Maximum Melting Temperature Research Articles

Phase Separation and Lamellae Assembly below UCST in Poly(l-lactic acid)/Poly(1,4-butylene adipate) Blend Induced by Crystallization

The crystalline/crystalline poly(l-lactic acid)/poly(1,4-butylene adipate) (PLLA/PBA) blend system exhibits upper critical solution temperature (UCST) behavior below the melting temperature of PLLA. When the PLLA/PBA (50/50) blend is cooled to crystallize below UCST, phase-separated domains appear prior to crystallization of PLLA, resulting in PLLA spherulites being overlapped with the phase-separated domains. However, phase-separated domains only appear in blends when they are crystallized after being subjected to maximum melting temperature (Tmax) for a short melting time (Δtmax 5 min). When PLLA/PBA (50/50) blend is melted for short Δtmax, the chain entanglement density is less; thus, the PBA chains are easy to be mutually expelled from the PLLA chains during crystallization of PLLA, resulting in PLLA-rich and PBA-rich separate domains. Apparently, phase separation in the PLLA/PBA blends is initiated by the ...

Solving Injection Molded Part Warpage under Asymmetric Mold Cooling Conditions by Corrugated Variations in Part Thickness

Abstract In this study, corrugated variations in part thickness designed to match the various mold temperature differences between the mold core and mold cavity sides, along with their effects on part warpage, were investigated. Part thickness was varied in a pitched manner and thickness reductions of 10%, 20%, 30%, and 40% of the original part thickness were implemented. The results show that when there is a mold temperature difference between core and cavity, using a corrugated thickness reduction design can greatly reduce or even eliminate part warpage. For a uniform thickness part molded under unbalanced cooling, the thermal neutral axis (where the maximum melt temperature is located) will deviate from the gap center toward the mold wall side with the higher temperature. Variation in thickness will pull the deviated thermal neutral axis back to the center line leading to a reduction in part warpage.

The critical analysis and mutual coherence of the thermodynamic data of the AIIIBV phases

The optimisation of thermodynamic properties for 17 AIIIBV compounds has been obtained for the first time by means of a novel correlative method. The maximum melting temperatures and sums of atomic numbers (ZA+ZB) of the isostructural phases are shown to be very important parameters in this approach to the thermodynamic analysis. A critical analysis and recommendations regarding the most realistic experimental and calculated data are presented. A set of the enthalpies of formation (±0.4kJ/mol-at), free enthalpies (±0.5–1.0kJ/mol-at), standard entropy (±0.5–1.0J/(Kmol-at)), entropies of formation, and heat capacity (±0.2–0.4J/(Kmol-at)) of AIIIBV compounds have been obtained. The similarity of the functions Cp(T) for the AIIIBV phases with the same sums of atomic numbers is also discussed. Thermodynamic properties of unknown semiconductor TlN, as well as insufficiently known compounds BP and BAs, have been evaluated with the proposed method. The proposed correlative method is also applicable for any isostructural phases (the systems AIII–BV, AII–BVI, RE-Me, and other compounds) and can be used for the critical analysis of their thermodynamic or physico-chemical properties.

Polymorphous transition wurtzite-sphalerite for nonstoichiometric cadmium and zinc chalcogenides

Cadmium and zinc chalcogenides are widely used as active components in displays (1), radiation detec� tors (1, 2), visible and IR lasers (3). Nonstoichiome� try defects and polymorphous transitions in nonstoi� chiometric phases have significant effect on crystal and polycrystalline film technology for these materi� als. The analysis of the structure and thermodynamic features of phase equilibrium diagrams of cadmium and zinc chalcogenides demonstrated that there were obtained crystal and polycrystalline preparations with both wurtzite ( P63mc) and sphalerite (F 3m) structure for all binary phases except CdTe and ZnTe under nor� mal pressure in the temperature range from the room temperature up to the maximum melting temperature. Herein for all compounds the sphalerite structure is stable at low temperatures while the wurtzite structure is stable at high temperatures. The native components solubility in nonstoichio� metric CdS, CdSe, ZnTe, ZnSe, CdTe was experi� mentally studied by direct methods of static tensime� tery and hightemperature equilibrium "quenching" technique with determination of nonstoichiometric composition by "extraction" technique (5). Using the study results the homogeneity limits of concerned compounds were defined more accurate in the wide

