A broad spectrum analysis of the dielectric properties of poly(2-hydroxyethyl methacrylate)

The dielectric permittivity, ε′, and the loss factor, ε″, of dry poly(2-hydroxyethyl methacrylate) were measured using a dielectric analyzer in the frequency range of 0.1 Hz to 100 kHz and between the temperature range of −150 to 275 °C. The dielectric response of the sub-T_gγ transition of PHEMA has been widely studied before but little to no DEA data above 50 °C is present in the literature. In this study the dielectric spectrum is presented up to and above the T_g. The electric modulus formalism is used to reveal the γ, β, α and conductivity relaxations. The apparent activation energies for the relaxations are presented.     

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 analyses of a series of poly(2-hydroxyethyl methacrylate-co-2,3-dihydroxypropyl methacylate) copolymers

The dielectric spectra of a series of copolymers of 2-hydroxyethyl methacrylate (HEMA) and 2,3-dihydroxypropyl methacrylate (DHPMA) were investigated. Recently, the full range dielectric spectrum of poly(2-hydroxyethyl methacrylate) was reported. This study looks at the effects on the dielectric behavior as a result of 2,3-dihydroxypropyl methacrylate addition. The dielectric permittivity, ε′, and the loss factor, ε″, were measured using a dielectric analyzer in the frequency range of 0.6 Hz to 100 kHz and between the temperature range of −150 and 275 °C. The electric modulus formalism was used to reveal the viscoelastic and conductivity relaxations present in the polymers. Several notable changes were observed as 2,3-dihydroxypropyl methacrylate concentration increased. It was observed through DSC and DEA that the glass transition temperature decreased as DHPMA content increased. The secondary dielectric relaxations were also affected as it was recorded that the activation energy for the γ transition increased and the β relaxation decreased with DHPMA content. Ionic conductivity data prove that DHPMA facilitates ionic mobility more efficiently than HEMA.     

Structural and ionic conductivity of Cu-doped titania (Ti0.95Cu0.05O2−δ) for high temperature energy devices

The Cu-doped titania (Ti_0.95Cu_0.05O_2-δ) is studied here as a solid-state ionic conductor for its possible application in high temperature energy devices such as an electrolyte for SOFC. The sample in the powder form was obtained by solid state method using TiO₂ and copper acetate by heating up to 1200 °C for 10 h. It was characterized by XRD, FT-IR, Raman, SEM/EDS, DRS-UV-Visible, photoluminescence, BET and ac-impedance techniques. The oxide ion conductivity (σ_t) values obtained from ac-impedance measurements showed a linear increase with temperature from 300 ? 700 °C. The σ_t values are similar to that of Ln-doped ceria, and the highest conductivity of 1.41 × 10^?4 Scm^?1 was recorded at 700°C. The activation energy for total conductivity was found to be 0.82 eV. The ionic and electronic transport numbers are 0.79 and 0.21, respectively. This study suggests the plausible use of rutile TiO₂ based (low-cost and structurally stable) materials as electrolytes in SOFC.     

Synthesis, proton conductivity and methanol permeability of a novel sulfonated polyimide from 3-(2′,4′-diaminophenoxy)propane sulfonic acid

A novel sulfonated diamine monomer, 3-(2′,4′-diaminophenoxy)propane sulfonic acid (DAPPS), was successfully synthesized and the sulfonated polyimide (SPI) was prepared from 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTDA) and DAPPS. The resulting SPI, NTDA-DAPPS, was soluble in common organic solvents. The SPI membrane displayed proton conductivity σ values of 0.12-0.35 S/cm at temperatures ranging from 35 to 90 °C in liquid water, which were similar to or higher than those of Nafion 117 and sulfonated hydrocarbon polymers. The σ of the SPI membrane decreased significantly with decreasing relative humidity (RH) and became much lower than that of Nafion 117 at 30% RH. The SPI membrane displayed good water stability at 80 °C and was thermally stable up to 240 °C. It showed reasonable mechanical strength of a modulus of 1.3 GPa at 90 °C and 90% RH. Its methanol permeability P_M was 0.57×10⁻⁶ cm²/s at 30 °C and 8.6 wt% methanol in feed, which was a fourth of that of Nafion 117. As a result, its ratio of σ/P_M was 21×10⁴ S cm⁻³ s, which was about 4 times larger than that of Nafion 117, suggesting potential application of the SPI membrane for direct methanol fuel cell.     

