Structure and ferroelectric phase transition of vinylidene fluoride-trifluoroethylene copolymers: 2. VDF 55% copolymer

Molecular and crystal structure changes in ferroelectric phase transition of vinylidene fluoride-trifluoroethylene (VDF-TrFE) copolymer with a VDF content of 55 mol% have been investigated by X-ray diffraction and infra-red and Raman spectroscopy. As the temperature rises from room temperature to the Curie point of ～60°C, the polar low-temperature phase consisting of all-trans chains experiences a first-order transition to the phase of tilted long trans segments connected by some skew bonds. At higher temperature this new phase transforms continuously and steeply to the non-polar high-temperature phase, where the molecular structure consists of a random combination of TG, T?, T₃G and $\text{T}{}_3\text{G}\text{?}$ rotational sequences. In the cooling process the high-temperature phase transforms to the cooled phase, which is essentially equivalent to the phase appearing intermediately in the heating process from the low-temperature to the high-temperature phase. The cooled phase gives a tilting X-ray fibre diagram and is found by X-ray analysis to contain an appreciable amount of so-called 60° domain structure. These are well interpreted by a conformational model of tilting trans structure containing skew linkages. The tensile stress along the fibre axis causes the transformation from the cooled phase to the low-temperature phase, where the probability of 60° domain structure and the degree of chain tilting are remarkably reduced.     

Dielectric loss of an oligomeric poly(propylene oxide) in the liquid region above Tg

Dynamic dielectric studies of oligomeric poly(propylene oxide) (PPO) of $\text{M}\text{?}{}_{\mathrm{n}}\text{=3034}$, between ?10° and 40°C at 0.1, 1, and 10 KHz, reveal a glass transition and a T >T_g liquid-liquid transition. Analysis of $\text{d}\text{?′}\text{d}\text{T}$ in the liquid region of PPO also indicates the presence of T₁₁. The activation enthalpies for the T_g and T₁₁ transitions have been calculated to be 39 and 18 kCal mol^?1, respectively. The T₁₁ transition in poly(propylene oxide) has been assigned to the motion of the entire polymer molecule.     

Pulsed n.m.r. of cis-polyisoprene: 1

Proton spin-spin (T₂), and spin-lattice (T₁) relaxation time measurements are reported for six monodisperse cis-polyisoprenes ($\text{M}\text{?}{}_{\mathrm{n}}$ from 2000 to 200 000) over the temperature range from ?50° to 170°C. At low temperatures (?30° to 10°C) T₁ and T₂ are determined by the short range segmental motions but above 10°C T₂ is sensitive to the long range motions. When $\text{M}\text{?}{}_{\mathrm{n}}\text{? 30 000 T}{}_2$ becomes influenced by the presence of entanglements which produce a transient network structure and this confers on the spin-spin relaxation a pseudo-solid-like response. Similar behaviour is observed in crosslinked networks produced by irradiation. The results are discussed in terms of the types of motion occurring in amorphous polymers above T_g and the analogy with dynamic mechanical measurements is discussed.     

Isothermal crystallization and chain mobility of poly(L-lactide)

Isothermal melt crystallization of poly(L-lactide) (PLLA) has been studied in the temperature range of 90 to 135°C. A maximum in crystallization kinetic was observed around 105°C. A transition from regime II to regime III is present around 115°C. The crystal morphology is a function of the degree of undercooling. At crystallization temperatures (T_c) below 105°C, further crystallization occurs upon heating; this behavior is not detected for T_c above 110°C. The analysis of the heat capacity increment at glass transition temperature (T_g) and of dielectric properties of PLLA indicates the presence of a fraction of the amorphous phase which does not relax at the T_g, and the amount of this so-called rigid amorphous phase is a function of T_c. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 64: 911–919, 1997     

Phase transition at a temperature immediately below the melting point of poly(vinylidene fluoride) from I: A proposition for the ferroelectric Curie point

