Concentration Of End Groups Research Articles

Radicals in Fuel Cell Membranes: Mechanisms of Formation and Ionomer Attack

Radical intermediates formed during polymer electrolyte fuel cell (PEFC) operation cause degradation of the proton exchange membrane. Generation and decay of the radicals HO•, H•, and HOO• in the membrane was simulated based on a kinetic framework with H2O2 as precursor. The degradation of the PFSA membrane was assumed to proceed via end-group attack. Two chemical environments were considered, a fuel cell test and an ex situ Fenton test. It was found that the reaction pathways and corresponding radical concentrations differ considerably. In particular, no H• is formed in a Fenton test due to the absence of H2. The influence of the concentration of Fe-ion impurities and end-groups on the radical concentration and rate of ionomer attack was studied. The calculated fluoride emission rate shows fairly good agreement with experimental results reported in the literature.

The Study of Thermolplastic Elastomer Based on Polyamide 6: Preparation, Morphology and Mechanical Performance

In this study,a new thermoplastic elastomer based on polyamid 6, and polytetrahydrofuran was prepared by continuous hydrolytic ring-opening polymerization process. The effective of adipic acid content on the number-average molecular and the content of amino end groups was investigated. The morphology deduce of TPMP, mechanical performances were also investigated.

A comprehensive single‐particle model for solid‐state polymerization of poly(L‐lactic acid)

AbstractWe propose here, a comprehensive model for the solid‐state polymerization (SSP) of a low to moderate molecular weight (MW) prepolymer of lactic acid, to produce high MW poly(L‐lactic acid) (PLLA). The reactions are rationally assumed to occur only in the amorphous region, and effective concentrations of end groups, vary with crystalinity, Xc, during SSP. We estimate byproduct diffusivities, D, using free volume theory. The effects of various parameters on the SSP of PLLA prepolymer have been examined with respect to the optimum MW, Xc and D. We introduce self‐consistently, scaling factors of ∼ 0.27, in the experimental procedure, to determine via 19F‐NMR, concentrations of the end groups, after converting them to fluorinated ester groups. The relevant reaction rate constants are obtained by fitting to early time data from representative SSP experiments at 150°C, under high vacuum, on PLLA prepolymer powder (i.e., spherical geometry) of number average MW, Mn0 ∼ 10,200 Da, which attains Mn ∼ 150,000 Da, via SSP. The subsequent successful comparison of the model predictions with experimental data throughout the entire SSP duration indicates that the model is comprehensive and accounts for all the relevant phenomena occurring during the SSP to synthesize high MW PLLA. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011

PET synthesis in the presence of new aluminum catalysts

A new polycondensation catalyst (ethylene glycol aluminum) was synthesized and applied in PET synthesis. It has been characterized by spectral studies (IR, NMR). The catalytic activity of organic aluminum compound was discussed in this paper. In this investigation, the synthesis of poly(ethylene terephthalate) (PET) is presented using ethylene glycol (EG) and terephthalic acid (TPA). The result proved that ethylene glycol aluminum was the real catalytic material. And it was proved to be the most efficient catalyst for the polycondesation reaction among the organic aluminum compounds. Similar catalytic activity has been observed for ethylene glycol aluminum compared to ethylene glycol antimony. The esterification reaction was monitored by measuring the distilled water as a function of time and the modeling of this process was carried out from these data. The received PET samples were characterized by viscometry, carboxyl end-group content and diethylene glycol (DEG) content. A mechanism of aluminum catalysis has been proposed to explain these observations.