Influence of surface layer conditions of multiwall carbon nanotubes on their electrophysical properties

We have investigated temperature σ(T) and magnetic field σ(B) dependences of the conductivity of multiwalled carbon nanotubes (MWNTs) with different average outer diameter d? and composite materials on the bases of these MWNTs in dielectric polymethyl methacrylate matrix (PMMA). It was shown that the percolation threshold of the electrical conductance for MWNT/PMMA composites lies in the range of concentration of MWNTs lower than 0.5 wt.%. From the joint analysis of σ(T) and σ(H) we have found that quantum corrections to the conductivity for interaction electrons is dominated in MWNTs with average outer diameters d? ～ 23 nm and MWNT/PMMA composite materials. We determined the constant of electron–electron interaction λ from quantum correction to σ(T) and σ(H). In the result we have found that λ depends on the concentration of MWNTs in MWNT/PMMA materials. This fact demonstrates that the interaction of the dielectric material of PMMA with surface atoms of MWNTs leads to the change of the λ in MWNTs. For MWNTs with average outer diameters d? ≤ 13 nm and for composite on the base of these MWNTs we have observed the domination of one dimensional variable range hopping conductivity (VRHC) at a temperature below 20 K. From the data of VRHC we estimated the density of localized states at the Fermi level N(E_F) and demonstrated that N(E_F) is decreasing in composite materials.     

Soft matter electrolytes based on polymethylmetacrylate dispersions in lithium bis(trifluoromethanesulfonyl)imide/1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ionic liquids

Ion transport in a polymer-ionic liquid (IL) soft matter composite electrolyte is discussed here in detail in the context of polymer-ionic liquid interaction and glass transition temperature. The dispersion of polymethylmetacrylate (PMMA) in 1-butyl-3-methylimidazolium hexafluorophosphate (BMIPF₆) and 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (BMITFSI) resulted in transparent composite electrolytes with a “jelly-like” consistency. The composite ionic conductivity measured over the range −30 °C to 60 °C was always lower than that of the neat BMITFSI/BMIPF₆ and LiTFSI-BMITFSI/LiTFSI-BMIPF₆ electrolytes but still very high (>1 mS/cm at 25 °C up to 50 wt% PMMA). While addition of LiTFSI to IL does not influence the glass T_g and T_m melting temperature significantly, dispersion of PMMA (especially at higher contents) resulted in increase in T_g and disappearance of T_m. In general, the profile of temperature-dependent ionic conductivity could be fitted to Vogel-Tamman-Fulcher (VTF) suggesting a solvent assisted ion transport. However, for higher PMMA concentration sharp demarcation of temperature regimes between thermally activated and solvent assisted ion transport were observed with the glass transition temperature acting as the reference point for transformation from one form of transport mechanism to the other. Because of the beneficial physico-chemical properties and interesting ion transport mechanism, we envisage the present soft matter electrolytes to be promising for application in electrochromic devices.     

Sulfonated multiblock copolynaphthalimides for polymer electrolyte fuel cell application

Sulfonated multiblock copolynaphthalimides (multiblock co-SPIs) were prepared by two-pot polymerization method from 1,4,5,8-naphthalenetetracarboxylic dianhydride, sulfonated diamine of 4,4′-bis(4-aminophenoxy)-3,3′-bis(4-sulfophenyl)biphenyl (BAPSPB) and nonsulfonated diamine of 4,4′-diaminophenyl hexafluoropropane. The multiblock co-SPI (BA1) with hydrophilic/hydrophobic block length of 20/10 and ion exchange capacity (IEC) of 1.67 meq g⁻¹ exhibited larger water uptake, larger in-plane and through-plane proton conductivity (σ∥ and σ⊥, respectively) than the random co-SPI with the similar IEC. The multiblock co-SPI (BA2) with the longer block length of 20/20 exhibited the large σ∥ and σ⊥ comparable to those of BA1, in spite of the smaller IEC of 1.35 meq g⁻¹. Both the multiblock and random co-SPIs showed the moderate anisotropic proton conductivity (σ_⊥/σ_//≒ 0.70) as well as anisotropic membrane swelling with about three times larger through-plane swelling than in-plane swelling. The TEM observation revealed that BA2 had an isotropic and inhomogeneous morphology with indistinct microphase-separated structure, whereas the random co-SPIs had a homogeneous morphology. The behavior of BAPSPB-based multiblock co-SPI membranes were quite different from that of the multiblock co-SPIs based on 2,2′-bis(4-sulfophenoxy)benzidine, which was due to the presence of two flexible ether bonds in BAPSPB moiety of the main chain. Even under the low humidification of 27/27% RH at 90 °C and 0.2 MPa, BA2 exhibited the fairly high PEFC performance; namely, cell voltage of 0.67 V at load current density of 0.5 A cm⁻² and maximum output of 0.51 W cm⁻², which were much larger than those of BA1 and the random co-SPI (RA1) with IEC of 1.84 meq g⁻¹, and have the high potential as PEM for PEFC applications.     