A phase transition at a temperature immediately below the melting point of poly(vinylidene fluoride) form I has been found by means of differential scanning calorimetry (d.s.c.) and infra-red (i.r.) vibrational spectroscopy. An endothermic d.s.c. shoulder has been observed at a temperature about 10°C below the melting point, in the vicinity of which the i.r. crystalline trans bands decrease in intensity steeply and the crystalline gauche bands increase in intensity, indicating the conformational change from all-trans to T₃GT₃G type. These observations have been found to be detectable more clearly for samples subjected to the poling treatment under a d.c. high voltage. The transition shows the characteristic behaviour essentially identical to those observed for ferroelectric copolymers of vinylidene fluoride and trifluoroethylene, except for the irreversibility of the structural change, suggesting that the phase transformation revealed here may be a ferroelectric-to-paraelectric phase transition of polar form I crystal and the the Curie point may be about 172°C. It is consistent with Micheron's measurement of the temperature dependence of the dielectric constant. Other structural changes in the form I sample occurring in the temperature range from 20° to 170°C have also been discussed based on the i.r. spectral measurements.     

The influence of phase transition on electrocaloric effect in lead‐free (Ba0.9Ca0.1)(Ti1−xZrx)O3 ceramics

In this paper, the influence of phase evolution on polarization change and electrocaloric response in lead‐free (Ba_0.9Ca_0.1)(Ti_1?xZr_x)O₃ ceramics (BCTZ) was systematically investigated. With increasing Zr/Ti ratio, the phase structure and phase transition behavior were greatly changed, resulting in various temperature and electric field dependence of electrocaloric responses. For x=0.05, a peak electrocaloric temperature change 1.64 K (at 130°C) and corresponding entropy change 1.78 J·kg^?1·K^?1 were obtained for 0‐7 kV·mm^?1 electric field. Negative electrocaloric temperature change in ?0.1 K was obtained below Curie temperature (T_c), which may be induced by the orthorhombic‐tetragonal ferroelectric phase transition. With the increase in x, the peak value of the electrocaloric response decreased but much better temperature stability was observed. Simultaneously the negative electrocaloric response gradually disappeared with the disappearance of the low temperature ferroelectric‐ferroelectric phase transition. For x=0.2, electrocaloric response showed good temperature stability ranging from room temperature to 130°C, attributing to the relaxor ferroelectric feature.     

Kinetic analysis of the crystallization of poly(p-phenylene sulphide)

Growth rates of spherulites were measured in poly(p-phenylene sulphide) crystallized from the melt and the quenched glass over the temperature range 100°C–280°C, possibly the most extensive overall range yet reported for any polymer and, as such, most propitious for study of régime III crystallization. For a medium M.wt. polymer, a régime II → III transition was obtained at 208°C using values of transport parameters common to many polymers ($\text{U1 = 1400}\text{cal mol}{}^{\mathrm{?1}}$, T_∞ ? T_g = 30°C) together with experimentally determined values of T⁰_m(315°C) and T_g(92°C). Under these conditions, the régime III/II slope ratio was found to be 2.07 (i.e. only 3.5% higher than predicted by régime theory), and reasonable estimates of surface free energies and of the work of chain folding were obtained. Other choices of the transport terms, including WLF and zero values, did not allow successful kinetic analyses. Although a régime I → II transition is predicted to occur at the high-temperature end of our growth-rate data, we found no experimental evidence for it. For a low M.wt. polymer, our analysis showed that régime III kinetics is obeyed at low temperatures, while at higher ones there is a continuous departure from that behaviour without, however, full attainment of régime II kinetics.     

Dielectric and impedance spectroscopic study of lithium doped potassium tantalum niobium oxide

The (K_0.90Li_0.10) (Nb_0.80Ta_0.20)_0.99Mn_0.01O₃ (KLTN) ceramic has been synthesized by the conventional solid-state reaction route. The Rietveld refinement X-Ray diffraction (XRD) patterns confirmed the single-phase orthorhombic crystal structure with space group Amm2. The frequency dependent electrical properties were examined by the complex dielectric and impedance spectroscopy in the temperature range 30 °C–500 °C where multiple structural phase transition was observed with high dielectric constant, low tangent loss and well saturated electric polarization. The shifting of the ferroelectric phase transition temperature by 30 °C in heating and cooling mode suggests the irreversible motion of the domains and domain walls and significant effects on grain boundaries on structural phase transition temperature. A series of phase transitions from orthorhombic to tetragonal (∼190 °C) and tetragonal to Cubic (∼390 °C in cooling and 420 °C in heating) have been obtained in the wide range of temperature. The different type of analogy such as Modulus formalism, complex impedance spectra, frequency dependent conductivity, the activation energy of charge carriers has been used to understand the microstructure-electrical properties relation.     

