Dielectric relaxations in phosphoric acid‐doped poly(2,5‐benzimidazole) and its composite membranes

Poly(2,5‐benzimidazole) (ABPBI)—a promising high‐temperature polymer electrolyte membrane—is characterized over a wide range of temperature (?50 to 220 °C) using broadband dielectric spectroscopy (BDS) to understand the various relaxation processes. The undoped ABPBI membrane shows two major secondary relaxations and a primary α relaxation. The effect of phosphoric acid (PA) and phosphotungstic acid grafted zirconium dioxide (PWA/ZrO₂) nanoparticles on the chain relaxation and the proton conductivity is investigated. The phosphoric acid alters the relaxation trends, increases the number of free ions in the polymer matrix, and therefore the conductivity. The shift in the peak frequencies of different chain relaxation processes in the presence of PA and PWA/ZrO₂ is attributed to the increase in free volume and the consequent easy motion of the polymer chains. The Fourier transform infra‐red (FTIR) spectroscopy of ABPBI and the acid‐doped composites show all the relevant peaks corresponding to C?C, C?N stretching, and phosphoric acid/phosphates, confirming the formation of ABPBI and doping with PA. The proton conductivity of the membranes is estimated from electrochemical impedance spectroscopy (EIS). To establish the effect of change in crystallinity on relaxations and proton conductivity, the undoped and PA‐doped membranes are characterized using thermogravimetric analysis and in situ XRD at high temperatures. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134 , 44867.     

Molecular dynamics of poly(glycolide‐co‐L‐lactide) copolymer during isothermal cold crystallization

Poly(glycolide‐co‐l ‐lactide) (PGA/PLLA) is a random copolymer with 92 wt % PGA, being the basic resin for Vicryl® suture. Molecular dynamics of PGA/PLLA in its wholly amorphous state and during isothermal cold crystallization at 70 and 80°C have been analyzed. Experimental results were generated over a wide range of frequency and temperature by broad‐band dielectric spectroscopy (DRS). The variation of the average relaxation time (defined as τ = [1/2]πf_max where f_max is the frequency at maximum loss for the main α relaxation) has been studied during cold crystallization and the temperature dependence of this average relaxation time for wholly amorphous and crystallized samples has been analyzed. This behavior has been modeled by Havriliak–Negami, Vogel–Fulcher–Tammann, and Kohlrausch–Williams–Watts equations. The evolution profile of the dynamics (frequency at which the maximum loss appears, f_max) depends on the crystallization temperature, being different at 80°C relatively to 70°C, which could reflect different progress of the spherulitic morphology, as it is shown by the evolution of the morphologies obtained during the crystallization processes, followed by optical microscopy. While the loss maximum (and consequently relaxation times) remains almost unmodified during the crystallization process at 70°C, for the process at 80°C the maximum first moves slightly to higher frequencies (shortening of relaxation times) and at the final stages of crystallization it moves to lower frequencies (increasing the relaxation times). Supporting evidence about the thermal behavior of the polymers has been obtained with DSC. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

Microwave dielectric relaxation and molecular dynamics in binary mixtures of poly(vinyl pyrrolidone)–poly(ethylene glycol)s in non‐polar solvent

