Equilibrium Melting Temperature Research Articles

Effect of the application of low shear rates on the crystallization kinetics of PLA

Flow-induced crystallization can effectively increase the crystallinity content in poly(lactic acid) parts, and this can be extremely significant since an increase of crystallinity for this material dramatically enhances many properties among which the heat resistance. This work analyzes nucleation and growth rate of spherulites formed during crystallization process of PLA at different temperatures and shear rates. The application of a shear flow results in an enhancement of the nucleation phenomena, with the formation of a new population of nuclei, and an improvement of the crystals growth already at low values of shear rates. Furthermore, it is possible to find a dependence of the equilibrium melting temperature upon the Weissenberg number.

Recalescence dynamics and solidification of a supercooled melt in a finite domain

We study the dynamics of supercooled solidification of a pure material in a finite domain subject to isothermal boundary conditions. At early stages when the liquid can effectively be treated as semi-infinite, we derive asymptotic solutions in the limits of both strong and weak latent-heat release, corresponding to large and small effective Stefan numbers, respectively. In particular, the solutions describing a rapid recalescence followed by a gradual change in the interfacial temperature are derived. Once the finite extent becomes effective, the system relaxes to an intermediate stage. For large Stefan numbers, the intermediate stage is quasi-steady, with the linear temperature profiles in the two phases and the interface temperature close to an equilibrium melting temperature. For Stefan numbers less than unity, the intermediate stage has a traveling-wave temperature profile in the liquid, similar to that in the one-sided problem, and a self-similar profile in the solid, where the temperature is close to the interface temperature through the whole solid except for a thermal boundary layer far from the interface. The model is applied to water, copper and salol, providing estimates for the freezing rates, interface position, and the recalescence and complete-freezing times in these pure systems.

Connecting glass-forming ability of binary mixtures of soft particles to equilibrium melting temperatures

The glass-forming ability is an important material property for manufacturing glasses and understanding the long-standing glass transition problem. Because of the nonequilibrium nature, it is difficult to develop the theory for it. Here we report that the glass-forming ability of binary mixtures of soft particles is related to the equilibrium melting temperatures. Due to the distinction in particle size or stiffness, the two components in a mixture effectively feel different melting temperatures, leading to a melting temperature gap. By varying the particle size, stiffness, and composition over a wide range of pressures, we establish a comprehensive picture for the glass-forming ability, based on our finding of the direct link between the glass-forming ability and the melting temperature gap. Our study reveals and explains the pressure and interaction dependence of the glass-forming ability of model glass-formers, and suggests strategies to optimize the glass-forming ability via the manipulation of particle interactions.

On the nucleation of polylactide by melt-soluble oxalamide based organic compounds

In this work, a series of oxalamide based organic compounds (OBOC) are synthesized and their capability to enhance the nucleation of polylactide is reported. The OBOCs are soluble in the polylactide melt and crystallize upon cooling, providing surface for heterogeneous nucleation to the polylactide matrix. One interesting observation is that the nucleation efficiency of the OBOCs increases at high cooling rates, making the use of OBOCs as a nucleating agent attractive for industrial processing conditions. The paper addresses the mechanism involved in the cooling rate dependence on the nucleation efficiency of the OBOC: The nucleation efficiency of polylactide is significantly enhanced in the presence of OBOC crystals, as a transcrystalline PLA morphology grows from the OBOC crystal surface (at relatively low supercooling from the equilibrium melting temperature of PLA). However, such crystallization of PLA occurs only when the OBOC crystals are formed at or below 145 °C while cooling at 10 °C/min. In contrast, when the OBOC crystals are formed above 145 °C (i.e. from a lower supersaturated state), the OBOC crystals display a significantly reduced capability for PLA nucleation. These findings override the possibility of both epitaxy and soft-epitaxy as a plausible nucleation mechanism. Supported by polarized optical microscopy, differential scanning calorimetry, plate-plate rheology and molecular modelling we evaluate the possibility of nucleation resulting from local stresses imposed on the polylactide melt invoking stress-enhanced nucleation. The imposed local shear rates, facilitated by the rapid growth of the OBOC crystals, are found high enough to facilitate contour orientation of the high molecular weight PLA chains next to the growing OBOC crystals, confirming the possibility for stress-enhanced nucleation. In addition, we identify surface roughness of OBOC crystals as a second parameter that influences the PLA nucleation process; the OBOC crystallization at high supersaturation is expected to yield smaller/defected OBOC crystals, which are presumed to provide high surface area and surface roughness. In contrast, when the OBOC crystals are grown at lowered supersaturation (>150 °C), they are bound to anneal during crystallization, providing a smoother surface and lower surface area – retrospectively exhibiting a decreased capability to promote the PLA nucleation, irrespective of the PLA supercooling. Interestingly, both the surface roughness of OBOC crystals and the local stresses they impose on the PLA melt increase when the OBOC crystal growth proceeds from a highly supersaturated state, providing an explanation to the cause of the favored crystallization of PLA at the high cooling rates in the presence of the chosen OBOCs.

