Synthesis and characterization of hyperbranched polyethylenes containing cross-linking structures by chain walking copolymerization of ethylene with diacrylate comonomer

We report the synthesis of higher-molecular-weight hyperbranched polyethylenes containing cross-linking structures via chain walking copolymerization of ethylene with a diacrylate cross-linker, 1,4-butanediol diacrylate, using a Pd-diimine catalyst, [(ArNC(Me)-(Me)CNAr)Pd(CH₃)(NCMe)]SbF₆ (Ar = 2,6-(iPr)₂C₆H₃). The hyperbranched chain topology of these polymers was achieved through the chain walking mechanism of the Pd-diimine catalyst. By controlling the diacrylate feed concentration in the polymerization system, three sets of hyperbranched polyethylenes having various cross-linking levels (including both intermolecular and intramolecular cross-linking) but without macrogelation were synthesized at three different temperatures, 15, 25, and 35 °C, respectively. The diacrylate content and cross-linking density in the copolymers were found to increase with the enhancements of diacrylate feed concentration and polymerization temperature. Triple-detection gel permeation chromatography (GPC) measurements confirmed the significant enhancement of polymer average molecular weight and broadening in molecular weight distribution with an increase of diacrylate feed concentration as a result of intermolecular cross-linking. The chain topology of the copolymers becomes more compact, compared to homopolymers, due to the presence of intermolecular cross-linking. Rheological studies show that the copolymers possess characteristic rheological properties typically found in polymers containing intermolecular cross-linking. Our results also indicate qualitatively the presence of intramolecular cross-linking in these hyperbranched copolymers, particularly in those synthesized at 35 °C, probably due to their highly compact chain topologies. This work demonstrates the capability of chain walking polymerization for synthesis of hyperbranched polyethylenes of enhanced molecular weights with the simple use of diacrylate as a cross-linker.     

Thermal cross-link behavior of an amorphous poly (para-arylene sulfide sulfone amide)

Cross-link behavior of an amorphous poly (para-arylene sulfide sulfone amide) synthesized via low temperature solution polycondensation was observed for the first time, when the polymer was subject to a series of thermal curing at 260 °C in air condition. The formation of cross-link network was demonstrated by the DSC and TGA results that T_g of the polymer enhanced from 259.17 °C to 268.89 °C, and the 1% weight loss temperature increased remarkably from 243.75 °C to 345.87 °C. EPR analysis further suggested that two kinds of free radicals, CO and C, induced by thermal curing were responsible for this cross-link behavior. According to FT-IR spectrum, the origin of these free radicals was confirmed as amide CO group in the polymer backbone. The cross-linking type was attributed to conventional radical cross-link reaction and the cross-link mechanism was discussed in detail subsequently.     

Determination of the interaction within polyester-based solid polymer electrolyte using FTIR spectroscopy

Characterization and interaction behavior between Li⁺ ion and CO groups of a series polyester electrolyte have been thoroughly examined using Fourier transform infrared (FTIR). The “free/Li⁺ bonded” CO absorptivity coefficient of the LiClO₄/polyester can be determined quantitatively using FTIR spectrum ranging from 1800 to 1650 cm⁻¹ at 80 °C. Results from curve fitting show that the “free/Li⁺ bonded” CO absorptivity coefficient is 0.144 ± 0.005. The CO group of polymer electrolyte shows strong interaction with Li⁺ ion and a limit value of 95% “Li⁺ bonded” CO is approached in the polymer electrolyte system when the Li⁺ ion equivalent fraction is about 0.28. The molecular structure of polyester electrolyte does not affect significantly the efficiency of interaction between Li⁺ ion and CO.     

Study on the oxidation of C4-phenolic-1,4-dihydropyridines and its reactivity towards superoxide radical anion in dimethylsulfoxide

Electrochemical characterization on glassy carbon electrode (GCE) and reactivity with superoxide radical anion in aprotic medium of three new synthesized C4-phenolic-1, 4-dihydropyridines is reported.Voltammetry, coulometry, controlled-potential electrolysis (CPE), UV-vis spectroscopy, ¹H NMR techniques were employed for the characterization of title compounds.The oxidation mechanism involves initially an oxidation process on the phenol moiety with the formation of the corresponding quinone followed by a second one affecting the dihydropyridine ring to give the pyridine derivative. Both processes appeared irreversible in character.Cyclic voltammetry was used to generate O₂⁻ by reduction on GCE of molecular oxygen in DMSO. The reactivity of DHPs towards O₂⁻ was directly measured by the anodic current decay of the radical in the presence of increasing concentration of tested 1,4-dihydropyridines and compared with the reaction of the reference antioxidant, Trolox. The linear correlations obtained between the anodic current of O₂⁻ and compound concentrations in the range between 0.01 mM and 1.00 mM allowed the determination of both the DHP antioxidant index (AI) and the concentrations needed to consume 50% of O₂⁻. Synthesized C4-phenolic 1,4-dihydropyridines exhibited significant scavenging capacity towards superoxide radical anion higher than Trolox and tested commercial 1,4-dihydropyridines.     

