Dielectric relaxation in montmorillonite/polymer nanocomposites

We report dielectric relaxation behavior in blends of sodium montmorillonite particles (MM) with a series of polymers (i.e., polyisoprene (PI), poly(propylene glycol) (PPG), and poly(butylene oxide) (PBO)). These polymers are known to exhibit the dielectric normal mode due to the fluctuation of the end-to-end vector as well as the segmental mode due to local, segmental fluctuations. The data indicate that all blend systems exhibit an additional relaxation process at a temperature region below the glass transition temperature, T_g, of the pure polymer component. The intensity of the new relaxation process increases with the content of MM and hence the relaxation process can be assigned to the segmental motion of the chains intercalated in the interlayers of MM. On the other hand, the relaxation time of the normal mode reflecting the fluctuation of the end-to-end vector is the same as the neat polymers but the intensity of the relaxation process increases due to enhancement of the internal electric field by MM.     

Influence of diamine structure on the low temperature dielectric relaxation of some poly(ether imide)s

Broadband dielectric relaxation spectra are reported on a range of poly(ether imide) polymers in which the chemical structure of the diamine used to create the polymer is systematically varied with the anhydride structure based on 2,2‐bis‐[4‐(3′,4′‐dicarboxyphenoxy)phenyl]hexafluoroisopropylidine dianhydride. In all the polymers examined, a dipole relaxation was observed below room temperature. The magnitude and activation energy associated with the relaxation process varied with the chemical structure reflecting the effects of steric hindrance on the conformational change associated with the N? C and C? O? C linkages. Values of the activation energies varied between 29 and 34 kJ/mol^?1, and are consistent with the observed relaxation being associated with constrained local oscillatory motions of small elements of the polymer backbone. The glass transition temperatures of these polymers are in the range 195–243°C and are associated with the large scale motion of the polymer backbone. Changes in the backbone structure influence the extent of inter chain–chain interaction and are reflected in the amplitude of the relaxation process and the high frequency limiting dielectric permittivity ε_∞ values which are important when these polymers are used in thin film electronic applications. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41684.     

Microwave dielectric relaxation and molecular dynamics in binary mixtures of poly(propylene glycol) 2000 and poly(ethylene glycol)s of varying molecular weight in dilute solution

Molecular dynamics of binary mixtures of poly(propylene glycol) (PPG) and poly(ethylene glycol)s (PEGs) of varying molecular weight due to molecular interactions, chain coiling and elongation in dilute solution under various conditions, ie varying number of monomer units of PEG, method of mixing of polymers and solvent environment, has been explored using microwave dielectric relaxation times. The average relaxation time τ_o, relaxation time corresponding to segmental motion τ₁ and group rotations τ₂, of a series of binary mixtures of poly(propylene glycol) 2000 and poly(ethylene glycol) of varying molecular weight (ie PPG 2000 + PEG 200, PPG 2000 + PEG 300, PPG 2000 + PEG 400, and PPG 2000 + PEG 600 mixed by equal volume in the pure liquid states, and PPG 2000 + PEG 1500, PPG 2000 + PEG 4000 and PPG 2000 + PEG 6000 mixed equal weights in solvent) have been determined in dilute solution in benzene and carbon tetrachloride at 10.10 GHz and 35 °C. A comparison of the results of these binary systems of highly associating molecules shows that the molecular dynamics corresponding to rotation of a molecule as a whole and segmental motion in dilute solutions are governed by the solvent density when the solutes are mixed in their pure liquid state. Furthermore, the molecular motion is independent of solvent environment when the polymers are added separately in the solvent for the preparation of binary mixtures. It has also been observed that there is a systematic elongation of the dynamic network of the species formed during mixing of pure liquid polymers in lighter environment of solvent with increasing PEG monomer units, while the elongation behaviour of the same species in the heavier environment of carbon tetrachloride solvent is in contrast to the elongation behaviour of the polymeric species formed in pure PEG. The role of rotating methyl side‐groups in the PPG molecular chain has been discussed in term of the breaking and reforming of hydrogen bonds in complex polymeric species for the segmental motion. In all these mixtures, the relaxation time corresponding to group rotations is independent of the solvent environment and constituents of the binary mixtures. The effect of chain flexibility and coiling in these binary mixtures is discussed by comparing the relaxation times of the mixtures with their individual relaxation times in dilute solutions measured earlier in this laboratory. © 2001 Society of Chemical Industry     

Thermal properties and dielectric relaxation of a multi‐component poly(ether‐co‐amide) based on polyamide‐12