Influence of melt structure on Form II to I phase transition of Polybutene-1 under shear flow

For decades, the influences of chemical structures and compositions, blend components and ambient environments on phase transition of polybutene-1 (PB-1) from Form II to I have been reported. Nevertheless, in terms of polymer processing, how to expedite the phase transition with a facile way is still a challenging task. In this study, four times the phase transition rate can be achieved by simply extruding the near melting point melt (NMP melt) of PB-1 through a capillary die. The melting survived Form I nanocrystals within the amorphous melt at the partially melting temperature (Tp) between the nominal melting temperature (Tm) and the equilibrium melting temperature (Tm0) allow accelerating the phase transition which was investigated by means of in situ small/wide angle X-ray scattering (SAXS/WAXS). Upon extruding of NMP melt, Form II transformed even faster than in the quiescent NMP melt. The number of flow-induced oriented crystalline structures (cylindrites) increased by reducing Tp. Kinetically, the half-transformation time (τ1/2) increased with rising Tp. Furthermore, τ1/2 of the core layer is large compared to τ1/2 of the skin layer. Our results suggest that both melting survived Form I nanocrystals and shear flow play integral roles in accelerating the phase transition of PB-1. We tentatively concluded that the higher nucleation probability of Form I during phase transition can be generated by more tie molecules distributed in the oriented lamellae caused by shear flow and the introducing extra internal stresses depended on the mismatch in the thermal expansion coefficients of Form I nanocrystals and Form II lamellar crystals.

Study on compatible characteristics of wheat and purple sweet potato starches

In this study, the compatibility of purple sweet potato (C-type) and wheat starches (A-type) was characterised by measuring glass transition temperature, equilibrium melting temperature and interaction parameter. The glass transition temperature of mixed starch containing 40% and 60% moisture drifted toward the high temperature with the increase in purple sweet potato starch content. The glass transition temperature of all mixed starches has a single peak between wheat and purple sweet potato starches. The equilibrium melting point temperature of all samples containing 60% moisture was lower than those containing 40% moisture. Under the same water content, the equilibrium melting point temperature of mixed starch decreased slightly as compared with that of wheat starch and no significant difference was observed among the mixed starches. The interaction parameters of wheat and purple sweet potato starches remained negative. The result indicated that wheat starch was compatible with purple sweet potato starch. The addition of 1%–15% purple sweet potato starch increased trough viscosity, breakdown, final viscosity, storage modulus (G’) and loss modulus (G”), decreased loss tangent (tan δ) and effectively improved the freeze-thaw stability of wheat starch gel. The honeycombed structure of wheat starch gel after lyophilisation became small and uniform with the increase in purple sweet potato starch content. Hence, wheat starch has good compatibility with purple sweet potato starch.

