C. Lacabanne

Study of cooperative relaxation modes in complex systems by thermally stimulated current spectroscopy

Abstract Cooperative relaxation modes associated with transitions of various systems have been experimentally resolved into elementary processes characterized by activation enthalpies and entropies following a compensation law. Two transitions with no kinetic effect-solid-solid transition and Curie transition have been studied. In both cases, the corresponding dielectric relaxations are characterized by a compensation phenomenon with transition temperature as the compensation temperature. Dielectric relaxations liberated at the glass transition, T g , have also been studied. They are also associated with a compensation law but, in that case, the compensation temperatures T g + 10/30°. This lag has been attributed to the kinetic character of T g . Note that it is lower in amorphous polymers than in semicrystalline polymers. The distribution function of activation parameters is broader in amorphous polymers than in semicrystalline ones.

α-Relaxation/retardation mode in semicrystalline polymers with flexible chains

Abstract The molecular mobility of two semicrystalline polymers with flexible chains—chemically crosslinked low density polyethylene (XLPE) and poly(vinylidene fluoride) (PVDF)—has been investigated in the temperature range between room temperature (above their glass transition temperature) and their melting point. D.s.c. was used to characterize the conformational evolutions related to crystalline domains. Thermostimulated currents and creep were used to study the dynamics of these movements. For both polymers, the α-relaxation/retardation mode is located in the temperature range of a small endothermic phenomenon. These low temperature d.s.c. peaks are not related to any melting process but rather to conformational disorder in the crystal-amorphous interphase. The fractional polarization/stress technique was applied to obtain the distribution of relaxation/retardation times. A compensation phenomenon was found above the α-relaxation mode of β-PVDF and XLPE. It has been ascribed to conformational movements associated with an order-disorder phase transition in the vicinity of the melting point. These high temperature phases would correspond to the paraelectric phase of β-PVDF and to the hexagonal phase of XLPE. Insofar as the α-mode is liberating molecular mobility at the crystal-amorphous interphase, it appears as a precursor of the order-disorder transition.

Study of the thermal and dielectric behavior of P(VDF-TrFE) copolymers in relation with their electroactive properties

Abstract On the basis of the pyroelectric spectra—pyroelectric coefficient p 3 versus temperature—of β-PVDF and P(VDF-TrFE) 75/25 mol.%, recorded between -100 and 70°C, we have investigated transition and relaxation phenomena involving both the crystalline and amorphous phases of P(VDF-TrFE) copolymers for TrFE unit content in the range 0–50 mol.%. To this aim, complementary techniques, DSC and TSC were used. The classical glass transition of the copolymers (Tgl) manifests by a clear change in slope in the p 3 versus T diagram. From its variation in DSC as a function of the weight fraction of TrFE units, we deduced a Tg of-12°C for PTrFE. An upper component of the glass transition, Tgu, has been found at ≈10–20°C in the pyroelectric spectrum. It has been associated to the amorphous phase constrained by crystallites. The pyroelectric coefficient increases strongly in the glass transitions range due to the large thermal expansion coefficient of the amorphous phase in the rubber-like state. The crystalline α...

Transitions in poly(vinylidene fluoride) by thermally stimulated techniques

Abstract Multiple transitions in poly(vinylidene fluoride)-PVF2 - have been studied by thermally stimulated techniques: Thermostimulated creep -TSCr - and thermostimulated current -TSCu -. The resolving power of these methods have allowed us to investigate the fine structure of transitions in PVF2. A relaxation mode is observed around the glass transition temperature -Tg -; it can be described by a distribution of relaxation times following an Arrhenius equation and it has been attributed to microbrownien motions of the amorphous chains. Another relaxation mode is revealed ∼50° higher than Tg. It corresponds to a distribution of relaxation times obeying a Vogel equation and has been assigned to the fluidification of the amorphous chains. In both cases, two sub-modes can be distinguished:The sub-mode observed at lower temperature has been attributed to the amorphous regions free from constraint, while the other one at higher temperature, has been assigned to amorphous chains under constraints from crystall...

