Study of the molecular mobility in polymers with the thermally stimulated recovery technique : a review

Thermally stimulated recovery (TSR) is a non-conventional mechanical spectroscopy technique that allows to analyse in detail the relaxation processes of polymeric systems in the low frequency region. This work reviews the main aspects and potentialities of this technique. The different kinds of TSR experiments that can be performed, global and thermal sampling (TS) experiments, are described and illustrated with several examples. Also, the different methods for the determination of the thermokinetic parameters (activation energy and pre-exponential factor) of the thermal sampling (TS) procedure are explained and compared. In this context, the compensation phenomenon, which always appears in TSR results when the studies are performed in the glass transition region of a given system, is discussed. Examples of the application of this technique to different polymeric systems during the last 20 years are provided. An emphasis will be made on the analysis of the effect of crystallinity degree and crosslink density on the TSR response. A comparison between the results (characteristic times and activation energies) obtained by different techniques, namely TSR, dynamic mechanical analysis (DMA), and differential scanning calorimetry (DSC), is made

Study of the viscoelastic properties of PET by thermally stimulated recovery

The conformational chain mobility of semi-crystalline poly(ethylene terephthalate), PET, is studied in the glass transition region by thermally stimulated recovery (TSR). The thermal sampling (TS) experiments are analysed according to a simple Voigt- Kelvin model. Both the activation energy (Ea) and the pre-exponential factor follow the usual trends with temperature and present the compensation behaviour, that is discussed in this work. The influence of crystallinity and orientation on the glass transition dynamics is analysed. In particular, the profile of variation of Ea with temperature is found to be very sensitive to these two effects. TSR results are compared with the results obtained by differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). In addition, this technique was successfully applied to the study of the structural relaxation process of semi-crystalline PET and it was found that the characteristic parameters of the TSR curves vary with the ageing temperature and ageing time. These variations are explained on the basis of the changes verified on the distribution of characteristic times with ageing

Enthalpy relaxation studies in polymethyl methacrylate networks with different crosslinking degrees

Structural relaxation of PMMA networks with distinct crosslink density has been studied by differential scanning calorimetry (DSC). The crosslinking agent used was ethylene glycol dimethacrylate (EGDMA). The experiments were carried out on heating after the samples have been subjected to distinct thermal histories, namely isothermal stages at different temperatures below the glass transition temperature for distinct times and cooling at different rates. These studies revealed a broadening of the glass transition with increasing crosslinking degree due to the constraints imposed by the crosslinks and suggested the presence of crosslink heterogeneity in the networks. A phenomenological model based on the configurational entropy concept was used to simulate the structural relaxation phenomenon and to evaluate the temperature dependence and distribution of the relaxation times of the conformational rearrangements for these networks. The agreement between the experimental results and the simulated thermograms was quite satisfactory. In addition, the kinetic fragility of the networks was evaluated from the results corresponding to the thermal treatments at distinct cooling rates. It was found an increase of the fragility index m with increasing crosslinking degree

Analysis of the thermal environment inside the furnace of a dynamic mechanical analyser

In this work, the thermal environment inside the furnace of a dynamic mechanical analyser was investigated by putting a standard sample in distinct positions inside the furnace. Penetration experiments were carried out in order to compare the measured melting temperature of the sample with the theoretical melting temperature. The thermal gradients were investigated for three distinct modes: compression, three-point bending and extension. In the compression mode a small variation of the measured melting point was found as a function of the radial distance. However, for both extension and three-point bending modes quite important variations were measured along the relevant directions. In the former case the bottom clamp was found to be warmer than the upper one and in the latter mode higher temperatures are found in the extremities of the samples (closer to the furnace wall). Other factors, such as the influence of the heating rate and the purge gas flow rate, were also investigated. In the particular case of the extension mode, it was found that the measured melting temperature decreased with increasing scanning rate. A simple model was used in order to investigate the influence of the temperature gradients on DMA measurements. For temperature gradients along the sample length below 10°C the differences in the viscoelastic parameters (tan d, D∗ and E∗) from the homogeneous case are small when compared with typical experimental errors

Influence of experimental variables on thermally stimulated recovery results: analysis of simulations and real data on a polymeric system

