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

Crystallization kinetics of a commercial poly(lactic acid) based on characteristic crystallization time and optimal crystallization temperature

This version of the article has been accepted for publication, after peer review and is subject to Springer Nature’s AM terms of use, but is not the Version of Record and does not reflect post-acceptance improvements, or any corrections. The Version of Record is available online at: https://doi.org/10.1007/s10973-020-10081-7[Abstract]: A model is proposed to fit differential scanning calorimetry (DSC) isothermal crystallization curves obtained from the molten state at different temperatures. A commercial 3D printing polylactic acid (PLA) sample is used to test the method. All DSC curves are fitted by a mixture of two simultaneous functions, one of them being a time derivative generalized logistic accounting for the exothermic effect and the other, a generalized logistic, accounting for the baseline. There is a rate parameter, which is allowed to vary across different temperatures. The rate parameter values obtained at different temperatures were jointly explained as a result of three crystallization processes, each one defined by a characteristic crystallization time, a characteristic temperature, and a dispersion or width factor. Apart from the very good fittings obtained at all temperatures, the results agree with the existence of a few crystal forms of PLA, which were demonstrated by other authors. Thus, the main significance of this work consists in providing a new approach in order to mathematically describe the isothermal crystallization kinetics of a polymer from the melt. Such a kinetic description is needed in order to predict the extent of a crystallization process as a function of time at any isothermal temperature. The approach used here allows to understand the overall crystallization of the PLA used in this work as the sum of three crystallization processes, each of them corresponding to a different crystal form. Each experimental crystallization exotherm, which may include more than one crystal form, can be reproduced by a generalized logistic function. The overall rate factor at a given temperature is the weighted sum of the rate factors of the different crystal structures at that temperature. The rate factor of each of these three processes is described by a Gaussian function whose parameters are a crystallization time, a characteristic temperature and a temperature dispersion factor. Therefore, the crystallization rate for each crystal form can be interpreted as a relative likelihood to crystallize at a given temperature. On the other hand, the characteristic crystallization time parameter refers to the time needed for a given crystal structure to be formed at the temperature at which the relative likelihood to crystallize of that form is highestThis research has been supported by the Spanish Ministry of Science and Innovation, MINECO Grant MTM2017–82724-

Thermal and Rheological Properties of Fischer–Tropsch Wax/High-Flow LLDPE Blends

[Abstract]: Waxes find use as processing aids in filled compounds and polyethylene-based masterbatches. In such applications, the thermal and physical property changes they impart to the polymer matrix are important. Therefore, this study details results obtained for blends prepared by mixing a Fischer–Tropsch (F–T) wax with a high-flow linear low-density polyethylene (LLDPE). The melting and crystallization behavior are studied using hot-stage polarized optical microscopy (POM) and differential scanning calorimetry (DSC). The calorimetry results are consistent with partial cocrystallization of the two components. The melting and crystallization exo- and endotherms for the wax- and LLDPE-rich phases remained separate. However, they change in shape and shift toward higher- and lower temperature ranges, respectively. It is found that increasing the wax content delays the crystallization, decreases the overall crystallinity, and reduces the size of the crystallites of the polyethylene-rich phase. Rotational viscosity is measured at 170 °C in the Newtonian shear-rate range. The variation of the zero-shear viscosity with blend composition is consistent with the assumption of a homogeneous melt in which the chains are in an entangled state. Therefore, it is concluded that the wax and LLDPE are, in effect, miscible in the melt and partially compatible in the solid state.Generous financial support from Sasol is gratefully acknowledged. Sasol research grant agreement SAP No. 126/20 G