Polyisobutylene with High exo-Olefin Content via β-H Elimination in the Cationic Polymerization of Isobutylene with H2O/FeCl3/Dialkyl Ether Initiating System

The cationic polymerizations of isobutylene (IB) with H2O/FeCl3/dialkyl ether initiating system were conducted in dichloromethane (CH2Cl2) at temperatures from −20 to +20 °C, in which the dialkyl ether includes diethyl ether (Et2O), dibutyl ether (Bu2O) or diisopropyl ether (iPr2O). The highly reactive polyisobutylenes (HRPIBs) with high content of exo-olefin end groups (−CH2−C(CH3)═CH2) 82−91 mol % and acceptable monomodal molecular weight distribution (Mw/Mn = 1.7−2.3) could be successfully synthesized at low concentration of FeCl3 at 0.005 mol·L−1 at 0 or even 10 °C. These results are comparable to those of commercial HRPIBs produced industrially at far below 0 °C. The directly rapid β-proton elimination from −CH3 of the growing chain ends and chain transfer reaction to monomer were dependent on concentration of iPr2O·FeCl3 complex (1:1), concentration of free iPr2O ([free iPr2O] = [iPr2O] − [FeCl3]) if (iPr2O·FeCl3> 1) and polymerization time. The much higher concentration of PIB chains formed in the ...

Self‐Assembled Arrays of Dendrimer–Gold‐Nanoparticle Hybrids for Functional Cell Studies

Engineered surfaces with nanoscale features of gold on silicon or glass have recently been used to improve the understanding of adhesion-mediated environmental sensing of cells. Often such surfaces present a cell-binding ligand, such as arginine–glycine–aspartic acid (RGD) peptide motifs, at controlled intramolecular distances on an inert background surface such as polyethylene glycol (PEG). The adhesion mechanism of macromolecular ligands in which direct interaction with cells is nonspecific is not known and the cell response is dictated by the type and the concentration of proteins adsorbed from solution. Dendrimers may increase the availability and multivalency of cell-interacting ligands as a consequence of their branched shape and inherently high concentration of end groups. It is therefore interesting to examine the eventual effect of the macromolecular architecture on the cell viability by the controlled reduction of ligands on a surface. Herein, we demonstrate the fabrication of selfassembledmacromolecular hybrid arrays in which the relative position of two anionic macromolecules of different architectures—a carboxy-functionalized dendrimer and a linear polymer—is straightforwardly controlled on a PEG surface. We also show how the interaction of primary human endothelial cells with these surfaces is modulated by the molecular spacing and how protein binding to the macromolecular arrays can be evaluated by using a standard surface plasmon resonance (SPR) technique. Self-assembled, short-range-ordered Au nanoparticle (NP) arrays were used as a versatile template to arrange polymeric entities at the nanometer scale (Figure 1a). This

Mechanism of Isomerization in the Cationic Polymerization of Isobutylene

The complex mechanism of the carbocationic rearrangements leading to a variety of olefin (exo-, endo-, tri-, and tetra-substituted) end groups in the cationic polymerization of isobutylene (IB) catalyzed by ethylaluminum dichloride (EtAlCl2) in nonpolar solvents in the temperature range of −40 to 25 °C was studied by model ionization experiments employing poly(isobutylene chloride) (PIB-Cl) of low molecular weight (Mn ∼ 1000−2000) obtained by living cationic polymerization. Ionizations were performed with EtAlCl2 in hydrocarbon media to mimic a conventional cationic polymerization of IB, but in the presence of a proton trap to suppress reprotonation of the first formed olefin and subsequent decomposition of the resulting PIB cation. In the absence of a proton trap, ionization of PIB-Cl resulted in about 70% of tri-substituted olefin end groups in the studied temperature range. The exo and endo olefin end-group content was negligible. MALDI and APPI TOF MS indicated that the tri-substituted PIB olefins con...

