Modelización de la transición vítrea con relajación entálpica a partir de datos térmicos

[Resumen] El objetivo de este trabajo es modelizar la curva de flujo de calor total que se obtiene a partir de ensayos de calorimetría diferencial de barrido (DSC) estándar en el rango de temperaturas de la transición vítrea y la recuperación entálpica. Para ello se propone un modelo matemático que permite la estimación de las curvas de flujo de calor reversing y nonreversing (se utiliza la denominación inglesa para evitar la confusión con procesos termodinámicamente reversibles o irreversibles) a partir de la curva de flujo de calor total obtenida en un ensayo de calorimetría diferencial de barrido. El modelo se ajusta de forma óptima a la curva de flujo de calor total mediante técnicas de regresión no lineal. Análogamente al MTDSC, el modelo propuesto permite separar los procesos reversing y nonreversing, pero con la diferencia importante de que al trabajar con datos de flujo de calor total, las señales separadas no se verán afectadas por la frecuencia de modulación. Teniendo en cuenta estas diferencias, la separación obtenida con el modelo se compara con la obtenida mediante MTDSC y se obtiene una estimación del efecto de la frecuencia. La posibilidad de aplicar este modelo resulta de especial interés en el estudio del envejecimiento físico de materiales amorfos o parcialmente amorfos, los cuales al ser almacenados a temperaturas inferiores a su temperatura de transición vítrea, evolucionan espontáneamente hacia un estado de equilibrio experimentando lo que se conoce como relajación entálpica. En esas situaciones, el estudio de la transición vítrea mediante DSC estándar es muy complicado, a no ser que se borre previamente la historia térmica, lo cual alteraría el material. Conviene destacar que la importancia del modelo no radica en poder obtener mediante DSC estándar algo similar a lo que se obtiene mediante MTDSC, sino en que la estimación de la temperatura de transición vítrea obtenida está libre, a diferencia del caso MTDSC, del efecto de la frecuencia.[Resumo] O obxectivo deste traballo é modelizar a curva de fluxo de calor total que se obtén a partir de ensaios de calorimetría diferencial de varrido (DSC) estándar no rango de temperaturas da transición vítrea e a recuperación entálpica. Para iso proponse un modelo matemático que permite a estimación das curvas de fluxo de calor reversing e non-reversing (utilízase a denominación inglesa para evitar a confusión con procesos termodinámicamente reversibles ou irreversibles) a partir da curva de fluxo de calor total obtida nun ensaio de calorimetría diferencial de varrido. O modelo axústase de forma óptima á curva de fluxo de calor total mediante técnicas de regresión non lineal. Analogamente ao MTDSC, o modelo proposto permite separar os procesos reversing e non-reversing, pero coa diferenza importante de que ao traballar con datos de fluxo de calor total, os sinais separados non se verán afectadas pola frecuencia de modulación. Tendo en conta estas diferenzas, a separación obtida co modelo compárase coa obtida mediante MTDSC e obtense unha estimación do efecto da frecuencia. A posibilidade de aplicar este modelo resulta de especial interese no estudo do avellentamento físico de materiais amorfos ou parcialmente amorfos, os cales ao ser almacenados a temperaturas inferiores á súa temperatura de transición vítrea, evolucionan espontáneamente cara a un estado de equilibrio experimentando o que se coñece como relaxación entálpica. Nesas situacións, o estudo da transisción vítrea mediante DSC estándar é moi complicado, a non ser que se borre previamente a historia térmica, o cal alteraría o material. Convén destacar que a importancia do modelo non radica en poder obter mediante DSC estándar algo similar ao que se obtén mediante MTDSC, senón en que a estimación da temperatura de transición vítrea obtida está libre, a diferenza do caso MTDSC, do efecto da frecuencia.[Abstract] The aim of this work is to model the total heat flow curve obtained by standard differential scanning calorimetry (DSC) in the glass transition-enthalpy recovery range of temperature. To this aim, a mathematical model is proposed, which allows to estimate the reversing and non-reversing curves from the total heat flow curve obtained in a standard DSC test. The model is optimally fitted to the total heat flow curve by non linear regression techniques. Similarly to modulated temperature-DSC (MTDSC), the model allows for separation of the reversing and non-reversing processes, but with the important difference consisting in that, since only total heat flow data are involved in the calculation, the separated signals will not be affected by the modulation frequency. Taking these differences into account, the separation obtained by the model is compared to the one obtained by MTDSC, and an estimation of the frequency effect is also obtained. The possibility of applying this model is of great interest for the study of the physical aging of noncrystalline or partially noncrystalline materials, which when stored at temperatures below its glass transition temperature, evolve spontaneously toward a state of equilibrium, experiencing what is known as enthalpy relaxation. In those situations, the study of the glass transition by standard DSC is very complicated, unless the thermal history is previously erased, which would alter the material itself. It should be mentioned that the importance of the proposed model does not lie on the possibility of obtaining by standard DSC something similar to what is obtained by MTDSC, but on the fact that the obtained estimation of the glass transition temperature is free, differently than in the MTDSC case, from the frequency effect