Thermally stimulated recovery (TSR) is a low frequency mechanical spectroscopy technique that allows investigation of conformational mobility in polymeric systems. In this study the effect of initial parameters chosen to perform experiments on the TSR response of a material in the thermal sampling mode is investigated. The studied experimental parameters are creep time (tσ) recovery time (tr) and window width (ΔTw); all are independently changed at one constant creep temperature. A simple model, able to describe global TSR and TS measurements, is used to evaluate the influence of each of the different parameters. The simulations are conducted for a system with a uniform distribution of activation energies and a fixed pre-exponential factor. These simulation results are qualitatively compared with some experimental data obtained for semicrystalline poly(ethylene terephthalate) under different conditions in the glass transition region. The tendencies resulting from the influence of the studied parameters on the intensity, the position of the TS peaks and the corresponding activation energies are found to be the same for the experimental and simulated results. Only the variation of the activation energy with tσ is opposite to that observed with the modelling results; this feature is explained on the basis of structural relaxation effects

Conformational mobility in a polymer with mesogenic side groups- dielectric and DSC studies

The relaxation times of the conformational rearrangements of the main-chain segments of a liquid crystalline side-chain polymer was determined from differential scanning calorimetry experiments in the temperature interval around and below its glass transition. Phenomenological models with fitting parameters were used to evaluate the temperature dependence of the relaxation times and the form parameter of the relaxation times distribution. These parameters were compared with its counterparts in the dielectric a relaxation process which appear in the temperature interval immediately above the calorimetric glass transition. For the temperature interval below the calorimetric glass transition the differential scanning calorimetry (DSC) results were compared with the dielectric results obtained by the thermally stimulated depolarisation technique

Water and protein dynamics in protein-water mixtures over wide range of composition

© 2012 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.[EN] Water and protein dynamics in two globular protein-water systems, water-lysozyme and water-BSA (bovine serum albumine), were studied by differential scanning calorimetry (DSC), dielectric relaxation spectroscopy (DRS) and thermally stimulated depolarization currents (TSDC) techniques. Water equilibrium sorption isotherms (ESI) measurements were also recorded at room temperature. The samples covered a wide range of composition, from practically dry solid pellets (2wt% of water) to dilute solutions (82wt% of water). Crystallization and melting events of water were studied by DSC and the amount of uncrystallized water was calculated. The evolution of dynamics with hydration level was followed for various dielectric relaxation processes, the emphasis being given to relaxation processes of polar groups on the surface of the proteins and of uncrystallized water molecules. A relationship between the formation of a conductive percolating water cluster and the saturation of the water process was found.This research has been co-financed by the European Union (European Social Fund - ESF) and Greek national funds through the Operational Program "Education and Lifelong Learning" of the National Strategic Reference Framework (NSRF) - Research Funding Program: Heracleitus II. Investing in knowledge society through the European Social Fund.Kyritsis, A.; Panagopoulou, A.; Pissis, P.; Sabater I Serra, R.; Gómez Ribelles, JL.; Shinyashiki, N. (2012). Water and protein dynamics in protein-water mixtures over wide range of composition. IEEE Transactions on Dielectrics and Electrical Insulation. 19(4):1239-1246. https://doi.org/10.1109/TDEI.2012.6259997S1239124619

Viscoelastic behaviour of polymethyl methacrylate networks with different crosslinking degrees

The influence of the cross-linking degree on the dynamics of the segmental motions close to the glass transition of poly(methyl methacrylate), PMMA, networks was investigated by three different mechanical spectroscopy techniques: thermally stimulated recovery (TSR), dynamic mechanical analysis (DMA), and creep. The application of the time-temperature superposition principle to isothermal DMA and creep results permitted to successfully construct master curves for PMMA networks with distinct cross-linking degrees. The former results were fitted to the KWW equation. The obtained variation of âKWW for the distinct networks indicated that the relaxation curves tend to broaden as the cross-linking degree increases. TSR results clearly revealed a significant shift of the R-relaxation to longer times and a broader relaxation as the cross-linking degree increases, what was also observed by DMA and creep. A change from a Vogel to an Arrhenius behavior was detected by the three techniques with the decrease of temperature below Tg. The temperature dependence of the apparent activation energies (Ea) was calculated from DMA, creep, and TSR experiments; above Tg a good agreement was seen between the Ea values for all the techniques. Furthermore, the effect of the cross-linking degree on the fragility of PMMA networks was evaluated. For these materials an increase of fragility with increasing cross-linking degree was observed