Dielectric relaxation study of binary mixtures of poly(vinyl pyrrolidone) (PVP) (M_w = 40 000 g mol^?1) and poly(ethylene glycol)s (PEGs) (M_n = 200, 400 and 600 g mol^?1) with concentration variation was carried out in dilute solutions of benzene at 10.1 GHz and 35 °C. The average relaxation time τ_o, corresponding to segmental motion τ₁ and group rotations τ₂ was determined for PVP–PEGs mixtures. A comparison of these mixtures relaxation times was made with the relaxation times of PEGs in benzene solvent. The evaluated τ_o values of PVP–PEGs mixtures in benzene solution are assigned to the reorientation of PEG molecules. It has been observed that the τ_o value of PVP–PEG200 mixtures increases with increasing concentration of PVP but their values are small in comparison with the τ_o value of PEG200 molecules. In the case of PVP–PEG400 and PVP–PEG600 mixtures, the evaluated values of τ_o are greater than the corresponding τ_o values of PEG400 and PEG600 molecules in benzene solvent. The variation in τ_o values in these systems has been discussed by considering the stretching effect in the PEGs molecular chains in PVP–PEGs mixtures in benzene solutions. The high value of distribution parameter α (≈0.4 to 0.7) suggests that in these mixtures there is a large contribution of segmental motion and group rotations to the relaxation processes. The nature of the formation of hydrogen‐bonded PVP–PEG complex heterogeneous network due to hydrogen bonding between carbonyl groups of PVP monomer units and terminal hydroxyl groups of PEGs is discussed. Furthermore, the elongation behaviour of PVP–PEG complex networks in benzene solvent and the molecular dynamics in the mixture due to breaking and reforming of hydrogen bonds has been explored by comparing the evaluated relaxation times and the Kirkwood correlation factor of pure PEG molecules for their possible use in drug control release systems. The relaxation times of these mixtures are independent of their viscosity, but the elongation of the mixture network is influenced by the PEG chain length and the number of hydroxyl groups in comparison with the number of carbonyl groups in the mixtures. Copyright © 2003 Society of Chemical Industry     

Salt‐leaching technique for the synthesis of porous poly(2,5‐benzimidazole) (ABPBI) membranes for fuel cell application

The first instance of synthesizing porous poly(2,5‐benzimidazole) (ABPBI) membranes for high‐temperature polymer electrolyte membrane fuel cells (HT‐PEMFCs), using solvent evaporation/salt‐leaching technique, is reported herein. Various ratios of sodium chloride/ABPBI were dissolved in methanesulfonic acid and cast into membranes by solvent evaporation, followed by porogen (salt) leaching by water washing. The membranes were characterized using SEM, FTIR, TGA, and DSC. The proton conductivity, water and acid uptake of the membranes were measured and the chemical stability was determined by Fenton's test. SEM images revealed strong dependence of sizes and shapes of pores on the salt/polymer ratios. Surface porosities of membranes were estimated with Nis Elements‐D software; bulk porosities were measured by the fluid resaturation method. Thermogravimetric analysis showed enhanced dopant uptake with introduction of porosity, without the thermal stability of the membrane compromised. Incorporating pores enhanced solvent uptake and retention because of capillarity effects, enhancing proton conductivities of PEMs. Upon acid doping, a maximum conductivity of 0.0181 S/cm was achieved at 130 °C for a porous membrane compared with 0.0022 S/cm for the dense ABPBI membrane at the same temperature. Results indicated that with judicious optimization of porogen/polymer ratios, porous ABPBI membranes formed by salt‐leaching could be suitably used in HT‐PEMFCs. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45773.     

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     

Broadband dielectric spectroscopy on the molecular dynamics in different generations of hyperbranched polyester

Dielectric spectroscopy (10^?2 Hz to 10⁶ Hz) was employed to investigate the molecular dynamics of hyperbranched polyesters where the number of the generation is systematically varied from 2 to 5. As a first result, the dielectric properties depends strongly on the generation of the hyperbranched polymers. For higher generations (3 to 5) at temperatures below T_g two relaxation processes are observed, a γ‐process at lower temperatures and a β‐process at higher ones. The apparent activation energies are around 100 kJ/mol which seems to be too high for truly localized processes. For the Generation 2, only the γ‐process is observed. For all investigated polymers the dielectric α‐relaxation could not be observed because of strong conductivity effects. Therefore, the conductivity is systematically analyzed which obeys the peculiarities found to be characteristic for semiconducting disordered materials. Especially, the Barton/Nakajima/Namikawa relationship is found to be valid. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Dielectric relaxation spectroscopy of poly[(R)‐3‐hydroxybutyrate] (PHB) during crystallization

