Non-reversible Component Research Articles

Effect of competitive adsorption between specialty admixtures and superplasticizer on structural build-up and hardened property of mortar phase of ultra-high-performance concrete

This study aims to enhance structural build-up of ultra-high-performance concrete (UHPC) without influencing the initial flowability, which is critical in repair applications (e.g., use of UHPC in thin bonded overlays for bridge deck rehabilitation). Specialty admixtures, such as a viscosity-modifying admixture (VMA), have been used to enhance the structural build-up at rest. However, the use of specialty admixtures can increase superplasticizer (SP) demand to maintain proper flowability; the synergistic effect of these admixtures on structural build-up is not well understood as the coupled admixture content can reverse the net effect on yield stress, viscosity, and thixotropy. In this study, VMAs including welan gum (WG), diutan gum (DG), and cellulose ether (CE) were used. Styrene-butadiene rubber (SBR) and acrylic ester (AE) latex polymers (LPs) that can enhance structural build-up and bond strength were also incorporated. The competitive adsorption between these specialty admixtures and SP and its effect on structural build-up, early-age hydration, compressive and pull-off strengths, porosity and entrapped air content of UHPC mortar (i.e., without fiber) were systematically investigated. Test results indicated that the incorporation of anionic WG and DG that exhibited high competitive adsorption with SP led to a 275%–450% enhancement in structural build-up despite the 55%–135% increase of SP demand. Such increase was limited to 130% when using CE, SBR, or AE that cannot effectively adsorb onto cement particles in the presence of SP. This was because the high competitive adsorption between specialty admixtures and SP strengthened the particle flocculation and promoted the cement hydration in the first hour; the latter resulted in 20%–40% enhancement in non-reversible component of structural build-up. Furthermore, the use of LP at low and moderate dosages secured high bond strength to normal strength mortar despite 8%–33% decrease in compressive strength given the increased capillary porosity. The use of VMAs increased the entrapped air and resulting 5%–15% lower compressive strength, while the bond strength was not influenced.

Reversible parallel communicating finite automata systems

We study the concept of reversibility in connection with parallel communicating systems of finite automata (PCFA in short). We define the notion of reversibility in the case of PCFA (also covering the non-deterministic case) and discuss the relationship of the reversibility of the systems and the reversibility of its components. We show that a system can be reversible with non-reversible components, and the other way around, the reversibility of the components does not necessarily imply the reversibility of the system as a whole. We also investigate the computational power of deterministic centralized reversible PCFA. We show that these very simple types of PCFA (returning or non-returning) can recognize regular languages which cannot be accepted by reversible (deterministic) finite automata, and that they can even accept languages that are not context-free. We also separate the deterministic and non-deterministic variants in the case of systems with non-returning communication. We show that there are languages accepted by non-deterministic centralized PCFA, which cannot be recognized by any deterministic variant of the same type.

Application of temperature modulation to FTIR spectroscopy: an analysis of equilibrium and non-equilibrium conformational transitions of poly(ethylene terephthalate) in glassy and liquid states

In this paper, the application of a temperature modulation to the temperature-resolved FTIR analysis is reported. The advantage offered by the spectroscopic investigation, able to follow the micro-structural and conformational sample modification involved in sample thermal transformation, was merged to that of temperature modulation, related to the possibility to separate the reversing (in-equilibrium within the experimental condition) to the non-reversing (non-equilibrium) processes. The potentiality of the technique (modulated temperature FTIR, MTFTIR) is highlighted through the study of the thermal transitions of amorphous poly(ethylene terephthalate) from 50 °C to the cold-crystallization. After the presentation of the theoretical framework and the experimental conditions, a step-by-step description of acquired data elaboration is given. The total variation of a selected band intensity as function of mean temperature as well as its reversing and non-reversing components are obtained. The evolution of the bands at 1340 and 971 cm−1, assigned to the trans conformation of the ethylenic unit and to the all-trans conformation of the repeating unit, respectively, are investigated. As expected, the glass transition is observed in the reversing components meanwhile the recovery of the glass relaxation and cold crystallization in non-reversing ones. Particularly interesting resulted the behaviour of the sample in the supercooled liquid state, between the glass transition and the cold-crystallization onset, in which the results show that the ethylenic conformers are in-equilibrium while the all-trans sequences are not. MTFTIR is confirmed to be a technique particularly suitable for the characterization of non-equilibrium conformational states of polymers.