A quantitative criterion for predicting solid-state disordering during high strain rate deformation

A quantitative criterion for predicting the onset of disordering during high strain rate deformation is defined that is based on the potential energy (PE) per atom (PE/atom). The criterion is a necessary, but not sufficient condition to predict disorder. The stress state and loading direction of the crystal must allow deviatoric displacements that can induce disordering and the strain rate must be sufficiently high. The criterion is tested using molecular dynamics (MD) simulations for Ag over a range of a stress states and loading directions relative to the crystal axis. It is found that, above a minimum PE per atom of −2.70 ± 0.01 eV/atom, the crystal becomes unstable and disorders at temperatures well below the equilibrium melting temperature. This criterion is found to be independent of stress state and loading direction, and results suggest that it can be applied broadly to other material systems and to scenarios where deformation is non-uniform and time dependent. An example is given for its application to Au in shear. We show that the minimum critical PE for disordering under high strain rate loading is estimated by finding the equilibrium PE per atom at melting, which can be obtained from a single MD simulation for each material. An example is provided that illustrates how PE/atom can be used to predict where a simulated system is with respect to the disordering threshold without conducting multiple simulations.

Thermodynamics of polymorphism in a bulk metallic glass: Heat capacity measurements by fast differential scanning calorimetry

The Au-based bulk metallic glass-forming alloy Au70Cu5.5Ag7.5Si17 is the first atomic system in which monotropic polymorphism has been experimentally found. In this study, strategies for measuring thermodynamic quantities such as heat capacity, transformation enthalpy and entropy, and Gibbs free energy for stable and metastable phases are evaluated based on this system. The transformation enthalpies between phases are determined via differential scanning calorimetry (DSC); the corresponding heat capacities are measured via stochastic temperature-modulated DSC (TOPEM); and the equilibrium melting temperatures of metastable phases, only detectable at high rates, are determined via fast DSC (FDSC). Whereas the slow heating rate of conventional DSC limits the characterization of metastable phases, the combination of both DSC and FDSC allows for their detailed heat-capacity determination. Furthermore, various approximation functions that have previously been used to determine the temperature dependence of the thermodynamic driving force for nucleation and crystallization are compared with the experimental data.

Thermal properties and degradation of enantiomeric copolyesteramides poly(lactic acid-co-alanine)s

Thermal properties and degradation of enantiomeric random copolyesteramides poly(l-lactic acid-co-l-alanine) [P(LLA-LAL)] and poly(d-lactic acid-co-d-alanine) [P(DLA-DAL)] copolymers with wide alanine unit content ranges from 0 to 21 and 22 mol% were investigated by differential scanning calorimetry and themogravimetry. The total enthalpy of cold crystallization enthalpy and melting enthalpy as an indicator of crystallinity decreased with a decrease in lactic acid unit content or an increase in alanine unit content. Equilibrium melting temperature values of α-form homo-crystallites for the unblended samples decreased with an increase in alanine unit content (157.3–179.6 °C) and those of δ-form homo-crystallites for the unblended L100 and D100 (190.7 and 185.1 °C) were much higher than the reported values (both 172.8 °C, Polymer 2017, 9, 625). Thermal stability was enhanced by enantiomeric polymer blending, although thermal degradation was accelerated by incorporated alanine units. The activation energy for thermal degradation (ΔEtd) of the blend samples decreased with an increase in alanine unit content, whereas that of the unblended samples showed complicated dependence on alanine unit content and became maximum at alanine unit contents of 21 and 22 mol%. The blend samples had higher ΔEtd values comparted to those of the unblended samples, except for the alanine unit contents of 21 and 22 mol%. The reason for the high ΔEtd values of the unblended samples compared with that of the blend sample at the alanine unit contents of 21 and 22 mol% is discussed.

Synthesis, Thermal Properties and Decomposition Mechanism of Poly(Ethylene Vanillate) Polyester.