Kinetic Simulation of the Chemical Stabilization Mechanism in Fuel Cell Membranes Using Cerium and Manganese Redox Couples

The attack by HO• on the ionomer in the polymer electrolyte fuel cell (PEFC) can be mitigated by the incorporation of regenerative radical scavengers, such as cerium and manganese, into the membrane. The influence of the presence of the Ce and Mn ions on the reaction of HO• with the perfluoroalkylsulfonic acid (PFSA) ionomer was studied in the framework of a kinetic model. Scavenging of HO• by Ce3+ and Mn2+ leads to a decrease in the steady-state concentration of HO• and, therefore, the rate of ionomer attack, where Ce is more effective than Mn. The HO• scavenging yield was found to decrease with increasing reactive end-group concentration of the PFSA ionomer. The oxidized metal ions are rapidly reduced by both and HOO•, which restores the scavenger and ensures that the majority (>99.99%) of metal ions is in the reduced, “HO•-scavenging-active” oxidation state. This stabilization mechanism is not expected to be operational in hydrocarbon membranes, because HO• reacts with aromatic units far too rapidly. Also, the effectiveness of the mitigation mechanism is limited under the conditions of an ex situ Fenton test due to the abundance of H2O2, which is confirmed by experimental data reported in the literature.

Hierarchical structure of phase‐separated segmented polyurethane elastomers and its effect on properties

AbstractPolyurethanes were prepared from 4,4′‐methylenebis(phenyl isocyanate), 1,4‐butanediol and poly(1,4‐butanediol adipate) polyester polyol. The hydroxyl functional group ratio of polyol/total diol was kept constant at 0.4, while the ratio of the isocyanate and hydroxyl groups (NCO/OH ratio) changed between 0.90 and 1.15. The polymers were prepared by one‐step bulk polymerization in an internal mixer. They were characterized using a number of methods including Fourier transform infrared spectroscopy, wide‐ and small‐angle X‐ray scattering, differential scanning calorimetry, dynamic mechanical analysis, light transmittance and tensile testing. Changing the stoichiometry modifies the molecular weight in accordance with the laws of stepwise polymerization, and the relative concentration of end groups also changes at the same time. Competitive interactions among various groups including chain‐end functional groups lead to the formation of slightly ordered phases of sub‐nanometre size at both ends of the composition range. These structures assemble into larger units at the 10 nm level, which associate further to even larger entities of micrometre size causing scattering of light and a decreased transparency of the samples. The order of the primary units, together with the number and size of assemblies at both higher levels, decrease as the composition approaches equimolar stoichiometry. The amount of less ordered amorphous phase has a maximum in this range. The stiffness of the polymers is determined by the amount of this phase, while ultimate properties are influenced also by molecular weight and the number of physical crosslinks formed. Copyright © 2010 Society of Chemical Industry

Chemical Modification of Polyamide 6 by Chain Extension with Terephthaloyl-biscaprolactam

The chain extension behaviors of terephthaloyl-biscaprolactam (TBC), 2,2′-bis(2-oxazoline)(BOZ), 2,2′-(1,3-phenylene)bis(2-oxazoline)(MPBO), TBC/BOZ, and TBC/MPBO, were studied to evaluate the coupling effect on polyamide 6 (PA6) in a Haake Rheocord mixer and an extruder. For the chain extension of PA6 with TBC only, the coupling results showed that there was an optimal dosage of chain extender, shortage of which caused incomplete chain extensions and excess of which led to more blocking reactions. When the added amount of TBC chain extender in PA6 was 0.684 wt%, the intrinsic viscosity of the PA6 increased from 1.630 to 1.848 dl/g, and the concentration of the amine end groups in the chain-extended PA6 decreased from the initial 5.12 × 10−5 to 1.96 × 10−5 mol/g. However, for the chain extension of PA6 with TBC/BOZ and TBC/MPBO mixtures, the intrinsic viscosity of PA6 increased to 2.189 dl/g and 2.097 dl/g, respectively, while at the same time both the concentration of carboxyl end-groups and amine end-groups of chain-extended PA6 decreased. The effect of temperature on the time needed for the completion of the chain extension reaction was not obvious, and the time needed for the completion of the reaction was about 200 sec. The chain-extended PA6 dissociated at high temperature, and the degradation is suggested to proceed simultaneously from the onset of the chain extension reaction. Furthermore, the effects of chain extenders on the thermal properties of chain-extended PA6 were investigated. The thermal stability of chain-extended PA6 was slightly improved.