DSC, dielectric relaxation and dynamic mechanical thermal analysis (DMTA) were carried out on two multi‐component poly(ether‐co‐amide) samples having different weight ratios of polyamide prepared by condensation polymerization with 12‐aminododecanoic acid, adipic acid and polyetherdiamine consisting of poly(tetramethylene oxide) and poly(propylene oxide). The melting temperature was lowered by an increase in the weight ratio of the polyamide segment. Three relaxation modes, α′, α_s and β, were found from dielectric relaxation measurements in different temperature ranges. The high temperature relaxation mode, α′, has a large dielectric constant, which disappears at the melting temperature of the polyamide crystal in the sample. The relaxation times for the segmental motion, α_s, were different for the samples, which is attributed to the difference in the composition of the uncrystallized polyamide segments in the amorphous domain. The glass transition temperature estimated from DMTA is located between those of constituting polymers. On the other hand, the activation energy of β‐mode observed at low temperatures is the same for samples with different polyamide ratios, which is attributed to the local motion of the polyether segments. The uncrystallized polyamide segments are miscible with the polyether segments, which results in a lowering of the glass transition temperature of the amorphous domain and enlarges the temperature range of the rubber state of the copolymer due to the high melting temperature of the polyamide segments. © 2016 Society of Chemical Industry     

Biodegradable blends based on polyvinyl pyrrolidone for insulation purposes

Polyvinyl pyrrolidone/polyvinyl alcohol (PVP/PVA) and polyvinyl pyrrolidone/starch (PVP/St) blends were prepared with different compositions. The compatibility studies indicate that PVP/PVA is compatible while PVP/St is incompatible. The addition of glycerol and glutaraldehyde can improve to some extent the phase separation behavior between PVP and St. The permittivity ε′ and the dielectric loss ε″ were measured in the frequency range 0.01 Hz up to 10 MHz and temperatures from 30 up to 90°C. It is found that the blend ratio (50/50) of both investigated systems is preferable for insulation purposes in comparable with the other blends under investigation. The data of the loss electric modulus M″ was calculated from the dielectric parameters ε′ and ε″and analyzed into three relaxation mechanisms ascribing the cooperative motion of the main and side chains τ₁ (αβ), the side chain motion τ₂ (β) and the segmental motion of the groups attached to the side chains τ₃ (βγ). The activation energy corresponds to the second relaxation process ΔH₂ was calculated using Arrhenius equation and found to be in the range which justifies the presumption of β‐relaxation process. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Relaxation processes in the surface layers of polymers at the interface

The proton spin-lattice relaxation and dielectric relaxation were studied in some polymers at the solid–polymer interface was constructed from several filled polymers. A useful model of surface layer which can be considered as consisting of a great number of small solid particles covered with a polymer layer. The following systems were studied: polystyrene, poly(methyl methacrylate), their copolymers and cellulose acetate in the presence of different content of fine particles of aerosil and Teflon. It was established that the decrease of surface layer thickness shifts the minimum of spin-lattice relaxation time T₁ of high temperature process to higher temperature and minimum T₁ of low temperature process to lower temperature. The same was found for dielectric losses reflecting the motion of side groups and of segments. From temperature dependence of T₁ and tan δ for both relaxation processes the apparent energies of activation were calculated. On the base of dielectric relaxation data the circular diagram of complex dielectric constant was constructed and by the Cole-Cole method the dispersion parameter α for polymers at the interface was calculated. These data also show the broadening of relaxation spectra in surface layers. The results are discussed in terms of the restriction of possible conformation of chains at the interface and their interaction with surface. It was established that character of molecular motion changes at the interface is dependent on the mode of molecular motion.     

Molecular motion in α-methylstyrene and styrene-alkane copolymers studied by 13C n.m.r. spectroscopy

The temperature dependence of the ¹³C n.m.r. relaxation times are reported for a series of copolymers formed between methyl styrene or styrene and various α, ω-alkyl dihalides. The polymers studied contained alkyl blocks with n (the number of CH₂ units) varying from 0–10. The relaxation data indicates that the motion of both the styrene and the alkyl blocks change with the value of n. A comparison of the ¹³C relaxation data with that obtained from ultrasonic relaxation measurements supports the hypothesis that in the higher members of each series the alkane chain moves essentially independently of the ‘styrene’ moiety. The variation in the activation energy of the relaxation of the styrene moiety reflects in part a decoupling of the neighbouring phenyl group motion, as a consequence of the introduction of the alkane block.     

