Cooperative relaxations in amorphous polymers studied by thermally stimulated current depolarization

Thermally stimulated current depolarization (t.s.c.) was used to study the relaxations in amorphous polymers including poly (ethyl methacrylate) (PEMA), poly (methyl methacrylate) (PMMA), polystyrene (PS), polycarbonate (PC) and polyarylate (PAR) over temperature ranges covering the β and (glass transition) regions. A.c. dielectric was used to obtain activation energies (E_a) for PS and PC to verify the accuracy of those values determined by the t.s.c. thermal sampling method. At temperatures below the glass transition (T_g) the values of E_a were found to agree with those predicted using an activated states equation with a zero activation entropy. This is evidence of the localized, non-cooperative nature of the low temperature secondary β relaxations which are found to be characterized by a continuous variation of activation energies as a function of temperature. The measured values of E_a depart from the zero activation entropy curve and exhibit a prominent maximum at T_g. This behaviour is known to be due to an enhanced degree of cooperativity of segmental relaxations near T_g. The results indicate that the main advantage of the thermal sampling method is the high sensitivity and high temperature resolution for cooperative relaxations. For the polymers studied here, only PEMA and PMMA show a substantial population of cooperative relaxations more than 60°C below T_g. This is tentatively explained in terms of structural heterogeneity due to variable tacticity in the methacrylates. Compensation of the t.s.c. relaxation spectra plotted in Arrhenius or Eyring plots was found for all polymers to differing degrees. Some discussion of compensation is made in terms of independently measured values of the coefficient of thermal expansion.     

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 α 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. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 2716–2723, 1999     

A critical phenomenon of phase transition in hydroxyapatite investigated by thermally stimulated depolarization currents

We observed a critical phenomenon of ferroelectric transition in hydroxyapatite (HAp) through measuring the thermally stimulated depolarization current of polycrystalline monoclinic HAp. Two peaks attributable to hydroxide (OH^?) ion dipole reorientations were observed. The one was very sharp, the other displayed a broad peak ranging from 350 to 550 K. While the broad peak can be attributed to the diffusive reorientation motions of OH^? ion dipoles, the sharp peak, related to the ferroelectric phase transition, implies the presence of ferroelectricity in HAp. The sharp peak was found near 380 K, the phase‐transition temperature from the monoclinic to hexagonal is ascribed to a critical phenomenon caused by the ferroelectric behavior of OH^? ion dipole reorientations.     

Thermally stimulated depolarization current studies on strain-induced relaxations in polymers

Thermally stimulated depolarization current (TSD) method has been used to study the molecular relaxation from room temperature up to the glass-transition temperature for three plastically deformed polymers. A well-defined distinct α′ peak has been observed in the vicinity of 70°C for polycarbonate, polysulfone, as well as poly(ethylene terephthalate), that had been annealed above T_g and subsequently cold-rolled. Since this relaxation is not observed after the initial measurements, it is a nonequilibrium relaxation. In the case of polycarbonate, the nonreversible α′ process had an activation energy ranging from 1.22 to 1.75 eV and an intensity proportional to the degree of deformation resulting from cold-rolling. The spontaneous polarization in the rolled material was believed to originate from the initial anisotropy which was subsequently enhanced by the rolling process. Density measurements, as well as the birefringence observation, have been performed on the rolled specimens. All the studies led to the conclusion that the molecular motion responsible for this α′ peak was the disorientation of local chain segments from their aligned conformation frozen in during the cold-work.     

TSDC法研究聚乙烯基吡咯烷酮驻极体的电荷存储性能

使用TSDC(热刺激去极化电流)法,在2℃/min和3.5℃/min两种加热速率下,温度范围30²50℃,测量了聚乙烯吡咯烷酮驻极体(PVP)的放电过程。结果表明,TSDC谱图中(230±5)℃位置的最高峰ρ峰,是来源于热刺激下被困空间电荷的退陷阱。ρ峰的电流非常大,是因为PVP分子的侧基是包含了内酰胺的吡咯烷酮环结构,属于强极性基团,这就导致分子内体陷阱增多,被困空间电荷量大大增加,热刺激下脱困的空间电荷也大幅增加;并且还加深了PVP的陷阱深度,这些特性使得PVP驻极体具有优良的存储电荷性能。     

The effect of spherulite structure of polypropylene on the thermally stimulated electret depolarization current

Summary The dependence of thermally stimulated depolarization current /TSC/ on morphological structure of polypropylene /PP/ films has been determined. Different morphologies in the investigated samples were arrised due to appropriate cooling conditions. The TSC maxima were correlated with the size of spherulites and attributed to the release of carriers traped at the crystalline and amorphous phase borders.Dedicated to Professor C. I. Simionescu on his 60th anniversary     

