Glass transition behaviour of poly(ether ether ketone)/poly(aryl ether sulphone) blends: dynamic mechanical and dielectric relaxation studies

Blends of poly(ether ether ketone) (PEEK) and poly(aryl ether sulphone) (PES) have been prepared in the whole composition range. The molecular dynamics and α-relaxation behaviour of these materials have been studied using dynamic mechanical and dielectric relaxation spectroscopy. From dynamic mechanical relaxation studies, two α-relaxation peaks corresponding to the segmental relaxation process of pure components in the blend was observed. Also, it was found that the temperature at which α-process of the homopolymers occurs, shows a slight change with blend composition, corresponding to a PEEK-rich and PES-rich phase. The relaxation intensities of the homopolymers in the blend compared to that in pure state were approximately proportional to their respective content in the blend. From the phase composition of the respective phases obtained using Fox equation, it has been inferred that PEEK dissolves more in PES than vice-versa. The α-relaxation of PES could not be detected from dielectric relaxation spectroscopy because of the possible influence of dc conduction and electrode polarization losses. Otherwise, the α-relaxation behaviour of PEEK-rich phase observed from dielectric relaxation studies agree with those inferred from dynamic mechanical relaxation studies. Furthermore, activation energies for molecular motions (E_a) at the α-relaxation have also been determined using an Arrhenius form of equation and it has been found that E_a for both PEEK-rich and PES-rich phase show variation with composition. Similarly, the relaxation times associated with the mobility of relaxing species in both PEEK and PES are influenced in the blends. It is likely that these observations are related to some interactions and a partial segmental mixing between the blend components, which result in changes in the local molecular environment on blending.     

Influence of azobenzene concentration on the dielectric behavior of amorphous comb-like copolymers with photochromic side groups

Dielectric spectroscopy in the frequency range from 10⁻² to 10⁶ Hz and in the temperature range from 190 to 440 K is employed to study the effect of azobenzene concentration on the dielectric relaxation processes of an amorphous comb-like copolymethacrylate. Four concentrations (x=29, 45.5, 54, and 74.5 mol%) of photochromic group 4-amino-azobenzene were investigated, where as comonomer a methacrylate unit having a derivative of benzanilide in the side group is used. Two prominent processes, the β-relaxation at low temperatures which is related to rotational fluctuations of the mesogenic unit around its long axes and the dynamic glass transition (α-relaxation, segmental dynamics) at higher temperatures are observed for all azobenzene concentrations. In addition in between the α- and the β-relaxation a β′-process is observed for the polymers with the two lowest azobenzene concentrations, which seems to be related to the azobenzene unit. The dependence of the dielectric strength and the relaxation rate of the relaxation processes on the azobenzene concentration is discussed and interpreted in a simple phenomenological model, where also data obtained by semi empirical quantum chemical calculations are used.     

Dielectric and dynamic-mechanical study of the mobility of poly(t-butylacrylate) chains in diblock copolymers: Polystyrene-b-poly(t-butylacrylate)

A calorimetric, dielectric and dynamic-mechanical study of the dynamics of the poly(t-butyl acrylate) (PtBa) chains has been carried out in a PtBa homopolymer and two polystyrene (PS)-b-PtBa block copolymers with different PtBa chain lengths. The DSC results show that the size of the cooperative rearranging regions is similar in the homopolymers and the copolymers, both for the PtBa rich- and the PS-rich regions. Therefore, no significant contributions are found arising from composition fluctuations in the copolymers. The relaxation map obtained from dielectric relaxation indicates that there are no differences in the temperature dependence of the α-relaxation of the PtBa block in the three samples studied. However, there are larger differences for the values obtained from DMTA experiments. Contrary to the α-relaxation, the relaxation map for the β-transition shows that the characteristic times for the PtBa blocks are smaller in the homopolymer than in the copolymers. In principle, these are unexpected results because the β-relaxations have a more local character than the α-ones. The width of the α-relaxation increases with T for all the samples, and it is slightly larger for the copolymers. The intensity of the α-relaxation is larger (between 3 and 4 times) for the homopolymer. Considering the molecular weights of the PtBa blocks, this effect has to be ascribed to the existence of frozen amorphous PtBa due to the existence of the glassy PS domains in the microphase separated copolymers.Molecular Dynamic Simulations (MDSs) for different sequences of the polymers under study were carried out. The conformational analysis was carried out between 1000 and 1700 K. The analysis of the variation of angles ?₁ and ?₂ of the ester group of PtBa points out the existence of a correlation between the conformational changes of the side group of the polymer chains and their relaxational behaviour.     