Ultrafast transient absorption spectroscopy of porphyrin-based NanoGUMBOS, silver-gold core-shell, and silicon nanoparticles

Event Abstract Back to Event Ultrafast transient absorption spectroscopy of porphyrin-based NanoGUMBOS, silver-gold core-shell, and silicon nanoparticles Jeewan C. Ranasinghe1, Asela S. Dikkumbura1, Rami A. Khoury1, Isiah Warner1 and Louis Haber1* 1 Louisiana State University, United States Ultrafast transient absorption spectroscopy provides information about excited state dynamics including electron transfer, energy transfer, internal conversion, and intersystem crossing providing important information relevant in optoelectronics, photovoltaics, and molecular sensing applications. The excited-state spectroscopy and ultrafast dynamics of porphyrin and zinc porphyrin-based nanoGUMBOS are reported. NanoGUMBOS are nanoparticles derived from a group of uniform materials based on organic salts or solid phase equivalents of ionic liquids. Porphyrin and zinc porphyrin dyes in water exhibit long-lived excited states on the order of several nanoseconds due to intersystem crossing to triplet states. However, these porphyrin lifetimes are significantly faster when in the nanoGUMBOS environment due to electronic delocalization, energy transfer, and enhanced intersystem crossing. In another study, the ultrafast spectroscopy of silver and silver-gold core-shell colloidal nanoparticles are investigated as a function of pump excitation power and nanoparticle size. The excited-state lifetimes of these plasmonic nanoparticles and their corresponding plasmon spectra show pump power dependence of electron-phonon and phonon-phonon scattering lifetimes. Additionally, the synthesis and spectroscopic characterization of silicon nanoparticles (SiNPs) are reported. As-synthesized SiNPs are terminated with allyl amine to protect the surface against oxidation. Temperature and pH-dependent luminescence is studied to determine the underlying mechanics of the corresponding excited-state relaxation. The photoluminescence intensity is observed to decrease under increasing temperature, indicating the presence of thermally-activated nonradiative relaxations with both reversible and nonreversible components. Overall, the synthesis, characterization, and ultrafast spectroscopy of a wide variety of colloidal nanomaterials are studied for the development of potential optoelectronic and molecular sensing applications. Acknowledgements Generous financial support for this work was provided from Louisiana State University. We would like to acknowledge Ying Xiao for help with transmission electron microscopy. Keywords: transient absorption, Nanoparticles, NanoGUMBOS, Silicon, core-shell, Electron-phonon and electron-electron interactions Conference: National Organization for the Professional Advancement of Black Chemists and Chemical Engineers (NOBCChE) 45th Annual Conference , Orlando, Florida, United States, 17 Sep - 20 Sep, 2018. Presentation Type: Poster Presentation Topic: Physical Chemistry Citation: Ranasinghe JC, Dikkumbura AS, Khoury RA, Warner I and Haber L (2019). Ultrafast transient absorption spectroscopy of porphyrin-based NanoGUMBOS, silver-gold core-shell, and silicon nanoparticles. Front. Chem. Conference Abstract: National Organization for the Professional Advancement of Black Chemists and Chemical Engineers (NOBCChE) 45th Annual Conference . doi: 10.3389/conf.fchem.2018.01.00015 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 09 Oct 2018; Published Online: 17 Jan 2019. * Correspondence: Mr. Louis Haber, Louisiana State University, Baton Rouge, United States, lhaber@lsu.edu Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers Jeewan C Ranasinghe Asela S Dikkumbura Rami A Khoury Isiah Warner Louis Haber Google Jeewan C Ranasinghe Asela S Dikkumbura Rami A Khoury Isiah Warner Louis Haber Google Scholar Jeewan C Ranasinghe Asela S Dikkumbura Rami A Khoury Isiah Warner Louis Haber PubMed Jeewan C Ranasinghe Asela S Dikkumbura Rami A Khoury Isiah Warner Louis Haber Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

On the use of Modulated Temperature Differential Scanning Calorimetry to assess wax crystallization in crude oils