Phase segregation of poly(3-dodecylthiophene)-block-poly(methyl methacrylate) copolymers

We investigated, via small angle X-ray scattering (SAXS), the ordered-to-disordered transition temperature (T_ODT) of symmetric poly(3-dodecylthiophene)-block-poly(methyl methacrylate) copolymers (P3DDT-b-PMMA) with different molecular weights synthesized by anionic coupling reaction. When the molecular weight of P3DDT-b-PMMA was properly chosen, the T_ODT was observed within experimentally accessible temperature range (higher than the glass transition and melting temperature for PMMA and P3DDT, respectively, but lower than the thermal degradation temperature). We also measured the temperature dependence of Flory–Huggins interaction parameter (χ) between P3DDT and PMMA as χ = 0.1109 + 76.63/T, in which T is the absolute temperature.     

Miscibility and viscoelastic properties of acrylic polyhedral oligomeric silsesquioxane-poly(methyl methacrylate) blends

We investigate the miscibility of acrylic polyhedral oligomeric silsesquioxanes (POSS) [characteristic size d≈2 nm] and poly(methyl methacrylate)(PMMA) in order to determine the effect of well-dispersed POSS nanoparticles on the thermomechanical properties of PMMA. Two different acrylic POSS species (unmodified and hydrogenated) were blended separately with PMMA at volume fractions up to ?=0.30. Both POSS species have a plasticizing effect on PMMA by lowering the glass transition temperature T_g and decreasing the melt-state linear viscoelastic moduli measured in small amplitude oscillatory shear flow. The unmodified acrylic-POSS has better miscibility with PMMA than the hydrogenated form, approaching complete miscibility for loadings ?<0.10. At a loading ?=0.05, the unmodified acrylic POSS induces a 4.9 °C decrease in the T_g of PMMA, far less than the 17.4 °C decrease in the glass transition temperature observed in a blend of 5 vol% dioctyl phthalate (DOP) in PMMA; however, the decrease in the glass transition temperature per added plasticizer molecule is nearly the same in the unmodified acrylic-POSS-PMMA blend compared with the DOP-PMMA blend. Time-temperature superposition (TTS) was applied successfully to the storage and loss moduli data and the resulting shift factors were correlated with a significant increase in free volume of the blends. The fractional free volume f₀=0.046 for PMMA at T₀=170 °C while for a blend of 5 vol% unmodified acrylic-POSS in PMMA f₀=0.057, which corresponds to an addition of 0.47 nm³ per added POSS molecule at ?=0.05. The degree of dispersion was characterized using both wide-angle X-ray diffraction (WAXD) and dynamic mechanical analysis (DMA). Diffraction patterns for both blend systems show clear evidence of phase separation at ?=0.20 and higher, but no significant phase separation is evident at ?=0.10 and lower. The storage modulus measured in DMA indicates appreciable phase separation for unmodified acrylic POSS loadings ?≥0.10, while no evidence of phase separation is present in the ?=0.05 blend in DMA.     