Dielectric relaxations in polyoxymethylene and in related nanocomposites: Identification and molecular dynamics

Broadband dielectric spectroscopy (BDS) is employed for the study of the dielectric response of Polyoxymethylene/boehmite alumina (POM/alumina) and Polyoxymethylene/Layered Silicates (POM/LS) semi-crystalline nanocomposites, together with the response of pure POM. The compilation of the dielectric responses of all systems reveals the existence of five dielectric relaxation mechanisms assigned, in terms of decreasing temperature at constant frequency, as IP (interfacial polarization), α-, β-, γ- and δ-relaxations. IP, α- and γ-relaxations are detected in all studied systems and they are well documented in the literature. δ-relaxation, been the fastest mechanism, is present only in the POM/LS system and is associated with defect dipoles in the crystalline phase of POM. Finally, β-relaxation is present only in the POM/alumina nanocomposite and its dynamics obey Vogel–Fulcher–Tamann temperature dependence. This work presents a complete mapping of the dielectric relaxation mechanisms of POM and signifies the connection of β-mode with the glass to rubber transition of POM, confirming its cooperative character.     

Influence of long-chain branching on linear viscoelastic flow properties and dielectric relaxation of polycarbonates

The dynamic viscoelastic property, creep and creep recovery behavior, and dielectric relaxation of long-chain branched Bisphenol A polycarbonates were measured in parallel plate rheometer and dielectric analyzer. The linear polycarbonate (PC-L) as reference and three branched polycarbonates (PC-Bs) have similar molecular weights and molecular weight distributions, while the PC-Bs have different branching degrees, below 0.7 branch points/chain and above twice of M_c. The long-chain branched polycarbonates exhibit higher zero-shear viscosities, more significant shear shinning, higher flow activation energies, and much longer relaxation times. It was also found that long-chain branches increase the elasticity of melt characterized by the steady-state recoverable compliance and the storage modulus. The ‘dissident’ rheological behavior of long-chain branching exhibiting mainly in addition polymers such as polyolefin, is confirmed in condensation polymers. These behaviors resulted from additional molecular entanglements of long-chain branches can be understood qualitatively in terms of the tube model for topological constraints. The dielectric α-relaxation of linear polycarbonate and branched polycarbonates has been fitted with Vogel-Fulcher-Tammann-Hesse (VFTH) equation and the shape of relaxation time curves is also analyzed. The long-chain branched polycarbonates present longer relaxation times, but divergent α-relaxation temperatures, because the latter is dominated by the free volume.     

Broadband dielectric spectroscopy of nanostructured maleated polypropylene/polycarbonate blends prepared by in situ polymerization and compatibilization