Dielectric properties and structures of melt‐compounded poly(ethylene oxide)–montmorillonite nanocomposites

The dielectric dispersion and relaxation process in melt‐compounded hot‐pressed poly(ethylene oxide) (PEO)–montmorillonite (MMT) clay nanocomposite films of 0–20 wt % MMT concentration were investigated over the frequency range 20 Hz to 1 MHz at ambient temperature. X‐ray diffraction study of the nanocomposites evidences that the PEO has been intercalated into the MMT interlayer galleries with a helical‐type multilayer structures, which results the formation of unique parallel plane PEO–MMT layered structures. The relaxation times corresponding to PEO chain segmental motion were determined from the loss peak frequencies of different dielectric formalisms and the same is used to explore the interactions compatibility between PEO molecules and the MMT nano platelets. It is revealed that the loading of only 1 wt % MMT in PEO matrix significantly increases the PEO chain segmental motion due to intercalation, which further varies anomalously with increase of MMT concentration. The real part of dielectric function at 1 MHz, relaxation time, and dc conductivity of these melt‐compounded nanocomposites were compared with the aqueous solution‐cast PEO–MMT films. Considering the comparative changes in the values of various dielectric parameters, the effect of synthesization route on the intercalated/exfoliated‐MMT structures and the PEO chain dynamics were discussed. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Mechanical, rheological, and thermal behavior assessments in HDPE/PA-6/EVOH ternary blends with variable morphology

Visco-elastic and dielectric spectra of multiwalled carbon-nanotube reinforced silicon elastomer nanocomposites were used to study relaxation behavior. SEM photomicrographs shows well dispersion of MWCNT in elastomer matrix. In visco-elastic analysis primary relaxation was studied as a function of temperature (?100 to 100 °C) at frequency 1Hz and strain 1 %. The effect of MWCNT loadings on storage modulus, loss modulus, and loss tangent has been studied. The non-linearity in loss tangent, storage modulus and loss modulus was explained on the basis of MWCNT-elastomer interaction and the inter-aggregate attraction of MWCNT. The secondary β relaxation was studied using dielectric relaxation spectra in the frequency range of 0.1 Hz to 10⁶ Hz. The effect of MWCNT loadings on the complex and real parts of impedance was distinctly visible which has been explained on the basis of interfacial polarization of fillers in a heterogeneous medium and relaxation dynamics of polymer chains in the vicinity of fillers. The dielectric formalism has been utilized to further investigate the conductivity and relaxation phenomenon. The ‘percolation limit’ of the MWCNT in the silicon elastomer was found to be in the range of 4 phr loading.     

An investigation on the molecular mobility through the glass transition of chlorinated butyl rubber

In this paper, dynamic mechanical analysis (DMA), dielectric spectroscopy (DS) and positron annihilation lifetime spectroscopy (PALS) were used to study chlorinated butyl rubber (CIIR), in order to shed light on its unique relaxation behaviors. The dynamic mechanical loss tangent of CIIR reveals an asymmetrical broad structure with a maximum peak on the high-temperature side and a shoulder peak on the low-temperature side. DS clarifies that the shoulder peak, which exactly corresponds to the ″ peak, is the α process originating from the local segmental motion. While the maximum peak is assigned to the slow process arising from the motion of longer chain segments. The slow process exhibits stronger frequency dependence than the α process. The PALS analysis also shows the two processes; moreover, it suggests that CIIR exhibits very effective chain packing. It is due to the effective chain packing that the motion of longer chain segments is retarded and separates from the local segmental motion in time scale. This effect is another reason for the two-peak structure of CIIR, besides the low intermolecular co-operativity.     

Effect of side chain architecture on dielectric relaxation in polyhedral oligomeric silsesquioxane/polypropylene oxide nanocomposites

Segmental and normal mode dynamics in polyhedral oligomeric silsesquioxane (POSS)/poly(propylene oxide) (PPO) non-reactive and reactive nanocomposites were investigated using a broadband dielectric relaxation spectroscopy (DRS) over wide ranges of frequency and temperature. Three POSS reagents with varying side chain architecture were selected for the study: OctaGlycidyldimethylsilyl (OG), TrisGlycidylEthyl (TG) and MonoGlycidylEthyl (MG). Spectra of OG and TG show a segmental (α) process at lower frequency and a local (β) relaxation at higher frequency, while MG displays only a local relaxation. Neat PPO has both segmental and normal mode (α_N) process. In POSS/PPO non-reactive nanocomposites, the presence of OG and TG causes a decrease in the time scale of α_N and α relaxation, while MG has no impact on the dynamics of PPO. Chemical reactions in POSS/PPO reactive nanocomposites lead to the formation of nanonetworks. Prior to the onset of reaction, POSS nanoparticles promote the motions of PPO chains, decrease the time scale of relaxation and give rise to thermodielectrically simple spectra. During the reaction, however, the network formation leads to spectral broadening and a gradual increase in the time scale of both segmental (α) and normal mode (α_N) relaxation. A detailed account of the effects of structure, concentration and dispersion of POSS in the matrix, molecular weight of PPO, extent of reaction and temperature on the molecular origin, temperature dependence and spectral characteristics of relaxation processes in POSS/PPO nanocomposites is provided.     