Electric and dielectric properties of some luminescent solar collectors based on phthalocyanines and hematoporphyrin doped PMMA

The electrical properties: DC-electrical conductivity (σ_DC), AC-electrical conductivity (σ_AC), dielectric permittivity (ɛ′) and dielectric loss (ɛ″) for pure polymethylmethacrylate (PMMA) and mixed with phthalocyanine, Ni-phthalocyanine and hematoporphyrin IX chloride laser dyes have been investigated. The conductivity values for all polymer–dye samples are higher than that of the pure PMMA. For all investigated samples, at the constant temperature, the frequency dependence of (σ_AC) was found to obey the relation (σ_AC) = Aω^s. The results are discussed according to the correlated barrier hopping CBH mechanism from which the Coulomb well barrier of charge carriers (W_M) was calculated. For the dye-doped samples, the hematoporphyrin IX chloride mixed sample showed the larger value of (W_M) compared with the others. At all frequencies, ɛ′ values for doped samples are higher than that of pure one. This is due to a free volume enhancement offered by the large size of the dye molecule. Also, the addition of dyes to PMMA causes an increase in the intensity of the loss peak (ɛ″) accompanied by a shift in its position. The effect of chemical structure of investigated dyes on the electrical properties is discussed.     

Synthesis and characterization of fully disulfonated poly(arylenethioethersulfone)s containing hexafluoroisopropylidene moiety

High molecular weight sulfonated poly(arylenethioethersulfone) homopolymer containing hexafluoroisopropylidene moiety (6F-SPTES-100) was synthesized from the monomers 3, 3′-disulfonated-4, 4′-difluorodiphenylsulfone and 4, 4′-(hexafluoroisopropylidene) diphenylthiol, using 4-fluorobenzophenone as the end-capping agent in polar aprotic solvents at temperatures up to 180 °C to provide the desired polymeric composition for utilization as proton exchange membrane (PEM) in fuel cells applications. Tough, ductile freestanding membranes were fabricated from N, N-dimethylacetamide (DMAc) by solvent-casting. The end-capped 6F-SPTES-100 polymer was fully characterized and the membrane was found to have proton conductivity as high as 180 mS/cm which was measured at 85 °C and 65% relative humidity. The proton conductivity of 6F-SPTES-100 was approximately two and half times higher than that of Nafion-117 under comparable conditions. The swelling and solubility characteristics of the 6F-SPTES-100 polymer in water are directly related to the high degree of sulfonation of the polymer backbone.     

Replies to comments contained in “Conductivity hysteresis in polymer electrolytes incorporating poly(tetrahydrofuran)” by O. Akbulut, et al., Electrochim. Acta 52 (2007) 1983

DSC indicates that first-heating endotherms at 95 and 100-115 °C in poly(tetramethylene oxide)-based polymers with LiClO₄ and LiBF₄, respectively, arise from the decomposition of phase-separated LiClO₄·3H₂O and a pre-melting transition in phase-separated LiBF₄ and not from organized adducts with poly(tetramethylene oxide) as asserted by Akbulut et al. and other literature. Water in the LiClO₄ system, at least (absent in freeze-dried samples), could account for higher conductivities reported by Akbulut et al. than observed by the present authors. Irreversibility of log σ versus1/T in these weakly ionophilic systems apparently arises from slow dissolution of lithium salts together with morphological changes in mixtures of the self-organising systems CmOn (I) with the ‘grain boundary bridging’ copolymer -[-(CH₂)₄-O-]_x-(CH₂)₁₂- (II). A three-component system I:II:LiBF₄ to which 9 wt% of tetrahydrofuran had been purposefully added showed deterioration in conductivity compared with the system without THF addition. This suggests that solvent-inhibition of self-organization is contrary to the suggestion by Akbulut et al. that irreversible transformation to a high ambient conductivity (σ = 10⁻⁴ to 10⁻³ S cm⁻¹) regime arises from plasticization by the 3 wt% of volatiles, generated by thermal decomposition of II in a three-component mixture, that they report. The irreversible transformation to higher conductivities is also observed in systems heated to maximum temperatures between 50 and 80 °C for which degradation was shown to be negligible.     

Electrical conductivity of hexagonal Ba(Ti0.94Ga0.06)O2.97 ceramics

Impedance spectroscopy is used to estimate the bulk conductivity, σ_b, of hexagonal (6H)-Ba(Ti_0.94Ga_0.06)O_2.97 ceramics in air, N₂ and 5%H₂/95%N₂ between 400 and 1000 °C. Isothermal plots of log σ_b vs log pO₂ in the temperature range between 700 and 1000 °C show the presence of a p–n transition with slopes of ∼−1/4 and +1/4 in the n- and p-type regions indicating that the conductivity obeys the ‘extrinsic’ model proposed by Smyth and co-workers for undoped and acceptor-doped cubic BaTiO₃-based materials [J. Am. Ceram. Soc. 64 (1981) 556; J. Am. Ceram. Soc. 65 (1982) 167]. The activation energy E_a for oxidation in the p-type region to produce free holes is similar for 6H-Ba(Ti_0.94Ga_0.06)O_2.97 and cubic BaTiO₃-based ceramics with an estimated value from σ_b data of ∼0.8 eV. The band gap for 6H-Ba(Ti_0.94Ga_0.06)O_2.97 ceramics is ∼3.2 eV.     