Nanocatalysis in Polyamide 6.6 Solid‐State Polymerization

AbstractSolid‐state polymerization (SSP) of a poly(hexamethyleneadipamide) (PA 6.6)/clay nanocomposite system was studied. SSP runs were performed in a fixed‐bed reactor, at temperatures 160–200 °C and reaction times up to 8 h. The influence of clay presence on the PA 6.6 SSP rate constant was herewith quantified for the first time to prove significant acceleration of the SSP process. A catalysis mechanism was suggested, according to which the positive effect of clay is of a synergistic origin attributed to nucleated crystal morphology, that increased the concentration of reactive end groups in the amorphous regions, to chain extension performed by clay SiOH groups, and to thermal protection of the polyamide matrix due to the presence of the nanoparticles. magnified image

Waterborne polyesters partially based on renewable resources

AbstractA new experimental approach for preparing biobased, water‐soluble polyesters (PEs) via titanium(IV) n‐butoxide‐catalyzed bulk polycondensation is presented. In the described method polymers were obtained from isosorbide, maleic anhydride and poly(ethylene glycol) (PEG). The chemical structure of the synthesized PEs was confirmed using 2D NMR spectroscopy and by titration methods. Careful analysis of 2D NMR spectra viz. correlation spectra (COSY), heteronuclear single quantum correlation spectra (HSQC) and heteronuclear multiple‐bond correlation spectra (HMBC) allowed to accomplish the complete proton assignment of isosorbide, PEG, and unsaturated acid residues in the PEs. Moreover, by using NMR spectroscopy the transformation of maleic anhydride into fumaric acid ester and the absence of maleic acid ester units in the final polymer were proven. However, during polycondensation part of the unsaturated bonds has reacted in a Michael addition with isosorbide or PEG. Gel permeation chromatography measurements revealed that the unsaturated PEs have Mn values in the range 3000–5000 g/mol. These PEs, with a low content of carboxylic acid end groups, exhibited sufficient thermal resistance for practical applications, for example, as free radical curable coatings. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2010

Kinetics and Mechanisms of Poly(poly(ethylene terephthalate)-co-poly(ε-caprolactone)) Catalyzed Randomization: Influence of End Groups

Hydroxy−ester interchanges were found to be the predominant reactions involved in poly(ethylene terephthalate)−poly(e-caprolactone) copolyester randomization. Even for high molar-mass copolyesters having low hydroxyl end group concentration, kinetic studies show that ester−ester interchanges play a minor role. Reaction rate was almost proportional to hydroxyl end groups concentration. The randomization kinetics was studied by 1H NMR on copolyesters obtained in situ by reacting a series of poly(ethylene terephthalate)s of various molar masses and end groups with e-caprolactone, in the molten state and in the presence of Ti(OBu)4 catalyst. A mechanism involving parallel catalyzed second-order hydroxy−ester and ester−ester interchanges is proposed for the randomization step and perfectly fits experimental data. This study clearly shows that the nature and concentration of hydroxyl end groups must be carefully controlled for reliable synthesis of copolyesters by homopolyester melt blending.

Synthesis and application of molybdenum (III) complexes bearing weakly coordinating anions as catalysts of isobutylene polymerization

AbstractAcetonitrile ligated molybdenum (III) complexes of the structure [MoCl(NCCH3)5]2+ bearing different weakly coordinating anions [B(C6F5)4]− (WCA a), [B{C6H3(m‐CF3)2}4]− (WCA b) and [(C6F5)3B‐C3H3N2‐B(C6F5)3]− (WCA c) were applied as homogeneous catalysts of the polymerization of isobutylene. High monomer conversions were obtained in short reaction times (<30 min). The molecular weight of the resulting polyisobutylene is nearly independent of parameters such as temperature, solvent, monomer concentration, but is strongly influenced by the type of WCA and by chain transfer reactions which were observed in these systems. Highly reactive low molecular weight polyisobutylenes (Mn < 2000 g/mol) were obtained with a high content of exo double bond end groups as shown by 1H NMR analysis. Furthermore, experiments were performed to reduce the isomerization of these exo end groups into other internal double bonds by varying the polymerization parameters. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 3775–3786, 2010