Review of concepts in therapeutic decision-making in HER2-negative luminal metastatic breast cancer

Purpose: Hormone receptor (HR)-positive, Human epidermal growth factor receptor 2 (HER2)-negative metastatic breast cancer (MBC) requires a therapeutic approach that takes into account multiple factors, with treatment being based on anti-estrogen hormone therapy (HT). As consensus documents are valuable tools that assist in the decision-making process for establishing clinical strategies and optimize the delivery of health services, this consensus document has been created with the aim of developing recommendations on cretiera for hormone sensitivity and resistance in HER2-negative luminal MBC and facilitating clinical decision-making. Methods: This consensus document was generated using a modification of the RAND/UCLA methodology, which included the definition of the project and identification of issues of interest, a non-exhaustive systematic review of the literature, an analysis and synthesis of the scientific evidence, preparation of recommendations, and external evaluation with a panel of 64 medical oncologists specializing in breast cancer. Results: A Spanish panel of experts reached consensus on 32 of the 32 recommendations/conclusions presented in the first round and were accepted with an approval rate of 100% about definition of metastatic disease not susceptible to local curative treatment, definition of hormone sensitivity and hormone resistance in metastatic luminal disease and therapeutic decision-making. Conclusion: We have developed a consensus document with recommendations on the treatment of patients with HER2-negative luminal MBC that will help to improve therapeutic benefits

Human mesenchymal stem cells growth and osteogenic differentiation on piezoelectric poly(vinylidene fluoride) microsphere substrates

The aim of this work was to determine the influence of the biomaterial environment on human mesenchymal stem cell (hMSC) fate when cultured in supports with varying topography. Poly(vinylidene fluoride) (PVDF) culture supports were prepared with structures ranging between 2D and 3D, based on PVDF films on which PVDF microspheres were deposited with varying surface density. Maintenance of multipotentiality when cultured in expansion medium was studied by flow cytometry monitoring the expression of characteristic hMSCs markers, and revealed that cells were losing their characteristic surface markers on these supports. Cell morphology was assessed by scanning electron microscopy (SEM). Alkaline phosphatase activity was also assessed after seven days of culture on expansion medium. On the other hand, osteoblastic differentiation was monitored while culturing in osteogenic medium after cells reached confluence. Osteocalcin immunocytochemistry and alizarin red assays were performed. We show that flow cytometry is a suitable technique for the study of the differentiation of hMSC seeded onto biomaterials, giving a quantitative reliable analysis of hMSC-associated markers. We also show that electrosprayed piezoelectric poly(vinylidene fluoride) is a suitable support for tissue engineering purposes, as hMSCs can proliferate, be viable and undergo osteogenic differentiation when chemically stimulated.The authors thank the Portuguese Foundation for Science and Technology (FCT) for financial support under project PTDC/EEI-SII/5582/2014, Strategic Funding UID/FIS/04650/2013 and grants SFRH/BPD/90870/2012 (C.R.) and SFRH/BPD/121526/2016 (D.M.C). The authors acknowledge funding by the Spanish Ministry of Economy and Competitiveness (MINECO) through the project MAT2016-76039-C4-3-R (AEI/FEDER, UE) and from the Basque Government Industry Department under the ELKARTEK program. JLGR, LC, RSS and AS acknowledge funding by the Conselleria de Educación, Investigación, Cultura y Deporte of the Generalitat Valenciana through PROMETEO/2016/063 project. CIBER-BBN is an initiative funded by the VI National R&D&i Plan 2008–2011, Iniciativa Ingenio 2010, Consolider Program, CIBER Actions and financed by the Instituto de Salud Carlos III with assistance from the European Regional Development. This work was partially financed with FEDER funds (CIBERONC (CB16/12/00284)). The authors acknowledge the assistance and advice of Electron Microscopy Service of the UPVinfo:eu-repo/semantics/publishedVersio