A new series of fluorinated alicyclic-functionalized polyimides derivated from natural-(D)-camphor: Synthesis,structure–properties relationships and dynamic dielectric analyses

A series of novel fluorinated alicyclic-functionalized polyimides (FPI-x) were prepared from a new alicyclic-functionalized diamine, 1,3-bis ((4-amino-2-(trifluoromethyl)phenoxylmethylene)-1,2,2-trimethyl cyclopentane (BAFMT), which was derivated from natural-(D)-camphor via multi-step process. The influence of the presence of the alicyclic and fluorinated structure on the general properties of the polymers was systematically investigated in terms of optical, thermal and mechanical properties, solubility and hydrophobicity, rendering the general structure–properties relationships for the FPI-x. Compared with the analogous control aromatic polyimide NPI-x, FPI-x shows better solubility, improved optical transparency, lower moisture absorption and surface energy, owing to the presence of the fluorine-containing group and alicyclic structure in the polymer chain. Although presenting a slight decrease as compared with NPI-x, the good thermal stability of polyimide was retained on the large in the fluorinated alicyclic-functionalized polymer FPI-x, with T_g ranging in 197–233 °C and T_d5 ranging in 374–420 °C. A particular research emphasis were placed on employing the dielectric relaxation technique to study the dielectric constant (ε′), dielectric loss (ε″) and electric modulus (M″) of the fluorinated alicyclic-functionalized polyimide film FPI-4 and its control sample NPI-4. The observed dielectric relaxations have been fully studied by integrating the contributions of the specific structure and functional groups. It is revealed that the presence of fluorinated alicyclic-functionalized group endows FPI-4 with lower dielectric constant ε′ (2.88 at 1 MHz and 25 °C) and dielectric loss ε″ (0.0058 at 1 Hz and 25 °C) than NPI-4 (3.49 and 0.0084) and commercial Kapton HN (3.19 and 0.006) with the same measurement frequency and temperature.     

Effect of calcination temperature on optical,magnetic and dielectric properties of Sol-Gel synthesized Ni0.2Mg0.8-xZnxFe2O4 (x = 0.0–0.8)

The Ni_0.2Mg_0.8-xZn_xFe₂O₄ (x = 0.0, 0.2, 0.4, 0.6 & 0.8) nanomaterials were prepared via sol-gel technique. These samples were calcined at three different temperatures (T) such as 400, 450 and 500 °C/5 h. Furthermore, the X-ray diffraction (XRD) patterns of all the calcined samples revealed the single phase cubic spinel structure. The lattice constants (a = b = c) were noticed to be increasing with increase of ‘x’. The grain shape, size and distribution of x = 0.0–0.8 contents were analyzed using field emission electron microscope (FESEM). The x = 0.2 content provided higher optical band gap energy (E_g) value than the remaining contents. Furthermore, the magnetization versus magnetic field (M − H) curves indicated the superparamagnetic nature of x = 0.0–0.8 contents. The high saturation magnetization (M_s) was noticed for x = 0.4 and 0.6 contents. In addition, the distribution of cations like Ni⁺², Mg⁺², Zn⁺², Fe⁺³ and Fe⁺² was performed between the tetrahedral (A) and octahedral (B) sites. The frequency dependence of dielectric constant (ε′), dielectric loss (ε") and ac-electrical conductivity (σ_ac) was investigated as a function of composition. Moreover, the temperature variation of ε′ showed the decreasing trend of dielectric transition temperature (T_e) with increase of ‘x’. The high ε′ of 163.1 (at 1 MHz) was noticed at x = 0.2 content calcined at 500 °C. Using the power law fit applied to the log σ_ac-log ω plots, the dc-electrical conductivity (σ_dc) and exponent (n) parameters were calculated.     