Universal Polymer Analysis by 1H NMR Using Complementary Trimethylsilyl End Groups

New degenerative chain transfer agents, namely 4-(trimethylsilyl)benzyl 4'-(trimethylsilyl)butane-dithioate, 4-(trimethylsilyl)benzyl 3'-(trimethylsilyl)propyl trithiocarbonate and their 3-(trimethylsilyl)benzyl isomers, that are two-fold labeled with complementary trimethylsilyl (TMS) markers, were designed and shown to be powerful tools for universal polymer analysis by conventional (1)H NMR spectroscopy. Their use in controlled free radical polymerization, here the reversible addition-fragmentation chain transfer (RAFT) method, resulted in polymers with low polydispersities up to high molar masses, as well as with defined complementary TMS end groups. Thus, routine (1)H NMR spectra allowed facile determination of the molar masses of polymers of various chemical structures up to at least 10(5) g/mol, and simultaneously provided crucial information about the content of end groups that is typically >95% when polymerizations are correctly performed. Polymerizations were carried out in various solvents for two standard monomers, namely n-butyl acrylate and styrene, as well as for two specialty monomers, so-called inimers, namely 2-(2-chloropropionyloxy)ethyl acrylate and 2-(2-chloropropionyloxy)ethyl acrylamide. The complementary end group markers revealed marked differences in the suitability of commonly used solvents for RAFT polymerization. The results demonstrate-beyond good polymerization control-that the new RAFT agents are universal, powerful tools for facile polymer analysis by routine (1)H NMR spectroscopy, of their absolute molar masses as well as of the content of end groups. This is crucial information, e.g., for the synthesis of high-quality telechelics and, in particular, of block copolymers, which is difficult to obtain by other methods. Preliminary screening experiments indicate that similar uses can be envisaged for analogous ATRP systems.

Preparation and characterization of high molecular weight poly(trimethylene terephthalate) by solid-state polymerization

Solid-state polymerization of poly(trimethylene terephthalate)(PTT) was carried out to obtain high molecular weight polymers. Two kinds of commercial PTT chips were polymerized in the solid state by the heat treatment at 190∼220°C for various times and they were characterized by end group content, molecular weight, thermal analysis, and X-ray diffraction. In the solid-state polymerization of PTT, the overall reaction rate was governed by the solid-state polymerization temperature and time, and pellet size. The content of carboxyl end groups decreased during the solid-state polymerization with increasing solid-state polymerization temperature and time. The melting temperature and crystallinity of the PTT were higher for the ones treated at higher temperature and longer time. The activation energy for the solid-state polymerization of PTT was in the range of 24∼25 kcal/mol for both chips. Through the solid-state polymerization of commercial PTT chips, high molecular weight polymers up to an intrinsic viscosity of 1.63 dl/g was obtained, which corresponded to about a 117,000 weight-average molecular weight.

Autocatalytic Equation Describing the Change in Molecular Weight during Hydrolytic Degradation of Aliphatic Polyesters

The autocatalytic equation derived in this study describes and even predicts the evolution of the number average molecular weight of aliphatic polyesters upon hydrolytic degradation. The main reaction in the degradation of aliphatic polyesters is autocatalytic hydrolysis of ester bonds, which causes the molecular weight to decrease. During hydrolysis of the ester bonds in the main chain of the polyester, the chains are cleaved and the end group concentrations will rise. The fundamentals of this equation are based on that principle. To validate the derived equation, the hydrolytic degradation of poly(4-methylcaprolactone), poly(epsilon-caprolactone), poly(d,l-lactide), and two different poly(d,l-lactide-co-glycolide) copolymers was monitored after immersion in a PBS buffer (pH = 7.4) at 37 degrees C. The number average molecular weight, mass loss, and crystallinity were determined after different time intervals. The experimental results confirm that hydrolytic degradation of aliphatic polyesters is a bulk erosion process. When comparing the M(n), calculated with the new autocatalytic equation, with the experimental results, it was found that the new model can predict the decrease of the M(n) upon hydrolytic degradation for semicrystalline and amorphous polymers, as well as for copolymers, without the need for complicated mathematics and excessive input parameters. This is a major improvement with respect to earlier proposed models in literature.

