Non-reversing Heat Flow Research Articles

Enthalpy relaxation of sodium aluminosilicate glasses from thermal analysis

AbstractThe sodium aluminosilicate (NAS) glass family is important for many different industrial applications, but glass relaxation has not yet been thoroughly studied in this system. Thermal analysis techniques such as differential scanning calorimetry (DSC) and modulated differential scanning calorimetry (MDSC) can provide insight into the enthalpy relaxation of glass by measuring the glass transition temperature (Tg), activation energy, and enthalpy of relaxation. MDSC is mostly used to study nonoxide and low Tg glasses, and there is much debate about whether the nonreversing heat flow analysis method is accurate. To the authors’ knowledge, this is the first paper using MDSC to study these NAS compositions, and one of few papers to report MDSC on high Tg oxide glasses. We report on one set of modulation conditions that obtain a linear response using Lissajous curves, as well as comparing the activation energy calculated from DSC with the enthalpy of relaxation obtained from MDSC. Our results show that the activation energy and enthalpy of relaxation do not give the same compositional minimum in relaxation, and therefore more work is needed to investigate the validity of the nonreversing heat flow approach for high Tg oxide glasses.

Structural relaxation in IR transmitting (GeSe4)100-x(As2Se3)x glasses

(GeSe4)100-x (As2Se3)x glasses in the GeSe4-As2Se3 pseudo-binary joint are prepared by the melt quenching method. In this tie-line, the entire composition 0 ≤ x ≤ 100 is characterized by fixed average network connectivity of 2.40, which is considered to be optimal connectivity as the number of constraints and the number degrees of freedom available to the atoms are balanced at this average connectivity. The glass forming ability of the glasses having this average connectivity of 2.40 is usually found to be maximum. Glass transition temperature (Tg) decreases till x = 30 and remains invariant in the range 40 ≤ x ≤ 70. For x > 70, Tg shows an increasing trend. The jump in specific heat capacity (ΔCp), non-reversing heat flow (ΔHnr), Tg and the optical bandgap (Eg) show a distinct change in the composition range 40 and 70 mol.% of As2Se3. The fragility index (m) lies in the range of 15 and 32 for all the prepared glasses, representing the strong nature of these glass forming melts in the (GeSe4)100-x (As2Se3)x glasses. The viscosity as a function of reduced temperature (Tg/T) shows almost an Arrhenius nature for 0 ≤ x ≤ 70 and for x > 70, a slight deviation from the Arrhenius behviour is observed. The structural network of these glasses primarily consists of GeSe4/2 and Se-Se chains for x ≤ 30. For 40 ≤ x ≤ 70, the network is dominated by AsSe3/2 pyramidal units and As-As homopolar bonds. For x > 70, the structural network is characterized by AsSe3/2 pyramidal units. All the prepared glasses are found to transmit infrared (IR) light over a wide wavelength range from 2 to 17 μm. The super strong nature of the melts indicates that optical devices can be easily made for IR applications.

Acoustic and mechanical testing of commercial cocoa powders

ABSTRACT In the present study, commercial cocoa powders with different cocoa fat contents were studied. It was found that the cocoa powders’ flow patterns were of a cohesive to highly cohesive characters. It was demonstrated, that the powders of higher crystalline structure were less flowable compared to the ones with the more amorphous ones. It was observed by SEM that the studied cocoa powders of higher cocoa fat content and the ones with the dietary fibers content (sample 2) exhibited more amorphous structure. The predominantly smooth surface structure of the higher fat content cocoa powder allowed its higher dense packing, triggering the decreased sound absorption typical for non-porous materials as quantified by NRC of 0.289 (sample 1, 100 mm material height) and 0.227 (sample 3) to 0.182 (sample 2). The latter conclusions were also supported by the observed increase of the structural mechanical stiffness of the freely poured powder bed of high cocoa fat amorphous powders, as resulting in the increasing magnitude of the Kl of 12.83 MPa (sample 1, 100 mm material height) and 19.29 MPa (sample 3) to 37.82 MPa (sample 2). Melting temperatures of the samples were determined by DSC. Results were directly corresponded to the cocoa butter content. The highest enthalpy of fusion (ΔH m) of (23.32 ± 0.21) J/g was obtained for the highest cocoa butter containing sample 2 (of 20–22 wt. %). Obtained values of ΔH m for samples 1 and 2 were of (12.38 ± 0.20) J/g and (10.27 ± 0.17) J/g. Tp (melt) for reversing heat flow was ranging from (30.16 ± 0.10) °C to (32.28 ± 0.10) °C indicating the melting of stable β polymorph. The melting peaks observed at distinct temperatures in the non-reversing heat flow patterns were indicating melting of the unstable α and metastable β’ and stable β cocoa butter polymorphic forms.

