On the high temperature TSDC peak in poly (ϵ-caprolactone)

In this work, thermally stimulated depolarization current (TSDC) and dielectric relaxation spectroscopy (DRS) techniques were performed in complementarities to characterize the dielectric relaxation processes in poly(methylmethacrylate) (PMMA) polymer. Three relaxations were highlighted. Based on correlation between TSDC and DRS data, theywere attributed respectively to dipolar β and α relaxations as well as interfacial ρ relaxation. New experimental procedure of fractional polarizationwas used to investigate space charge trap depth distribution within the band-gap of PMMA. Then the individual peaks were fitted to Bucci–Fieschi model, to evaluate the relaxation parameters. The obtained values were discussed in correlation to those evaluated from DRS data fitted to Havriliak–Negami and Arrhenius models. The various observed peaks in the thermograms are discussed on the basis of space charge polarization. The trap energies were evaluated from the fit to the Bucci–Fuschi model. Similarly other parameters such as relaxation time and charge release are evaluated.

2013

The complex permittivity of films of polyether ether ketone (PEEK) has been investigated over a wide range of frequency. There is no relaxation peak in the range of 1Hz to 10 5 Hz but in the lowfrequency side (10-4 Hz) there is an evidence of a peak that also can be observed by thermally stimulated discharge current measurements. That peak is related with the glass transition temperature (T g) of the polymer. The activation energy of the relaxation was found to be 0.44 eV, similar to that of several synthetic polymers. Space charges are important in the conduction mechanism as shown by discharging transient.

Material Research Innovations, 2001

The Thermally Stimulated Discharge Current (TSDC) technique is widely used for the study of main and secondary dielectric relaxations in polymers. The TSD current is described by different equations that can be arranged in a unique three-parameters (the activation energy W, A and B) general form. The physical meaning of A and B depends on the origin of the discharge currents. In this paper a method is proposed to obtain these parameters by fitting the experimental data with the analytical expression of the current, in the range around the maximum. Simulations were carried out to underline the relative importance of the parameters. A method is proposed for the decomposition of experimentally determined complex bands into a limited number of elementary peaks, each of them characterized by average values for W and B. The errors resulting from different approximations used in the analytical current expression or by the utilization of various expressions for the relaxation time are analyzed. The method is applied for the analysis of the TSDC spectra in the glass-rubber transition temperature regions of PET and PMMA, yielding several peaks characterized by narrow distributions of W (∆W ≈ ± 0.06 eV).

Polymer International, 2012

The relaxation properties of polymethacrylates of the n-alkyl series with n = l, 2 and 4 (poly(methyl methacrylate) (PMMA), poly(ethyl methacrylate) (PEMA) and poly(n-butyl methacrylate) (PnBMA)) have been measured and analyzed in order to relate their properties to the size of the lateral side chains. The n-alkyl series has been regarded as a model system and was used in this work to test a graphical data analysis method. Essentially, four relaxation processes were detected in the three polymers: the γ , β, α and αβ relaxations, in increasing order of temperature. It was found that the γ relaxation has a low activation energy, of around 36.3-38.5 kJ mol −1 , independent of the side chain, exhibiting low entropy of activation values when referring to the Eyring description of the activation parameters. The β relaxation was found to be similar in PMMA and PEMA, showing an activation energy of 88.8 kJ mol −1 , increasing to 112.8 kJ mol −1 in PnBMA. The activation entropy was also found to be low for this relaxation, although greater than that for the γ relaxation. In contrast, the α relaxation is quite different in these polymers. We observed a gradual shift in the glass transition temperature towards lower temperatures as the side chain increases in length. The manner in which the α transition makes its way into the dielectric spectra is more abrupt in PMMA than in PnBMA, denoting a higher fragility in the former polymer. Finally, there is a significant difference in the coalescence scenarios of the α and β relaxations for temperatures higher than the glass transition temperature, where they give rise to the so-called αβ relaxation.

Thermally stimulated depolarization current (TSDC) technique was used to study the dielectric relaxations of chlorinated polyvinyl chloride (CPVC) blended with poly (ethylene succinate) (PES) over the temperature range 300-400 K, covering a and p regions. a-relaxation was suggested to be due to cooperative motion of both C-C1 and -COO groups of the blend, while p-relaxation is of space-charge nature. Thermal peak cleaning was used to deconvolute the global TSDC spectra, giving apparent activation energies (E,) over the temperature range 360 -400 K. Maximum value of E, was found at Tz. Thermal peak cleaning data were fitted using Frohlich method.