In this paper, measurements of the Wax Precipitation Temperature (WPT) of waxy crude oil samples by a type of Modulated Temperature Differential Scanning Calorimetry, commercially termed StepScan™ (SSDSC), are reported. Repeatable and reliable measurements of WPT by SSDSC are found to depend on specific experimental cooling parameters used with this technique. When appropriate parameters are used, two exothermic peaks can be detected on the non-reversing component and one peak on the reversing component (RC) of SSDSC thermograms. We also present indications that the onset of the peak on RC is related to the change in phase angle detected by sinusoidal temperature modulation programs and that it likely does not correspond to a measurement of the WPT. Additionally, by further adjusting SSDSC experimental parameters, because SSDSC is not based on a sinusoidal temperature modulation, reproduction of SSDSC cooling programs by rheometry and Polarized Light Microscopy (PLM) is possible. It was found that the different values used for the SSDSC cooling parameters, when reproduced on the rheometer, do not impact the oil’s rheology significantly at the applied shear rate. Moreover, it is shown that the onset of the peak detected on the RC portion of SSDSC thermograms is associated with a significant increase in average wax crystal number, as indicated by microscopy; while the consequences on average wax crystal size are found to depend on the initial cooling temperature. Furthermore, comparisons of WPT determination by SSDSC and by conventional DSC analysis show the potential of SSDSC in obtaining comparable and more reliable WPT measurements depending on crude oil composition and SSDSC cooling parameters.

Variance Reduction Using Nonreversible Langevin Samplers.

A standard approach to computing expectations with respect to a given target measure is to introduce an overdamped Langevin equation which is reversible with respect to the target distribution, and to approximate the expectation by a time-averaging estimator. As has been noted in recent papers [30, 37, 61, 72], introducing an appropriately chosen nonreversible component to the dynamics is beneficial, both in terms of reducing the asymptotic variance and of speeding up convergence to the target distribution. In this paper we present a detailed study of the dependence of the asymptotic variance on the deviation from reversibility. Our theoretical findings are supported by numerical simulations.

Modeling strain and dielectric hysteretic type dependences in polycrystalline ferroelectrics by methods of two-level continuum

A one-dimensional model is represented for electrical and mechanical response of polycrystalline ferroelectric material when exposed to it electric field and coaxial him mechanical stress great intensity. The equations describing the residual polarization and deformation are obtained in the form of equations with differentials, which can also be regarded as finite-difference equations. The reversible or induced components par with residual or nonreversible components of polarization and deformation were introduced. The combination of these components allows us to construct the dielectric and deformation hysteresis loops. It is shown that for a specific choice of the model parameters, it is possible to describe the experimental curves not only qualitatively but also quantitatively. Model can be used in finite element analysis to describe the irreversible processes of polarization and depolarization of ferroelectric ceramics.

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.

Modulated Differential Scanning Calorimetry (MDSC) studies on low-temperature freezing of water adsorbed on clays, apparent specific heat of soil water and specific heat of dry soil

Abstract Low temperature exothermic peaks (LTEP) were previously observed in the DSC experiments on cooling of homoionic clays (Kozlowski, 2011). According to a straightforward hypothesis these thermal events might be attributed to the crystallization of the adsorbed “unfreezable” water. In the present paper, results of additional experiments by use of Modulated Differential Scanning Calorimetry (MDSC) are presented. Decomposition of the total heat capacity measured on cooling into the reversible and nonreversible components indicates that the LTEP are most likely a manifestation of freezing or a similar transition, yet quantitative determination of this freezable fraction would require realistic data on the specific heat of the adsorbed water as a function of its relative content and temperature. Detailed analysis of the apparent specific heat of soil water as a function of temperature shows that a model, in which the amount of water approximately equal to the hygroscopic water content w0.95 (equivalent to 2–3 pseudo-layers) remains unfrozen to − 70 °C, proves consistent with the obtained results and referenced data on the specific heat of the adsorbed water. The hypothesis that the LTEP are related to the freezing of the water hydrating exchangeable cations seems the most reasonable in view of the obtained results and some recent data from references.