Modulated differential scanning calorimetry study of wax-doped asphalt binders

In order to better understand how wax constituents affect thermoreversible aging phenomena, three kinds of n-alkanes were doped into a base asphalt binder. Modulated differential scanning calorimetry (MDSC) analysis was performed on a series of binders containing increasing wax contents. It was discovered that the addition of n-alkanes not only changes the non-reversible thermal signal but also the reversible one. For asphalt doped with n-C20, the thermal behavior shows significant time-dependent behavior, while those doped with n-C32 and n-C40 do not. The size of both endothermic peaks in the total heat flow and non-reversible heat flow curves decreases with oxidation of wax-doped asphalt binders. The appearance of crystallizable fractions in wax-doped asphalt binders with varying thermal history can be modeled in both isothermal Avrami and non-isothermal Ozawa theoretical frameworks. A possible mechanism for the thermoreversible aging phenomenon has been proposed but it will need further work for validation and refinement.

Toward a better understanding of the low-temperature reversible aging phenomenon in asphalt binder

ABSTRACT Asphalt aging with time is one of the main causes that leads to the performance deterioration of asphalt pavement. Compared with non-reversible oxidation aging, reversible aging which resulted from molecular structuring gets little attention in asphalt materical specification, especialy at low-temperature. This paper aims to compare the effect of oxidation and isothermal conditioning on original asphalt binder’s low-temperature properties. Firstly, the mechanisms of asphalt reversible aging are systematically reviewed in this paper. Secondly, extended bending beam rheometer (Ex-BBR) test and modulated differential scanning calorimetry (MDSC) test are carried out on original asphalt and its oxidative residue. The results show that oxidation has apparently different influence on two physical hardening indicators used in this study. Only three days of non-accelerated physical hardening may produce a more brittle material than accelerated oxidative residue. There is an obvious peak around 25°C in non-reversing heat flow curve of asphalt binder after long-term isothermal conditioning, and it is caused by some imperfect crystals forming fully crystallized structure after long-term conditioning. Both long-term isothermal conditioning and oxidation will increase the amount of crystalline phase and rigid amorphous phase.

Short- and Long-Term Epoxy Modification of Bitumen: Modification Kinetics, Rheological Properties, and Microstructure.

Aiming to gain knowledge in the epoxy-bitumen modification mechanisms, this work explores the effects that epoxy concentration and ambient curing exert on the physico-chemistry and thermo-rheological properties of epoxy-modified binders. Process rheokinetics of epoxy-bitumen blends indicates that binder short-term modification (i.e., during processing) is accelerated by epoxy concentration. Furthermore, a synergistic effect of epoxy concentration and ambient curing is found during long-term modification (i.e., during curing at ambient conditions). As a result, viscous and viscoelastic rheological properties of binders are enhanced at medium/high in-service temperatures, at least, after one month of curing. FTIR (Fourier Transform Infrared spectroscopy) tests and SARAs (Saturates, Aromatics, Resins and Asphaltenes) analysis confirm the existence of esterification/etherification reactions between epoxy oxirane groups and the carbonyl groups available in aromatic and resin molecules. Thus, the new high molecular weight compounds increase the asphaltenic fraction of modified bitumen. Likewise, nonreversing heat flow curves obtained by modulated calorimetry corroborate the formation of such highly structured domains responsible for the final binder performance.

