Detailed Differential Scanning Calorimetry Research Articles

Influence of welding parameters on mechanical, microstructure, and corrosion behavior of friction stir welded Al 7017 alloy

Fusion welding of age-hardenable high strength aluminum (Al) alloys can be challenging due to the propensity of these alloys to develop solidification-related issues such as porosity and microcracking. A solid-state welding technique, such as friction stir welding (FSW), offers an excellent alternative to fusion welding by minimizing solidification-related defects in high strength Al alloys. While high quality welds of high strength Al alloys are possible with FSW, the process is highly dependent on the welding parameters. The present study attempts to correlate welding parameters to weld quality using a systematic approach spanning microscopic to macroscopic length scales. Mechanical and corrosion testing shows the influence of the traverse speed on the different weld zones. Higher traverse speed shows higher yield strength (∼287 MPa) and better re-passivation corrosion resistance (∼ −0.958V) as compared to slower traverse speed weld yield strength of ∼264 MPa and re-passivation corrosion resistance of ∼ −0.97V. The dissolution, reprecipitation, growth, and coarsening of precipitates in the weld zones have been validated via detailed differential scanning calorimetry (DSC) and microstructural evaluation using transmission electron microscopy (TEM), revealing the effect of welding parameters. In addition, the mechanical and corrosion properties of the weld improved with post-weld heat treatment, further substantiated with microscopy detailing the changes in precipitate morphology in the weld zones. Finally, some guidelines are provided to assist in selecting parameters for FSW of age hardenable high strength Al alloys to control the width of the HAZ.

Insights into the Bead Fusion Mechanism of Expanded Polybutylene Terephthalate (E-PBT).

Expandable polystyrene (EPS) and expanded polypropylene (EPP) dominate the bead foam market. As the low thermal performance of EPS and EPP limits application at elevated temperatures novel solutions such as expanded polybutylene terephthalate (E-PBT) are gaining importance. To produce parts, individual beads are typically molded by hot steam. While molding of EPP is well-understood and related to two distinct melting temperatures, the mechanisms of E-PBT are different. E-PBT shows only one melting peak and can surprisingly only be molded when adding chain extender (CE). This publication therefore aims to understand the impact of thermal properties of E-PBT on its molding behavior. Detailed differential scanning calorimetry was performed on neat and chain extended E-PBT. The crystallinity of the outer layer and center of the bead was similar. Thus, a former hypothesis that a completely amorphous bead layer enables molding, was discarded. However, the incorporation of CE remarkably reduces the crystallization and re-crystallization rate. As a consequence, the time available for interdiffusion of chains across neighboring beads increases and facilitates crystallization across the bead interface. For E-PBT bead foams, it is concluded that sufficient time for polymer interdiffusion during molding is crucial and requires adjusted crystallization kinetics.

Thermodynamic Analysis of Point Mutations Inhibiting High-Temperature Reversible Oligomerization of PDZ3

Differential scanning calorimetry (DSC) indicated that PDZ3 undergoes a peculiar thermal denaturation, exhibiting two endothermic peaks because of the formation of reversible oligomers at high temperature (N↔I6↔D). This contrasts sharply with the standard two-state denaturation model observed for small, globular proteins. We performed an alanine scanning analysis by individually mutating three hydrophobic residues at the crystallographic oligomeric interface (Phe340, Leu342, and Ile389) and one away from the interface (Leu349, as a control). DSC analysis indicated that PDZ3-F340A and PDZ3-L342A exhibited a single endothermic peak. Furthermore, PDZ3-L342A underwent a perfect two-state denaturation, as evidenced by the single endothermic peak and confirmed by detailed DSC analysis, including global fitting of data measured at different protein concentrations. Reversible oligomerization (RO) at high temperatures by small globular proteins is a rare event. Furthermore, our present study showing that a point mutation, L342A, designed based on the crystal structure inhibited RO is surprising because RO occurs at a high-temperature. Future studies will determine how and why mutations designed using crystal structures determined at ambient temperatures influence the formation of RO at high temperatures, and whether high-temperature ROs are related to the propensity of proteins to aggregate or precipitate at lower temperatures, which would provide a novel and unique way of controlling protein solubility and aggregation.