The miscibility and molecular dynamics of nanostructured maleated polypropylene (mPP)/polycarbonate (PC) blends prepared by in situ polymerization of macrocyclic carbonates with polypropylene modified with 0.5 wt% of maleic anhydride-reactive groups were investigated over a wide range of frequencies (10⁻²-0.5 × 10⁷ Hz) at different constant temperatures using broadband dielectric spectroscopy and scanning transmission electron microscope (STEM). The molecular dynamics of the glass relaxation process of the blend (α-relaxation process) appeared at a lower temperature range compared with that of the pure PC. This shift in the molecular relaxation process is attributed to the partial miscibility of the two polymer components in the blends as previously confirmed by the morphology via STEM. Nanoscale morphologies with average domain diameters as small as 50 nm were obtained for the different blend compositions studied. The STEM photographs show that the graft mPP-g-PC prefers to locate at the interfaces as previously reported. The relaxation spectrum of pure PC and mPP/PC blends was resolved into α- and β-relaxation processes using the Havriliak-Negami equation and ionic conductivity. The dielectric relaxation parameters, such as relaxation peak broadness, maximum frequency, f_max, and dielectric strength, Δ? (for the α- and β-relaxation processes), were found to be blend composition dependent. The kinetics of the α-relaxation processes of the blends were well described by Vogel-Fulcher-Tammann (VFT) equation. The local process of PC was resolved into two relaxation processes β₁ and β₂, associated with the carbonyl groups' motion and the combined motions of carbonyl and phenylene groups, respectively. Only β₂ shifted to lower frequency in the blend while β₁ was relatively not affected by blending. The electric modulus of the blends was used to get a sufficient resolution of the different relaxation processes in the samples, i.e., α-, β-relaxation processes, ionic conductivity, and interfacial polarization. In addition, the blending method used was found to increase the d.c. conductivity without affecting the charge carrier transport mechanism, making it possible to develop novel polymer blends with tunable dielectric properties and morphology from existing polymers.     

Dielectric and dynamic mechanical relaxation behaviour of poly(ethylene 2,6-naphthalene dicarboxylate). II. Semicrystalline oriented films

The dielectric and dynamic mechanical behaviour of bi-stretched non-treated and annealed semicrystalline poly(ethylene 2,6-naphthalene dicarboxylate) (PEN) films are studied as a function of different morphologies obtained by thermal treatments at temperatures close to the melting temperature of a semicrystalline film. Differential scanning calorimetry (DSC) shows that the glass transition temperatures do not change significantly with the thermal treatment for bi-stretched films. However, the melting temperatures and the degree of crystallinity increase with the value of annealing temperature. Both dielectric relaxation spectroscopy (DRS) and dynamic mechanical analysis (DMA) display three relaxation processes. In order of decreasing temperature, can be observed: the α-relaxation due to the glass transition, the β^∗-process assigned to cooperative molecular motions of the naphthalene groups which aggregate and the β-relaxation due to local fluctuations of the carbonyl groups. The α-relaxation process shifts to higher temperatures for the 250 and 260 °C treated bi-stretched semicrystalline samples compared to the sample thermally treated at 240 °C according to DRS data but shifts to lower temperatures according to the DMA measurements for the three annealed samples. This discrepency results from the different sensitivity of each methods with regards to the release of orientation. At a fixed frequency the temperature associated to β^∗-relaxation is lower for the non-treated bi-stretched semicrystalline samples than for the treated ones using DMA but no difference can be seen in DRS. The associated apparent activation energies are rather high which suggest cooperative motions. It is assumed that the orientation of the samples prevents coupling between the naphthalene groups due to the stretched chain configuration in the amorphous phase. The activation energy for the β-process given by DRS is independent of the thermal treatment and the value agrees with those found for poly(ethylene terephthalate) (PET) and amorphous PEN. Evidence of the decrease of orientation in the sample with thermal treatment can be seen via the onset of mobility, both by DRS and DMA. Thus, the orientation induces a greater change of properties compared to the crystalline samples obtained from the thermal treatment of an amorphous sample. Finally, a three phase model is proposed since there is evidence of a rigid amorphous phase present in PEN biaxially stretched samples which was favoured by the dependence of dielectric relaxation strengths on the degree of crystallinity for the β^∗- and α-relaxation.     

