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

Biodegradable poly(L-lactic acid) scaffolds with internal hyaluronic acid coating. biological response in vitro

[Excerpt] Poly(L-lactic acid), a synthetic biodegradable polyester, is widely accepted for many tissue engineering applications. Hyaluronic acid (HA), as a polysaccharide of the extra cellular matrix (ECM), besides exhibiting an excellent biocompatibility, influences cell signaling, growth and differentiation. A combination between these materials might be interesting for tissue engineering studies. However, HA must be chemically modified for this use because of its easy dissolution in water and quick degradation in biological environments. Glutaraldehyde, GA, has been proposed as crosslinking agent to produce HA hydrogels. However, accordingly to the literature, there are contradictory results about the experimental protocol to be used, and relating to the cytotoxicity caused by glutaraldehyde remaining in the sample after crosslinking reactions. [...]info:eu-repo/semantics/publishedVersio

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

Chitosan-Silica Hybrid Porous Membranes

Chitosan–silica porous hybrids were prepared by a novel strategy in order to improve the mechanical properties of chitosan (CHT) in the hydrogel state. The inorganic silica phase was introduced by sol–gel reactions in acidic medium inside the pores of already prepared porous scaffolds. In order to make the scaffolds insoluble in acidic media chitosan was cross-linked by genipin (GEN) with an optimum GEN concentration of 3.2 wt.%. Sol–gel reactions took place with Tetraethylorthosilicate (TEOS) and 3-glycidoxypropyltrimethoxysilane (GPTMS) acting as silica precursors. GPTMS served also as a coupling agent between the free amino groups of chitosan and the silica network. The morphology study of the composite revealed that the silica phase appears as a layer covering the chitosan membrane pore walls. The mechanical properties of the hybrids were characterized by means of compressive stress–strain measurements. By immersion in water the hybrids exhibit an increase in elastic modulus up to two orders of magnitude.The research project is implemented within the framework of the Action "Supporting Postdoctoral Researchers" of the Operational Program "Education and Lifelong Learning" (Action's Beneficiary: General Secretariat for Research and Technology), and is co-financed by the European Social Fund (ESF) and the Greek State, Grant Number: NARGEL-PE5(2551). JFM thanks the Portuguese Foundation for Science and Technology (FCT) for financial support through the PTDC/FIS/115048/2009 project. JLGR acknowledges the support of the Ministerio de Economia y Competitividad, MINECO, through the MAT2013-46467-C4-1-R project

Biological evaluation of macroporous scaffolds with different surface energies for regeneration of the central nervous system

[Excerpt] Central nervous system trauma and neurodegenerative diseases involve massive neuronal and glial cell death and loss of the threedimensional spatial organization and connectivity of the neuronal networks. In this work we employ a biostable model system to study differentiation and viability of neural precursors in 3D scaffolds. Polymer scaffolds with interconnected porous with 90 microns of pore size were produced varying the hydrophobic-hydrophilic monomeric units ratio along the polymer chain. The materials studied, biocompatible and biostable, were polymer or copolymer networks based on the hydrophobic homopolymer poly(ethyl acrylate), PEA, and its copolymers with hydroxyethyl acrylate, p(EA-co-HEA) and methacrylic acid, p(EA-co-MAAc). In these biomaterials, the survival of differentiated functional neurons derived from cultured subventricular zone (SVZ) postnatal neural stem cells was investigated. [...]info:eu-repo/semantics/publishedVersio

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