Enhanced thermal stability of piezoelectricity in lead-free (Ba,Ca)(Ti,Zr)O3 systems through tailoring phase transition behavior

Good thermal stability in lead-free BaTiO₃ ceramics is important for their applications above room temperature. In this study, thermal stable piezoelectricity in lead-free (Ba,Ca)(Ti,Zr)O₃ ceramics was enhanced by tailoring their phase transition behaviors. Comparison between (1-x)Ba(Ti_0.8Zr_0.2)O₃-x(Ba_0.65Ca_0.35)TiO₃ and (1-y)Ba(Ti_0.8Zr_0.2)O₃-y(Ba_0.95Ca_0.05)TiO₃ revealed that latter system at y?=?0.80 had much better thermal stable piezoelectric coefficient than the former at x?=?0.45. Both systems crystalized in tetragonal to orthorhombic phase boundary at room temperature. The phase transition temperature and degree of diffusion were adjusted by Ca and Zr ions contents and demonstrated great influence on temperature dependent dielectric permittivity, hysteresis loops, and in-situ domain structures. The improved thermal stability of (1-y)Ba(Ti_0.8Zr_0.2)O₃-y(Ba_0.95Ca_0.05)TiO₃ prepared at y?=?0.80 was linked to its higher paraelectric to ferroelectric phase transition temperature (T_m?=?115.7?°C) and less degree of diffusion (degree of diffusion constant γ?=?1.35). By comparison, (1-x)Ba(Ti_0.8Zr_0.2)O₃-x(Ba_0.65Ca_0.35)TiO₃ prepared at x?=?0.45 revealed T_m?=?81.3?°C and γ?=?1.65. Overall, these findings look promising for future stimulation of phase transition behaviors and design of piezoelectric materials with good thermal stabilities.     

Poly(ortho-vinylbenzophenone)

This article reports the synthesis and free radical polymerization of ortho-vinylbenzophenone. The glass transition temperature T_g of the homopolymer is 136°C. The products synthesized appeared to be atactic and amorphous. The Mark-Houwink constants for poly (o-vinylbenzophenone) in tetrahydrofuran are K = 4.2 × 10^?2 cm³ g^?1 and a = 0.765. The pre-exponential constant under theta conditions, K_θ, is estimated to be 5.93 × 10^?2 cm³ g^?1. The ratio of unperturbed dimensions of the actual polymer and free rotating analogue chain is 3.93, which is almost double that of polystyrene. The Flory-Huggins interaction parameter for poly $\text{(o-}\text{vinylbenzophenone}\text{)}\text{tetrahydrofuran}$ is 0.48 at room temperature. The $\text{k}{}_{\mathrm{<mtext>p</mtext>}}\text{k}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}{}_{\mathrm{<mtext>t</mtext>}}$ ratio at 60°C is $\text{1.1 × 10}{}^{\mathrm{?2}}\text{l}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{mol}{}^{\mathrm{<mtext>?1</mtext><mtext>2</mtext>}}\text{s}{}^{\mathrm{<mtext>?1</mtext><mtext>2</mtext>}}$. In free radical copolymerizations with styrene at 70°C, r₁ (o-vinylbenzophenone) = 1.216, r₂ = 0.751. This copolymerizations is virtually random.     

Properties of polyurethane oligomeric blends versus high molecular weight block copolymers