The dielectric response of poly [(R)‐3‐hydroxybutyrate] (PHB) was investigated as a function of time after quenching a film from the melt to a crystallization temperature of 20 °C. In the frequency range investigated (20 to 10⁶ Hz) a relaxation maximum was observed–attributable to the glass transition–which could be analysed by the Havriliak–Negami relation. Changes in the complex dielectric constant were monitored both during spherulite growth (primary crystallization) and subsequently during a period of progressive crystallization (secondary crystallization) at room temperature. The relaxation strength changed only slightly during primary crystallization and its peak position remained at a constant frequency. Subsequently a continuous decrease in relaxation strength occurred, indicating considerable changes in molecular mobility after spherulite growth had been completed. The results provide further evidence that molecular mobility in the amorphous regions decreases significantly with time, and that this would be the reason why PHB shows embrittlement on ageing at room temperature. Copyright © 2004 Society of Chemical Industry     

Broadband dielectric spectroscopy of Nafion‐117, sulfonated polysulfone (sPSF) and sulfonated polyether ketone (sPEK) membranes

Nafion^®‐117, sulfonated polysulfone (sPSF) and sulfonated polyetherketone (sPEK) are characterized using broadband dielectric spectroscopy in the frequency range of 10 MHz–100 mHz. Overall, there are 4–5 relaxation processes in these sulfonated membranes and a comparison of their spectral features allows assigning the relaxation processes. At an optimum amplitude of ～100 mV_rms, all the relaxations are clearly defined as the electrode polarization is minimized. At low temperatures (?130 °C), these membranes show a broad relaxation peak in the mid‐frequency region, which quickly shifts towards the high‐frequency region as the temperature is increased to ?90 °C. This peak is observed in proton exchange membranes for the first time due to the use of low ac amplitude, and it is assigned to the relaxation of the confined water in the micro‐pores. With all the membranes, the peak associated with ? SO₃H group relaxation is observed in the same frequency range at a temperature of ～?80 °C. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134 , 44790.     

Gas permeation properties of poly(2,5‐benzimidazole) derivative membranes

In this article, three novel polymers based on poly(2,5‐benzimidazole) (ABPBI) were synthesized by introducing propyl, isobutyl or n‐butyl groups to its side chain through an alkyl substitution reaction. FTIR and ¹³C NMR were applied to confirm the formation of corresponding chemical groups. Their physical properties including crystallinity, thermal stability, mechanical strength, and micro‐morphology were also characterized. Their solubility in common solvents were also tested to see if the modification will bring any improvement. Gas permeation properties of three derivative membranes prepared by a casting and solvent‐evaporation method were tested with pure gases including H₂, N₂, O_2, CH₄, and CO₂. It has been revealed that gas with a smaller molecular size owned a larger permeability. This means gas permeation in all prepared membranes should be diffusivity selective. Among all three modified ABPBI membranes, isobutyl substitution modified ABPBI (IBABPBI) showed the best selectivity of H₂ over other gases such as N₂ (～185) and CO₂ (～6.3) with a comparable permeability (～9.33 barrer) when tested at 35°C and 3.0 atm. Testing temperature increase facilitated gas permeation for all three membranes obviously; while in term of gas selectivity temperature increase showed diverse alteration because it brought variable impact on gas solubility of different gases. Even so, IBABPBI membrane still owned acceptable selectivity of H₂ over N₂ (～118) and CO₂ (～6.3) with an almost doubled permeability (～17.5 barrer) when tested at 75°C and 3.0 atm. Additional tests showed that running at high pressure did not bring any obvious deterioration to gas separation performance of IBABPBI membrane. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40440.     

Dielectric and viscoelastic relaxations in poly(dicyclohexyl itaconate)

The relaxation spectrum of poly(dicyclohexyl itaconate) (PDCHI) was studied by dynamic mechanical, DC dielectric and thermally stimulated current measurements. Four relaxations, α, β, γ and δ, were obtained. The only method by which all four peaks were observed is that of dynamic dielectric measurements because of the broad range of frequencies employed. The β, γ and δ relaxations were characterized by the activation energy in a relaxation map. A tentative explanation of the molecular origin of each absorption is proposed. In the case of the α relaxation we have used two transformations, one from the permittivity to polarizability and the second from compliance to deformability in order to make evident the existence of this relaxation.     