Thermal-alteration interphase transformations in natural and synthetic arsenic sulfide polymorphs

Abstract Thermal-alteration interphase transformations in natural (realgar α-As4S4 of two mineral origins) and synthetic (commercial powdered high-temperature β-As4S4 modification synthesized from elemental constituents and subjected to high-energy mechanical ball milling) arsenic monosulfide polymorphs are studied exploring temperature-modulated DSC TOPEM® method. Specific heat capacity and non-reversing heat flow variations in realgar α-As4S4 demonstrate two endothermic events, these being ascribed to interphase α → β transformation at ∼(540–550) K, and melting of this newly-formed high-temperature β-As4S4 phase at 581–582 K. This polymorph originated from thermal alteration of mineral realgar possesses congruent melting in contrast to synthetic β-As4S4 polymorph, which shows non-equilibrium melting due to accompanied generation of compositionally-authentic amorphous phase. Calorimetric studies on synthetic β-As4S4 in powdered coarse-grained and milled states demonstrate complicated non-equilibrium melting in principally different crystalline-amorphous environments along with crystal-to-glass transformation. Structural-chemical heterogeneity of β-As4S4 crystallites results in incongruent double-peak melting through two endothermic events at ∼578 K and ∼588 K. The amorphous phase formed under high-energy milling of synthetic β-As4S4 possesses a dual nature due to stabilization of As-rich glassy substances with low- and high-temperature glass transition mid-points. This process in the powdered synthetic β-As4S4, identified as re-amorphization of initial amorphous phase and direct vitrification from β-As4S4 crystallites, was parameterized as compared to calorimetric thermal-alteration events in orpiment As2S3 mineral.

Analysis of Multiple Melting of High-Speed Melt Spun Polylactide Fibers Based on Kinetic Modeling

ABSTRACTMultiple melting behavior of high-speed melt-spun polylactide (PLA) fibers was investigated by temperature modulated differential scanning calorimetry (TMDSC) in the heating process with various modulation periods in the calorimeter. In the case of the as-spun PLA fibers taken-up at 1 km/min, a melting endothermic peak and a recrystallization exothermic peak appeared at the same peak temperature of 151°C in the reversing and non-reversing heat flows (RHF and NRHF), respectively, whereas at 168°C, an endothermic peak was observed in both the RHF and NRHF. On the other hand, the as-spun PLA fibers taken-up at a high-speed of 6 km/min showed the melting in both the RHF and NRHF, but the recrystallization behavior was not obvious in the NRHF at the shorter modulation period conditions. The obtained data were analyzed based on the kinetic modeling of melting proposed by Toda et al. The real and imaginary parts of the complex apparent heat capacity in the melting region showed a strong modulation period dependence for both the low- and high-speed spun fibers. The endothermic heat flow of melting was separated by extrapolating the frequency to zero. For the PLA fibers spun at 1 km/min, a set of melting and recrystallization peaks in the RHF and NRHF appeared even for the melting at 168°C. In other words, the simultaneous occurrence of melting and recrystallization was confirmed through this extrapolation. For the 6 km/min PLA fibers, the presence of an exothermic heat of recrystallization was clearly confirmed at a peak temperature of 164°C.

Investigation of Morphology of Recycled PET by Modulated DSC

During research injection molded samples were made from recycled poly (ethylene terephthalate) (PET). Morphological properties of samples were investigated by modulated differential scanning calorimetry (MDSC). Total heat flow was separated in two parts, reversing and non-reversing heat flow during measurements. Relationships were found between crystallization and melting processes: the initial crystallinity equals to the non-reversing melting, and the post-crystallization processes equals to reversing melting.