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

Comparison by thermal analysis of Joule‑cured versus oven‑cured composites

Financiado para publicación en acceso aberto: Universidade da Coruña/CISUG[Abstract]: The current technology for curing high-performance composites, such as those used in industries like such as aeronautics and the automotive industry, is based on the use of autoclaves, where the material is cured by external heating, in large ovens. This type of curing requires enormous amounts of energy, of which only a small part is invested in the actual curing of the material, and the rest is mainly used for heating and maintaining the temperature of the autoclave. An alternative method that entails a lower energy cost compared to the traditional methodology is curing through the Joule effect, in which an electric current is passed through the material, so that it acquires temperature from the inside due to the passage of current through the carbon fibres, triggering and accelerating the curing process of the composite. While Joule curing may provide a much more efficient and faster curing, a control technology is needed to ensure that temperatures all throughout the composite match the temperature programme. In this work, a procedure has been developed to control the Joule effect curing of carbon fibre/epoxy composites in order to compare, by means of differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA), the curing obtained by this method with that obtained by the traditional oven curing method.We acknowledge the financial support provided by the Ministerio de Ciencia e Innovación (Spain), under grant PID2020-113578RB-100, and the Programa de Doutoramento Industrial 2022, funded by Xunta de Galicia, through the grant number 07_IN606D_2022_2695330.Xunta de Galicia: 07_IN606D_2022_269533

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

Application of the Time–Temperature Superposition Principle to Predict Long-Term Behaviour of an Adhesive for Use in Shipbuilding

Financiado para publicación en acceso aberto: Universidade da Coruña/CISUG[Abstract]: The use of adhesives in the marine sector is rather limited at the time being, but their use in specific areas of the ship would be an advantage due, among other things, to their low weight and low stress concentration along the bonding joint. The aim of this work is to predict the long-term behaviour of the material, as this is a critical factor when using adhesive as a bonding method in ships, since its durability must be guaranteed throughout a previously defined life cycle. This can be predicted by applying the time–temperature superposition principle (TTS), which involves carrying out a test at different temperatures for each specimen, considerably reducing the test time. Two types of experiments have been carried out according with operation modes in dynamic mechanical analysis (DMA): a dynamic frequency sweep and a stationary creep test under constant stress, to check the behaviour of the adhesive under both dynamic and sustained loading. The master curve for the frequency study will be constructed in such a way as to cover the whole range of relevant vibrations that can occur on the vessel, while that for the creep test the curve obtained covers a range of 25 years, which is usually used as the minimum service life in shipbuilding. For both, a temperature range from room temperature to the maximum operating temperature of the material established by the manufacturer shall be studied

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

Thermal and rheological comparison of adhesives

This version of the article has been accepted for publication, after peer review and is subject to Springer Nature’s AM terms of use, but is not the Version of Record and does not reflect post-acceptance improvements, or any corrections. The Version of Record is available online at: https://doi.org/10.1007/s10973-019-08882-6[Abstract]: In some industrial sectors such as naval construction, the use of adhesives is still limited to some specific applications. However, shipbuilders, academia and classification societies are cooperating to expand the field of certificated applications of adhesive joints. As a part of a validation study, thermal and rheological studies of the curing process and of the cured adhesives should be included. While a neat glass transition and other relaxation processes can be normally identified by ramp temperature tests performed both on a differential scanning calorimeter or on a rheometer, there are some adhesive systems in which several glass transitions or melting or crystallization processes overlap. Applying a thermal treatment to delete the thermal history and conditioning are common practices to clarify what happens in complex systems. However, although that practices usually help, there are still some complexities due to overlapping processes that cannot be easily understood. An important point of this work is to show how differential scanning calorimetry (DSC) and rheology complement to each other in order to demonstrate several thermal relaxations and to obtain a better understanding of the cure process. The use of two different techniques along with a careful election of the setup parameter values allows to better interpret the thermal events. In addition, thermogravimetry (TG) helps to understand some rheological behaviors. In the end,this work shows how a good insight of the adhesive properties can be obtained by means of the combined use of DSC, rheology and TG.This research has been partially supported by the Spanish Ministry of Science and Innovation, Grants MTM2014-52876-R and MTM2017–82724-R and by UMI UDC-Navanti