Estimation of standard reduction potentials of alkyl radicals involved in atom transfer radical polymerization

The redox properties of some alkyl radicals, which are important in atom transfer radical polymerization both as initiators and mimics of the propagating radical chains, have been investigated in CH₃CN by an indirect electrochemical method based on homogeneous redox catalysis involving alkyl halides (RX) and electrogenerated aromatic or heteroaromatic radical anions (D⁻). Dissociative electron transfer between RX and D⁻ yields an intermediate radical (R), which further reacts with D⁻ either by radical coupling or by electron transfer. Examination of the competition between these reactions, which depends on ED/D−°, allows determination of the standard reduction potential of R as well as the self-exchange reorganization energy λR/R⁻. The standard reduction potentials obtained for the radicals CH₂CN, CH₂CO₂Et and CH(CH₃)CO₂Me are −0.72 ± 0.06, −0.63 ± 0.07 and −0.66 ± 0.07 V vs. SCE, respectively. Quite high values of λR/R⁻ (from 122 to 164 kJ mol⁻¹) were found for all radicals, indicating that a significant change of structure accompanies electron transfer to R.     

Two new siloxanic proton-conducting membranes: Part I. Synthesis and structural characterization

The development of stable polymer electrolytes having good proton conductivity, low cost and operating at medium temperatures represent a crucial step in the evolution of polymer electrolyte fuel cells. We describe two new siloxanic proton-conducting membranes that were synthesized through a two-stage protocol. In the first stage, a poly(methyl hydrosiloxane) precursor (P) bearing siloxane side chains with sulfonic acid groups was prepared. In the second step, the hydrolysis of pristine precursor or its derivative obtained by grafting siloxane chains on P yielded two types of membranes with the formulas {Si(CH₃)₃O[Si(CH₃)HO]_21.26[Si(CH₃)((CH₂)₃SO₃H)O]_1.8[Si(CH₃)((CH₂)₃Si(CH₃)₂O)O]₁₄Si(CH₃)₃}_n (A) and {Si(CH₃)₃O[Si(CH₃)HO]_21.26[Si(CH₃)((CH₂)₃SO₃H)O]_1.8[Si(CH₃)((CH₂)₃(Si(CH₃)₂O)_w)O]_v[Si(CH₃)((CH₂)₃Si(CH₃)₂O-)O]_14 − vSi(CH₃)₃}_n (B), with w = 20.31. Polymer membranes of A and B were prepared by means of a hot-pressing process at 80 °C and 10 t/cm². Scanning electron microscopy showed that A and B are rubbery materials with rough and transparent surfaces. Thermogravimetric investigations performed under air atmosphere disclosed that A and B are thermally stable up to at least 198 °C. DSC measurements yielded T_g(s) of −44 and −60 °C for A and B, respectively. The polymers exhibit ionic exchange capacities of 0.33 (A) and 0.15 meq/g (B). FT-IR and FT-Raman investigations revealed that the polymers consist of reticulated siloxane networks with pendant silicone chains having sulfonic acid groups.     

Synthesis and high electrochemical activity of poly(aniline-co-2-amino-4-hydroxybenzenesulfonic acid)

Poly(aniline-co-2-amino-4-hydroxybenzenesulfonic acid) (PAAHB) was synthesized using chemical oxidative copolymerization of aniline and 2-amino-4-hydroxybenzenesulfonic acid (AHB) in the presence of an ionic liquid at 50 °C. The conductivity of the PAAHB copolymer synthesized at the optimum conditions is 0.47 S cm⁻¹ that is lower than that of polyaniline, but is slightly affected by water. The cyclic voltammograms demonstrate that the PAAHB copolymer has excellent redox activity from highly acidic solution to pH 12.0 in a wider potential range. This is attributed to the synergistic effect of the SO₃⁻ and OH functional groups in the copolymer chain and the ionic liquid incorporated into the PAAHB film. It is evident that the pH dependence of the redox activity and conductivity of the PAAHB copolymer prepared chemically is much better than that of polyaniline, and is further improved, compared to the PAAHB copolymer prepared electrochemically. The proton NMR spectrum of the PAAHB copolymer demonstrates that the SO₃⁻ group exists in the copolymer chain instead of the SO₃H group. The ESR spectra show that the ESR signal intensity is a function of the monomer concentration ratio of AHB to aniline in the mixture. The morphology of the PAAHB copolymer is also dependent on the monomer concentration ratio in the mixture.     