Molecular dynamics of poly(butylene tert-butyl isophthalate) and its copolymers with poly(butylene terephthalate) as revealed by broadband dielectric spectroscopy

The dynamic behaviour of a set of copolyesters made of 1,4-butanediol, terephthalic acid and 5-tert-butyl isophthalic acid, as well as the two parent homopolymers was studied. Although the insertion of the tert-butyl isophthalate units in the main chain of poly(butylene terephthalate) modified the glass transition, the β relaxation did not change dramatically. By using broadband dielectric spectroscopy we have obtained a detailed picture of the dynamic behaviour of these systems. In all cases, a strong correlation between the α relaxation and the β relaxation above T_g was observed in accordance with recent proposals based on the coupling model.     

Structural and AC impedance studies on nanocomposite polymer electrolytes based on poly(ε‐caprolactone)

Nanocomposite polymer electrolyte (NCPE) films of [75 wt % poly(ε‐caprolactone) : 25 wt % zinc triflate] + x wt % nanofiller Al₂O₃ (x = 1, 3, 5, 7) were prepared by solution cast technique. Such NCPE films were characterized using Fourier transform infrared and AC impedance spectroscopic techniques. Complexation of polymer with salt and nanofiller was revealed from FTIR analysis. On the other hand, an apparent increase in the number density of charge carriers upto 5 wt % loading of the nanofiller was also confirmed. Furthermore, AC impedance spectroscopic studies have shown that ionic conductivity increases with the addition of Al₂O₃ and reaches a maximum of 2.5 × 10^?5 S cm^?1 at room temperature for 5 wt % loading of nanofiller. The dielectric behavior of all the synthesized samples has also been analyzed and presented. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40524.     

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     

Study of the dielectric relaxation of poly(phenylpropyl acrylate) and poly(phenylpropyl methacrylate): effect of slight differences in chemical structure

A comparative study of the dielectric relaxational behaviour of two structurally close polymers, containing aromatic side groups, was carried out in order to analyse how slight differences in the chemical structure affect the molecular responses to perturbation field. Specifically, poly(phenylpropyl acrylate) (P3Ph1PA) and poly(phenylpropyl methacrylate) (P3Ph1PM) were studied using differential scanning calorimetry and dielectric relaxation spectroscopy in the frequency range 10^?2–10⁶ Hz and temperature window of ?80 to 120 °C. Both techniques show one glass–rubber transition for P3Ph1PA and two for P3Ph1PM, which evidence the great effect of the methyl groups on the segmental motions of the polymer. Phenomenological analysis of the data was carried out in order to establish the strength, width and fragility parameters of the glass–rubber transitions. In the case of P3Ph1PA, the strength is found to be larger than for P3Ph1PM, pointing out that the methyl group disturbs the mobility. Conductive processes dominate the dielectric spectra at high temperatures and low frequencies. © 2015 Society of Chemical Industry     

Proton spin-lattice and spin-spin relaxation times in annealed poly(ethylene terephthalate)

Proton spin-lattice, T₁, and spin-spin, T₂, relaxation times of poly(ethylene terephthalate) film annealed at various temperatures were measured using a broad line pulse spectrometer. The value of T₁ is closely related to the crystallinity of the sample and only one T₁ was observed for each sample, indicating that the spin diffusion is effectively operating. Even in the amorphous sample there are some nuclear spins, the motion of which is strongly restricted.     