Multiple Melting Behavior of High-Speed Melt Spun Polylactide Fibers

ABSTRACTHigh-speed melt spinning of polylactide (PLA) was conducted and the structure and multiple melting behavior of the as-spun fibers were investigated. In the analysis of temperature modulated differential scanning calorimetry (TMDSC) thermograms for the as-spun PLA fibers taken-up at 1 and 6 km/min, the peaks around the melting temperature region in the reversing heat flow (RHF) and nonreversing heat flow (NRHF) curves were mainly separated into (1) a pair of an endothermic peak (Peak L) in RHF and an exothermic peak (Peak R) in NRHF in a low temperature region, (2) an endothermic peak (Peak M) both in RHF and NRHF (only in RHF for PLA fiber spun at the low-speed) in an intermediate temperature region, and (3) an endothermic peak (Peak H) both in RHF and NRHF in a higher temperature region. Wide-angle X-ray diffraction (WAXD) measurements were conducted during the heating process of the as-spun fibers cut into powders. In the case of fibers obtained at 1 km/min, disordered crystals, i.e. α′-form crystals, were formed through cold crystallization followed by a disorder-to-order phase transition, i.e. α′ to α crystalline modification, with partial melting of the α′ crystals around 148.5°C in the temperature range of Peaks R and L. Finally, the α form crystals melted above 169.4°C, in the temperature range of Peak H. On the other hand, the PLA crystals generated by the orientation-induced crystallization during the spinning process at a spinning velocity of 6 km/min did not show a WAXD profile of perfect α form crystals but showed an intermediate structure having lattice spacings between the α′ and α forms. Such intermediate crystals did not transformed into α form crystals during the heating process.

Effect of Compression on the Molecular Arrangement of Itraconazole-Soluplus Solid Dispersions: Induction of Liquid Crystals or Exacerbation of Phase Separation?

Predensification and compression are unit operations imperative to the manufacture of tablets and capsules. Such stress-inducing steps can cause destabilization of solid dispersions which can alter their molecular arrangement and ultimately affect dissolution rate and bioavailability. In this study, itraconazole-Soluplus solid dispersions with 50% (w/w) drug loading prepared by hot-melt extrusion (HME) were investigated. Compression was performed at both pharmaceutically relevant and extreme compression pressures and dwell times. The starting materials, powder, and compressed solid dispersions were analyzed using modulated differential scanning calorimetry (MDSC), X-ray diffraction (XRD), small- and wide-angle X-ray scattering (SWAXS), attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), and broadband dielectric spectroscopy (BDS). MDSC analysis revealed that compression promotes phase separation of solid dispersions as indicated by an increase in glass transition width, occurrence of a peak in the nonreversing heat flow signal, and an increase in the net heat of fusion indicating crystallinity in the systems. SWAXS analysis ruled out the presence of mesophases. BDS measurements elucidated an increase in the Soluplus-rich regions of the solid dispersion upon compression. FTIR indicated changes in the spatiotemporal architecture of the solid dispersions mediated via disruption in hydrogen bonding and ultimately altered dynamics. These changes can have significant consequences on the final stability and performance of the solid dispersions.

Thermogenesis-triggered seed dispersal in dwarf mistletoe.

Lodgepole pine dwarf mistletoe (DM), Arceuthobium americanum, is a parasitic flowering plant and forest pathogen in North America. Seed dispersal in DM occurs by explosive discharge. Notably, slight warming of ripe DM fruit in the laboratory can trigger explosions. Previously, we showed that alternative oxidase, a protein involved in endogenous heat production (thermogenesis) in plants, is present in DM fruit. These observations have led us to investigate if thermogenesis induces discharge. Here, infrared thermographs reveal that ripe DM fruits display an anomalous increase in surface temperature by an average of 2.1±0.8 °C over an average time of 103±29 s (n=9, 95% confidence interval) before dehiscence. Furthermore, both non-isothermal and isothermal modulated differential scanning calorimetry consistently show an exothermic event (~1 J g−1) in the non-reversible heat flow just prior to discharge. These results support thermogenesis-triggered seed discharge, never before observed in any plant.