Hydrogen‐Bonded Liquid Crystals in Confined Spaces—Toward Photonic Hybrid Materials

AbstractA series of hybrid materials based on chiral nematic mesoporous organosilica (CNMO) films infiltrated with liquid crystalline hydrogen‐bonded assemblies is prepared and characterized with respect to the mutual manipulation of the photonic properties of the host and the liquid‐crystalline behavior of the guest. Detailed differential scanning calorimetry studies reveal the impact of confinement on the mesomorphic behavior of the liquid crystalline assemblies in the pores of the CNMO films. The photonic properties of the chiral nematic mesoporous host can be controlled by changing the temperature or irradiating the films with UV light. These stimuli‐induced phase transitions are accompanied by changes in the orientational order of the mesogens as revealed by 19F NMR spectroscopy. The combination of confinement and changes in the molecular orientation in a unique hybrid material based on hydrogen‐bonded liquid crystals and a porous host with a chiral nematic mesostructure is an interesting concept for the design of optical sensors, reflectors, or filters.

Synchrotron X-ray investigation of the layer spacing in a series of low molar mass bi-mesogen organosiloxane smectic materials

The temperature dependence of the layer spacing of a series of chiral bi-mesogen organosiloxane liquid-crystal materials is presented. The detailed measurements were taken at the ELETTRA radiation source in Trieste on thin specimens contained between glass coverslips. In the materials with 10 and 11 spacers between the siloxane and the mesogenic moiety, it is observed that the temperature dependence of the layer spacing is not monotonous. In the material with six spacers, there is an abrupt change of 0.01 nm in the layer spacing between 35 °C and 36 °C. In this temperature range, domains with both layer spacing coexist. This observation is in agreement with polarised light microscopy observations; however, detailed differential scanning calorimetry (DSC) measurements show no heat associated with this transition.

High molecular weight poly(ethylene-2,5-furanoate); critical aspects in synthesis and mechanical property determination

Furan-2,5-dicarboxylic acid (FDCA) is a widely advocated renewable substitute for terephthalic acid (TA). Preparation of high molecular weight FDCA based polyesters by an industrially common combination of melt polymerization and subsequent solid state post condensation is described. Ultimately, poly(ethylene 2,5-furanoate) (PEF) with absolute Mn = 83,000 g mol−1 is obtained, determined by triple detection Size Exclusion Chromatography. The bulk polymer properties of FDCA based polyesters, necessary to evaluate their industrial potential were determined the Young's modulus of PEF is determined to be 2450 ± 220 MPa and the maximum stress 35 ± 8 MPa. The influence of crystallinity on the mechanical properties as function of temperature was determined by dynamic mechanical thermal analysis. A detailed differential scanning calorimetry study on the crystallization behavior of high molecular weight PEF allowed to calculate the equilibrium melting temperature (Tm0) of 239.3 and 239.7 °C for the first and second melting peak, respectively. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 4191–4199

Thermoanalytical Evidence of Metastable Molecular Defects in Form I of Benzamide

Friedrich Wohler and Justus von Liebig discovered the first case of molecular polymorphism when crystallizing benzamide in 1832.(1) Thus far only microcrystalline mixtures of metastable phases with the thermodynamically stable form I were available. Optimizing the conditions for crystal growth now gave access to single crystals of the metastable form III which allowed for a detailed comparison of form I and form III by Raman and IR spectroscopy and thermoanalysis. Surprisingly, in the course of detailed differential scanning calorimetry (DSC) measurements for form I an endothermic peak prior to melting was observed. As proven by temperature dependent powder diffraction, thermal microscopy, molecular dynamic (MD) simulations, and solid-state NMR, this premelting thermosignal is not related to an enantiotropic phase transition of form I into form III with a transition temperature close to the melting point. Rather the thermosignal is related to a significant population of metastable defects in form I. While...

Curing studies of bisphenol a based bismaleimide and cloisite 15a nanoclay blends using differential scanning calorimetry and model‐free kinetics

AbstractBlends of 2,2‐bis[4‐(4‐maleimidophenoxy phenyl)]propane [bismaleimide (BMIX)] with different proportions (1, 2, 3, 4, 5, 7, and 9%) of the nanoclay Cloisite 15a were prepared with ultrasonication. Fourier transform infrared studies reveal the existence of interactions between the clay particles and the imide rings in BMIX. The difference in the melting characteristics and the decrease in the curing window caused by the incorporation of the clay particles in BMIX, as evidenced by detailed differential scanning calorimetry investigations, confirmed the existence of interactions between the nanoclay particles and BMIX molecules. The Flynn–Wall–Ozawa, Vyazovkin, and Friedman kinetics methods were used to calculate the activation energies (Ea's) for the curing of the BMIX materials.Eafor the polymerization varied, depending not only on the amount of clay loaded in the BMIX but also on the extent of the curing reaction. Because of the loss of interaction between the clay platelets and the imide rings of BMIX, a decrease inEaat higher reaction extents was noted when there was lower clay loadings (1–4% Cloisite 15a) in BMIX. A reversal in the previous behavior was noted at higher clay loadings (7 and 9% Cloisite 15a) in BMIX and was attributed to the restriction of the molecular mobility due to the presence of increased concentrations of clay platelets and the decreased availability of reaction sites for polymerization. These two opposing factors played were equal at the optimum level of Cloisite 15a loading (5%) in BMIX, which was reflected in the constancy ofEavariation noted with increasing reaction extent. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013