Plastics are perceived as modern and versatile materials, but their use is linked to numerous environmental issues as their production is based on finite raw materials (petroleum or natural gas). Additionally, their low biodegradability results in the accumulation of microplastics. As a result, there is extensive interest in the production of new, environmentally friendly, bio-based and biodegradable polymers. In this context, poly(ethylene vanillate) (PEV) has a great potential as a potentially bio-based alternative to poly(ethylene terephthalate); however, it has not yet been extensively studied. In the present work, the preparation of PEV is reported. The enthalpy and the entropy of fusion of the pure crystalline PEV have been estimated for the first time. Additionally, the equilibrium melting temperature has also been calculated. Furthermore, the isothermal and non-isothermal crystallization behavior are reported in detail, and new insights on the thermal stability and degradation mechanism of PEV are given.

Interlude of metastability in the melting of polymer crystals.

We have studied the process of melting of polymer crystals using Langevin dynamics simulations with a coarse-grained united atom model. We have considered two ideal situations: one in which a single crystal melts and the other in which a multichain crystal melts. We show that the melting of the single crystal proceeds through a globular metastable state, which is followed by expansion to a more random coil-like state. Similarly, the melting of the multichain crystal reveals a special mechanism comprising two steps: one in which a long-lived partially molten metastable state is formed, followed by a second step in which the chains peel off from the crystalline core to a free state. We elucidate the nature of the metastable state close to the equilibrium melting temperature and show that the multichain crystals equilibrate to states of intermediate order, with the extent of ordering decreasing as we increase the melting temperature. We quantify the kinetics of melting by estimating a free energy landscape using parallel tempering Langevin dynamics simulations. These simulations reveal a metastable state in the single molecule systems, allowing us to estimate the free energy barriers. Additionally, the melting of the multichain crystals reveals the existence of two barriers, with the preference for the intermediate state reducing with increasing temperature. We compare our findings to the existing experimental evidence and find qualitative agreements.

ReaxFF reactive force field for molecular dynamics simulations of liquid Cu and Zr metals.

We develop a ReaxFF reactive force field used for the molecular dynamics simulations of thermophysical properties of liquid Cu and Zr metals. The ReaxFF parameters are optimized by fitting to the first-principles density-functional calculations on the equations of state for bulk crystal structures and surface energies. To validate the force field, we compare the ReaxFF results with those from experiments and embedded-atom-method (EAM) potentials. We demonstrate that the present ReaxFF force field well represents structural characteristics and diffusion behaviors of elemental Cu and Zr up to high-temperature liquid regions. It reasonably reproduces the thermodynamic processes associated with crystal-liquid interface. In particular, the equilibrium melting temperatures show better agreement with experimental measurements than the results from EAM potentials. The ReaxFF reactive force field method exhibits a good transferability to the nonreactive processes of liquid systems.

Reexamination of the equilibrium melting temperature of natural rubber crystals

The equilibrium melting temperature ($$T_{\mathrm{m}}^0$$) of natural rubber crystals was reexamined by the Hoffman–Weeks plot and the Gibbs–Thomson plot using the lamellar thickness obtained from the same sample as the one used for the measurement of melting temperature. The sample was prepared from deproteinized latex. As a result, the $$T_{\mathrm{m}}^0$$ of NR was estimated to be 61 °C. At the same time, the fold-surface free energy was estimated to be 0.046 Jm−2. These values should be more reliable than the previously assumed values. The equilibrium melting temperature ($$T_{\mathrm{m}}^0$$) of natural rubber crystals was reexamined by the Hoffman–Weeks plot and the Gibbs–Thomson plot using the lamellar thickness obtained from the same sample as the one used for the measurement of melting temperature. The sample was prepared from deproteinized latex. As a result, the $$T_{\mathrm{m}}^0$$ of NR was estimated to be 61 °C. At the same time, the fold-surface free energy was estimated to be 0.046 Jm−2. These values should be more reliable than the previously assumed values.

Spontaneous solid-solid interface melting driven by concentration gradient.