Immobilized Polymer Fraction in Hyperbranched Polymer/Silica Nanocomposite Suspensions

The glass transition behavior of hyperbranched polymer (HBP) molecules with suspensions of silica and glass particles up to the concentrated regime (25 vol %) was analyzed by modulated differential scanning calorimetry (MDSC). The reversing and nonreversing components of the MDSC signal were measured on suspensions of untreated and silylated particle of size in the nanometer and micrometer ranges. The heat capacity step (ΔCp) at the glass transition of the HBP was found to be independent of silica loading for microparticles, whereas it decreased with increasing particle amount in the case of nanoparticles. A similar behavior was observed for the enthalpy relaxation. These changes in chain dynamics and the progressive suppression of aging were attributed to immobilization effects of the HBP at the surface of the particles, which became detectable only in the case of a very high specific surface. The immobilized HBP fraction was assumed to form a shell of constant thickness around individual particles and was calculated from the ΔCp at the transition. In the case of untreated particles with a silanol surface, the immobilized shell was formed by HBP molecules H-bonded to the particles. The thickness of the shell was found to be equal to 1.9 nm, which corresponded to half the size of the HBP. In the case of methacrylsilane-treated silica, the immobilized shell thickness was found to be equal to 1.3 nm, which corresponded to a monolayer of covalently bound silane. © 2010 American Chemical Society.

The prevalence and incidence of neurocognitive impairment in the HAART era

HAART suppresses HIV viral replication and restores immune function. The effects of HAART on neurological disease are less well understood. The aim of this study was to assess the prevalence and incidence of neurocognitive impairment in individuals who initiated HAART as part of an AIDS clinical trial. A prospective cohort study of HIV-positive patients enrolled in randomized antiretroviral trials, the AIDS Clinical Trials Group (ACTG) Longitudinal Linked Randomized Trials (ALLRT) study. We examined the association between baseline and demographic characteristics and neurocognitive impairment among 1160 subjects enrolled in the ALLRT study. A history of immunosuppression (nadir CD4 cell count < 200 cells/microl) was associated with an increase in prevalent neurocognitive impairment. There were no significant virological and immunological predictors of incident neurocognitive impairment. Current immune status (low CD4 cell count) was associated with sustained prevalent impairment. The association of previous advanced immunosuppression with prevalent and sustained impairment suggests that there is a non-reversible component of neural injury that tracks with a history of disease progression. The association of sustained impairment with worse current immune status (low CD4 cell count) suggests that restoring immunocompetence increases the likelihood of neurocognitive recovery. Finally, the lack of association between incident neurocognitive impairment and virological and immunological indicators implies that neural injury continues in some patients regardless of the success of antiretroviral therapy on these laboratory measures.

Numerical simulation of DSC and TMDSC curves as well as reversing and nonreversing curve separation

The basic physical meaning of temperature modulation for DSC is an arguable research topic, and its interpretation affects the development of thermal analysis and polymer science. This article studies the basic physical meaning of TMDSC by numerical simulation. DSC and TMDSC output curves are computed for a sample with step changes in its specific heat and for a sample with crystallites melting over the temperature range. The TMDSC curves are further analyzed to obtain the reversing and nonreversing components. It is shown that separation of the reversing and nonreversing components from the underlying heat flow cannot be justified. Some common misconceptions regarding TMDSC are discussed as well. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007

