaxing matrix were not influenced by the increasing crystallinity, while the interconnectivity slightly increased. This behavior was attributed to the release of the quenched-in stresses and to the consequent slight increase in the structural interconnectivity. For the 50–125 µm powder, distinctly different relaxation dynamics were observed. This leads to a conclusion that the crystalline phase grows throughout the bulk glassy matrix along the internal micro-cracks. At higher crystallinity, a sharp increase in T_g, an increase in interconnectivity, and an increase in the variability of structural units engaged in the relaxation motions were observed. Full article

(This article belongs to the Special Issue Glass Transition and Related Phenomena 2.0)

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18 pages, 3833 KiB

Open AccessArticle

Thermodynamic and Dynamic Transitions and Interaction Aspects in Reorientation Dynamics of Molecular Probe in Organic Compounds: A Series of 1-alkanols with TEMPO

by Josef Bartoš and Helena Švajdlenková

Int. J. Mol. Sci. 2023, 24(18), 14252; https://doi.org/10.3390/ijms241814252 - 18 Sep 2023

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Abstract

The spectral and dynamic properties of 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) in a series of 1-alkanols ranging from methanol to 1-decanol over a temperature range 100–300 K were investigated by electron spin resonance (ESR). The main characteristic ESR temperatures connected with slow to fast motion [...] Read more.

The spectral and dynamic properties of 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) in a series of 1-alkanols ranging from methanol to 1-decanol over a temperature range 100–300 K were investigated by electron spin resonance (ESR). The main characteristic ESR temperatures connected with slow to fast motion regime transition; T_50G ‘s and T_X1^fast ‘s are situated above the corresponding glass temperatures, T_g, and for the shorter members, the T_50G ‘s lie above or close to melting point, T_m, while the longer ones the T_50G < T_m relationship indicates that the TEMPO molecules are in the local disordered regions of the crystalline media. The T_50G ‘s and especially T_X1^fast ‘s are compared with the dynamic crossover temperatures, T_X^VISC = 8.72M^0.66, as obtained by fitting the viscosity data in the liquid n-alkanols with the empirical power law. In particular, for N_C > 6, the T_X1^fast ‘s lie rather close to the T_X^VISC resembling apolar n-alkanes [PCCP 2018,20,11145-11151], while for N_C < 6, they are situated in the vicinity of T_m. The absence of a coincidence for lower1-alkanols indicates that the T_50G is significantly influenced by the mutual interaction between the polar TEMPO and the protic polar medium due to the increased polarity and proticity destroyed by the larger-scale melting transition. Full article

(This article belongs to the Special Issue Glass Transition and Related Phenomena 2.0)

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15 pages, 2210 KiB

Open AccessArticle

The Flow of Glasses and Glass–Liquid Transition under Electron Irradiation

by Michael I. Ojovan

Int. J. Mol. Sci. 2023, 24(15), 12120; https://doi.org/10.3390/ijms241512120 - 28 Jul 2023

Cited by 4 | Viewed by 781

Abstract

Recent discovery and investigation of the flow of glasses under the electron beams of transmission electron microscopes raised the question of eventual occurrence of such type effects in the vitrified highly radioactive nuclear waste (HLW). In connection to this, we analyse here the [...] Read more.

Recent discovery and investigation of the flow of glasses under the electron beams of transmission electron microscopes raised the question of eventual occurrence of such type effects in the vitrified highly radioactive nuclear waste (HLW). In connection to this, we analyse here the flow of glasses and glass–liquid transition in conditions of continuous electron irradiation such as under the e-beam of transmission electron microscopes (TEM) utilising the configuron (broken chemical bond) concept and configuron percolation theory (CPT) methods. It is shown that in such conditions, the fluidity of glasses always increases with a substantial decrease in activation energy of flow at low temperatures and that the main parameter that controls this behaviour is the dose rate of absorbed radiation in the glass. It is revealed that at high dose rates, the temperature of glass–liquid transition sharply drops, and the glass is fully fluidised. Numerical estimations show that the dose rates of TEM e-beams where the silicate glasses were fluidised are many orders of magnitude higher compared to the dose rates characteristic for currently vitrified HLW. Full article

(This article belongs to the Special Issue Glass Transition and Related Phenomena 2.0)

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9 pages, 3078 KiB

Open AccessArticle

Vortex Glass—Vortex Liquid Transition in BaFe₂(As_1-xP_x)₂ and CaKFe₄As₄ Superconductors from Multi-Harmonic AC Magnetic Susceptibility Studies

by Ion Ivan, Alina M. Ionescu, Daniel N. Crisan and Adrian Crisan

Int. J. Mol. Sci. 2023, 24(9), 7896; https://doi.org/10.3390/ijms24097896 - 26 Apr 2023

Cited by 1 | Viewed by 1026

Abstract

For practical applications of superconductors, understanding the vortex matter and dynamics is of paramount importance. An important issue in this context is the transition of the vortex glass, which is a true superconducting phase, into a vortex liquid phase having a linear dissipation. [...] Read more.