Crystallization kinetics of PCL tagged with well-defined positional triazole defects generated by click chemistry

Well-defined poly(caprolactone) PCL samples of narrow molecular weight distribution, tagged with a triazole junction in predetermined position of the chain, were synthesized by an alkyne-azide click reaction. The crystallization kinetics of these original PCL’s, having a triazole “defect” in central position or at the chain end, were investigated by WAXD and by differential scanning calorimetry (DSC) under nonisothermal and isothermal conditions. Major kinetic differences could be observed between the triazole end-capped PCL and the one having a triazole “defect” in central position of the chain. The crystallization rate is highest for the triazole end-capped PCL, whereas a PCL with a triazole junction in more central position forms irregular shaped spherulites and is characterized by a longer induction period, a lower crystallization rate and a reduced maximum melt temperature. Its thermostability as demonstrated by thermogravimetry analysis TGA, is however increased with respect to the corresponding triazole end-capped sample.

Six types of spherulite morphologies with polymorphic crystals in poly(heptamethylene terephthalate)

Six types of spherulite morphologies packed with polymorphic crystals and their growth kinetics in melt-crystallized poly(heptamethylene terephthalate) (PHepT) were characterized using polarized-light optical microscopy (POM), Fourier transformed infrared microspectrometry (micro-FTIR), differential scanning calorimetry (DSC) and atomic-force microscopy (AFM). Two maximum melting temperatures (Tmax), a higher 150 °C and a lower 110 °C, were used to melt the initially crystallized PHepT of either α- or β-crystal. The high Tmax was enough to melt all nuclei, but the lower Tmax was considered as near or slightly below the equilibrium melting temperatures of these two cells (if estimated by nonlinear methods). When crystallized at various Tc from these two Tmax’s, PHepT can exhibit as many as six types of spherulites (Ring Type-I, -II, -III, Maltese-cross Type-1, -2, and -3) owing to different nucleations. Ring Type-I, Maltese-cross Type-1 and -3 spherulites are packed of the sole β-crystal, while Ring Type-II, -III and Maltese-cross Type-2 spherulites are attributed to the sole α-crystal. However, as the PHepT polymorphic cells are related to Tc, such correlations between the crystal cells and spherulite types (ring or ringless) cannot be ruled out to be a coincidence.

Stereo-Complex Crystallization of Poly(lactic acid)s in Block-Copolymer Phase Separation

Block-copolymer containing a poly(L-lactic acid) (PLLA) segment was blended with pure poly(D-lactic acid) (PDLA) chain in chloroform solution and casted into the dried film. This method could form the stereocomplex (Sc) crystal of PLLA/PDLA within the nanometer-sized phase separation self-assembled by block-copolymer. Differential scanning calorimetry measurements of the prepared and annealed films gave the maximum achievable melting temperature of 245 °C, which was highest among those previously reported for PLA Sc-crystals. Surprisingly, all of added PDLA chains were Sc-crystallized, depending on the blend composition with block-copolymer containing PLLA.

THE EFFECT OF MECHANICAL VIBRATION ON CRYSTALLIZATION OF POLYPROPYLENE

The testing specimens of an injected polypropylene sample were subjected to heat treatment for 30 min at various temperatures around its melting peak,and then the treated samples were characterized by DSC.It was found that there existed an temperature window at which the resulting melting temperature and crystallinity of the treated sample increased obviously.For heat treated specimens under static conditions the temperature window occured only below the peak melting temperature.For heat treated specimens under vibration in a dynamic mechanical analyzer with definite amplitude and frequency,this temperature window covered the whole melting range.Heat treating the PP sample under vibration in a temperature range of 5℃ above its melting peak,the resulting specimen possessed a maximum melting temperature.