Role of epoxidation on segmental motion of polyisoprene as studied by broadband dielectric spectroscopy

In this article, the broadband dielectric spectroscopy (BDS) is used to investigate the relaxation behavior of polyisoprene (PI) epoxidized to various levels and the special consideration is devoted to the effect of epoxidation degree on segmental relaxation. To associate segmental relaxation with molecular structure, several empirical equations are used to fit experimental data. Furthermore, we want to discuss the origin of the change of segmental relaxation. Thermal measurement reveals that glass transition temperature of epoxidized PI (EPI) increase with the increase of epoxidation degrees. The epoxidation modification increases the breadth of the dielectric dispersion. Oxirane groups as steric interference enhance the cooperativity of segmental relaxation process and retard the relaxation process, which results in the increase of coupling parameter. Specifically, Vogel–Fulcher–Tammann equation shows the relationship between correlation length and temperature. These results further illustrate the effect of epoxidation group on segmental motion of PI. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43879.     

Ultrasonic studies of aqueous solutions of polyvinylalcohol

Ultrasonic absorption and velocity measurements are reported on aqueous solutions of polyvinylalcohol of different molecular weights, degrees of hydrolysis and concentration. The absorption measurements cover a frequency range from 2–1000 MHz and the velocity data a frequency range from 2–200 MHz, the data were obtained over a temperature range from 273 to 333 K. Comparison of the data obtained as a function of the degree of hydrolysis indicate the magnitude of the contributions due to relaxation of the backbone and of the side chain acetyl groups. In the case of the 100% hydrolysed material, the relaxation spectrum is simplified and corresponds to that of motion of the pendant hydroxyl groups. Deviations of the variation of the attenuation with concentration from the ‘ideal’ dilute solution behaviour are ascribed to the effects of polymer-polymer interactions. The magnitude of the volume change associated with polymer-solvent interactions is estimated and discussed.     

Dielectric properties of nylon 6/clay nanocomposites from on-line process monitoring and off-line measurements

Nylon 6/clay nanocomposites were studied by dielectric relaxation spectroscopy (DRS) to correlate morphology and microstructure with relaxation behavior of the polymer matrix at the molecular level. Partially exfoliated clay microstructure was achieved by extruding nylon 6 with surfactant-treated montmorillonite clays. A new on-line dielectric slit die sensor was used to examine the melt state properties during extrusion compounding. Solid state properties were probed by off-line DRS over a temperature range from −50 to 180 °C in a frequency range from 10⁻³ to 10⁶ Hz. Using non-linear regression methods in conjunction with the temperature-frequency positions of relaxations observed in the dielectric loss data, the experimental data were fit with the Havriliak-Negami and Cole-Cole dielectric relaxation functions corrected for electrode polarization and DC conductivity. Characteristic frequency, relaxation strength, and DC conductivities were extracted from curves with overlapping relaxation modes. Two dielectric relaxations were observed in the composite melt: the α relaxation associated with molecular segmental motion, and a Maxwell-Wagner relaxation (MW) resulting from interfacial polarization at the resin/clay interface. Analysis of the solid-state data yielded a comprehensive master plot of dielectric relaxations attributed to segmental and local molecular dynamics and other relaxations resulting from water and Maxwell-Wagner interfacial polarization. The impact of clay fillers is seen in nearly all relaxation processes changing both characteristic frequency and strength of the relaxation.     

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.     

Glassy dynamics of hydrogen-bonded heteroditopic molecules

A self-complementary heteroditopic molecule composed of thymine and diamidopyridine end groups and a flexible aliphatic interconnecting chain has been synthesized. The glassy dynamics of this hydrogen-bonded supramolecule have been investigated by using dielectric and rheological measurements, in combination with infra-red spectroscopy and solid-state ¹³C NMR experiments. Decoupling of main dielectric relaxation from viscosity has been found in the vicinity of the glass transition and the temperature dependence of viscosity appears to be stronger than that of dielectric relaxation. The unusual dynamic decoupling phenomenon is ascribed to the chemical/dynamic heterogeneity and formation of hydrogen bonds in the supramolecules.     

