Melting Point Transition Research Articles

Correlation of Structure with Crystalline to Amorphous Phase Transitions of 1,3,6-Substituted Fulvene-Derived Molecular Glasses.

An investigation into the crystalline to amorphous phase transitions of prepared 1,3,6-substituted pentafulvenes showed the expected reversible heated melt and cooling recrystallization in only a few examples. Systematic incorporation of bulky substituents at the 6-position of the fulvene ring led to the nonreversible thermal behavior, rendering phases that were locked into glassy, vitrified states. These molecular glasses produced physically translucent and amorphous features with glass transition temperatures in the range of 61-77 °C, comparable with high-strength plastics such as polyethylene terephthalate. Additionally, the melting point transitions and the resulting heat of fusion values were found to be directly influenced by the nature of the 6-position substituent. Single crystal X-ray crystallography showed that in some cases, fulvenes possessing fused aromatics exhibited a high degree of intermolecular π-π stacking. These results point to a class of molecular glass formers as host materials possessing tunable bulk properties for potentially new optical applications.

Identifying structural changes with unsupervised machine learning methods

Unsupervised machine learning methods are used to identify structural changes using the melting point transition in classical molecular dynamics simulations as an example application of the approach. Dimensionality reduction and clustering methods are applied to instantaneous radial distributions of atomic configurations from classical molecular dynamics simulations of metallic systems over a large temperature range. Principal component analysis is used to dramatically reduce the dimensionality of the feature space across the samples using an orthogonal linear transformation that preserves the statistical variance of the data under the condition that the new feature space is linearly independent. From there, k-means clustering is used to partition the samples into solid and liquid phases through a criterion motivated by the geometry of the reduced feature space of the samples, allowing for an estimation of the melting point transition. This pattern criterion is conceptually similar to how humans interpret the data but with far greater throughput, as the shapes of the radial distributions are different for each phase and easily distinguishable by humans. The transition temperature estimates derived from this machine learning approach produce comparable results to other methods on similarly small system sizes. These results show that machine learning approaches can be applied to structural changes in physical systems.

Thermal and viscoelastic properties of entangled supramolecular polymer networks as a powerful tool for prediction of their microstructure

Thermal and viscoelastic properties of entangled supramolecular polymer networks, SPNs, depend strongly on binary and collective assembly of associative groups. The collective assemblies can phase separate from polymer matrix chains and form domains with different sizes and shapes, which have different melting point transitions. By increasing content of associative groups along the polymer chains, their high-order association leads to formation of domains, which have higher melting temperatures than other ones. We prepared a SPN system that contains three networks. All networks have similar precursor polymer backbone, but different content of ureidopyrimidinone, UPy, moiety as strong hydrogen bonding associative group. Thermal and viscoelastic properties of these networks were studied by differential scanning calorimetry and dynamic mechanical thermal analysis measurements. Binary-associated UPy groups phase separate into collective assemblies by π–π stacking. These stacks melt at different temperatures upon heating. The presence of transient assemblies is considerably hinder the relaxation of polymer backbone chains. These domains can be observed in the microstructure of networks by optical microscopy. Thermal and viscoelastic properties of sample with highest content of UPy groups are significantly different from the other networks. This is related to the high-order association of UPy motifs, as can be captured by presence of needle-like domains in its microstructure.

Temperature-Dependent Excited State Absorption in DNA and LNA Oligomers Supports an Emerging Model of Excited State Dynamics in DNA

Transient absorption measurements of excited states in DNA and LNA were performed using a femtosecond pump-probe arrangement with excitation at 266 nm and absorption monitored at 400 nm while varying the sample tem- perature between 5 °C and 70 °C. Samples consisted of adenine monophosphate monomer, polyadenine 12-mer in single- stranded form, and polyadenine 12-mer in hybridized form. Excited states decayed in a biphasic manner with short-lived ( 1) and long-lived ( 2) components, while the monomer had only a 'single' short-lived decay time. Temperature increases increased absorption intensities and reduced  1 until they approached those of the monomer at high temperatures (where stacking is minimal). These results suggest that the initial excitation in stacked regions is cooperative and involves several bases and that the number of bases involved is reduced with increasing temperature. In contrast, increasing temperatures had little effect on  2 while absorption intensities decreased, suggesting that very few, perhaps only two, stacked bases are involved and that their number is reduced at higher temperatures. We found no clear evidence of melting point transitions indicating that those excited states probed with our arrangement were not dependent on base pairing. Our results are con- sistent with and strengthen an emerging consensus model of excited state dynamics in DNA wherein a UV photon is ab- sorbed collectively by electronically coupled and thus well-stacked intrachain bases. This collective excitation results in a Frenkel exciton that is delocalized over these bases, and the Frenkel exciton then decays rapidly to a long-lived, lower en- ergy, dark intrachain exciplex.