Study of dielectric relaxation of an epoxide oligomer by a direct current transient method

Dielectric α-relaxation of a bisphenol-A type epoxide oligomer has been investigated in the vicinity of the glass transition temperature (T_g) by the direct current (DC) transient method. The logarithm of the DC transient current for the oligomer was well approximated by the third order function of the logarithm of time. The complex dielectric constant was calculated through the Fourier transformation of that approximation function according to Simpson's integration rule in a frequency range of 10^?5 ? 1 Hz. At the temperature around the T_g (45°C), the dielectric α-relaxation process of the oligomer was found to be governed by the Havriliak-Negami equation. The relationship between the DC conductivity (σ) and the dielectric relaxation time (τ), σ·τ^m = const, is valid near and above the T_g of the oligomer. The DC transient current method combined with the DC conduction and the dielectric bridge measurements is considered to be a practical tool for analyzing the dielectric α-relaxation process of the epoxide oligomer over a wide frequency and temperature range.     

Dynamic mechanical and dielectric relaxations in poly(di-n-chloroalkylitaconates)

Dielectric and viscoelastic relaxation measurements have been carried out on poly(2-chloroethyl diitaconate) (PDCEI) and poly(3-chloropropyl diitaconate) (PDCPI) between 123 K and temperatures about 293 K above the glass transition temperatures.The two polymers exhibit three peaks, a γ-relaxation in the range from 133 to 193 K (at 1 Hz), a broad β-process, in the range from 193 to 293 K and a third peak observed in mechanical measurements at 323 K (PDCEI) and 293 K (PDCPI) probably corresponding to the α dynamic glass transition phenomena. In dielectric measurements, conductive contributions overlap the α-relaxation precluding the observation of peaks at temperatures above room temperature. The apparent activation energies for the γ-relaxation according to the mechanical and dielectric measurements are close to the values derived from the empirical force field molecular mechanics calculations. A comparison is made between the relaxational data of PDCEI and PDCPI by one hand and poly(di-n-propyl itaconate) (PDPI) and poly(di-n-butyl itaconate) (PDBI) by other. This comparison allows us to conclude the relevant role played by the chlorine atoms not only in the γ relaxations but also in the β relaxations of PDCEI and PDCPI.     

Dielectric relaxation of poly(tetrafluoroethylene–perfluorovinyl ether) films

Dielectric constant and dielectric loss have been studied for poly(tetrafluoroethylene–perfluorovinyl ether) (PFA) films over a wide temperature range in the frequency range 0.1–100 kHz. Two relaxation peaks were observed, one at room temperature (α_a-relaxation) and the other in the range 170–140 K (β-relaxation), with activation energies of 143·2 and 16·4 kcal/mol, respectively. The β-absorption is attributed to the short segmental local mode motion of the main chains. The α_a-relaxation can be interpreted as due to large-scale conformational rearrangement. The Cole–Cole diagrams are given at different temperatures and the distribution parameters (ϵ₀–ϵ_∞) and (1–α) of the relaxation times were calculated. The X-ray diffraction pattern of PFA shows both a diffuse halo and sharp reflections, characteristic of amorphous and crystalline phases of conventional semicrystalline polymers. Also, no evidence of crystallinity in the films due to thermal treatment during dielectric measurements was observed. IR spectra revealed the absence of any new peaks after the heat treatment.     

Dielectric and dynamic mechanical measurements of poly(4-methyl pentene-1)

The α-relaxation process of poly(4-methyl pentene-1) was studied by dielectric and dynamic mechanical means. The complex dielectric constant was determined at nine discrete frequencies from 100 to 10,000 Hz and over a temperature range of ?50–90°C. The complex dynamic mechanical Young's modulus was determined over the audiofrequency range of 10–22,000 Hz and a temperature range of 21–76°C, from which a master curve was constructed. The relaxation process was studied by comparing the activation energies and width of the dispersion curves. The results of a logarithmic frequency vs. reciprocal temperature plot of the loss peak maxima show that both the dielectric and mechanical curves are roughly linear but have different slopes. From the slopes the activation energies were determined. For the dielectric data an activation energy of 39 kcal/mol was obtained, whereas for the mechanical data a value of 106 kcal/mol was found. The width of the dispersion curves was determined by using a Cole–Cole empirical fit. The width of the dielectric dispersion curve is narrower by as much as a factor of 3 than the mechanical dispersion curve. It is concluded that the energy to cause the large scale molecular motion involved in the α-relaxation is lower when excited by an alternating electric field than by an alternating stress field. Also the number of repeat units involved is smaller in the dielectric case than in the mechanical case.     