Reversible melting and crystallization of high‐molar‐mass poly(oxyethylene)

AbstractThe heat capacity of poly(oxyethylene) (POE) with a molar mass of 900,000 Da has been analyzed with differential scanning calorimetry and quasi‐isothermal, temperature‐modulated differential scanning calorimetry. The crystal structure, lattice parameters, and coherently scattering domain sizes have been measured with wide‐angle X‐ray diffraction as a function of temperature. The high‐molar‐mass POE crystals are in a folded‐chain macroconformation and show some locally reversible melting starting already at about 250 K. At 335 K, the thermodynamic heat capacity reaches the level of the melt. The reversible crystallinity depends on the modulation amplitude and has been varied in the melting range from ±0.2 to ±3.0 K. Before melting, there is neither a change in the crystal structure nor a change in the domain size, but the expansivity of the crystals increases at about 320 K. These observations support the interpretation that the monoclinic POE crystals possess a glass transition temperature with a midpoint at about 324 K, whereas the maximum melting temperature is 341 K. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 475–489, 2007

High-pressure / High-temperature Behavior of the Methane-Ammonia- Water System up to 3 GPa

Melting phase relations in the methane-ammonia-water system up to 3 GPa have been obtained in a series of in situ experiments in externally heated diamond anvil cells. The melting temperature of methane clathrate hydrates increases rapidly above pressures of ~ 1.5 GPa, and does not appear to be significantly affected by the presence of ammonia. The reaction of the hydrate formation at pressures 2 - 3 GPa is kinetically impeded. Our data show that the high-pressure methane hydrate has the maximum melting temperature among the clathrate hydrates studied so far.

Methodology for adaptation of a conjugate mathematical model of hydrodynamics and heat exchange in a glass-melting furnace with horseshoe-shaped flame direction

The sequence for adapting a conjugate mathematical model describing external heat exchange in the work space and heat exchange and hydrodynamics of the melt in a glass-melting furnace melting tank with horseshoe-shaped flame direction was examined. It was shown that it is sufficient to use the experimental dependences of the air heating temperature and temperature of the glass melt at the outlet of the melting tank on the furnace efficiency for adapting the model when the basic modeling boundary conditions have been correctly assigned. Equations were obtained that establish the correlation of the specific heat rate and maximum melting temperature and the specific output of glass melt, and they can be used in constructing an algorithm for controlling the thermal work of a glass-melting furnace of the design studied.

Effect of nano-CaCO 3 on polymorphic behavior in syndiotactic polystyrene for non-isothermal crystallization

Syndiotactic polystyrene ( sPS)/nano-CaCO 3 composites were prepared on a twin screw extruder for the first time. Differential scanning calorimeter and wide-angle X-ray diffraction were used to investigate the effect of maximum melting temperature ( T max) and the content of nano-CaCO 3 on the crystal forms and melting behavior of sPS for non-isothermal crystallization. The results indicated that the increasing T max decreases the crystallization temperature of sPS and facilitates the formation of β-crystal. The addition of nano-CaCO 3 changes the melting behavior and crystal form of melt-crystallized sPS and facilitates the formation of β-crystal. The increase in the content of nano-CaCO 3 decreases crystallization temperature of sPS and increases the content of β-crystal, which was similar to the effect of increasing T max. Pure β-crystal in sPS/nano-CaCO 3 composites can be obtained at lower melting temperature due to the effect of nano-CaCO 3.

Crystallization behavior of polycarbonate/poly(ethylene oxide) blends studied by DSC

Blends of polycarbonate (PC) with poly(ethylene oxide) (PEO), both semicrystalline polymers, were prepared by co-dissolving in chloroform and casting the mixture. Crystallization of PEO/PCL blends was studied by differential scanning calorimetry (DSC) and analyzed by the Avrami equation. Their miscibility was confirmed not only by the decreasing crystallization temperature, melting temperature, crystallinity and crystallization rate of PEO, but also by the equilibrium melting temperature depression of PC. PEO induced the crystallization of PC within the entire composition range. At a predetermined crystallization temperature, the crystallization of PC was controlled by both the PC content and molecular mobility of the blends, which could induce a maximum melting temperature or enthalpy of PC at a certain PEO content. The analysis of the equilibrium melting temperature of PC seemed to show the Flory–Huggins interaction parameter for these blends might be composition-dependent. PC could prevent the crystallization process of PEO. With the addition of PC, the increasing Avrami exponent and the decreasing crystallization rate of PEO were related with the dilute effect of PC and decreasing molecular mobility of the blends.

