Influence of fragility and molecular symmetry on the formation of stable glasses

The successful formation of vapor-deposited glasses with high kinetic stability of ethylcyclohexane, a strong glass former of m=60, and tetrachloromethane, with a pseudo isotropic molecular structure, indicates that fragility and molecular asymmetry are not prerequisites for stable glass formation. The AC chip nanocalorimetry was also improved with laser modulation reaching frequency 1MHz, allowing determination of the dynamic glass transition in a frequency range of 11 orders of magnitude together with measurements from four different temperature modulated differential scanning calorimeters

New experimental melting properties as access for predicting amino-acid solubility

The properties of melting are required for the prediction of solubility of solid compounds. Unfortunately, direct determination of the enthalpy of fusion and melting temperature by using conventional DSC or adiabatic calorimetry is often not possible for biological compounds due to decomposition during the measurement. To overcome this, fast scanning calorimetry (FSC) with scanning rates up to 2 × 104 K s−1 was used in this work to measure the melting parameters for L-alanine and glycine. The enthalpy of fusion and melting temperature (extrapolated to zero heating rate) were ΔfusH = (22 ± 5) kJ mol−1 and Tfus = (608 ± 9) K for L-alanine, and ΔfusH = (21 ± 4) kJ mol−1 and Tfus = (569 ± 7) K for glycine. These melting properties were used in the modeling framework PC-SAFT to predict amino-acid solubility in water. The pure-component PC-SAFT parameters and one binary parameter were taken from literature, in which these parameters were fitted to solubility-independent thermodynamic properties such as osmotic coefficients or mixture densities. It was shown that this allowed accurately predicting amino-acid solubility in water over a broad temperature range. The combined methodology of PC-SAFT and FSC proposed in this work opens the door for predicting solubility of molecules that decompose before melting

Unravelling the nature of citric acid:l-arginine:water mixtures: The bifunctional role of water

This project has received funding from the European Union's Horizon 2020 (European Research Council) under grant agreement no. ERC-2016-CoG 725034. This work was supported by the Associate Laboratory for Green Chemistry-LAQV which is financed by national funds from FCT/MCTES (UID/QUI/50006/2019). Further, C. H. and Y. Z. C. thank German Science Foundation (DFG) for funding (HE 7165/6-1 and CH 1922/1-1).The use of water as a component of deep eutectic systems (DES) has raised some questions regarding its influence on the nature of the mixture. Does it form a DES or an aqueous solution and what is the role of water? In this work, the nature of citric acid:l-arginine:water mixtures was explored through phase equilibria studies and spectroscopic analysis. In a first step, PC-SAFT was validated as a predictive tool to model the water influence on the solid liquid equilibria (SLE) of the DES reline using the individual-component approach. Hence, activity coefficients in the ternary systems citric acid:l-arginine:water and respective binary combinations were studied and compared using ePC-SAFT. It was observed that the water-free mixtures citric acid:l-arginine showed positive deviation from Raoult's law, while upon addition of water strong negative deviation from Raoult's law was found, yielding melting depressions around 100 K. Besides these strong interactions, pH was found to become acidic (pH = 3.5) upon water addition, which yields the formation of charged species ([H2Cit]- and [l-arg]+). Thus, the increased interactions between the molecules upon water addition might be caused by several mechanisms such as hydrogen bonding or ionic forces, both being induced by water. For further investigation, the liquid mixtures citric acid:l-arginine:water were studied by FTIR and NMR spectroscopy. FTIR spectra disproved a possible solubility enhancement caused by salt formation between citric acid and l-arginine, while NMR spectra supported the formation of a hydrogen bonding network different from the binary systems citric acid:water and l-arginine:water. Either being a DES or other type of non-ideal solution, the liquefaction of the studied systems is certainly caused by a water-mediator effect based on the formation of charged species and cross interactions between the mixture constituents. This journal ispublishersversionpublishe

Glass transition cooperativity from broad band heat capacity spectroscopy

Molecular dynamics is often studied by broad band dielectric spectroscopy (BDS) because of the wide dynamic range available and the large number of processes resulting in electrical dipole fluctuations and with that in a dielectrically detectable relaxation process. Calorimetry on the other hand is an effective analytical tool to characterize phase and glass transitions by its signatures in heat capacity. In the linear response scheme, heat capacity is considered as entropy compliance. Consequently, only processes significantly contributing to entropy fluctuations appear in calorimetric curves. The glass relaxation is a prominent example for such a process. Here, we present complex heat capacity at the dynamic glass transition (segmental relaxation) of polystyrene (PS) and poly(methyl methacrylate) (PMMA) in a dynamic range of 11 orders of magnitude, which is comparable to BDS. As one of the results, we determined the characteristic length scale of the corresponding fluctuations. The dynamic glass transition measured by calorimetry is finally compared to the cooling rate dependence of fictive temperature and BDS data. For PS, dielectric and calorimetric data are similar but for PMMA with its very strong secondary relaxation process some peculiarities are observed

Determination of Cooperativity Length in a Glass-Forming Polymer

To describe the properties of glass-forming liquids, the concepts of a cooperativity length or the size of cooperatively rearranging regions are widely employed. Their knowledge is of outstanding importance for the understanding of both thermodynamic and kinetic properties of the systems under consideration and the mechanisms of crystallization processes. By this reason, methods of experimental determination of this quantity are of outstanding importance. Proceeding in this direction, we determine the so-called cooperativity number and, based on it, the cooperativity length by experimental measurements utilizing AC calorimetry and quasi-elastic neutron scattering (QENS) at comparable times. The results obtained are different in dependence on whether temperature fluctuations in the considered nanoscale subsystems are either accounted for or neglected in the theoretical treatment. It is still an open question, which of these mutually exclusive approaches is the correct one. As shown in the present paper on the example of poly(ethyl methacrylate) (PEMA), the cooperative length of about 1 nm at 400 K and a characteristic time of ca. 2 μs determined from QENS coincide most consistently with the cooperativity length determined from AC calorimetry measurements if the effect of temperature fluctuations is incorporated in the description. This conclusion indicates thataccounting for temperature fluctuationsthe characteristic length can be derived by thermodynamic considerations from the specific parameters of the liquid at the glass transition and that temperature does fluctuate in small subsystems

First Clear-Cut Experimental Evidence of a Glass Transition in a Polymer with Intrinsic Microporosity: PIM‑1

Polymers with intrinsic microporosity (PIMs) represent a novel, innovative class of materials with great potential in various applications from high-performance gas-separation membranes to electronic devices. Here, for the first time, for PIM-1, as the archetypal PIM, fast scanning calorimetry provides definitive evidence of a glass transition (<i>T</i>_g = 715 K, heating rate 3 × 10⁴ K/s) by decoupling the time scales responsible for glass transition and decomposition. Because the rigid molecular structure of PIM-1 prevents any conformational changes, small-scale bend and flex fluctuations must be considered the origin of its glass transition. This result has strong implications for the fundamental understanding of the glass transition and for the physical aging of PIMs and other complex polymers, both topical problems of materials science