Low frequency ac conduction and dielectric relaxation behavior of solution grown and uniaxially stretched poly(vinylidene fluoride) films

The measurements of ac conductivity [σ_m(ω)], dielectric constant [?′(ω)] and loss [?″(ω)] have been performed on solution grown (thickness ∼85 μm) and uniaxially stretched (thickness ∼25, 45 and 80 μm) films of poly(vinylidene fluoride) (PVDF) in the frequency range 0.1 kHz-10 MHz and in the temperature range 77-400 K. The σ_m(ω) can be described by the relation σ(ω) = Aω^s, where s is close to unity and decreases with increase in temperature. Three relaxations, observed in the present investigation, have been designated as the α_c-, the α_a- and the β-relaxations appearing from high temperature side to the low temperature side. The α_c-relaxation could not be observed in the case of uniaxially stretched poly(vinylidene fluoride) films. The α_c- and α_a-relaxations are associated with the molecular motions in the crystalline regions and micro-Brownian motion in the amorphous regions of the main polymer chain, respectively, whereas the β-relaxation is attributed to the rotation of side group dipoles or to the local oscillations of the frozen main polymer chain.     

Dielectric analysis of cellulose and its vinylic copolymers

The real and imaginary parts of the complex dielectric permittivity, ε′ and ε″, for some vinylic copolymers of cellulose [prepared with vinyl acetate (VA) and methyl acrylate (MA) and Ce (IV) ions as initiator ] and for cellulose were measured over a frequency band of 0.1–10² kHz and a temperature range from ?40 to 100°C. In vinylic copolymers of cellulose, we observed one dielectric relaxation attributed to the α-relaxation of the vinylic side chain grafted on cellulose. In cellulose dielectric spectra, this relaxation did not appear, but we detected one relaxation that may correspond to the β-relaxation. For these vinylic copolymers of cellulose, the ε″ against ε′ plot gives a skewed are that closely resembles that of the Davidson–Cole model, with a broader distribution for high frequencies that shows the overlap of several relaxations in the process considered. Some differences observed between the vinylic copolymers of cellulose may be due to the composition and the length of the vinylic side chains and to the frequency of grafting on the cellulose.     

Relaxation in poly(alkyl methacrylate)s: Change of intermolecular coupling with molecular structure, tacticity, molecular weight, copolymerization, crosslinking, and nanoconfinement

The voluminous amount of data in the literature on the structural α- and the Johari-Goldstein β-relaxations of the poly(n-alkyl methacrylate)s allows a systematic study of the interrelation between the two important relaxation processes. The data bring out the systematic changes in the interrelation between the structural α- and the Johari-Goldstein β-relaxations with changes in molecular structure, molecular weight, tacticity and size (by nanoconfinement), and modifications by copolymerization, and crosslinking. The results can all be interpreted as primarily due to changes in intermolecular coupling, which have significant effects on the many-molecule dynamics constituting the structural α-relaxation, but not on the precursory Johari-Goldstein β-relaxation. Theoretically, the Coupling Model predicts a relation of intermolecular coupling (or degree of cooperativity of the α-relaxation) to the ratio of the α- and the β-relaxation times, and a correlation of intermolecular coupling to the steepness or “fragility” index. The predicted relation and correlation are compared with experimental data of the poly(alkyl methacrylate)s.     