For practical applications of superconductors, understanding the vortex matter and dynamics is of paramount importance. An important issue in this context is the transition of the vortex glass, which is a true superconducting phase, into a vortex liquid phase having a linear dissipation. By using multi-harmonic susceptibility studies, we have investigated the vortex glass—vortex liquid phase transitions in CaKFe₄As₄ and BaFe₂(As_0.68P_0.32)₂ single crystals. The principle of our method relates the on-set of the third-harmonic susceptibility response with the appearance of a vortex-glass phase in which the dissipation is non-linear. Similar to the high-critical temperature cuprate superconductors, we have shown that even in these iron-based superconductors with significant lower critical temperatures, such phase transition can be treated as a melting in the sense of Lindemann’s approach, considering an anisotropic Ginzburg-Landau model. The experimental data are consistent with a temperature-dependent London penetration depth given by a 3D XY fluctuations model. The fitting parameters allowed us to extrapolate the vortex melting lines down to the temperature of liquid hydrogen, and such extrapolation showed that CaKFe₄As₄ is a very promising superconducting material for high field applications in liquid hydrogen, with a melting field at 20 K of the order of 100 T. Full article

(This article belongs to the Special Issue Glass Transition and Related Phenomena 2.0)

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11 pages, 1115 KiB

Open AccessArticle

Universal Evolution of Fickian Non-Gaussian Diffusion in Two- and Three-Dimensional Glass-Forming Liquids

by Francesco Rusciano, Raffaele Pastore and Francesco Greco

Int. J. Mol. Sci. 2023, 24(9), 7871; https://doi.org/10.3390/ijms24097871 - 26 Apr 2023

Cited by 4 | Viewed by 1045

Abstract

Recent works show that glass-forming liquids display Fickian non-Gaussian Diffusion, with non-Gaussian displacement distributions persisting even at very long times, when linearity in the mean square displacement (Fickianity) has already been attained. Such non-Gaussian deviations temporarily exhibit distinctive exponential tails, with a decay [...] Read more.

Recent works show that glass-forming liquids display Fickian non-Gaussian Diffusion, with non-Gaussian displacement distributions persisting even at very long times, when linearity in the mean square displacement (Fickianity) has already been attained. Such non-Gaussian deviations temporarily exhibit distinctive exponential tails, with a decay length

λ

growing in time as a power-law. We herein carefully examine data from four different glass-forming systems with isotropic interactions, both in two and three dimensions, namely, three numerical models of molecular liquids and one experimentally investigated colloidal suspension. Drawing on the identification of a proper time range for reliable exponential fits, we find that a scaling law

λ (t) \propto t^{α}

, with

α ≃ 1 / 3

, holds for all considered systems, independently from dimensionality. We further show that, for each system, data at different temperatures/concentration can be collapsed onto a master-curve, identifying a characteristic time for the disappearance of exponential tails and the recovery of Gaussianity. We find that such characteristic time is always related through a power-law to the onset time of Fickianity. The present findings suggest that FnGD in glass-formers may be characterized by a “universal” evolution of the distribution tails, independent from system dimensionality, at least for liquids with isotropic potential. Full article

(This article belongs to the Special Issue Glass Transition and Related Phenomena 2.0)

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15 pages, 4011 KiB

Open AccessArticle

Time–Temperature Superposition Principle in Shearing Tests Compared to Tension Conditions for Polymers Close to Glass Transition

by Noëlle Billon, Carlos Eloy Federico, Guilhem Rival, Jean Luc Bouvard and Alain Burr

Int. J. Mol. Sci. 2023, 24(4), 3944; https://doi.org/10.3390/ijms24043944 - 15 Feb 2023

Cited by 2 | Viewed by 1303

Abstract

The well-known principle of time–temperature superposition (TTS) is of prime interest for polymers close to their glass transition. First demonstrated in the range of linear viscoelasticity, it has been more recently extended to large deformations in tension. However, shear tests were not yet [...] Read more.

The well-known principle of time–temperature superposition (TTS) is of prime interest for polymers close to their glass transition. First demonstrated in the range of linear viscoelasticity, it has been more recently extended to large deformations in tension. However, shear tests were not yet addressed. The present study depicted TTS in shearing conditions and compared it to results in tensile conditions both for low and high strains for a polymethylmethacrylate (PMMA) of different molar masses. The main objectives were to enlighten the relevance of the principle of time–temperature superposition for shearing at high strain and to discuss the way shift factors should be determined. It was suggested that shift factors could be dependent on compressibility, which should be taken into account when addressing various types of complex mechanical loadings. Full article

(This article belongs to the Special Issue Glass Transition and Related Phenomena 2.0)

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Review

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21 pages, 3626 KiB

Open AccessReview

Dielectric Spectroscopy Studies of Conformational Relaxation Dynamics in Molecular Glass-Forming Liquids

by Michela Romanini, Roberto Macovez, Sofia Valenti, Wahi Noor and Josep Lluís Tamarit

Int. J. Mol. Sci. 2023, 24(24), 17189; https://doi.org/10.3390/ijms242417189 - 06 Dec 2023

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Abstract

We review experimental results obtained with broadband dielectric spectroscopy concerning the relaxation times and activation energies of intramolecular conformational relaxation processes in small-molecule glass-formers. Such processes are due to the interconversion between different conformers of relatively flexible molecules, and generally involve conformational changes [...] Read more.

We review experimental results obtained with broadband dielectric spectroscopy concerning the relaxation times and activation energies of intramolecular conformational relaxation processes in small-molecule glass-formers. Such processes are due to the interconversion between different conformers of relatively flexible molecules, and generally involve conformational changes of flexible chain or ring moieties, or else the rigid rotation of planar groups, such as conjugated phenyl rings. Comparative analysis of molecules possessing the same (type of) functional group is carried out in order to test the possibility of assigning the dynamic conformational isomerism of given families of organic compounds to the motion of specific molecular subunits. These range from terminal halomethyl and acetyl/acetoxy groups to both rigid and flexible ring structures, such as the planar halobenzene cycles or the buckled saccharide and diazepine rings. A short section on polyesters provides a generalisation of these findings to synthetic macromolecules. Full article

(This article belongs to the Special Issue Glass Transition and Related Phenomena 2.0)

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