Optimization of Homogenization Parameters of Al-Cu-Li Alloy Cast Ingots Using Calorimetry and Metallographic Techniques

In the present study, thermal treatments for homogenizing cast structures of Al-Cu-Li alloy AA2195 for improved workability are developed chiefly by empirical methods and detailed Differential Scanning Calorimetry (DSC) and microstructural characterization. DSC has been carried out on as-cast samples to establish the homogenization temperatures and avoid incipient melting. Homogenization time has been calculated empirically and microstructural characterization and DSC has been carriedout to after each cycle to validate the empirically established homogenization cycle. Homogenization cycle (435°C/8hrs+495°C/12hrs+525°C/32hrs) has been established for AA2195 alloy having an average grain size of 500μm based on calorimetric studies and microstructural examination.

DSC Study on Phase Transitions and Their Correlation with Properties of Overaged Al-Zn-Mg-Cu Alloys

This article investigates the phase transitions of complex quaternary Al-Zn-Mg-Cu alloys with Zr addition at overaged conditions. Differential scanning calorimetry (DSC) is employed to quantitatively analyze the phase transformation phenomena of a wide range of 7xxx series alloys through endothermic and exothermic reactions. The DSC observations detailing heat effect peaks and thermal parameters of η′ dissolution contain valuable information on the presence of equilibrium phases and the optimum alloying element contents. Based on DSC experimental data and phase diagrams, the balance of critical properties such as strength and electrical conductivity of Al-Zn-Cu-Mg 7xxx series alloys has been studied by considering the formation, dissolution, and incipient melting of S and T phase, dissolution of η′ phase as well as the formation of η phase. Nine Al-Zn-Mg-Cu alloys have been studied through microstructural examination and detailed DSC analysis. The correlation between the properties and the DSC data of the selected alloys has been analyzed.

Preparation and properties of one‐pack polybenzoxazine‐modified polyurethanes with improved thermal stability and electrical insulating properties

AbstractA novel method has been developed for the combination of polyurethanes and polybenzoxazines. For this purpose, firstly, p‐nitrophenol blocked polyurethanes (BPUs) were prepared via the reaction of poly(tetramethylene ether) glycol of various molecular weights, 2,4‐tolylene diisocyanate and p‐nitrophenol. The BPUs were then mixed with 2,2‐bis(3,4‐dihydro‐3‐phenyl‐2H‐1,3‐benzoxazine)propane (Ba) at various weight ratios. To prepare poly(urethane‐co‐benzoxazine) networks, the BPU/Ba mixtures were subjected to a heating programme derived from detailed differential scanning calorimetry and gel content measurements. Results showed about 30 to 40 °C reduction of polymerization temperature for complete curing of BPU/Ba mixtures in comparison to neat Ba. This phenomenon was related to catalytic action of librated p‐nitrophenol molecules. The thermal, mechanical, viscoelastic and electrical properties of prepared thermoset polymers were measured and correlated with their chemical structures. A significant improvement of thermal stability and dielectric strength in comparison to neat polyurethanes was found. Also, enhancement of tensile properties, ease of curing and ability to be transformed into thin films are fascinating features of these newly developed materials in comparison to neat polybenzoxazines. Therefore, these polymers have potential applicability as high‐performance materials in modern electrical industries. Copyright © 2010 Society of Chemical Industry

DSC and morphological studies on the crystallization behavior of β-nucleated isotactic polypropylene composites filled with Kevlar fibers

The crystallization behavior of β-nucleated isotactic polypropylene (PP) composites filled with Kevlar fibers (KFs), as well as that of non-nucleated PP/KF composites for comparison, was investigated using differential scanning calorimetry (DSC) and polarized optical microscopy (POM). The morphological observations revealed that the KF addition could induce thick α-transcrystalline layer around their surfaces in PP/KF composites, while no obvious transcrystalline layer could be detected in β-nucleated PP/KF composites. Detailed DSC investigations suggested that for the PP/KF composites, the dominant modification was α-form, and the crystallization process of matrix was promoted by KF addition, as illustrated by faster isothermal crystallization rate, shorter induction time, and higher crystallization temperature. However, for β-nucleated PP/KF composites, the main modification was β-form, and their crystallization characteristics were independent of KF addition, indicating that the α-nucleating effect of KFs was absent in this system. The DSC results were confirmed by further rheological and wide angle X-ray diffraction (WAXD) studies. The mechanism of the formation of transcrystalline layer was also discussed.