The effect of micro-fibrillated cellulose upon the dielectric relaxations and DC conductivity in thermoplastic starch bio-composites

Thermoplastic starch (TPS) bio-composites were prepared with natural rubber latex (NR) and/or cellulose microfibers of variable size, as reinforcements. The relaxation processes are investigated via broadband dielectric spectroscopy in a broad temperature and frequency range. By means of the electric modulus formalism, the dynamic glass-to-rubber transition process of the starch-rich regions in TPS (α-relaxation), and the presence of water molecules at the interfaces between the TPS matrix and the fillers (MWR-IP), is observed and discussed. The dielectric behavior and energy storage were found to be affected by the presence of the aforementioned water molecules, hindering the polarizability of the samples due to hydrogen bonding with the hydrophilic constituents. Following the temperature dynamics of the α-relaxation, good adhesion between the TPS matrix and the cellulose microfibers was revealed, although the addition of natural rubber latex microparticles has the opposite effect attributed to their hydrophobic character. Finally, the dc conductivity was estimated from the ac dielectric spectra with a new mathematical formulation that is introduced here, and showed nonlinear temperature dependence for all the samples under study. The purpose of this study is to investigate the use of biodegradable eco-friendly thermoplastic starch bio-composites as dielectric materials for capacitor applications. Microfibrillated cellulose and/or natural rubber latex are employed as reinforcements and insight upon their dielectric relaxation behavior is provided by broadband dielectric spectroscopy in a broad temperature and frequency range.     

Calorimetric and dielectric study on poly(trimethylene terephthalate)/polycarbonate blends

Poly(trimethylene terephthalate) (PTT)/polycarbonate (PC) blends with different compositions were prepared by melt blending. The miscibility and phase behavior of melt-quenched and cold-crystallized blends were studied using differential scanning calorimetry (DSC) and dielectric relaxation spectroscopy. The blends of all compositions display only one glass transition (T _g ) in both states. The melting temperature and the crystallinity of PTT in the blend decrease with increasing PC content. The dielectric results for the melt-quenched blends, for PC content up to 60 wt.%, exhibited two merged relaxation peaks during the heating scan; the lower temperature relaxation peak represent the normal glass-transition (α) relaxation of the mixed amorphous phase and the higher temperature relaxation due to the new-constrained mixed amorphous phase after crystallization. Cold-crystallized blends displayed only one glass transition α-relaxation whose temperatures varied with composition in manner similar to that observed by DSC. The dielectric α-relaxation of cold crystallized blends has been analyzed. Parameters relating to relaxation broadening, dielectric relaxation strength, and activation energy were quantified and were found to be composition dependent. The PTT/PC blends could be considered as two-phase system, a crystalline PTT phase and a mixed amorphous phase consisting of a miscible mixture of the two polymers. However, the crystallinity was only detected for blends containing greater than 40 wt.% PTT.     

Dielectric Relaxation Spectroscopy of Poly (Vinyl Chloride-co-Vinyl Acetate-co-2-Hydroxypropyl Acrylate)/Poly (Acrylonitrile-Butadiene-Styrene) Polymer Blend

The dielectric techniques used to investigate the relaxation behavior of poly (vinyl chloride-co-vinyl acetate-co-2-hydroxypropyl acrylate) (PVVH), poly (acrylonitrile-butadiene-styrene) (ABS), and its polyblends include broadband AC dielectric relaxation spectroscopy (DRS) in the frequency range from 10^?2 to 10⁵ Hz, and thermally stimulated depolarization current (TSDC) technique in the temperature range from 300 K to 413 K. It was observed that PVVH is characterized by a dipolar relaxation peak around 347 K and a space charge peak in the temperature range 353–383 K, whereas pure ABS is characterized by a dipolar relaxation peak at 389 K. On the other hand, polyblend samples are found to be characterized by two different relaxation peaks at 359 K and 387 K, respectively. The dielectric properties of pure materials and their polyblend are investigated. All samples are characterized by high dielectric constant (ε′) at very low and high temperature. The dielectric loss shows a single peak for pure materials, whereas polyblends show a broad peak at low frequency which could be attributed to the Maxwell-interfacial polarization (MWS) and another peak at high frequency which could be attributed to the dipolar relaxation. The temperature dependence of AC conductivity was investigated for all samples. The values of the exponent n suggest that the hopping mechanism dominates at lower temperatures.     