A microfluidic device with a linear temperature gradient for parallel and combinatorial measurements.

Methods for obtaining combinatorial and array-based data as a function of temperature are needed in the chemical and biological sciences. It is presently quite difficult to employ temperature as a variable using standard wellplate formats simply because it is very inconvenient to keep each well at a distinct temperature. In microfluidics, however, the situation is very different due to the short length scales involved. In this article, it is shown how a simple linear temperature gradient can be generated across dozens of parallel microfluidic channels simultaneously. This result is exploited to rapidly obtain activation energies from catalytic reactions, melting point transitions from lipid membranes, and fluorescence quantum yield curves from semiconductor nanocrystal probes as a function of temperature. The methods developed here could quite easily be extended to protein crystallization, phase diagram measurements, chemical reaction optimization, or multivariable experiments.

New cermet solar coatings for solar thermal electricity applications

On the basis of successful research into new metal–aluminium nitride (M–AlN) cermet solar selective coatings, a commercial-scale cylindrical direct current (dc) magnetron sputter coater has been developed for the deposition of these cermet solar coatings onto batches of tubes. The coater vacuum chamber has a diameter of 0.8 m and is 1.8 m in height. 32 glass tubes may be accommodated in the chamber. Stainless steel–aluminium nitride (SS–AlN) cermet material has been investigated as a solar absorbing layer. Three cylindrical post cathodes, consisting of stainless steel, aluminium and copper tubes, are installed in the chamber. During the deposition of an SS–AlN cermet layer, both the Al and SS targets are sputtered simultaneously in the gas mixture of argon and nitrogen. The nitrogen partial pressure is set sufficiently high at the aluminium target side to ensure that a nearly pure AlN ceramic sublayer is deposited by dc reactive sputtering. The planetary rotation of the 32 tubes results in a multi-layered system consisting of alternating SS and AlN sublayers. This multi-sublayer system can be considered as a macro-homogeneous cermet layer. Varying SS target current achieves different SS volume fractions in the cermet layers. A solar absorptance of 0.94–0.95 and emittance of 0.04–0.05 at room temperature has been achieved for the SS–AlN cermet solar coatings. These solar tubes are stable at 330–400°C. Using this commercial-scale coater, it should be possible to produce solar tubes stable at even higher temperatures (450–550°C), based on a high melting point transition metal–aluminium nitride cermet.

Modification of poly(vinyl alcohol) with aliphatic and aromatic epoxides in the molten phase

A new method for improving the processability and thermal stability of commercially available poly(vinyl alcohol) is presented involving the reaction of the hydroxyl group on the polymer backbone with long-chain aliphatic, cycloaliphatic, and aromatic epoxides. The reaction was performed in the molten phase using a laboratory-scale thermostated reactor. As expected, the extent of the reaction varied with the chemical structure of the epoxide, the properties of polymers obtained being dependent on the amount of the incorporated side group. Specifically, the reactivity of long aliphatic chain epoxides was low and the polymers obtained exhibited a small decrease in the melting points, being directly proportional to the length of the aliphatic chain. They displayed, however, improvement in thermal stability compared to the parent polymer. Cyclohexene oxide was appreciably more reactive and it exhibited a larger melting point reduction and satisfactory thermal stability. Polymers functionalized with aromatic rings and prepared under the same conditions were mostly amorphous, not showing melting point transition or improvement in their thermal stability. Finally, the reactions with aliphatic epoxides were catalyzed with phosphoric acid and the modified polymer exhibited a large decrease in the melting point but not a concomitant improvement in thermal stability.

Anharmonic Oscillations and Tg, Tm Transitions

A working hypothesis has been developed to account for a change in character of thermal motion at glass transition. According to this hypothesis the pronounced onset of anharmonic vibrations is responsible for a stepwise increase in the thermal expansion coefficient alpha as well as in the similar temperature change in specific heat coefficient cp. In this paper the both transitions (the first order transition at melting point and the second order transition at Tg) are investigated on the basis of the viewpoint connected with the change in characteristics of motion of the particles, so typical for an onset of a liquid state. At present, two different definitions for the coefficient of thermal expansion are used. One is usually adopted in polymer physics, where also the statistical approach to polymer chains configurations plays the major role in Tg definition. However, the statistical configurational approach of polymer physics cannot be applied directly to the inorganic glasses and also does not provide any explanation of relatively small changes in cp values at melting point transition which sometime occur. Using the interpretation of solid state physics, the present paper intends to make a first step and bridge over the gap between these two approaches and to explain the Tg transition in dynamic terms, common to the polymers as well as to the low molecular weight substances.

A compact electron-beam metal vapour source with integral cooling and shutter

A simple compact ultra-high-vacuum (UHV) electron-beam evaporator, to deposit ultra-thin layers of low vapour pressure, high melting point transition metals is described. It has an integral cryoshield and shutter which will fit a UHV system with a standard 70 mm flange.