Enhancement of ionic conductivity by mixing lithium borate with lithium aluminate

Several lithium borates (Salt A and Salt B) and a lithium aluminate (Salt C) with electron-withdrawing groups, OC₆F₅, OCOCF₃ or N(SO₂CF₃)₂, and oligoether chains (O(CH₂CH₂O)_nCH₃) directly bonded to the ate complex center, B or Al, were prepared. Lithium borate and lithium aluminate were mixed to get mix-salt electrolytes. Higher ionic conductivity was observed for the mix-salt than for the pure-salt. Conductivity as high as 1.1 × 10⁻⁴ S/cm at ambient temperature (25 °C) was achieved for the electrolyte in the optimized composition. The reason for such mixing effect on enhancement of ionic conductivity was discussed. Other electrochemical properties including electrochemical stability, compatibility with lithium anode and cyclic performance were also investigated for the mix-salt electrolytes.     

Synthesis and characterization of liquid crystalline copolymers with dual photochromic pendant groups

In order to study the photoreactivity and the optical properties of liquid crystalline copolymers with multiple photochromic groups, a series of novel liquid crystalline binary and ternary polyacrylates consisting of one (CC or NN) or dual (CC and NN) photochromic segments were synthesized and characterized considering their liquid crystalline, optical, and photochromic properties and their thermal stability. Achiral homopolymer P1 shows a smectic A phase (fan-shaped texture), and all chiral copolymers CP1-CP6 exhibit chiral nematic phases (cholesteric, oily streaks textures). The polymers show excellent solubility in common organic solvents such as chloroform, toluene, and THF. These polymers also exhibit good thermal stability, with decomposition temperatures (T_ds) greater than 373 °C at 5% weight loss, and beyond 440 °C at 50% weight loss under nitrogen atmosphere. UV irradiation caused E/Z photoisomerization at NN and CC segments of the synthesized photochromic copolymers leading to reversible and irreversible isomerizations, respectively. The synthesized liquid crystalline ternary copolymer CP6, containing two different photochromic NN and CC groups, is sensitive to different UV wavelengths and is notably interesting from the viewpoint of photochromic copolymers.     

Electrochemical investigation of superoxide anion scavenging ability of 1,2,3-triketohydrindene hydrate in aprotic solvents

1,2,3-Triketohydrindene hydrate (NHy) shows well-defined redox electrochemistry in the formation of monoanionic radical (NHy⁻) and dianion (NHy²⁻) in nitrogen saturated aprotic solvents such as acetonitrile and dimethylsulfoxide. Cyclic voltammetry reveals that in an oxygen-saturated solution of DMSO, the oxidation peak of superoxide anion (O₂⁻) at −0.7 V versus Ag/AgCl wire electrode, decreases systematically with increasing NHy concentration. The similar behaviour is observed in the rotating disk voltammetry. On Pt disk, oxygen is reduced to O₂⁻ at a constant potential of −0.8 V and at Pt ring, O₂⁻ is oxidised to oxygen and the corresponding limiting current plateau in the ring voltammogram is decreased linearly as [NHy] is increased. In aqueous solutions, NHy is found to exhibit completely different redox chemistry due to its structural changes and hence showed no favourable redox potentials for efficient quenching of O₂⁻.     

Polysulfones tethered with benzimidazole

Benzimidazole units have been grafted onto a polysulfone (PSU) backbone via long alkyl thio-ether chains using a two-step procedure. In the first step, lithiated PSU was reacted with 10-undecenoyl chloride to graft PSU with undecenoyl side chains. The second step involved a free-radical thiol-ene coupling reaction between the CC bonds of the pendant undecenoyl chains and 2-(2-benzimidazolyl)ethanethiol. In this reaction, all the CC bonds were converted into thio-ether linkages without any detectable structural degradation, as confirmed by ¹H NMR spectroscopy and size-exclusion chromatography. The procedure constitutes a convenient and general pathway to attach functional or mesogenic groups to PSU via long flexible spacers. Thermogravimetry showed that the benzimidazole-functionalized polymers were stable up to 250 °C under nitrogen atmosphere, and that the first degradation step was attributed to the cleavage of the thio-ether bond. While the grafting of the undecenoyl side chains was found to significantly decrease the glass transition temperature (T_g), the subsequent tethering of the benzimidazole only slightly increased the T_g of the grafted PSU backbone. The concentration of benzimidazole was probably too low for the formation of a percolating benzimidazole domain. This explains the quite modest proton conductivity measured under completely dry conditions, e.g. 34 nS/cm at 180 °C for a polymer functionalized with 1.7 benzimidazole units per repeating unit of PSU.     

