Eric Dargent

Three phase model in drawn thermoplastic polyesters: comparison of differential scanning calorimetry and thermally stimulated depolarisation current experiments

Abstract Differential scanning calorimetry and thermally stimulated depolarisation current measurements are performed to quantify various phases present in amorphous and semi-crystalline polyester samples uniaxially drawn above their respective glass transition temperature. Results show the appearance of a crystalline phase induced by stretching and of a part of the amorphous phase which does not participate in glass transition. The existence of this phase—called rigid amorphous phase—is enhanced by the presence of crystallites rather than by the drawing.

Thermal behaviour of drawn semicrystalline poly(ethylene terephthalate) films

Behaviours of drawn semi-crystalline poly(ethylene terephthalate) films are investigated by DSC, X-ray diffraction and birefringence measurements. The comparison of the different results confirms the coexistence of two structures into the amorphous part of the material: a completely disordered amorphous phase and a mesomorphic amorphous one. Moreover, for the strongest draw ratio, the calorimetric results show that the drawing effect on the strain induced crystalline structure proceeds by a better orientation of this structure rather than by nucleation and growth of new oriented crystallites.ZusammenfassungMittels DSC, Röntgendiffraktion und Messung der Doppelbrechung wurde das Verhalten von gezogenen semikristallinen Poly(ethylenterephthalat)-Filmen untersucht. Ein Vergleich der verschiedenen Ergebnisse bestätigt die Koexistenz zweier Strukturen im amorphen Teil der Substanz: einer vollständig ungeordneten amorphen Phase und einer mesomorphen Phase. Weiterhin zeigen die kalorimetrischen Ergebnisse für das größte Zugverhältnis, daß der Zugeffekt auf die dehnungsinduzierte Kristallstruktur durch eine bessere Ausrichtung dieser Struktur gekennzeichnet ist, als dies durch Keimbildung und das Wachstum neu ausgerichteter Kristallite der Fall ist.

Fragility index of drawn or annealed poly(ethylene terephthalate) films studied by thermally stimulated depolarisation currents

Abstract According to the ‘strong-fragile’ glass forming liquid concept, amorphous poly(ethylene terephthalate) (PET) must be classified as a ‘fragile’ supercooled liquid. In this work we show that the value of the fragility index of the glassy material (mg) can be obtained by means of Thermally Stimulated Depolarisation Currents measurements. Experiments were carried out on different PET for which the molecular mobility of the vitreous domain was constrained by the existence of a crystalline phase. This crystalline phase was obtained in two different ways, i.e. by thermal cold crystallisation or by a uniaxial hot-drawing method. Depending on the degree of crystallinity, but whatever the nature of the crystalline morphology (spherulitic or oriented crystallites), it is found that the initial fragile character becomes stronger as the degree of crystallinity increases.

THE GLASS TRANSITION Correlation of DSC and TSDC investigations

This work deals with a comparison of data obtained from differential scanning calorimetry (DSC) and thermally stimulated depolarization current (TSDC) investigations. Measurements were performed on various poly(ethylene terephthalate) films: a wholly amorphous, a thermally crystallized and drawn samples.For each specimen, the TSDC complex spectra, resolved into elementary ones, led to the determination of the classical compensation temperature (Tc). The glass transition temperature (Tg) and the fictive equilibrium temperature (Tf) were determined by means of DSC. It appears thatTc is different fromTg and very close toTf.

Dielectric relaxations in drawn semi-crystalline poly(ethylene terephthalate)

Abstract The dielectric relaxations of the amorphous phase of drawn poly(ethylene terephthalate) are investigated by thermo-stimulated current measurements. The results show the dependence of these relaxations with the draw ratio and with the strain induced crystallized phase.

Combining Flash DSC, DSC and broadband dielectric spectroscopy to determine fragility

New experimental results focused on Flash DSC, DSC and broadband dielectric spectroscopy investigations are reported in this work. The fictive temperatures and fragility indexes are estimated from Flash DSC experiments and compared to values obtained from classical DSC. The consistency of the Tool–Narayanaswamy–Moynihan model and fragility concept is then investigated over a large range of cooling rates. Indeed, the Flash DSC allows exploring thermal properties of materials over a continuous and broad range of heating and cooling rates, complementary to rates usually available with DSC. The reliability of investigations is also demonstrated by comparing results obtained from two model amorphous polymeric systems: polystyrene and poly(ethylene terephthalate)-glycol. The temperature dependence of the cooling rate obtained by Flash DSC and DSC is also compared to the temperature dependence of the relaxation times obtained from broadband dielectric spectroscopy, experiment considered as the reference concerning the fragility measurements. The comparison of these two dependencies implies a better understanding about the origin of the temperature dependence of the cooling rate.