Morphology and properties of Nafion membranes prepared by solution casting

In this study, dilute Nafion solutions consisting of solvents with various dielectric constants ? and solubility parameters δ, i.e. N,N′-dimethyl acetamide, N,N′-dimethyl formamide, N-methyl formamide, methanol-water mixture (4/1 g/g), ethanol-water mixture (4/1 g/g), and isopropanol-water mixture (4/1 g/g), were freeze dried and the conformations of Nafion molecules in dilute solutions were observed using transmission electron microscope. The membranes were prepared by solution casting from these solutions and evaporating the solvents at temperatures below T_G of Nafion, then annealing the membranes at 150 °C which was ∼50 °C above T_G of Nafion. We show Nafion molecular conformations in dilute solutions are strongly influenced by δ and ? of solvents. And, thus the morphology, water uptake, proton conductivity, and methanol permeability of membranes prepared by solution casting are also influenced by δ and ? of solvents.     

Electrical performance of orthotropic and isotropic 3YTZP composites with graphene fillers

3 mol% yttria tetragonal zirconia polycrystal (3YTZP) composites with orthotropic or isotropic microstructures were obtained incorporating few layer graphene (FLG) or exfoliated graphene nanoplatelets (e-GNP) as fillers. Electrical conductivity was studied in a wide range of contents in two configurations: perpendicular (σ_?) and parallel (σ_//) to the pressing axis during spark plasma sintering (SPS). Isotropic e-GNP composites presented excellent electrical conductivity for high e-GNP contents (σ_? ～ 3200 S/m and σ_// ～ 1900 S/m for 20 vol% e-GNP), consequence of their misoriented distribution throughout the matrix. Optimum electrical performance was achieved in the highly anisotropic FLG composites, with high electrical conductivity for low contents (σ_? ～ 680 S/m for 5 vol%), percolation threshold below 2.5 vol% FLG and outstanding electrical conductivity for high contents (σ_? ～ 4000 S/m for 20 vol%), result of the high aspect ratio and low thickness of FLG.     

Influence of DGEBA crosslinking on Li ion conduction in poly(ethyleneimine) gels

We characterize the DC conductivity (σ₀) of solution electrolytes prepared by adding LiCF₃SO₃ (LiTf) salt to a (50/50 w/w) solution of branched poly(ethyleneimine) (PEI) in N,N-dimethylformamide (DMF). The value of σ₀ increases with decreasing LiTf concentration over the range of compositions studied due to the formation of contact ion pairs at higher LiTf concentrations, with the highest value of σ₀ exceeding 10⁻³ S cm⁻¹ at 20 °C. Rubber-like gel electrolytes are prepared by epoxide-amine crosslinking of selected solutions by addition of diglycidyl ether of bisphenol A (DGEBA). Holding the [N]:[Li] mole ratio fixed, increasing the crosslink density dramatically decreases σ₀ at all temperatures studied. The decrease in σ₀ cannot be attributed to an increase in the glass transition temperature, as little variation in T_g is noted amongst the samples due to their high solvent content. Rather, we propose that the decrease in conductivity is due to loss of fast segmental motions associated with chain ends, which become tethered to the network upon crosslinking.     

Synthesis and properties of sulfonated copoly(p-phenylene)s containing aliphatic alkyl pendant for fuel cell applications

A series of novel copoly(p-phenylene)s (PPs) containing an alkyl pendant were successfully synthesized via Ni(0)-catalyzed coupling polymerization. Sulfonated copolymers (SPPs) were achieved by postsulfonation from concentrated H₂SO₄. SPPs showed good solubility in polar aprotic solvents and gave flexible, tough, and transparent free-standing films by solvent casting. The ion exchange capacities (IECs) of the membranes ranged from 2.50 to 2.65 meq/g. All SPP membranes displayed proton conductivity similar to or higher than that of Nafion, especially at high relative humidity (>70% RH) (SPP-1: 0.271 Scm⁻¹, SPP-2: 0.284 Scm⁻¹, SPP-3: 0.212 S cm⁻¹, Nafion: 0.127 Scm⁻¹; at 80 °C and 95% RH). They also exhibited acceptable water uptake in the range of 52-56 vol% at 80 °C with little dimensional change. The gas permeability of the SPP membranes was much lower than that of Nafion 112. Therefore, these materials are promising for fuel cell application.