Effects of Residual Nuclei on Crystals and Equilibrium Melting Points of Dual Crystals in Poly(ethylene 2,6‐naphthalene‐dicarboxylate)

AbstractSummary: The crystal structures and possible transformations in poly(ethylene naphthalate) (PEN) were examined using differential scanning calorimetry (DSC) and wide‐angle X‐ray diffraction (WAXD). The crystal cells (α‐ and β‐form) were dependent on several factors related to their thermal histories. For PEN subjected to Tmax = 280 °C (where Tmax is the maximum melt temperature at which the samples were treated before crystallization), only the α‐form developed regardless of the temperature of crystallization or cooling rate. However, for PEN subjected to Tmax = 300 °C, PEN developed mixed dual crystals, with greater fractions of the α‐form at low temperatures of crystallization (220 °C or lower) or higher cooling rates, but higher β‐form fractions at higher crystallization temperatures (>230 °C). The difference in the relative stability of these two crystals was utilized to prepare PEN containing only the α‐form or only the β‐form. The equilibrium melting temperatures of these two crystal forms in PEN were then measured separately. For PEN containing only the β‐form, T (289.3 °C) was found by using the Hoffman‐Weeks technique, while for PEN containing only the α‐form, T was determined using the same technique but was found to be lower (282.6 °C).Comparison of Hoffman‐Weeks extrapolations for the α‐crystal and β‐crystal, respectively, in PEN.magnified imageComparison of Hoffman‐Weeks extrapolations for the α‐crystal and β‐crystal, respectively, in PEN.

Equilibrium Melting Temperature of Isotactic Polypropylene with High Tacticity. 2. Determination by Optical Microscopy

In part 1 of this series, we proposed a new method to determine the correct equilibrium melting temperature (Tm0). Effects of the “melting kinetics” and lamellar thickening were omitted from Tm. The correct Tm0 of isotactic polypropylene (iPP) ([mmmm] = 99.6%, Mn = 64 × 103, and Mw/Mn = 2.4) was observed to be 186.1 °C. In this paper, the rigorous Gibbs−Thomson plot was obtained by using the direct correspondence between maximum melting temperature (Tm,max) and maximum lamellar thickness (lmax). Tm,max and lmax were observed by means of optical microscope and transmission electron microscope (TEM), respectively. The validity of the Gibbs−Thomson plot obtained by means of a differential scanning calorimeter (DSC) (part 1 of this series) was confirmed by comparing it with the rigorous Gibbs−Thomson plot in this paper. The Hoffman−Weeks plot is widely used as one of the methods to obtain Tm0. It was shown that the Hoffman−Weeks plot is correct only when l ∝ 1/ΔT, where ΔT is the degree of supercooling, is sa...

Phase equilibrium and crystallization behavior of mixed lipid systems

AbstractAs complex lipid systems, the phase and crystallization behavior of mixtures of a high‐melting milk fat fraction with a low‐melting milk fat fraction or canola oil was studied. A turbidity technique was developed to estimate solubility and metastability conditions of these lipid mixtures. Both solubility and metastability of the high‐melting milk fat fraction in liquid lipids increased exponentially with temperature. At a given equilibration temperature, liquid phases and solid fractions with nearly identical melting profiles and TAG compositions were obtained regardless of the original concentration of the lipid mixture. The maximum melting temperature (MMT), as measured by DSC, of the liquid phase increased dramatically in the equilibrium temperature range of 27.5–35.0°C but did not change at temperatures below and above this range (down to 25.0°C and up to 40°C in this study). The content of long‐chain TAG (C46−C52) increased and short‐chain TAG (C36−C40) decreased in the liquid phases as the equilibrium temperature increased. A plot of the TAG group ratio (i.e, long‐short‐chain TAG) vs. equilibrium temperature was generated to illustrate the phase behavior of the complex lipid system and to represent a solubility curve, from which the supersaturation level for crystallization kinetics was determined. Higher supersaturation and lower temperature resulted in higher nucleation and crystallization rates. Compared to the system with a low‐melting milk fat fraction, mixtures of the high‐melting milk fat fraction with canola oil had higher nucleation and crystallization rates due to the lower solubility found for this system.