Morphology and properties of injection-moulded carbon-nanofibre poly(etheretherketone) foams

Poly(ether ether ketone) (PEEK) is a high performance polymer that cannot usually be foamed reliably using conventional injection-moulding processes. Here, vapour-grown carbon nanofibres (CNFs) are introduced to stabilise the foaming process, and the resulting morphology of injection-moulded integral foams is investigated in detail. Different image analysis techniques revealed the positive effect of the nanofiller on the cellular structure. Electron microscopy confirmed a homogeneous dispersion of the nanofibres in the cellular PEEK cores. The mechanical properties of the foam injection-moulded samples, in bending, showed an increase in yield strength and elastic modulus with nanofibre loading fractions up to 15 wt%. Although the compressive properties of the foams were reduced as compared to the solid-polymer, the CNFs clearly offset this reduction in properties. Detailed differential scanning calorimetry (DSC) and dynamic mechanical analysis provide further evidence of an interaction between the matrix and the nanoscale filler.

Physicohemical characterization of the freezing behavior of mannitol-human serum albumin formulations

The goal of the study was to analyze the impact of human serum albumin (HSA) quality (stabilized or nonstabilized HSA), the addition of NaCl, and the HSA stabilizers Na-octanoate and Na-N-acetyltryptophanate on the freezing behavior of mannitol-HSA formulations. The focus was on crystallization, Tg' (glass transition temperature of the maximally freeze-concentrated phase), and Tc (collapse temperature). Differential scanning calorimetry (DSC), cryomicroscopy, and low-temperature x-ray powder diffraction (LTXRD) were used to study the frozen state. In mannitol-HSA formulations, mannitol crystallization was inhibited and Tg' lowered to a greater extent by stabilized HSA (containing Na-octanoate, Na-N-acetyltryptophanate, and NaCl) than by unstabilized HSA. Detailed DSC and LTXRD studies showed that in the concentrations used for stabilizing HSA, NaCl led to changes in the freezing behavior, an effect that was less pronounced for the other stabilizers. NaCl further lowered the Tc, which was determined by cryomicroscopy. As the freezing behavior governs the lyophilization process, the changes have to be taken into consideration for the development of a lyophilization cycle, to avoid collapse and instabilities.

Novel polymers with a high carboxylic acid loading

AbstractClick chemistry has been used to prepare a range of novel polymers with pendant carboxylic acid side groups. Four azido carboxylic acids, either mono‐ or difunctional and aliphatic or aromatic, have been prepared and thoroughly characterized. Extensive model reactions with 1‐ethyl‐4‐hydroxybenzene, the simplest model for poly(4‐hydroxystyrene), and the four azido carboxylic acids have been conducted to establish the proper reaction conditions and provide an analytical frame for the corresponding polymers. Poly(4‐hydroxystyrene) moieties in three different polymers—poly(4‐hydroxystyrene), poly(4‐hydroxystyrene‐co‐methyl methacrylate), and poly(4‐hydroxystyrene‐b‐styrene)—have been quantitatively transformed into oxypropynes by the use of either Williamson or Mitsunobu strategies and subsequently reacted with the azido carboxylic acids. Detailed differential scanning calorimetry investigations of all the polymers in general exhibit [when poly(4‐hydroxystyrene) is a substantial part] significant changes in the glass‐transition temperature from the polar poly(4‐hydroxystyrene) (120–130 °C) to the much less polar alkyne polymers (46–60 °C). A direct correlation between the nature of the pendant groups in the derivatized polymers and the glass‐transition temperature has emerged: the aromatic carboxylic acids give high glass‐transition temperatures (90–120 °C), and the aliphatic carboxylic acids give lower glass‐transition temperatures (50–65 °C). © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44:6360–6377, 2006

Influence of DPH on the structure and dynamics of a DPPC bilayer.