Discovery of Colossal Breathing-Caloric Effect under Low Applied Pressure in the Hybrid Organic–Inorganic MIL-53(Al) Material

Financiado para publicación en acceso aberto: Universidade da Coruña/CISUG[Abstract] In this work, “breathing-caloric” effect is introduced as a new term to define very large thermal changes that arise from the combination of structural changes and gas adsorption processes occurring during breathing transitions. In regard to cooling and heating applications, this innovative caloric effect appears under very low working pressures and in a wide operating temperature range. This phenomenon, whose origin is analyzed in depth, is observed and reported here for the first time in the porous hybrid organic–inorganic MIL-53(Al) material. This MOF compound exhibits colossal thermal changes of ΔS ∼ 311 J K–1 kg–1 and ΔH ∼ 93 kJ kg–1 at room temperature (298 K) and under only 16 bar, pressure which is similar to that of common gas refrigerants at the same operating temperature (for instance, p(CO2) ∼ 64 bar and p(R134a) ∼ 6 bar) and noticeably lower than p > 1000 bar of most solid barocaloric materials. Furthermore, MIL-53(Al) can operate in a very wide temperature range from 333 K down to 254 K, matching the operating requirements of most HVAC systems. Therefore, these findings offer new eco-friendly alternatives to the current refrigeration systems that can be easily adapted to existing technologies and open the door to the innovation of future cooling systems yet to be developed.This work was financially supported by Ministerio de Economía y Competitividad MINECO and EU-FEDER (projects MAT2017-86453-R and PDC2021-121076-I00), Xunta de Galicia and IACOBUS Programme. Funding for open access fee was provided by Universidade da Coruña/CISU

A relatively simple look at the rather complex crystallization kinetics of PLLA

[Abstract] This work demonstrates that, despite the existence of a significant number of works on PLA crystallization, there is still a relatively simple way, different from those already described, in which its complex kinetics can be observed. The X-ray diffraction (XRD) results presented here confirm that the PLLA under study crystallizes mostly in the α and α′ forms. An interesting observation is that at any temperature in the studied range of the patterns, the X-ray reflections stabilize with a given shape and at a given angle, different for each temperature. That means that both α and α′ forms coexist and are stable at the same temperatures so that the shape of each pattern results from both structures. However, the patterns obtained at each temperature are different because the predominance of one crystal form over the other depends on temperature. Thus, a two-component kinetic model is proposed to account for both crystal forms. The method involves the deconvolution of the exothermic DSC peaks using two logistic derivative functions. The existence of the rigid amorphous fraction (RAF) in addition to the two crystal forms increases the complexity of the whole crystallization process. However, the results presented here show that a two-component kinetic model can reproduce the overall crystallization process fairly well over a broad range of temperatures. The method used here for PLLA may be useful for describing the isothermal crystallization processes of other polymers

The Complexity of Lignin Thermal Degradation in the Isothermal Context

[Abstract] Thermal degradation of lignin in nitrogen atmosphere is evaluated by linear heating and isothermal tests. While linear heating suggests that thermal decomposition in the 200–400 °C range mainly consists of a single step, a careful analysis of isothermal tests points to different lignin fractions having different stabilities. This is an important point for practical predictions, since kinetics obtained as if the degradations at different temperatures were the same would lack practical utility. Instead, stairway type tests are proposed to evaluate the degradation rates and sample quantities involved at the temperatures of interest.This research was funded by MINECO, grant number MTM2017-82724-R and by Xunta de Galicia (Grupos de Referencia Competitiva ED431C-2020-14 and Centro de Investigación del Sistema universitario de Galicia ED431G 2019/01), all of them through the ERDFXunta de Galicia; ED431C-2020-14Xunta de Galicia; ED431G 2019/0