Di-ureasil xerogels containing lithium bis(trifluoromethanesulfonyl)imide for application in solid-state electrochromic devices

In this study we report the characterization of a prototype solid-state electrochromic device based on poly(ethylene oxide) (PEO)/siloxane hybrid networks doped with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI). The polymer networks prepared, designated as di-ureasils and represented as d-U(2000), were produced by a sol-gel procedure and are composed of a siliceous framework to which both ends of polyether chains containing about 40 CH₂CH₂O units are covalently bonded through urea linkages. Samples with compositions of 200 ≥ n ≥ 0.5 (where n is the molar ratio of CH₂CH₂O to Li⁺) were characterized by thermal analysis, complex impedance measurements and cyclic voltammetry at a gold microelectrode. Electrolyte samples were obtained as self-supporting, transparent, amorphous films and at room temperature the highest conductivity was observed with the d-U(2000)₃₅LiTFSI composition (3.2 × 10⁻⁵ Ω⁻¹ cm⁻¹). We report the results of preliminary evaluation of these polymer electrolytes as multi-functional components in prototype electrochromic displays. Device performance parameters such as coloration efficiency, optical contrast and image stability were also evaluated. The electrolytes with n > 8 presented an optical density above 0.56 and display assemblies exhibited good open-circuit memory and stable electrochromic performances.     

Electron transfer reaction and mechanism of oxidation of Inosine

Electrochemical oxidation of Inosine has been studied in the phosphate buffers of pH range 3.3-10.9 at pyrolytic graphite electrode. In the entire pH range a single well-defined oxidation peak (I_a) was observed, when the sweep was initiated in the positive direction. In the reverse sweep no cathodic peak was obtained. The peak potential of the oxidation peak was dependent on pH and shifted to less positive potential with increase in pH. The kinetics of the UV absorbing intermediate was followed spectrophotometrically and the decay occurred in a pseudo first order reaction having k values in the range 0.50-0.92 × 10⁻³ s⁻¹ in the entire pH range studied. The value of n was found to be 2.95 ± 0.3. The products of oxidation were silylated and characterized by using GC-Mass. Two tetramers having CC, CN, NN, CON and COOC linkages were identified. A plausible mechanism for the electrooxidation of Inosine has been suggested.     

Kinetic study on reversible addition-fragmentation chain transfer (RAFT) process for block and random copolymerizations of styrene and methyl methacrylate

The degenerative (exchange) chain transfer constant C_ex was determined for the dithioacetate-mediated living radical block and random copolymerizations of styrene (St) and methyl methacrylate (MMA) at 40 °C. The addition of the polystyrene (PSt) radical to a polymer-dithioacetate adduct (P-X) to form the intermediate radical (PSt-(X)-P) was (about twice) faster than that of the poly(methyl methacrylate) (PMMA) radical to form the intermediate radical PMMA-(X)-P. The fragmentation (release) of the PMMA radical from the PSt-(X)-PMMA intermediate formed at the initiating stage of block copolymerization was much (about 100 times) faster than the release of the PSt radical, explaining why the block copolymerization of MMA from a PSt-dithiocarbonate adduct is not so satisfactory as that of St from a PMMA-dithiocarbonate adduct. In the random copolymerization, there was implicit penultimate unit effect on the exchange chain transfer process, which appeared in the addition process but not in the fragmentation process.     

Diffusivity of anion vacancies in WO3 passive films

The WO₃ films were grown in 0.1 M HClO₄ aqueous solution, at different formation potentials (E_f) in the range of 2.0-7.0 V versus sce, on W electrode. The anion diffusion coefficient (DO) of WO₃ films was calculated from EIS spectra, following the surface charge approach (at high-field limit approximation), the Point Defect Model and the Mott-Shottky analysis. Among the parameters necessary to evaluate DO, the half-jump distance (a) is very relevant, given that a small variation in a has a great impact in the calculation of DO. In this work, it is proposed the half-jump distance (a) should be evaluated from spectroscopic data (available in the literature). The value of a (∼1.9 Å) is taken from lattice constants of a-WO₃ (amorphous-WO₃), with different values of N (coordination number), and the lattice constants of m-WO₃ (monoclinic-WO₃). The calculated value of DO was ∼3 × 10⁻¹⁷ cm²/s.     