Peculiarities of δ- and α-relaxations in thermotropic side chain liquid crystalline polymers with and without nematic reentrant phase

We have performed a dielectric spectroscopy study of four homologous cyanobiphenyl polyacrylates with long side chains. The α- and δ-relaxation times were found to be sensitive to the sequential transformations between mesophases. The τ^δ in the isotropic phase exhibits the characteristics that obeys VFT relation and depend strongly on spacer length. The relaxation times, τ^δ, for the crossover from short range intermolecular interactions to long range LC ordering, decreases with increasing side chain length, implying that the cooperative motions of mesogenic dipoles arrange long range order at shorter time scales, as the spacer length is increased. In the SmA mesophases of CBPAn compounds with n = 8, 9 and 11 α-relaxation times were found to be nearly temperature independent at high temperatures. Thus, segmental motions take place in the state of diminished dynamical constraints of backbones, which can be attributed to the plasticization effects of polymeric layers in the case of long methylene spacers. In CBPA6 and CBPA8, with even numbers of methylene groups, anomalies of δ-relaxation processes were observed at the nematic reentrant (N_re) transitions. The anomalies of α-relaxation processes in the SmA mesophases were found to be precursors of N_re transitions as temperature decreases. The changes in backbone conformations of the SmA layers with decreasing temperature create the conditions for molecular corrugations of the side chains leading to the formation of the nematic order of N_re phases.     

Physical crosslinking effects in α,ω-dihydroxy terminated polybutadienes

Structure and molecular dynamics of α,ω-dihydroxy terminated polybutadienes of varying number averaged molecular weight (1320-10?500 g/mol) have been investigated by Fourier-transformed infrared (FTIR) spectroscopy, nuclear magnetic resonance (NMR) spectroscopy, dielectric relaxation spectroscopy (DRS), differential scanning calorimetry (DSC) and wide-angle X-ray scattering (WAXS). DSC and DRS revealed an increase in the glass transition temperature upon decrease of the molecular weight, accompanied by an increasing dynamic fragility m (or steepness index) of the dielectric α-process. This correlation between T_g and m for different molecular weights indicates the presence of a physical network, where H-bonded end-group clusters act as temporary crosslinks. From the dielectric relaxation strength Δ?_α(M_n), the fraction of associated hydroxy groups (f_bond) was estimated showing a peak value for the two but shortest polymers, a behaviour that strongly resembles the molecular weight dependence of the fragility. By considering the quantity f_bond(M_n) in a modified Fox-Flory approach, the measured T_g(M_n) behaviour could be reproduced in a satisfying way. FTIR results support this general picture and show a considerable dependence of the extent of hydrogen bonding and formation of hydroxy groups associates on the molecular weight. Further, WAXS and DSC results disprove the idea of formation of pseudo-crystalline hydrophobic microdomains in these compounds as suggested by other authors.     

Ultrasonic absorption and relaxations in ABS composite polymers

We investigated ultrasonic attenuation, dynamic Young's modulus, Izod impact strength, and dielectric relaxation of acrylonitrile-butadiene-styrene composite polymers (ABS) in which styrene-co-butadiene rubber particles are dispersed in styrene-co-acrylonitrile random copolymer. High ultrasonic attenuation was observed around 240 K and the intensity of attenuation increased with increasing rubber content. Temperature dependence curves of dielectric loss in the frequency range from 12 Hz to 200 kHz and temperature range from 80 to 420 K exhibited four relaxation processes designated as the α, β′, β, and γ. From the relaxation map produced for the mechanical and dielectric relaxations, the ultrasonic absorption was attributed to the β and β′ processes. It was deduced that inter-grain thermoelastic process, also contribute to the ultrasonic attenuation. Correlation was found between the ultrasonic absorption and the Izod impact strength indicating that the high impact strength of ABS is partly due to the effective absorption of impact energy through those relaxation processes.