Reducing cholinergic constriction: the major reversible mechanism in COPD

The airway narrowing in chronic obstructive pulmonary disease (COPD) has often been misunderstood as being irreversible. However, a large proportion of patients with COPD do respond to bronchodilator agents with significant changes in lung function. Unlike in asthma, abnormalities in airway smooth muscle structure or function are not believed to play a key role in COPD airway narrowing. Although there are only limited data suggesting that cholinergic tone may be increased in COPD, the well-documented efficacy of antimuscarinic agents in increasing airway calibre suggests that cholinergic tone represents the major reversible component of airflow obstruction in these patients. Airway wall thickening and loss of airway-to-parenchyma interdependence are nonreversible components of airflow obstruction in COPD that may amplify the effect of changes in airway smooth muscle tone. Thus, keeping airway smooth muscle tone to a minimum might offer patients long-lasting airway patency and protection against breathlessness, which is the major complaint of patients with COPD. Receptor antagonism by anticholinergic agents can achieve effective relaxation of airway smooth muscle in COPD. According to a classical view of cholinergic receptor function and distribution, the ideal anticholinergic bronchodilator would be one that blocks both M 1 and M 3 receptors, which mediate airway smooth muscle contraction, but not the M 2 receptor, stimulation of which reduces acetylcholine release from vagus nerve endings and prevents the airway smooth muscle from contracting by excessive increments. Agents with such pharmacodynamic selectivity are not available, but effective and prolonged inhibition of airway smooth muscle tone has been obtained with tiotropium, which binds to all three major muscarinic receptor subtypes, but for much longer to M 3 than to M 2 receptors. Recent data show that long-term treatment with tiotropium for 1 yr helps sustain 24-h airway patency. This sustained effect may help to explain the improvements in both exacerbation rate and lung function observed in chronic obstructive pulmonary disease patients treated with tiotropium.

A STEPSCAN Differential Scanning Calorimetry Study of the Thermal Behavior of Chocolate

STEPSCAN differential scanning calorimetry, a form of temperature-modulated differential scanning calorimetry (DSC), was used to study the thermal transitions occurring during the heating of chocolate of varying thermal histories. Conventional DSC thermograms acquired during heating of chocolate can be complex, with the observation of a series of overlapping endothermic and exothermic events. STEPSCAN DSC was used to deconvolute the total heat flow into reversing (rapid) and nonreversing (slow) components, which were assigned to melting and recrystallization events, respectively. Such a separation is usually difficult using conventional DSC. The recrystallization events were more pronounced in rapidly cooled samples where the polymorphic form V had been nucleated through tempering. Because of the presence of artifacts, STEPSCAN can only provide a crude separation of reversing and nonreversing signals in this system. The general applicability and limitations of STEPSCAN DSC as well as the effects of prenucleation and rate of cooling of chocolate are discussed.

Reversing and Nonreversing Heat Capacity of Poly(lactic acid) in the Glass Transition Region by TMDSC

A study of the glass transition of an amorphous and a semicrystalline poly(lactic acid) (PLA) is performed with adiabatic calorimetry, differential scanning calorimetry (DSC), and temperature-modulated DSC (TMDSC). The reversing, total, and nonreversing apparent heat capacities of samples with different contents of l- and d-lactic acid and with various thermal histories were evaluated. Different modes of TMDSC analyses of amorphous and semicrystalline PLA were compared to the total heat capacity from standard DSC. The enthalpy relaxation and the cold crystallization in the glass transition region are largely irreversible. The melting is largely irreversible, but a 100% reversing fraction is observed at low temperatures from 375 to 420 K, which becomes small inside the major melting peak at about 440 K. From the TMDSC of amorphous PLA, the combined information on endothermic and exothermic enthalpy relaxation and glass transition were deconvoluted into the reversing and nonreversing components. The glass t...

Effect of soft segment length on the thermal behaviors of fluorinated polyurethanes

Differential scanning calorimetry (DSC) and temperature modulated DSC (MDSC) have been applied to investigate the thermal behaviors of fluorinated polyurethanes (FPU), which were obtained using 2,2,3,3-tetrafluoro-1, 4-butanediol as the chain extender and based on various soft segments—polytetramethyl oxides (PTMO) with molecular weights of 650, 1000, 1400 and 2000. An exothermic peak and/or multiple melting endotherms were observed during the heating to melting temperature of soft and hard segments. Attributed to the simultaneous recrystallization and melting processes during heating, these features have been confirmed via MDSC, where an endotherm and an exotherm were noted in reversing and non-reversing components of the heat flow. Separating the non-reversing components from the reversing curves, the dependencies of polyurethane morphology on the length of the soft segment could be clarified using MDSC analysis. Soft segment lengthening significantly influences the morphology of soft segment domains in FPUs. The phase separation and crystallinity of the soft segment increased with its length. However, soft segment length exerted a minor influence on the dissociation temperature of the short-range ordered hard segment domain and on the melting temperature of hard segment crystals. Examination of the heats of melting based on the quasi-isothermal MDSC experiments indicated that the crystallinity of hard segment domains declined with increasing soft segment length.