FTIR investigation of the influence of diisocyanate symmetry on the morphology development in model segmented polyurethanes

Novel segmented polyurethanes with hard segments based on a single diisocyanate molecule with no chain extenders were prepared by the stoichiometric reactions of poly(tetramethylene oxide)glycol (M_n=1000 g/mol) (PTMO-1000) and 1,4-phenylene diisocyanate (PPDI), trans-1,4-cyclohexyl diisocyanate (CHDI), bis(4-isocyanatocyclohexyl)methane (HMDI) and bis(4-isocyanatophenyl)methane (MDI). Time dependent microphase separation and morphology development in these polyurethanes were studied at room temperature using transmission FTIR spectroscopy. Solvent cast films on KBr discs were annealed at 100 °C for 15 s and microphase separation due to self organization of urethane hard segments was followed by FTIR spectroscopy, monitoring the change in the relative intensities of free and hydrogen-bonded carbonyl (CO) peaks. Depending on the structure of the diisocyanate used, while the intensity of free CO peaks around 1720-1730 cm⁻¹ decreased, the intensity of H-bonded CO peaks around 1670-1690 cm⁻¹, which were not present in the original samples, increased with time and reached saturation in periods ranging up to 5 days. Structure of the diisocyanate had a dramatic effect on the kinetics of the process and the amount of hard segment phase separation. While PPDI and CHDI based polyurethanes showed self-organization and formation of well ordered hard segments, interestingly no change in the carbonyl region or no phase separation was observed for MDI and HMDI based polyurethanes. Quantitative information regarding the relative amounts of non-hydrogen bonded, loosely hydrogen bonded and strongly hydrogen bonded and ordered urethane hard segments were obtained by the deconvolution of CO region and analysis of the relative absorbances in CO region.     

Electrochemical behaviour of gold and stainless steel under proton irradiation and active RedOx couples

Model experiments are reported where proton beams delivered by the cyclotron located at CERI (CNRS-Orléans) are used for irradiating AISI 316L/water and Au/water high purity interfaces with 6 MeV protons. The free exchange potentials at the interfaces are recorded as a function of time at room temperature in situ before, under, and after proton irradiation. The evolutions are compared to those calculated for the Nernst potentials associated with the radiolytic RedOx couples. It is shown how the comparison gives evidence that five radiolytic species - O₂, H₂O₂, HO₂⁻, HO₂ and O₂⁻ - exchange electrons at the Au interfaces in a range of dose rates that vary over three orders of magnitudes, i.e. 0.0048 < dr(10⁷ Gy h⁻¹) < 4.8. The balance between the electron exchanges at Au interfaces is adjusted by the RedOx reactions associated with the above species. The free exchange potential reaches the same steady value for Au and AISI 316L interfaces irradiated at high doses, ≥2.5 × 10⁷ Gy, (0.020 ± 0.025) V versus NHE. Such low values are the first ones to be reported. The HO₂ and O₂⁻ radical disproportionations play a key role and control the potential at the interfaces under 6 MeV proton flux. This role is generally mostly overlooked for gamma irradiation.     

Gas crossover and membrane degradation in polymer electrolyte fuel cells

Hydrogen gas crossover measurements and durability tests of a single cell under open-circuit conditions were carried out to investigate membrane degradation in polymer electrolyte fuel cells (PEFCs). The limiting current density for hydrogen crossover was approximately 0.8 mA cm⁻² at 80 °C under atmospheric pressure, and gas crossover increased with an increase in cell temperature, humidity and hydrogen gas pressure. Under open-circuit conditions, the perfluorinated ionomer electrolyte membrane deteriorated significantly although no net electrochemical reactions took place at the cathode and anode. The mechanism for membrane degradation was discussed in terms of heat generation and hydrogen peroxide formation upon gas crossover and the resulting catalytic combustion, and it was concluded that the latter is the primary reason, in which hydrogen peroxide is most probably formed by gas crossover of oxygen and the resulting catalytic combustion at the anode side. In addition, it was inferred that reactive oxygen radicals (HO and HO₂) were formed in the presence of minor impurities such as Fe²⁺ and Cu²⁺ ions, which could accelerate the membrane degradation.