The contact of heterogeneous materials with different atomic structures and concentrations becomes omnipresent in modern materials. The differences across the interface introduce various gradients that could alter thermodynamic and kinetic behaviors of the materials. Here, we report a spontaneous melting of the interface formed by a crystalline metal and an amorphous solid at the temperature below their respective equilibrium melting temperatures. The transformation can even become continuous with disappearance of all known features of the first-order melting transition. We show that the change in the nature of melting in the interface region is caused by random disordering induced by interdiffusing atomic elements or impurities in the crystalline phase and the presence of a strong concentration gradient constrained by the sample geometry.

Premelting increases the rate of regelation by an order of magnitude

ABSTRACTGlacier sliding over small obstacles relies on melting on their upstream sides and refreezing downstream. Previous treatments have appealed to ‘pressure melting’ as the cause of the spatial variations in melting temperature that drive thisregelationprocess. However, we show that typical liquid pressure variations across small obstacles are negligible and therefore variations in ice pressure closely approximate variations in effective stress. For a given change in effective stress, the equilibrium melting temperature changes by an order of magnitude more than when the pressure of ice and liquid both change by an equal amount. In consequence, the temperature gradients that drive heat flow across small obstacles are larger than previously recognized and the rate of regelation is faster. Under typical conditions, the transition wavelength at which ice deformation and regelation contribute equally is of m-scale, several times longer than previous predictions, which have been reported to underestimate field inferences.

Isotope Effects on the Crystallization Kinetics of Selectively Deuterated Poly(ε‐Caprolactone)

ABSTRACTDeuterium labeling of semi‐crystalline polymers can dramatically affect their crystallization behaviors. However, the influence of different labeled positions in a partially deuterated polymer on its crystallization is still far from understood. Here, we synthesized a series of selectively deuterated poly(ε‐caprolactones) (PCLs) through ring‐opening polymerization of ε‐caprolactone with controlled deuteration sites, including fully protiated (D0), fully deuterated (D10), tetra deuteration at the 3‐ and 7‐ caprolactone ring positions (D4) and hexa deuteration at the 4‐, 5‐, and 6‐ caprolactone ring positions (D6). All the PCLs showed a similar lamellar structure and parameters. Differential scanning calorimetry (DSC) analysis revealed that the equilibrium melting temperature , melting temperature Tm, crystallization temperature Tc, and crystallization kinetics changed systemically with the deuterium content except for D4, which indicates that the presence of CD2 moieties on either side of ester group in the polymer chain combined with isotopic inhomogeneity could influence the chain packing. The nonmonotonic trend of Tm as a function of deuterium content could be attributed to the difference in a hydrogen‐bond like intermolecular interaction between different PCLs. Partially deuterated PCLs (D4 and D6) showed an Avrami index near 2. After analyzing the parameters at the same supercooling temperature ΔTc, the existence of two crystallization regimes of PCLs were detected. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019, 57, 771–779

Isothermal Crystallization Kinetics of Poly (3-hydroxybutyrate/ Poly(ethylene-co-vinyl acetate) Blends Enhanced by NH4Cl as a Nucleating Agent

The isothermal crystallization kinetics of poly (3-hydroxybutyrate) (PHB)/NH4Cl composites, PHB/poly(ethylene-co-vinyl acetate), with vinyl acetate contents of 80% (EVA80) blends and PHB/EVA80/1 wt% NH4Cl composites were investigated by using differential scanning calorimetry (DSC). The Avrami approach was applied to estimate the kinetic parameters of the PHB samples under isothermal crystallization conditions. It was found that the values of The Avrami rate constant, of PHB were increased with the incorporation of NH4Cl, declining EVA80 content in the PHB blends, and decreasing crystallization temperature. Moreover, the Avrami exponent, n, remained unchanged in spite of the change in the crystallization temperature, EVA80 content, and presence of NH4Cl. The activation energy, calculated using the Arrhenius equation, for PHB/NH4Cl was much lower than that of pure PHB and PHB/EVA80, suggesting that the NH4Cl induced heterogeneous nucleation. The values for PHB/EVA80/1 wt% NH4Cl increased as the EVA80 content was increased, indicating a retardation effect of EVA80. Furthermore, a modified Lauritzen–Hoffmann treatment (LH) was used to evaluate the nucleation constant, of the PHB samples based on the assumption that U* was 13.8 kJ/mol and the equilibrium melting temperatures were 470 K, intercepts of HL equation, were 25.9 for all PHB samples. The values indicated that the crystallization of PHB in its composites and blends, described here, occurred in regime III. The maximum degradation rate temperatures of the PHB/NH4Cl composites were higher than that of PHB, by 9 oC. No significant change in thermal stability was detected by the addition of EVA80.