A Logistic Approach for Kinetics of Isothermal Pyrolysis of Cellulose

[Abstract] A kinetic model is proposed to fit isothermal thermogravimetric data obtained from cellulose in an inert atmosphere at different temperatures. The method used here to evaluate the model involves two steps: (1) fitting of single time-derivative thermogravimetric curves (DTG) obtained at different temperatures versus time, and (2) fitting of the rate parameter values obtained at different temperatures versus temperature. The first step makes use of derivative of logistic functions. For the second step, the dependence of the rate factor on temperature is evaluated. That separation of the curve fitting from the analysis of the rate factor resulted to be very flexible since it proved to work for previous crystallization studies and now for thermal degradation of celluloseMinisterio de Asuntos Económicos y Transformación Digital; MTM2017-82724-RXunta de Galicia; ED431C-2020-14Xunta de Galicia; ED431G 2019/0

Properties of Mosquito Repellent-Plasticized Poly(lactic acid) Strands

This article belongs to the Special Issue Thermal and Rheological Characterization of Polymeric Materials[Abstract] Poly(lactic acid) (PLA) is an attractive candidate for replacing petrochemical polymers because it is fully biodegradable. This study investigated the potential of PLA as a sustainable and environmentally friendly alternative material that can be developed into commercially viable wearable mosquito repellent devices with desirable characteristics. PLA strands containing DEET and IR3535 were prepared by twin screw extrusion compounding and simultaneously functioned as plasticizers for the polymer. The plasticizing effect was investigated by thermal and rheological studies. DSC studies showed that the addition of DEET and IR3535 into PLA strands reduced the glass transition temperature consistent with predictions of the Fox equation, thus proving their efficiency as plasticizers. The rheology of molten samples of neat PLA and PLA/repellents blends, evaluated at 200 °C, was consistent with shear-thinning pseudoplastic behaviour. Raman studies revealed a nonlinear concentration gradient for DEET in the PLA strand, indicating non-Fickian Type II transport controlling the desorption process. Release data obtained at 50 °C showed initial rapid release followed by a slower, near constant rate at longer times. The release rate data were fitted to a novel modification of the Peppas-Sahlin desorption modelThis work was financially supported from the Deutsche Forschungsgemeinschaft (DFG), Germany [Grant number AN 212/22-2] and University of Pretoria (UP) Postdoctoral Fellowship programme. The research has been also supported by Spanish MINECO grant MTM2017-82724-R through the ERDFDeutsche Forschungsgemeinschaft (DFG); AN 212/22-2https://www.mdpi.com/journal/molecules/special_issues/Thermal_Rheological_Polymeric_Material

Impact estimates of the actions for the rehabilitation of energy efficiency in residential building

The prediction of energy savings and indoor air quality resulting from the implementation of rehabilitation measures for energy efficiency in residential buildings is paramount to invest in the right technology and optimize available resources. In this paper, we propose a new methodology, based on actual data collection, to quickly and effectively assess energy efficiency and air quality of housings, considering variables such as the customs of the occupants, regulations and the constructive typology. The effect corresponding to the reforms carried out for energy efficiency rehabilitation in four homes built at different periods, under different rules, with different levels of occupancy and characteristics of its constructive elements is evaluated successfully

Multiple phase and dielectric transitions on a novel multi-sensitive [TPrA][M(dca)3] (M: Fe2+, Co2+ and Ni2+) hybrid inorganic–organic perovskite family

The hybrid inorganic–organic [TPrA][M(dca)3] (M: Fe2+, Co2+ and Ni2+) compounds, where TPrA is the tetrapropylammonium cation and dca is the dicyanamide anion, are unique multi-sensitive compounds that display multiple phases and dielectric transitions. These materials exhibit up to three first-order structural transitions (between the polymorphs I, Ia, Ib and II) associated with the same number of dielectric transitions in the temperature range of 210–360 K. The mechanisms responsible for these dielectric responses are found to be novel within the hybrid perovskites, involving ionic displacements of the A-site cations (TPrA) and order/disorder processes of the X anions (dca). In addition, the phase transitions and dielectric transition temperatures can be tuned by applying external hydrostatic pressure or by inducing internal pressure by modifying the tolerance factor through ionic substitution in the B-sites. This multi-sensitive response towards temperature, external and internal pressure opens up promising technological applications for this family of materials, such as dielectric transductors or multistimuli-sensors, whose response can be modulated in a wide range of temperatures and pressuresThe authors are grateful for the financial support from Ministerio de Economía y Competitividad MINECO (MINECO) ENE2014-56237-C4-4-R and Xunta de Galicia under the project GRC2014/042. J. M. B.-G. also wants to thank Barrié Foundation for a predoctoral fellowship and S. Y.-V. to the Xunta de Galicia for a postdoctoral grant (Plan I2C)S