Dielectric relaxation spectroscopy of poly(ethylene terephthalate)

Contour maps of dielectric loss tangent within the ranges 0.1 Hz to 3 MHz and ?175 °C to +190 °C are presented for a commercial poly(ethylene terephthalate) (PET) in two initial states of crystallinity. Individual absorption regions resemble those for poly(butylene terephthalate) and are attributed to carbonyl‐driven α‐ and β‐relaxation processes and to Maxwell–Wagner–Sillars polarizations. Possible causes are considered for the asymmetry and structure apparent in the α‐peak of partially crystalline PET. © 2001 Society of Chemical Industry     

Effects of blending on the molecular dynamics of highly interacting binary polymer blends of poly(methyl methacrylate) and poly[styrene‐co‐(maleic anhydride)]

The molecular dynamics and miscibility of highly interacting binary polymer blends of poly(methyl methacrylate) (PMMA) and poly[styrene‐co‐(maleic anhydride)] random copolymer with 8 wt% maleic anhydride content (SMA) were investigated as a function of composition over a wide range of frequency (10^?2–10⁶ Hz) at different constant temperatures (30–160 °C). Only one common glass relaxation process (α‐process) was detected for all measured blends, and its dynamics and broadness were found to be composition dependent. The existence of only one common α‐relaxation process located at a temperature range between those of the pure polymer components indicated the miscibility of the two polymer components over the entire range of composition. The miscibility was also confirmed by measuring the glass transition temperatures of the blends, T_g, using differential scanning calorimetry. The composition dependence of T_g of the blends showed a positive deviation from the linear mixing rule and well described by the Gordon–Taylor–Kwei equation. The relaxation spectrum of the blends was resolved into α‐ and β‐relaxation processes using the Havriliake–Negami (HN) equation and ionic conductivity. The dielectric relaxation parameters obtained from HN analysis, such as broadness of relaxation processes, 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 process of the blends were well described by the Meander model, while an Arrhenius‐type equation was used to evaluate the molecular dynamics of the β‐relaxation process. Blending of PMMA and SMA was found to have a considerable effect on the kinetics and broadness of the β‐relaxation process of PMMA, indicating that the strong interaction and miscibility between the two polymer components could effectively change the local environment of each component in the blend. © 2013 Society of Chemical Industry     

Proton conducting polymer blends from poly(2,5‐benzimidazole) and poly(2‐acrylamido‐2‐methyl‐1‐propanesulfonic acid)

Proton conducting polymer electrolyte membranes were produced by blending of poly(2,5‐benzimidazole) (ABPBI) and poly(2‐acrylamido‐2‐methyl‐1‐propanesulfonic acid) (PAMPS) at several stoichiometric ratios with respect to polymer repeating units. The membranes were characterized by using Fourier transform infrared spectroscopy for interpolymer interactions and scanning electron microscope for surface morphology. Thermal stability of the materials was investigated by thermogravimetric analysis. Glass transition temperatures of the samples were measured via differential scanning calorimetry. The spectroscopic measurements and water uptake studies indicate a complexation between ABPBI and PAMPS that inhibited polymer exclusion up on swelling in excess water. Proton conductivities of the anhydrous and humidified samples were measured using impedance spectroscopy. The proton conductivity of the humidified ABPBI:PAMPS (1 : 2) blend showed a proton conductivity of 0.1 S/cm, which is very close to Nafion 117, at 20°C at 50% relative humidity. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Dielectric relaxation and electrical conduction in (BixNa1−x)0.94Ba0.06TiO3 ceramics

The dielectric relaxation and electrical conduction were investigated in (Bi_xNa_1?x)_0.94Ba_0.06TiO₃ (Abb. xBNBT6, x = 0.5, 0.495, 0.485, and 0.475) ceramics prepared by solid state reaction. With a decrease in x, the dielectric properties of the ceramics decreased, whereas the electrical conduction increased, resulting in a transition from insulator to oxide‐ions conductor. When x = 0.475, the ceramics exhibited large conductivity (~10^?3 S cm^?1 at 575°C) and low activation energy (~0.45 eV), indicating their potential application in solid oxide fuel cells. A mixed conduction mechanism with oxide‐ions, electrons, and holes was proposed. With a decrease in x from 0.495 to 0.475, it was found that the p‐type conduction was switched to n‐type conduction. The dielectric relaxation of the x = 0.495 sample was associated with short‐range hopping of oxygen vacancies. However, the dielectric properties of the x = 0.485 and 0.475 samples can be explained by Maxwell‐Wagner interface relaxation.