Temperature dependence of the pyroelectric coefficient in polyvinylidene fluoride

The pyroelectric properties of bioriented β-PVDF were studied in the temperature range [- 100-70°C]. The temperature dependence of the pyroelectric coefficient was obtained, and its variation was compared with the DSC thermogram and the TSC complex spectrum. Three transitions have been observed in the investigated temperature range. They have been associated with the glass transition of the free and constrained amorphous phases (Tgl = -40°C and Tgu = 15°C) and to conformational reorganizations at the crystallites surface (Tac = 47°C). A correlation has been established between these pyroelectric transitions and volumetric variations. Dimensional effects have been found to reasonably describe the increase of the pyroelectric coefficient through the glass transition range. These changes in the thickness of the film correspond to secondary pyroelectricity and explain nearly 50% of the room temperature coefficient. In the glassy state, primary pyroelectricity is dominant.

Compositional variation of the glass transition and the associated dielectric relaxation in copolymers of vinylidene fluoride and trifluoroethylene

Abstract The glass transition temperature ( T g ) of a series of statistical copolymers of vinylidene fluoride and trifluoroethylene has been determined experimentally. Its variation as a function of the chemical composition of the copolymers has been discussed based on the Fox, Di Marzio and Couchman equations. Using T g = −42°C for poly(vinylidene fluoride) and a value of T g usually reported for polytrifluoroethylene ( T g = 31°C, a significant discrepancy has been obtained between experimental and predicted values. Thus, it has been suggested that the glass transition temperature for polytrifluoroethylene is located in the range of −20 to −10°C, which is consistent with Wunderlichs thermal analysis data.

Thermally Stimulated Current spectroscopy for the study of thermoplastics

Relaxation phenomena in polycarbonate have been explored by Thermally Stimulated Current (TSC) spectroscopy. A comparative study of transitions by differential scanning calorimetry has also been undertaken. In the sub-Tg region, the observed relaxations have been associated with the diffusion of local defects along the chains. In the glass transition region, the annealing induces the segregation of two relaxation modes: the lower temperature component associated with the unstressed amorphous phase, the higher temperature component attributed to stressed amorphous phase responsible for the modules observed in electron microscopy and X-ray diffraction.ZusammenfassungRelaxationsphÄnomene von abgeschrecktem und getempertem Polycarbonat (Lextan) wurden mit Thermisch Stimulierten Feldpolarisations Spektroskopie [Thermally Stimulated Current (TSC) Spectroscopy] untersucht. Unterhalb der Glasumwandlung wird eine Relaxation beobachtet, die mit einem molekularen Kettenmechanismus in Zusammenhang gebracht wird. Temperung verursacht eine Trennung in eine spannungsfreie und eine unter Belastung durch die Kristalle stehende amorphe Phase. Experimentelle Befunde werden diskutiert und Modellbetrachtungen zur dielektrischen Relaxation vorgestellt.

Thermally stimulated current changes of irradiated skin

Radiation effects on collagen and skin have been characterized by means of thermally stimulated current (TSC) spectroscopy. X-rays, β-rays and UV radiation have similar effects on the molecular mobility of dermal collagen and skin. They induce a decrease of intra and intermolecular mobility. The restriction of molecular movement can be explained by an increase of collagen cross-links.

Chain dynamic of calcified tissue

Thermally stimulated current spectroscopy has been applied to the investigation of molecular mobility in human calcified tissue. A comparative study of extracts and residues at various stages of demineralization is presented. Results show that:• the matrix (collagen) is in a glassy state at physiological temperature;γ the filler (apatite) increases the static modulus;• the interfaces/interphase (non-collagenous proteins and particularly proteoglycans) ensure cohesion and ductability for the composite. Biomaterials for orthopaedic prostheses require the same morphology in order to phenomenologically reproduce the same dynamic behaviour.

A depolarization thermocurrent study of polyethylene

Abstract A comparative study of low density polyethylene (LDPE) and high density polyethylene (HDPE) of different surface morphologies was carried out using the depolarization thermocurrent (DTC) technique. The DTC spectra show the existence of four relaxation modes. A DTC peak is observed at +50°C in HDPE moulded on an aluminium support and is attributed to the diffusion of free carriers which become mobile at this temperature. A DTC peak is observed at about −20°C in all the polyethylenes studied. Its intensity is greatest in HDPE with amorphous skins. This relaxation mode is attributed to the “upper” glass transition of polyethylene. The corresponding molecular motions take place in the restricted amorphous regions, i.e. they are subject to constraints from crystallites. The DTC peak observed at about −90°C has the greatest intensity (like that at −20°C) in HDPE with amorphous skins. This mode is related to the “lower” glass transition; it is located in the true amorphous regions, i.e in those which are free from constraints. The DTC peak observed at −150°C is probably due to a bulk relaxation in crystallites.