Molecular motions in poly(N-vinylsuccinimide)

The dielectric and pulsed n.m.r. relaxation of poly(N-vinyl succinimide) has been investigated as a function of frequency and temperature. Five loss processes (α to ?) have been observed and assigned to main chain motion (α, β) and ring deformations (γ, δ, ?) on the basis of comparative work with chemically analogous polymers previously reported^1–4.     

Effect of chlorination and bromination on the dielectric characteristics of bisphenol-A polyesters

The variation of dielectric behaviour with temperature of three crosslinked polyester resins based on the condensates of chloro-methyl acetate and bisphenol-A, 2,2-di-(4-hydroxy 3,5-dichloro phenyl) propane and 2,2-di-(4-hydroxy 3,5-dibromo phenyl) propane were studied in the microwave frequency region at wave length 3.21 cm. The dielectric constants of these polyesters are 2.93, 2.91 and 2.77 respectively and do not change with temperature in the 30–85°C region. The decrease in dielectric constant after incorporating 2,2-di-(4-hydroxy 3,5-dichloro phenyl) propane and 2,2-di-(4-hydroxy 3,5-dibromo phenyl) propane in the polyester chain is attributed to the decrease in the number of ester and ether groups in the polyester chain per unit weight of the cured polyesters. It is found that the loss factor for bisphenol-A based polyester resin is maximum at 55°C and the relaxation time of this polyester is 1–7 · 10^-11 sec at 55°C. It can be assigned to the free motion of ether linkages due to the absence of halide groups at 3,5 position of bisphenol-A incorporated in the polyester chain. This can also be attributed to the absence of secondary forces between the chains due to the presence of bulky p-phenylene groups in the polyester chain and crosslinking by bulky molecules like styrene. In case of chlorinated and brominated polyesters, the motion about ether and ester linkages is difficult. The chlorinated bisphenol-A polyester has less variation in loss factor as compared to that of bromo polyester. It may be due to the homo polystyrene or large polystyrene segments present in bromo polyester. These results are also supported by the studies on hydrolytic stability of their base resins and by the heat distortion point of the crosslinked polyesters.     

Molecular motion in melt samples of poly(propylene glycol) studied using dielectric and Kerr effect relaxation techniques

Dielectric relaxation times and electro-optical Kerr-effect relaxation times have been measured for melt samples of pure poly(propylene glycol) of nominal molecular weight 1025, 2025 and 4000, over the temperature range 209K–255K. Kerr-effect relaxation curves were analysed in terms of two main components: a ‘fast’ primary process (A), associated with a negative optical birefringence; and a ‘slow’ secondary process (B), associated with a positive optical birefringence. Measurement of dielectric loss at different frequencies for poly(propylene glycol) of molecular weight 2025 and 4000 indicated two main relaxation processes: a ‘fast’ process (the normal α-process), which carried with it most of the dielectric loss; and a ‘slow’ secondary process of much smaller amplitude. The close correspondence between dielectric and Kerr-effect relaxation times, for the primary and secondary relaxation processes, indicated that these techniques are probing effectively different aspects of the same molecular motion. Dielectric and Kerr-effect relaxation times for the secondary process depended greatly on the molecular weight of the polymer and were compared with the predictions of the models for the reptational motions of chains in the bulk polymer, as proposed by De Gennes and by Doi and Edwards.     

The influence of the type of substituents on the polymer dynamics of different regioselective 2-O-esters of starch using dielectric spectroscopy

The dielectric relaxation spectra of different regioselective 2-O-acyl-esters of amylomaize starch with a degree of substitution DS=1 were measured in an extended temperature (−135 to 0 °C) and frequency range (0.01 Hz to 3 MHz). For all starch esters, two relaxation processes were found, which are from the type of a secondary relaxation (constant activation energy). One process can be assigned to the local backbone motion of the polymer and the other one with the ester side group motion in position C2 at the anhydro glucose unit. The dynamic parameters of both relaxations were determined and compared. The double bond in the crotonoyl-ester side group has a little influence on both the segmental and the side group motion in relation to the butanoyl-ester. The bulky benzoyl-ester side group shows an inverse influence on the local chain dynamics than the alkyl-ester side group.