We have conducted extensive molecular dynamics (MD) simulations together with differential scanning calorimetry (DSC) and nuclear magnetic resonance (NMR) experiments to quantify the influence of free 1,6-diphenyl-1,3,5-hexatriene (DPH) fluorescent probes on the structure and dynamics of a dipalmitoylphosphatidylcholine bilayer. Atomistic MD simulations show that in the membrane-water interface the influence of DPH is minor, whereas in the acyl-chain region DPH gives rise to major perturbations. In the latter case, DPH is found to influence a wide range of membrane properties, such as the packing and ordering of hydrocarbon tails and the lateral diffusion of lipid molecules. The effects are prominent but of local nature, i.e., the changes observed in the properties of lipid molecules are significant in the vicinity of DPH, but reduce rapidly as the distance from the probe increases. Long-range perturbations due to DPH are hence not expected. Detailed DSC and (2)H NMR measurements support this view. DSC shows only subtle perturbation to the cooperative behavior of the membrane system in the presence of DPH, and (2)H NMR shows that DPH gives rise to a slight increase in the lipid chain order, in agreement with MD simulations. Potential effects of other probes such as pyrene are briefly discussed.

Two-state irreversible thermal denaturation of anionic peanut ( Arachis hypogaea L.) peroxidase

Detailed differential scanning calorimetry (DSC), steady-state tryptophan fluorescence and far-UV circular dichroism (CD) studies, together with enzymatic assays, were carried out to monitor the thermal stability of anionic peanut peroxidase (aPrx) at pH 3.0. The spectral parameters were seen to be good complements to the highly sensitive but integral method of DSC. Thus, changes in far-UV CD corresponded to changes in the overall secondary structure of the enzyme, while changes in intrinsic tryptophan fluorescence emission corresponded to changes in the tertiary structure of the enzyme. The results, supported with data concerning changes in enzymatic activity with temperature, show that thermally induced transitions for aPrx are irreversible and strongly dependent upon the scan rate, suggesting that denaturation is under kinetic control. It is shown that the process of aPrx denaturation can be interpreted with sufficient accuracy in terms of the simple kinetic scheme, N → k D , where k is a first-order kinetic constant that changes with temperature, as given by the Arrhenius equation; N is the native state, and D is the denatured state. On the basis of this model, the parameters of the Arrhenius equation were calculated.

Thermally induced conformational changes in horseradish peroxidase.

Detailed differential scanning calorimetry (DSC), steady-state tryptophan fluorescence and far-UV and visible CD studies, together with enzymatic assays, were carried out to monitor the thermal denaturation of horseradish peroxidase isoenzyme c (HRPc) at pH 3.0. The spectral parameters were complementary to the highly sensitive but integral method of DSC. Thus, changes in far-UV CD corresponded to changes in the overall secondary structure of the enzyme, while that in the Soret region, as well as changes in intrinsic tryptophan fluorescence emission, corresponded to changes in the tertiary structure of the enzyme. The results, supported by data about changes in enzymatic activity with temperature, show that thermally induced transitions for peroxidase are irreversible and strongly dependent upon the scan rate, suggesting that denaturation is under kinetic control. It is shown that the process of HRPc denaturation can be interpreted with sufficient accuracy in terms of the simple kinetic scheme N -->k D where k is a first-order kinetic constant that changes with temperature, as given by the Arrhenius equation; N is the native state, and D is the denatured state. On the basis of this model, the parameters of the Arrhenius equation were calculated.

Synthesis and properties of the addition polymer of benzenedithiol with divinylbenzene

AbstractPolyaddition of 1,4‐benzenedithiol (BDT) to 1,4‐divinylbenzene (DVB) was carried out with 2,2′‐azobisisobutyronitrile initiator in toluene at 75°C under a nitrogen atmosphere. The polymerization proceeded without an induction period, to give a white polymer with a high molecular weight (M̄w = 110,000) in ca. 90% yield for 2 hr. It was confirmed by 1H‐NMR (nuclear magnetic resonance), IR (infrared) and sulfur contents that the polymer had an alternating structure of DVB and BDT units. The end‐capping reaction of the polymer was also achieved by addition of thiophenol and/or styrene to the polymerization solution at a final stage of the polymerization. The polymer film exhibited a reversible phase transition between a transparent state and an opaque one by thermal mode. The thermal property of the polymer was studied by differential scanning calorimetry (DSC) analysis and polarized optical microscope observation with the polymer film. The detailed DSC analysis showed that the end‐capped polymer with a relatively low molecular weight (M̄W = 4400–9600) exhibited similar to liquid crystalline behavior. A diffuse reflectance spectrum of the polymer coated on an aluminum plate showed a marked difference in reflective light intensity in the ultraviolet and visible regions: the reversible phase transition between an opaque and a transparent polymer layers was induced by thermal mode. The light transmittance of the polymer film, which was measured by depolarized light intensity method, was very sensitive toward the temperature variation.