Undercooled Melt Formation by Melting of Metastable Eutectic Structure in Al<SUB>2</SUB>O<SUB>3</SUB>-Y<SUB>2</SUB>O<SUB>3</SUB>-ZrO<SUB>2</SUB> System

This paper presents a selection of the eutectic systems in the Al 2 O 3 -Y 2 O 3 -ZrO 2 pseudo-ternary system and the production of the undercooled melt due to melting of the metastable eutectic structure at the metastable eutectic temperature. In the pseudo-ternary system, there are two eutectic systems; one is the Al 2 O 3 -YAG(Y 3 Al 5 O 12 )-ZrO 2 equilibrium system and the other is the Al 2 O 3 -YAP(YAlO 3 )-ZrO 2 metastable system. The eutectic system was selected by maximum melt temperature before solidification. YAG nucleation was inhibited in the melt when heated above 2250 K and consequently the metastable eutectic solidification path was selected. The selection controlled by the melt temperature before solidification was similar to that of the Al 2 O 3 -Y 2 O 3 system. Furthermore, the undercooled melt formation was found when the Al 2 O 3 -YAP-ZrO 2 metastable eutectic structure was heated up to temperatures above the metastable eutectic temperature. The addition of ZrO 2 to the Al 2 O 3 -Y 2 O 3 system inhibited the YAG nucleation remarkably and enhanced the undercooled melt formation in comparison with the Al 2 O 3 -Y 2 O 3 pseudo-binary system. Ease of the undercooled melt formation in the pseudo-ternary system will be beneficial for the melt shaping processes.

Melting curve equations at high pressure

A number of equations for the melting temperature dependence on pressure, ${T}_{m}(P),$ based on the two- and one-phase approaches to melting have been obtained. All melting curves have the form ${T}_{m}=F(P)D(P),$ where $F(P)$ is the Simon (rising) melting equation and $D(P)$ is the damping function, which asymptotically slopes down under pressure. This form predicts that each solid phase has a maximum melting temperature at positive or negative pressure. The simplest equation of this form is ${T}_{m}{=T}_{0}(1+\ensuremath{\Delta}{P/a)}^{b}\mathrm{exp}(\ensuremath{-}c\ensuremath{\Delta}P),$ where $\ensuremath{\Delta}P=P\ensuremath{-}{P}_{0},$ ${P}_{0}$ is some reference pressure, and a, b, and c are parameter which are identified in terms of thermodynamic values. All melting curve data obey this equation. This implies that there exist no anomalous melting curves. All melting curves, rising, falling, and flattening, as well as curves with a maximum, are normal insofar as all of them can be described by the unified equation.

Computer aided rheological design of extrusion dies for profiles

A global methodology for the rheological design of profile extrusion dies is proposed. This methodology accounts for: (i) flow defects due to maximum admissible stresses, pressure drop and melt temperature increase; (ii) post-extrusion phenomena (shrinkage upon cooling, draw-down promoted by pulling and swelling after die exit) and (iii) flow balancing. The part of the methodology, that is concerned with flow balancing, was implemented and is here illustrated in two case studies, each one leading to the adoption of a different constructive solution. The software is based on a finite-volume method, which performs the required three-dimensional simulations, and is also briefly described.

Selection of eutectic systems in AI2O3–Y2O3 ceramics

There are two eutectic reactions in the Al203–rich portion of the AI2O3-Y2O3 pseudo–binary system; one is the equilibrium A1203–YAG eutectic reaction at 1826%C, and the other is the metastable A1203–YAP eutectic reaction at 1702%C. Selection of the A1203–YAG and the A1203–YAP eutectics was examined in terms of cooling rate, nucleation temperature and maximum melt temperature. When the melt was cooled from 2100%C at any cooling rate, it always nucleated below the Al203–YAP eutectic temperature, therefore the Al203–YAP eutectic was selected. The Al203–YAG eutectic was selected when the melt was cooled from 1900%C at a cooling rate of less than 1 K s-1. The selection of the two eutectic systems was determined by the nucleation temperature, although the maximum holding temperature of the melt and the cooling rate significantly affected the nucleation temperature. The structure of the melt, such as coordination of oxygen and chemical order when being heated to 2100%C may affect the nucleation behavior.