Rigidity transitions in glasses driven by changes in network dimensionality and structural groupings

Calorimetric, Raman and electrical conductivity properties of alkali borates (, Na) are studied as a function of composition and these show the presence of stiffness transitions and an intermediate phase which are driven by a combination of network dimensionality change and usual topological constraint changes. This picture is confirmed by a detailed Raman analysis showing that specific modes of molecular structural groupings dominate the network structure in the intermediate phase. Their evolution shows a one-to-one correspondance with the observed non-reversing heat flow at the glass transition, and are correlated with thresholds in ionic conductivity that allows identifying a flexible phase at high alkali content, whereas the mildly stressed-rigid -rich glasses are driven by the conversion of planar 2D boroxol ring structures into the 3D structural groupings. These findings deeply modify the usual picture of these archetypal glasses, and reveal the very first example of the onset of rigidity tuned by network dimensional conversion.

Determination of the Voltage-Dependence of Parasitic Heat Flow in Lithium Ion Cells Using Isothermal Microcalorimetry

Parasitic reactions that occur in lithium ion batteries are well known to result in cell failure [1], and therefore being able to measure and reduce these reactions is of utmost importance. Isothermal microcalorimetry has been previously used to determine the relative contribution of parasitic heat flow between cells varying in electrolyte composition [2]. However, the absolute magnitude of the voltage-dependent parasitic heat flow for individual cells has not previously been determined. Here, by varying the current over narrow voltage ranges, the relative contributions of each of the heat flow sources as a function of state of charge can be isolated. When a current is applied to a cell there are three sources of heat flow [3]: Joule heating due to polarization, changes in entropy, and parasitic reactions. Polarization produces a non-reversible heat flow that is proportional to the square of the current, changes in entropy during intercalation and deintercalation produces a reversible heat flow that is proportional to the current, and parasitic reactions produce a heat flow that is thought to be independent of the current. The relative effect of entropy, polarization, and parasitics can therefore be determined over small voltage ranges by varying the current. The data can be fit using a simplistic model where each contribution is modeled with a simple function, typically linear, of the state of charge. The fitting results then give the relative contributions of each term, with particular importance to the function associated with the parasitic heat flow for an individual cell, allowing for the extraction of the voltage-dependence of the parasitic heat flow for an individual cell.Figure 1 shows an example of such an analysis for a 180 mAh machine-made high voltage-LiCoO2/graphite pouch cell with a 1 M LiPF6 in 3:7 EC:EMC electrolyte. The heat flow was measured using a TA instruments TAM III isothermal calorimeter equipped with twelve microcalorimeters with an accuracy of < ±1 mW. There is an excellent agreement between the simplistic model and the experimental data, showing the ability of isothermal microcalorimetry to accurately extract the voltage-dependence of parasitic heat flow in individual cells.This analysis is particularly useful for drawing conclusions about the behaviours of electrolyte additives across different cell chemistries where the method of relative differences in heat flow cannot be used. Results will be presented for three different machine-made pouch cell chemistries: high voltage-LiCoO2/graphite to 4.4 V, Li[Ni0.33Mn0.33Co0.33]O2 (NMC)/graphite to 4.2 V, and NMC/graphite to 4.4 V. The effect of a variety of different additives such as vinylene carbonate, methylene methanedisulfonate, ethylene sulfate, trimethylene sulfate, etc, will also be explored.

Formation of a Rigid Amorphous Fraction in Poly(3-(2'-ethyl)hexylthiophene).

Herein, we detail the formation of a rigid amorphous fraction in poly(3-(2'-ethyl)hexylthiophene) (P3EHT) at high relative crystallinity, yielding a more complete picture of the solid-state structure. In the differential scanning calorimetry (DSC) heating scans of isothermally crystallized P3EHT a distinct endothermic peak appears slightly above the crystallization temperature. This previously undescribed endothermic feature of P3EHT's thermal behavior is observed consistently ∼20 °C above the crystallization temperature-shifting to higher temperatures with increasing crystallization temperature-and increases in magnitude with both time and crystallization temperature. Here, we determine the origins of this endothermic peak with DSC and temperature-modulated DSC (TMDSC). TMDSC reveals that the annealing peak observed in the total heat flow (THF)-heat flow equivalent to that of conventional DSC-is a consequence of an enthalpic relaxation observable as an endothermic peak in the nonreversible heat flow (NHF) and a glass transition evident as a step increase in the reversible heat flow (RHF). In conjunction with conventional DSC observations, these results indicate that the observed annealing peak is a consequence of the formation of distinct amorphous regions-a mobile amorphous fraction (MAF) and a rigid amorphous fraction (RAF)-during the isothermal crystallization process and not the melting of a distinct crystallite population or melt recrystallization.