Genomic characterization of escherichia coli isolates belonging to a new hybrid aepec/expec pathotype o153:H10-a-st10 eae-beta1 occurred in meat, poultry, wildlife and human diarrheagenic samples

Different surveillance studies (2005–2015) in northwest Spain revealed the presence of eae-positive isolates of Escherichia coli O153:H10 in meat for human consumption, poultry farm, wildlife and human diarrheagenic samples. The aim of this study was to explore the genetic and genomic relatedness between human and animal/meat isolates, as well as the mechanism of its persistence. We also wanted to know whether it was a geographically restricted lineage, or whether it was also reported elsewhere. Conventional typing showed that 32 isolates were O153:H10-A-ST10 fimH54, fimAvMT78, traT and eae-beta1. Amongst these, 21 were CTX-M-32 or SHV-12 producers. The PFGE XbaI-macrorestriction comparison showed high similarity (>85%). The plasmidome analysis revealed a stable combination of IncF (F2:A-:B-), IncI1 (STunknown) and IncX1 plasmid types, together with non-conjugative Col-like plasmids. The core genome investigation based on the cgMLST scheme from EnteroBase proved close relatedness between isolates of human and animal origin. Our results demonstrate that a hybrid MDR aEPEC/ExPEC of the clonal group O153:H10-A-ST10 (CH11-54) is circulating in our region within different hosts, including wildlife. It seems implicated in human diarrhea via meat transmission, and in the spreading of ESBL genes (mainly of CTX-M-32 type). We found genomic evidence of a related hybrid aEPEC/ExPEC in at least one other countryThis study was supported by projects: AGL2013-47852-R from the Ministerio de Economía y Competitividad (MINECO, Spain) and Fondo Europeo de Desarrollo Regional (FEDER); AGL2016-79343-R from the Agencia Estatal de Investigación (AEI, Spain) and FEDER; PI16/01477 from Plan Estatal de I+D+I 2013-2016, Instituto de Salud Carlos III (ISCIII), Subdirección General de Evaluación y Fomento de la Investigación and FEDER; and ED431C2017/57 from the Consellería de Cultura, Educación e Ordenación Universitaria of Xunta de Galicia and FEDERS

Development and validation of a clinical score to estimate progression to severe or critical state in Covid-19 pneumonia hospitalized patients

The prognosis of a patient with Covid-19 pneumonia is uncertain. Our objective was to establish a predictive model of disease progression to facilitate early decision-making. A retrospective study was performed of patients admitted with Covid-19 pneumonia, classified as severe (admission to the intensive care unit, mechanic invasive ventilation, or death) or non-severe. A predictive model based on clinical, analytical, and radiological parameters was built. The probability of progression to severe disease was estimated by logistic regression analysis. Calibration and discrimination (receiver operating characteristics curves and AUC) were assessed to determine model performance. During the study period 1,152 patients presented with Covid-19 infection, of whom 229 (19.9%) were admitted for pneumonia. During hospitalization, 51 (22.3%) progressed to severe disease, of whom 26 required ICU care (11.4); 17 (7.4%) underwent invasive mechanical ventilation, and 32 (14%) died of any cause. Five predictors determined within 24 hours of admission were identified: Diabetes, Age, Lymphocyte count, SaO2, and pH (DALSH score). The prediction model showed a good clinical performance, including discrimination (AUC 0.87 CI 0.81, 0.92) and calibration (Brier score = 0.11). In total, 0%, 12%, and 50% of patients with severity risk scores ≤5%, 6-25%, and >25% exhibited disease progression, respectively. A simple risk score based on five factors predicts disease progression and facilitates early decision-making according to prognosis.Carlos III Health Institute, Spain, Ministry of Economy and Competitiveness (SPAIN) and the European Regional Development Fund (FEDER)Instituto de Salud Carlos II