Partitioning and phase equilibria of PEGylated excipients in fluorinated liquids

Mixtures of common polymeric excipients and hydrofluoroalkane (HFA) liquids show rich and complex phase behaviour. Phase diagrams and phase compositions are reported for poly(ethylene glycol)s with varying levels of end-group methylation in mixed solvent systems consisting of the model propellant 2H,3H-perfluoropentane (HPFP) and the fully fluorinated analogue perfluoropentane (PFP). Studies have been performed as a function of molecular weight as well as end group chemistry (monomethyl, MM; dimethyl, DM; and dihydroxyl, DH), and for binary polymer mixtures in HPFP/PFP solvent systems. The solvent composition required to induce phase separation by addition of the non-hydrogen bonding PFP is strongly dependent on end-group concentrations. It shows a linear increase with increasing methylation, whilst remaining insensitive to OH group concentration in dihydroxylated PEG systems. For single polymer systems it is observed that strong partitioning of the polymer is observed, and changes in polymer concentration occurring across the phase diagram are a result of changing solvent partitioning between upper and lower phases. These solvent effects are dependent on the composition (wt% PFP) in the solvent mixture. The linear dependence of solvent composition required to induce phase separation at fixed polymer concentration on end group concentrations can be used to predict the phase behaviour for mixtures of monomethylated PEG with either dimethyl or dihydroxyl PEGs, whereas mixtures of dihydroxyl with dimethyl end-capped PEGs show a deviation from linear behaviour with dominance of the dihydroxyl end groups, which is reflected in the obtained phase diagrams. This study hence progresses understanding of factors that influence solubility of PEG-type polymers in HFAs and will facilitate the identification of predictive methodologies for formulation.

1H‐NMR spectroscopic study of the effect of aging vascular prostheses made of poly(ethylene terephthalate) on the macromolecular weight

Trichloroacetyl isocyanate reacts rapidly and quantitatively with both acid and hydroxyl chain ends to form derivatives that can be readily determined by (1)H-NMR spectroscopy. This method provides a convenient mean for characterization of polyethylene terephthalate (PET) end-groups. The (1)H-NMR spectroscopy has been applied to describe the chemical aging of the PET vascular prostheses by determination of the hydroxyl and carboxyl end-group concentrations and therefore the macromolecular weight. To validate (1)H-NMR results, we used chemical titration of the end-groups and classical viscosimetric method as complementary techniques. The analyses made on the explants of different lifetime demonstrated a significant deterioration compared with the virgin prostheses. A high degradation of macromolecular weight is observed. This phenomenon is explained by a random scission of the ester linkages.

Factors affecting degradation of polyethylene terephthalate (PET) during pre-flotation conditioning

In general, plastics are exposed to different degrading agents in every procedure involved in their recovery from waste mixture and from subsequent recycling. In this study, two methods of pre-flotation conditioning were used to determine how these methods affect the general properties of the pre-conditioned PET particles to be recovered from the PET-PVC mixture. The first method comprised the conditioning of PET samples using an alkaline solution of nonionic surfactant (Triton X-100) based on the patent by the Goodyear Tire and Rubber Company. The second method, developed in this study, was a conditioning process which used an alkali-less solution of the same nonionic surfactant (Triton X-100) used in the first method. The following analytical methods were used to characterize properties of the pre-conditioned PET samples that were correlated to relative degradation of the samples: differential scanning calorimetry (DSC), for thermal behavior of the samples; FT-IR spectroscopy, for functional groups present in the samples; and, Pohl's method, for carboxyl end-group concentration count. Results show that in addition to water the presence of NaOH in the conditioning solution contributes to the further degradation of the polymer.