Segmental compatibility has been investigated in both oligomeric polyurethane blends and polyurethane block copolymers. The block copolymers are formed by linking a hard segment, composed of three MDI and two butane diol units on average with various macroglycols. The monodisperse oligomeric hard segment, H₃, with its chain ends reacted with ethanol is used as the urethane component in blends with macroglycols. The macroglycols used in both the blend and block copolymer systems include polyethylene oxide (PEO), polypropylene oxide (PPO), polytetramethylene oxide (PTMO), and polybutadiene (PBD). Blends of H₃ and PEO form a eutectic at a weight ratio of $\text{≈20}\text{80}\text{(}\text{H}{}_3\text{/}\text{PEO}\text{)}$ with a T_m,e = 34°C. H₃ and PTMO blends also give rise to a eutectic composition at $\text{≈20}\text{80}\text{(}\text{H}{}_3\text{/}\text{PTMO}\text{)}$ but with a T_m,e = 10°C. Both PPO and PBD mix with H₃ to form a crystalline—amorphous blend. The miscibility of H₃ and the soft segments at the melting point of H₃ is in the order of PEO > PTMO > PPO > PBD. In the block copolymer systems, stress—strain and dynamic mechanical testing indicate that the block copolymerization of a hard segment with each soft segment results in a microphase separated elastomer as expected. The extent of phase separation increases in the order of PBD > PTMO > PPO > PEO which is coincident with the trend predicated by the application of Hilderbrand's solubility parameter concept. All the soft segments used occur in an amorphous phase in the block copolymers while PEO and PTMO crystallize in a blend with H₃. The differences between the properties of the blends and block copolymers suggest that the phase separation, segment crystallization and domain coalescence are substantially restricted by the urethane—polyol junction points.     

LCST behaviour in blends of poly(vinylidene fluoride) and isotactic poly(ethyl methacrylate)

Blends of poly(vinylidene fluoride) (PVF₂) with isotactic, atactic or syndiotactic poly(ethyl methacrylate) (it-, at- or st-PEMA) were studied by calorimetry and light microscopy. The occurrence of single glass transitions over a broad composition, as well as the lowering of the crystallization temperature upon cooling from the melt, indicate a complete compatibility in the amorphous state in blends of PVF₂ with at- and st-PEMA up to high temperatures. With it-PEMA, however, phase separation took place when the temperature was raised, sugesting LCST behaviour. Cloud points appeared in the temperature range of 150–200°C, making this system suitable for phase separation studies. The occurrence of double glass transitions as well as double crystallization exotherms in some $\text{PVF}{}_2\text{it-PEMA}$ blends could be explained from the phase diagram and the slowness of phase mixing upon cooling.     

Dielectric relaxation in vinylidene fluoride–hexafluoropropylene copolymers

The molecular mobility in copolymers of vinylidene fluoride–hexafluoropropylene VDF/HFP of 93/7 and 86/14 ratios has been investigated by means of broadband dielectric relaxation spectroscopy (10^?1–10⁷ Hz), differential scanning calorimetry DSC (?100 to 150°C), and of wide angle X‐ray diffraction WAXS. Four relaxation processes and one ferroelectric‐paraelectric phase transition have been detected. The process of the local mobility β‐ (at temperatures below glass transition point) is not affected by chemical composition of the copolymer and the formed structure. Parameters of segmental mobility in the region of glass transition (α_a‐relaxation) depend on the ratio of comonomer with lower kinetic flexibility. α_c‐relaxation is clearly observed only in VDF/HFP 93/7 copolymer, which is characterized by a higher crystallinity and a higher perfection of crystals of α‐ (α_p‐) phase. Diffuse order–disorder relaxor type ferroelectric transition connected with the destruction of the domains in low‐perfect ferroelectric phase in the amorphous regions has been detected for both copolymers. An intensive relaxation process (α‐process) was observed for both copolymers in high‐temperature region. DSC data shows that it falls on the broad temperature region of α‐phase crystals melting. It is considered to be connected with the space charge relaxation. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

X-ray diffraction study of crystal transformation and molecular disorder in poly(tetrafluoroethylene)

The intermolecular and intramolecular disorders, and the crystalline phase transition in poly(tetrafluoroethylene) are examined by X-ray diffraction between 20° and 250°C. A presence of the crystalline phase transition around T_t = 150°C is confirmed by the rapid decrease in the intensity of the Bragg reflection 1015. This phase transition is ascribed to the abrupt increase of the intermolecular translational disorder along the chain. From the quantitative analysis of the profile of the 0015 reflection, it is found that the increase in the translational disorder described above is accompanied by the excitation of the intramolecular disorder in conformation. The intramolecular disorder in conformation below T_t is found to be due mainly to the thermal excitation of trans conformation in the usual 15-7 helix. Further increase in disorder in conformation is found above T_t, which is due to the excitation of gauche conformation. The fraction of trans conformation increases up to 40% with the increase of temperature, in good agreement with the theoretical calculation. The fraction of gauche conformation above T_t is also estimated to give, for example, 7.5 × 10^?3 at T = 214°C.     