Determination of the Voltage-Dependence of Parasitic Heat Flow in Lithium Ion Cells Using Isothermal Microcalorimetry

Parasitic reactions that occur in lithium ion batteries are well known to result in cell failure [1], and therefore being able to measure and reduce these reactions is of utmost importance. Isothermal microcalorimetry has been previously used to determine the relative contribution of parasitic heat flow between cells varying in electrolyte composition [2]. However, the absolute magnitude of the voltage-dependent parasitic heat flow for individual cells has not previously been determined. Here, by varying the current over narrow voltage ranges, the relative contributions of each of the heat flow sources as a function of state of charge can be isolated. When a current is applied to a cell there are three sources of heat flow [3]: Joule heating due to polarization, changes in entropy, and parasitic reactions. Polarization produces a non-reversible heat flow that is proportional to the square of the current, changes in entropy during intercalation and deintercalation produces a reversible heat flow that is proportional to the current, and parasitic reactions produce a heat flow that is thought to be independent of the current. The relative effect of entropy, polarization, and parasitics can therefore be determined over small voltage ranges by varying the current. The data can be fit using a simplistic model where each contribution has one linear term and one that scales with the state of charge. The fitting constants then give the relative contributions of each term, with particular importance to the parameters associated with the parasitic heat flow for an individual cell, allowing for the extraction of the voltage-dependence of the parasitic heat flow for an individual cell.Figure 1 shows an example of such an analysis for a 225 mAh machine-made LiCoO2/graphite pouch cell with a 1 M LiPF6 in 3:7 EC:EMC electrolyte. The heat flow was measured using a TA instruments TAM III isothermal calorimeter equipped with twelve microcalorimeters with an accuracy of < ±1 mW. There is an excellent agreement between the simplistic model and the experimental data, showing the ability of isothermal microcalorimetry to accurately extract the voltage-dependence of parasitic heat flow in individual cells.This analysis is particularly useful for drawing conclusions about the behaviours of electrolyte additives across different cell chemistries where the method of relative differences in heat flow cannot be used. Results will be presented for four different machine-made pouch cell chemistries: LiCoO2/graphite to 4.2 V, Li[Ni0.33Mn0.33Co0.33]O2 to 4.2 V, Li[Ni0.33Mn0.33Co0.33]O2 to 4.4 V, and Li[Ni0.4Mn0.4Co0.2]O2 to 4.7 V. The effect of a variety of different additives such as vinylene carbonate, methylene methanedisulfonate, ethylene sulfate, trimethylene sulfate, etc, will also be explored.

Thermo-analytical study on transitions in styrene–maleic anhydride copolymers with low- and high-molecular weights

Abstract The thermal behaviour near the glass transition temperature Tg was studied for poly(styrene-co-maleic anhydride) or (SMA) copolymers with a broad range of molecular weight (Mw = 5500–180,000 g/mol) and amount of maleic anhydride (22–50 mol%). The influences of molecular structure for low- and high-molecular weight copolymers were detailed by using thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and temperature modulated differential scanning calorimetry (TMDSC). Data from TGA provide evidence for variations in thermal stability, mainly over the first degradation step up to 350 °C. Most interesting data results from TMDSC for high-molecular weight SMA, where the transition in reversible heat flow and Δcp remains almost constant, while the transition in non-reversible heat flow and Δcp gradually increases with amount MA. Comparing low- and high-molecular weight SMA, a linear model for Tg cannot be applied for the global amount of maleic anhydride, while it can be successfully implemented if a more detailed molecular structure with two constituents is considered. As such, further evidence is provided for the intrinsic heterogeneous molecular structure of high-molecular weight SMA with “styrene-rich” and “maleic-anhydride-rich” polymer segments.