Chinese Journal of Polymer Science, 2018

Dielectric relaxation spectroscopy (DRS) of poly(ε-caprolactone) with different draw ratios showed that the mobility of polymer chains in the amorphous part decreases as the draw ratio increases. The activation energy of the α process, which corresponds to the dynamic glass transition, increases upon drawing. The enlarged gap between the activation energies of the α process and the β process results in a change of continuity at the crossover between the high temperature a process and the α and β processes. At low drawing ratios the a process connects with the β process, while at the highest drawing ratio in our measurements, the a process is continuous with the α process. This is consistent with X-ray diffraction results that indicate that upon drawing the polymer chains in the amorphous part align and densify upon drawing. As the draw ratio increases, the α relaxation broadens and decreases its intensity, indicating an increasing heterogeneity. We observed slope changes in the α traces, when the temperature decreases below that at which τα ≈ 1 s. This may indicate the glass transition from the 'rubbery' state to the non-equilibrium glassy state.

Macromolecular Symposia, 2014

The aim of this work is to propose a reliable approach to estimate values of the relaxation time constant of polymers at the glass transition temperature. This approach consists of using a new thermo stimulated depolarisation current TSDC protocol which analyzes the relaxation current kinetics obtained after submitting the sample to an electrical field shock at constant temperature. The measurements have been performed on a series of polyethylene terephthalate (PET), polycarbonate (PC) and polyethylene terephthalate glycol (PETg) samples. Different temperatures were needed to explore the range of the glass transition domain of each polymer. The results compared to calorimetric and dielectric data show a good agreement. So we confirm that the value t ms ¼ 100 s often used for characterizing the molecular dynamic of the liquid glass former at T g is not universal.

Macromolecules, 2000

The dielectric behavior of polyacrylonitrile (PAN) in its pristine and cyclized stages was investigated. A PAN homopolymer displays a strong secondary relaxation at an activation energy of Ea ) 111 kJ/mol in the pristine stage and a weak relaxation at Ea ) 48 kJ/mol in the cyclized stage. By contrast, a PAN copolymer with itaconic acid, which is used in carbon fiber production, displays a strong secondary relaxation at Ea ) 134 kJ/mol in the pristine stage and two relaxations with activation energies of Ea ) 24 kJ/mol for the lower frequency relaxation and Ea ) 9 kJ/mol for the higher frequency relaxation. No relaxations were found for either polymer in the aromatized stage. The relaxation spectra were interpretated in terms of the Havriliak-Negami (HN) function.

Journal of Polymer …, 1975

The dielectric behavior of poly(diethy1 siloxane) supports the adiabatic calorimetric findings of Beatty and Karasz. In particular, a sub-!/', transition is observed near-180°C at 100 Hz, the glass transition near-135°C at 100 Hz, and a first-order transition near-70°C (crystal-crystal transformation). This glass-transition temperature is the lowest reported polymeric glass transition for polymers.

Journal of Polymer Science Part B: Polymer Physics, 1999

A family of four thermotropic polyesters starting with the poly(tetramethylene terephthaloyl bis-4-oxybenzoate), substituting either asymmetrically in the tetramethylene spacer or incorporating polar substituents onto the aromatic units of the mesogen, is studied by the Thermally Stimulated Depolarization Currents (TSDC) technique. The results are compared to the Dynamic-Mechanical Analysis at frequencies ranging from 0.1 to 1 Hz from 123 to 423 K. The unsubstituted polymer has a complex low-temperature TSDC spectrum corresponding to local reorientation modes due to the motion of the COO groups with different locations along the main chain. By comparing the effect of the Cl and CH 3 substituents on the relative intensity and on the mean energies of the distribution of relaxation times determined by the DSA procedure, the lowest temperature mode is attributed to the COO peripheral groups and the modes located at higher temperatures, to the internal COO groups, which may be accompanied by adjacent segments. The mechanic and dielectric ␣-transitions are also very sensitive to the substituents, the addition of CH 3 in the 1,4 flexible spacer shifting the glass transition temperature above room temperature. The existence of the three-dimensional order that is present in the materials with a linear spacer significantly broadens the ␣-relaxation.