Revisiting the thermal relaxations of poly(vinyl alcohol)

The molecular dynamics of poly(vinyl alcohol) (PVA) were studied by dielectric spectroscopy and dynamic mechanical analysis in the 20–300°C range. The well-established plasticizing effect of water on the glass-transition temperature (T_g) of PVA was revisited. Improper water elimination analysis has led to a misinterpretation of thermal relaxations in PVA such that a depressed T_g for wet PVA films (ca. 40°C) has been assigned as a secondary β relaxation in a number of previous studies in the literature. In wet PVA samples, two different Vogel–Fulcher–Tammann behaviors separated by the moisture evaporation region (from 80 to 120°C) are observed in the low- (from 20 to 80°C) and high- (>120°C) temperature ranges. Previously, these two regions were erroneously assigned to two Arrhenius-type relaxations. However, once the moisture was properly eliminated, a single non-Arrhenius α relaxation was clearly observed. X-ray diffraction analysis revealed that the crystalline volume fraction was almost constant up to 80°C. However, the crystallinity increased approximately 11% when temperature increased to 180°C. A secondary β_c relaxation was observed at 140°C and was related to a change in the crystalline volume fraction, as previously reported. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Mechanical anisotropy in oriented polychlorotrifluoroethylene

The dynamic tensile modulus (E) and loss (tan δ) of various oriented samples of polychlorotrifluoroethylene have been measured at 0°, 45° and 90° to the draw direction over the range of ? 100 to 160°C. For the cold-drawn samples the mechanical anisotropy at the lowest temperature is determined by the overall chain orientation resulting in E₀ >E₄₅ >E₉₀. Above the γ relaxation (20°C) a shear process is activated leading to a change in the anisotropy pattern to E₀ >E₉₀ >E₄₅. Cold-drawing followed by annealing gives rise to further changes so that E₉₀ >E₀ >E₄₅ above the β relaxation (120°C). The effect of annealing has been attributed to the relaxation of the amorphous regions and the development of lamellar texture, and the anisotropy in tan $\text{δ}\text{E}$ at the β relaxation is found to be consistent with the interlamellar shear model.     

Mechanical properties of polysiloxanes: 1. Poly(tetramethyl-p-silphenylene siloxane) homopolymer and tetramethyl-p-silphenylene siloxane/dimethyl siloxane block copolymers

The mechanical behaviour of poly(tetramethyl-p-silphenylene siloxane) [poly(TMPS)] homopolymer and random block copolymers of tetramethyl-p-silphenylene siloxane-dimethyl siloxane [poly(TMPS-DMS)] of mean DMS block of 12 monomer units have been investigated over a wide range of composition and temperature using a Rheovibron viscoelastometer. The compositions ranged from TMPS/DMS wt % ratio of $\text{100}\text{0}$ to $\text{30}\text{70}$. The temperature intervals spanned from just above ?120°C to the point where melting became evident as dictated by the molecular architecture of each system. Two amorphous relaxation transitions, corresponding to DMS and TMPS phases, were found for copolymers with high TMPS content (?80 wt %). At lower TMPS compositions (?50 wt %) these transitions merge together. All dynamic transitions are a function of composition and crystallinity. The ‘hard’ TMPS phase provides crosslinks and acts as filler for the rubbery amorphous phase. The percentage elongation under tensile loading increases with the DMS amorphous content, which parallels an increase in clarity and decrease in density and crystallinity. A morphological model which depends on composition is proposed for these polysiloxanes. The model advanced is consistent with other physical evidence derived from other techniques. Changes in mechanical behaviour parallel changes in specimen morphology.     