The thermally stimulated current technique applied to side-chain liquid-crystal polymers

The thermally stimulated current (t.s.c.) technique is applied to three side-chain liquid-crystal polymers (LCP) and an LCP that contains a highly dipolar dye. The t.s.c. trace is shown to be very sensitive to a number of LCP properties including glass transition, degree of mesogenic alignment and the amount of polarization that can be stored. A strong correlation between such properties and the results of high-frequency dielectric spectra is established. These parameters are directly relevant for the application of these materials as optical storage media and in non-linear optics. In particular, the technique is able to define the optimum conditions for poling these LCP systems. The technique can also be used to determine a number of fundamental molecular properties such as activation energy, molecular relaxation distribution and polymer flexibility and, as such, allows fundamental examination of the relationships between main-chain and mesogenic motions.     

Molecular motion and detrapping behavior of trapped space charges in polyvinyl pyrrolidone: a thermally stimulated depolarization current study

The discharging process of polyvinyl pyrrolidone electret under two different heating rates 3.5 and 2 °C/min has been investigated by the measurement of thermally stimulated depolarization current (TSDC) in the temperature range from 30 to 250 °C. The TSDC spectra of PVP shows three peaks located at 70 ± 10, 150 ± 10, and 230 ± 10 °C, respectively. When explaining the TSDC spectra, this thesis deduced that the first peak, β peak, originates from side group relaxation; the second peak, α peak, corresponds to the relaxation of the frozen dipoles on the molecule chain segments during glass transition; the third peak, ρ peak, is attributed to the detrapping of the trapped space charge under thermal stimulation. The current of the ρ peak is remarkably strong due to the fact that PVP molecules’ side groups—the pyrrolidone rings containing lactam structure—are strong polar groups which cause the great increase in the number of bulk traps, trapped space charge, as well as detrapped space charge under thermal stimulation.     

Transient and thermally stimulated depolarization currents in polyvinyl formal films

Transient currents in double-sided vacuum aluminized polyvinyl formal (PVF) film samples of thickness 40 μm were investigated in the charge and discharge modes at different temperatures and fileds. The results observed suggest that the transient currents are governed by charge injection processes leading to space charge effects. Comparative studies of the isochronal characteristics (i.e. current–temperature plots at fixed times) with the thermally stimulated discharge current (TSDC) indicated a strong resemblance between the two techniques. These studies suggest that the simultaneous mechanism of movement of charges through microscopic distances coupled with dipolar orientation may be responsible for the TSDC peak observed at 100°C.     

A new method for torsional pendulum analysis of polymers through the glass transition region

A freely oscillating torsional pendulum has been used to measure the dynamic-mechanical loss of polymers through the softening temperature by encapsulating the polymer in a rigid matrix of cured phenolic resin. The method is illustrated with loss curves through the glass transition region of poly(vinyl chloride), polystyrene, a styrene–maleic anhydride copolymer, poly(phenylene oxide), and two binary alloys involving these polymers. The advantages and simplicity of the technique are emphasized.     

Thermally stimulated depolarization (TSD) current study of plasticized PVC

Summary Plasticized PVC samples of different plasticizer concentration were tested by the thermally stimulated discharge current (TSD) method. Processing conditions were also varied. It has been found that plasticized PVC consists of at least three structural units having different molecular mobility. Their amount and characteristics strongly depend on the processing conditions, i.e. the thermal- and shear history of the material. TSD method because of its excellent resolving power proved to be a useful tool detecting these structural units. Received: 9 June 1998/Revised version: 2 November 1998/Accepted: 2 November 1998     

Thermally stimulated depolarization current (TSDC) study of molecular motions in the glass-transition region of polyarylate (PAr)

Dipolar relaxation processes in the glass-transition region of a commercial polyarylate (PAr) have been studied by means of the thermally stimulated depolarization current (TSDC) technique. Complementary differential scanning calorimetry (d.s.c.) and thermogravimetry (t.g.) measurements have also been carried out for comparison. The TSDC global spectrum obtained is resolved into two thermocurrent peaks. The elementary processes involved in these broad peaks were separated by application of the fractional polarization procedure. The Arrhenius-like kinetic parameters corresponding to the different elementary processes have been obtained fitting each elementary TSDC peak by a Debye-like expression. Empirical distribution function of activation energies has been also deduced. The origin of the different studied dipolar relaxation processes is discussed in relation to the molecular motions in the glass-transition region.     