Crystallization kinetics and molecular mobility of an amorphous active pharmaceutical ingredient: A case study with Biclotymol

The present case study focuses on the crystallization kinetics and molecular mobility of an amorphous mouth and throat drug namely Biclotymol, through differential scanning calorimetry (DSC), temperature resolved X-ray powder diffraction (TR-XRPD) and hot stage microscopy (HSM). Kinetics of crystallization above the glass transition through isothermal and non-isothermal cold crystallization were considered. Avrami model was used for isothermal crystallization process. Non-isothermal cold crystallization was investigated through Augis and Bennett model. Differences between crystallization processes have been ascribed to a site-saturated nucleation mechanism of the metastable form, confirmed by optical microscopy images. Regarding molecular mobility, a feature of molecular dynamics in glass-forming liquids as thermodynamic fragility index m was determined through calorimetric measurements. It turned out to be around m=100, describing Biclotymol as a fragile glass-former for Angells classification. Relatively long-term stability of amorphous Biclotymol above Tg was analyzed indirectly by calorimetric monitoring to evaluate thermodynamic parameters and crystallization behavior of glassy Biclotymol. Within eight months of storage above Tg (T=Tg+2°C), amorphous Biclotymol does not show a strong inclination to crystallize and forms a relatively stable glass. This case study, involving a multidisciplinary approach, points out the importance of continuing looking for stability predictors.

Permeation Properties of Poly(m-xylene adipamide) Membranes

The permeation properties of a semiaromatic polyamide, the poly(m-xylene adipamide) (MXD6), were investigated by water and carbon dioxide permeation experiments (pervaporation and gas permeation tests). Complementary microstructure informations were obtained from calorimetric measurements. Amorphous and semicrystalline MXD6 membranes were studied. The analysis of the water flux through amorphous MXD6 membranes showed a plasticization phenomenon followed by a water-induced crystallization. It resulted that the role played by water in these materials was complex because of the dependence of the water diffusivity on water concentration and time. Because of the presence of crystalline phase, a significant reduction of water and gas permeability of MXD6 and an increase in the delay of diffusion were observed. In terms of barrier properties for water and carbon dioxyde, MXD6 membrane crystallized at high temperature were more performant than water-induced crystallized ones. Correlations between microstructure and transport properties had been so established.

Relationship between Draw Ratio and Strain-Induced Crystallinity in Uniaxially Hot-Drawn PET MXD6 Films:

The evolutions of the strain-induced crystalline phase of poly(ethylene terephthalate) (PET), poly(m-xylene adipamide) (MXD6), and PET–MXD6–ionomer blend films are investigated by differential scanning calorimetry, X-ray diffraction, and birefringence measurements. Initially wholly amorphous, the films are uniaxially hot-drawn above the glass transition temperature Tg. During drawing and depending on the draw ratio, an induced crystalline phase appears in the three materials. The comparison of the degree of crystallinity shows that the maximum degree of crystallinity is close to 40% for the three materials and the DSC analysis shows a similar thermal behavior between PET, MXD6, and blend samples. The critical draw ratios for the crystalline phase appearance are equal to 2, 3, and 3 for PET, MXD6, and blend, respectively.

Compensation effect observed in thermally stimulated depolarization currents analysis of polymers

Thermally stimulated depolarization currents (TSDC) and differential scanning calorimetry (DSC) are performed on thermoplastic polyesters and dimethacrylate resins over temperature ranges covering the α and β relaxation regions. The noncooperative β relaxation is characterized by a continuous variation of activation energies as a function of temperature and follows the activated states equation with a zero activation entropy. The cooperative a relaxation shows a prominent maximum of the activation energies at the glass transition temperature. Compensation behavior is often observed by TSDC for many polymers in the glass transition temperature range. We show that this behavior is not systematic and that it appears for the α relaxation if the increase of the apparent activation enthalpy is strong and if the width of the glass transition is weak.