Thermal properties of poly(vinyl alcohol)–solute blends studied by TMDSC

AbstractA thermal analysis study of blends of semicrystalline poly(vinyl alcohol) (PVA) with a pharmaceutical substance, buflomedil pyridoxal phosphate (BPP) is presented. Temperature‐modulated DSC (TMDSC) was used to determine the Tg as well as the crystallinity of blends with various polymer to drug ratios, for different annealing procedures. Positive deviations from a simple expression for the composition dependence of the glass transition of the blend were found. This result, together with the increased thermal stability of PVA–BPP blends, evidenced by TGA analysis, indicates the existence of specific interactions between the polar groups of the two components. The incorporation of dispersed BPP in the PVA matrix results in a composition‐dependent lowering of the polymer's Tm and degree of crystallinity. In addition, we found that, while melting of pure PVA is predominantly reversing, its melting in the blends acquires an increasingly higher nonreversing component with increasing BPP content in the blend. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 1151–1156, 2004

Przejscia fazowe mieszanin poli(tlenek etylenu)/kwasy karboksylowe zdolnych do akumulowania energii

Melting - solidification phase transitions of various, differing in molecular weight, poly(ethylene oxide) (PEOX) samples and their blends with selected carboxylic acids (capric, lauric, myristic, palmitic or stearic one) have been investigated. The methods of differential scanning calorimerry (DSC) and modulated temperature DSC (MT-DSC) were used (Fig. 4, Table 1 and 2). Theoretical values of enthalpies of phase transitions of PEOX/stearic acid (KS) blends were calculated and compared with experimental data (Fig. 3, Table 3). For some ranges of both substances' concentrations the synergetic effect, expressed as increase in phase transition heat value, was found. The effect observed stimulates further application research works on new organic materials being able to storage of energy. The use of MT-DSC method allowed separating the reversing and nonreversing components of heat flow of melting and solidification processes (Figs. 5-8). The results obtained let interpret the effect of PEOX and KS interaction on the mechanism of crystallization of these substances or their mixtures. This way additional information (in comparison with classical DSC method) concerning phase transitions occurring in PEOX/carboxylic acid systems during heating and cooling was obtained.

Step-scan alternating DSC study of melting and crystallisation in poly(ethylene oxide)

Step-scan alternating differential scanning calorimetry (SSA-DSC) method was applied to investigate the phase behaviour of well-characterised poly(ethylene oxide) (PEO). Influence of the three main measurement's parameters of SSA-DSC method: length of the isothermal segment ( t iso/s), temperature jump between two subsequent isothermal segments (step/deg) and linear heating rate in dynamic segments ( b/K/min), on the shape of reversing and non-reversing components during the melting and crystallisation of PEO, has been evaluated. It was found that the reversing component during melting of PEO is increasing with an increase of the isothermal segment length. This effect is due to the existence of defected polymer crystal structures that form metastable regions between crystal phase and already melted polymer. Reversible recrystallisation in the presence of still existing polymer crystals is facilitated by longer isothermal segments. By increasing the step, the equilibrium of reversible processes is shifted towards products and activation of rate-controlled processes takes place; molecular nucleation is hampered and partial melting and/or recrystallisation proceed slower—this effect can be observed as a decrease of reversing signal with increasing step.

Phase Behavior of Poly(Ethylene Oxide) Studied by Modulated‐Temperature DSC—Influence of the Molecular Weight

Melting and crystallization behavior of poly(ethylene oxide) (PEO) with different molecular weight was investigated by modulated‐temperature differential scanning calorimetry (MT‐DSC)—step‐scan alternating DSC. It was found that by separating the reversing and nonreversing components of the (total) heat flow, PEO 10000, which exhibits the highest degree of crystallinity, shows the smallest nonreversing signal during crystallization. This effect can be attributed to the favorable structural features associated with spacial alignment. On the other hand, the crystallization process of PEO with molecular weight of 3400 is hindered by a relatively high content of end groups that may cause defects in the crystal lattice. For PEO 35000, low segmental mobility and chain entanglements lower the rate of crystallization. The area of the reversing component of PEO melting for different molecular weight fractions confirms that for PEO 10000, recrystallization is less intensive than for both the lower and higher molecular weight analogues.