Temperature-modulated differential scanning calorimetry for the study of reversing and nonreversing heat flow of shape memory alloys

The measurement of the thermophysical parameters associated with the thermoelastic martensitic transformation (TMT) of shape memory alloys (SMAs) is fundamental for the correct simulation of the process describing the outcome of SMA actuation. In particular, there are two important energy components during a TMT: the first one is a reversing form related to the nucleation and accommodation of martensite twin boundaries upon cooling, and it is restored when the martensite is heated up to austenite finishing transformation temperature. The second one is represented by a nonreversing form associated with the movement of the grain interfaces, with the defect forming and with the work raised when a plastic transformation occurs. This nonreversing component is dissipated in the form of heat and internal frictional work. Standard differential scanning calorimetry (DSC) measures the sum of these two contributions; therefore, DSC is not able to identify the reverse component that is the one useful in simulation and mathematical modeling of the SMA actuation. Temperature-modulated DSC (MDSC) is really able to separate the two energy forms by the superimposition of a sinusoidal temperature signal to the standard DSC linear temperature ramp. In this paper, MDSC is used to investigate the evolution of reversing and nonreversing heat flows of several NiTi and NiTi-based wires and sheets depending on the microcrystalline state. Observations on the reversing specific heat capacity trend are also drawn.

Behaviors of Endothermic and Exothermic of Wood and Wood Components between 100℃ and 200℃

To clarify the mechanism of the heat-treatment for wood between 100°C and 200°C, the behaviors of endothermic and exothermic of wood and wood components (cellulose, hemicellulose, lignin) between 100°C and 200°C under the dried air condition were measured using the temperature-modulated DSC. The measurement were carried out in the heating processes under stepwise heating and cooling cycles (100°C→140°C→100°C→160°C→100°C→180°C→100°C→200°C, ±5°C/min). The total heat flow (THF) and the nonreversing heat flow (NRHF) curves of wood, cellulose and hemicellulose in the 1st heating process (100°C→140°C) were found in the exothermic side compared with the curves in other heating processes. The reversing heat flow (RHF) curves of wood in the 4th heating process (100°C→200°C) were found in the exothermic side compared with the curves in other heating processes. However the RHF curves of cellulose and hemicellulose were observed on the almost same line. The THF and NRHF curves of lignin were shifted to the exothermic side with the stepwise heating and cooling cycles. On the other hand, the RHF curves of lignin were shifted to the endothermic side with the cycles. These results suggest that the effects of the heat-treatment for wood between 100°C and 200°C are based on the thermodynamic stabilization of the conformation of cellulose, hemicellulose and especially lignin due to the heating history.

Insight into the annealing peak and microstructural changes of poly(l-lactic acid) by annealing at elevated temperatures

An endothermic annealing peak appears in DSC heating scans of cold-crystallized poly(l-lactic acid) after annealing above the cold-crystallization temperature. The annealing peak shifts to higher temperature and its magnitude increase with increasing annealing temperature or time. Furthermore, the origin of the annealing peak and microstructural changes of poly(l-lactic acid) were investigated by temperature-modulated differential scanning calorimetry (TMDSC), Wide-angle X-ray diffraction (WAXD), small-angle X-ray scattering (SAXS) and dynamic mechanical analysis (DMA). During TMDSC scan, the observed annealing peak on the total heat flow (THF) is separated into an endothermic peak of enthalpy relaxation on the nonreversible heat flow (NHF) and a glass transition (devitrification) step on the reversible heat flow (RHF). SAXS results showed that the significant increase in long period mainly arises from the increase in the amorphous layer after annealing. The storage modulus is enhanced with increasing annealing temperature, accompany with glass transition temperature shifts to lower temperature, and the softening is also observed above annealing temperature in DMA measurements. It is showed that the annealing at elevated temperature promotes the chain mobility of the constraint amorphous phase and divides them into more orderly rigid amorphous fraction (RAF) and more disorderly mobile amorphous fraction (MAF). The above results indicate that the annealing peak is attributed to the nonreversible enthalpy relaxation of RAF, which formed by annealing process, rather than the melting of thin lamellae or imperfect crystals formed by secondary crystallization.