Dielectric relaxation study of low-density polyethylene by thermally stimulated depolarization currents and thermal sampling

The dielectric relaxation zone of a low-density polyethylene has been studied by thermally stimulated depolarization currents (TSDC) and thermal sampling (TS). Three relaxations have been found in this zone, which have been labeled γ_I, γ_II, and γ_III in decreasing order of temperature. Samples of different crystallinities were subjected to several thermal treatments in order to study the molecular origin of each relaxation. The γ_I relaxation intensity changes with the thermal treatment, while the intensity of the γ_I relaxation is only slightly modified. Using TS it has been proved that the γ_I relaxation fulfils the compensation law with a high level of significance. Nevertheless, the experimental data are not enough to verity the γ_I and γ_III relaxation.     

A comparative study of the kinetic theories of the glass transition phenomenon in high polymers

The effect of thermal history on the glass transition temperature (T_g) of poly(vinyl chloride) was studied. The parameters like the hole energy (E_h), the activation energy for the disappearance of holes (E_j), the activation enthalpy for the structural relaxation (Δh1* and Δh2*) and the activation energy for the glass transition process (E) were calculated. The increase of E_h value with the increase in T_g value shows that there exists a holesize distribution. The E_j (26.9 ± 0.5 kcal/mol) value calculated according to Wunderlich's treatment, the Δh1* (27.8 ± 0.6 kcal/mol) value obtained by Moynihan's procedure and the E (35.3 kcal/mol) value from Barton-Critchley's method agree well with one another in the poly(vinyl chloride) system. The Δh2* quantity, obtained through Moynihan's formulations, increase as the rate of heating was increased, a result similar to the variation of E_h value with the rate of heating.     

Molecular relaxation mechanisms of tyrosine-derived polycarbonates by thermally stimulated depolarization currents

A series of tyrosine-derived polycarbonates with different lengths (2 ≤ n ≤ 8) for the alkyl ester pendent chain were studied by measuring thermally stimulated depolarization currents (TSDC). The observed spectra could be separated into three regions: the low-temperature zone with a broad, complex β band (80–240 K), the intermediate zone (250–300 K), and the high-temperature zone (300–400 K) with a sharp α peak. The application of direct signal analysis (DSA) to decompose the complex peaks into elementary processes led to the determination of the relaxation time distribution and temperature dependence of each process. The variation of the relaxation parameters as a function of the pendent chain length facilitated the tentative identification of the relaxation mechanisms responsible for the observed current peaks. It is proposed that as the temperature increases one observes, first, the individual motion of each polar group, then the concerted motion of the entire pendent chain, and, last, the movement of the polymer backbone. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 63: 1457–1466, 1997     

Molecular motions in poly(vinyl acetate) revisited. A thermally stimulated current study

The dipolar relaxation mechanisms in poly(vinyl acetate) have been studied in detail using the technique of thermally stimulated currents. The papers published in the literature about this subject are very contradictory, particularly with respect to the assignment of the observed discharges to the corresponding motions at the molecular level. This work aims at clarifying these problems. We detected and characterised three different relaxation mechanisms: (1) a low temperature one (around ?140°C) which was attributed to local internal rotations in the acetate side-groups; (2) a relaxation whose maximum occurs at 42°C, which corresponds to the glass transition relaxation, and shows a compensation behaviour; (3) an upper glass transition relaxation whose maximum appears at 87°C and was attributed to a liquid-liquid transition. These assignments have been made on the basis of the analysis of the behaviour of the samples when submitted to different thermal and electrical treatments.     

Molecular mobility in polymers studied with thermally stimulated recovery. II. Study of the glass transition of a semicrystalline PET and comparison with DSC and DMA results

The glass transition of a semicrystalline PET is studied by dynamic mechanical analysis (DMA) and thermally stimulated recovery (TSR). The DMA results allowed us to construct a master curve for E′ and D′, where the shift factors were modelled with the Vogel-Fulcher-Tamman-Hesse (VFTH) equation above T_g. Using the thermal sampling procedure, the TSR technique allowed us to decompose the complex distribution of retardation times into a set of quasi-elementary processes. At the compensation point a broad distribution of retardation times is observed using DSC data. The TSR results show a typical increase of the activation energies in the glassy state, followed by a decrease above T_g that may be modelled with the VFTH equation. This observation was explained taking into account both the typical time scale range of TS experiments and the evolution of the retardation times distribution with temperatures below and above T_g, under the Adams-Gibbs theory. The evolution of the activation energies from the TSR results was shown to be compatible with the DMA and DSC data.