Regime III crystallization in melt-crystallized polymers: The variable cluster model of chain folding

The kinetic nucleation theory of chain folding, including the effects of reptation, is extended to predict the increase in crystal growth rate G that is implied by measurements on PE and POM at moderately large undercoolings. The increased growth rate denotes the rather abrupt transition from Regime II where $\text{G∝i}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ to Regime III where G∝ i (i = surface nucleation rate). The distance between the niches on the growth front in Regime II diminishes rapidly with falling crystallization temperature T_x, and approaches the molecular width at a specified undercooling where Regime III begins. In PE, the Regime I → Regime II transition occurs at $\text{ΔT ? 16°}\text{C}$, and the Regime II → Regime III transition is predicted to occur at ΔT ～ 23°C (ΔT based on T°_m(∞) = 145°C). Growth rate data on PE and POM crystallized from the melt suggest conformity with the theoretical predictions. The implications of Regime III crystallization to chain morphology are discussed. The kinetic theory, which predicts narrowly spaced niches on the growth front, taken together with the restrictions on the degree of non-adjacent re-entry imposed by the ‘Gambler's Ruin’ treatment, leads directly to the ‘variable cluster’ model as the relevant morphology in Regime III. Here runs of adjacently chain-folded stems of varying size (averaging about three or so stems) are laid down interspersed with non-adjacent re-entries, leading to a lamellar surface that is about two-thirds ‘regular’ or ‘tight’ folds, most or all of these representing strictly adjacent re-entries. The steady-state reptation process operative in Regimes I and II in PE is impaired at temperatures just below the inception of Regime III, and it is suggested that at lower temperatures the ‘slack’ portions of the chains engage in forming the small clusters of adjacent stems. The variable cluster model leads in a natural way to the amorphous component found in quench-crystallized polymers, and is consistent with neutron scattering data on quench-crystallized PEH-PED.     

Solid and liquid state relaxations in spin-labelled poly)ethylene glycol) at high temperatures (T>Tg)

Free and covalently bonded (esterified) nitroxyl radicals experienced in poly(ethylene glycols) (PEG; $\text{M}\text{?}{}_{\mathrm{n}}$ 200–22 000) at temperatures T >T_g several different isotropic rotational relaxation regions. As a first approximation it was assumed, that in the polyglycols $\text{M}\text{?}{}_{\mathrm{n}}\text{? 1000}$ there are at least three rotational relaxation regions: the liquid state (I), the melting state (II) and the solid state (III). The existence of the fourth region, the frozen solid state (IV), was also concluded. The existence of the relaxation region (II) indicated the close interaction between radicals and the crystalline phase. The order of rotational activation energies (E_a) was E^II_a >E^III_a >E^I_a >E^IV_a ($\text{M}\text{?}{}_{\mathrm{n}}\text{? 1000}$). In the polymer melts (I) E_a values of free and bonded radicals first diminished as a consequence of the decrease of the end group effect and they achieved constant high molecular weight values (～15 and 25 kJ respectively). E_a changed in the solid state as a function of $\text{M}\text{?}{}_{\mathrm{n}}$ principally in the same manner except of the higher numerical values (～40 kJ).E_a of free and covalently bonded radicals in the transition region (II) gained a maximum at $\text{M}\text{?}{}_{\mathrm{n}}$ 1550 (125 and 170 kJ) and another at $\text{M}\text{?}{}_{\mathrm{n}}\text{> 9500}$ (130 and 165 kJ) expressing the high degree of order in these polymers in the solid state.The results obtained correlated well with the proton magnetic resonance measurements but they did not correlate with the amorphous dielectric relaxation measurements.It was concluded that the following factors may affect the rotational relaxations of nitroxyl radicals in PEG: the free volume of the polymer, the crystallinity, the chain packing and the end-group effect. The segmental character of the relaxation process was clearly indicated.     

Comparison of the dielectric and viscoelastic properties of two poly (propylene glycol) liquids

The complex relative permittivities of two poly(propylene glycol) liquids, of molecular weights 380 and 3030, have been determined. Measurements were made in the frequency range 60 Hz to 200 MHz and at temperatures from ?64° to +25°C. The lower molecular weight liquid showed only one relaxation region, which could be described by the Williams-Watts function with $\text{}\text{?}\text{gb = 0.51}$. The higher molecular weight liquid exhibited a similar main relaxation region and a secondary relaxation, of smaller amplitude, at lower frequencies. The relaxation times and their temperature dependencies are compared with previously determined viscoelastic relaxation and retardation times for the same liquids, and with the results from earlier dielectric studies by Baur and Stockmayer, and by Williams. The differences between the dielectric and mechanical times are discussed.