Crystalline Structures and Structural Transitions of Copolyamides Derived from 1,4-Diaminobutane and Different Ratios of Glutaric and Azelaic Acids.

Copolyamides derived from even 1,4-butanediamine and different mixtures of odd dicarboxylic acids with a great difference in the number of methylene groups (i.e., glutaric and azelaic acids with 3 and 7 groups, respectively) have been synthesized, characterized and structurally studied. Calorimetric analyses revealed a complex behavior with multiple melting peaks associated to lamellar reordering and the presence of defective crystals. Equilibrium melting temperatures were evaluated and showed a eutectic behavior with composition. Copolymers were able to crystallize even for samples with comonomer percentages close to 50%. Negative and ringed spherulites from the melt state and small lath-like lamellar crystals from dilute solution crystallizations were attained. Furthermore, calorimetric data pointed out the exclusion of the less abundant monomer from the lattice of the predominant structure. All samples at room temperature showed a similar crystalline structure (form I) defined by two predominant reflections at spacings close to 0.430 and 0.380 nm, which has been related for even-odd nylons with a two-hydrogen bonded structure. Real time synchrotron experiments showed that melt crystallized samples have two polymorphic transitions on heating, which were practically reversible and consequently were also detected during cooling from the melt state. Interestingly, a different behavior was detected among solution crystallized samples and specifically the transition to the intermediate structure (form II) was not detected during heating for samples enriched on the azelate component or more precisely when they were exclusively crystallized in the form I.

Effects of shear temperature-controlled entanglement network on the structure evolution of Poly(ethylene oxide) in shear flow

This study investigated the structure evolution of poly (ethylene oxide) (PEO) samples sheared at different temperatures above the equilibrium melting temperature followed by cooling to an isothermal temperature of 50 °C. We found that the higher shear temperature tended to decrease the number of entanglements to generate a larger network deformation, which was characterized by the deviation angle. It improved the orientation of chains and noticeably accelerated the crystallization process, however the final values of the long period, lamellae thickness and amorphous thickness remained almost similar. Based on the statistical theory of rubber elasticity, we proved that numerous entanglements, which were characterized by the longest relaxation time or the zero shear viscosity, indeed prevented the extension of chains when the concentration of the long chains was much higher than the long chain overlap concentration.

Thermodynamic features of perfectly crystalline poly(3‐hexylthiophene) based on Flory's relation

ABSTRACTPoly(3‐hexylthiophene) (P3HT) as a conjugated polymer has many electrical and photovoltaic applications. However, reliable values for the equilibrium melting temperature () and for the enthalpy of fusion of a perfect P3HT crystal () are still in doubt. In the published works, ranges from 33 to 99 J/g. Here, the melting point of P3HT in P3HT‐diluent mixtures is measured with changing the diluent concentration. By satisfying the requirements of Flory relation, “working with the same polymer crystals” and “working in the equilibrium state,” usually neglected in the literature, a reliable value of = 83 ± 2 J/g is obtained. The reported values for of P3HT are also in a wide range, 260‑300 °C. In this work, accurate applying of the Hoffman‐Weeks relation, against the effects of lamellar thickening, yields = 272 ± 2 °C rather than 300 ± 2°C reported in the literature. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019, 57, 431–437