Scanning Small-Angle X-ray Scattering of Injection-Molded Polymers: Anisotropic Structure and Mechanical Properties of Low-Density Polyethylene

Injection molding is known to create a layered anisotropicmorphologyacross the sample thickness due to varying shear and cooling ratesduring the manufacturing process. In this study, scanning small-angleX-ray scattering was used to visualize and quantify the distributionof hierarchical structures present in injection-molded parts of low-densitypolyethylene (LDPE) with varying viscosities. By combining scatteringdata with results from injection molding simulations and tensile testing,we find that oriented shish-kebab structures, as well as elongatedspherulite structures consisting of semicrystalline ellipsoids, contributeto high ultimate tensile strength along the flow direction. Furthermore,we show that a higher degree of orientation is found close to theinjection gate and in LDPE with higher viscosity, consequently fromelevated shear and cooling rates present during the injection moldingprocess

Ab initio investigation of monoclinic phase stability and martensitic transformation in crystalline polyethylene

We study the phase stability and martensitic transformation of orthorhombic and monoclimic polyethylene by means of density functional theory using the nonempirical consistent-exchange vdW-DF-cx functional [Phys. Rev. B 89, 035412 (2014)]. The results show that the orthorhombic phase is the most stable of the two. Owing to the occurrence of soft librational phonon modes, the monoclimic phase is predicted not to be stable at zero pressure and temperature, but becomes stable when subjected to compressive transverse deformations that pin the chains and prevent them from wiggling freely. This theoretical characterization, or prediction, is consistent with the fact that the monoclimic phase is only observed experimentally when the material is subjected to mechanical loading. Also, the estimated threshold energy for the combination of lattice deformation associated with the T1 and T2 transformation paths (between the orthorhombic and monoclimic phases) and chain shuffling is found to be sufficiently low for thermally activated back transformations to occur. Thus, our prediction is that the crystalline part can transform back from the monoclimc to the orthorhombic phase upon unloading and/or annealing, which is consistent with experimental observations. Finally, we observe how a combination of such phase transformations can lead to a fold-plane reorientation from {110} to {100} type in a single orthorhombic crystal

Proton-Conducting Sulfonated Aromatic Ionomers and Membranes by Chemical Modifications and Polycondensations

Proton-exchange membrane fuel cells (PEMFC)s are increasingly regarded as promising environmentally benign power sources. Intensive development is today directed towards reducing the cost and increasing the durability of the fuel cell, as well as expanding the operational window for the PEMFC for a range of applications. The proton-conducting membrane is one of the key components in the PEMFC. The need for improved performance of the proton-conducting membrane has led to an extensive worldwide research, from which aromatic ionomers have emerged as promising candidates. One of the major challenges is to prepare functional high-performance proton-conducting membranes, with optimized properties. By concentrating the sulfonic acid groups, which facilitate the proton transport, to specific chain segments, the nanophase separation between hydrophilic and hydrophobic domains may be enhanced, which may provide membranes with balanced water sorption characteristics. In the present work, the sulfonic acid groups were concentrated to specific segments in either the polymer backbone or on pendant side chains. Based on the former approach, polysulfones with fully tetrasulfonated aryl-SO2-aryl-aryl-SO2-aryl segments were prepared by lithiation, reaction with sulfur dioxide, followed by oxidation of the resulting sulfinates. The possibility to fully tetrasulfonated these segments offer possibilities to prepare various aromatic copolymers and membranes with locally very high densities of hydrolytically stabile sulfonic acid groups. As a second approach to enhance the phase separation, the sulfonic acid groups were separated from the polymer backbone and were concentrated to side chains. PSUs carrying various mono-, di – and trisulfonated side chains were synthesized by chemical modification. Moreover, aromatic ionomer with various polymer backbones with pendant benzoyl side chains were synthesized by polycondensations. SAXS measurements showed that longer side chains and higher local acid concentrations, gave larger characteristic separation length between the ionic clusters accompanied with narrower distribution. Proton conductivity measurements showed that larger characteristic separation lengths resulted in higher proton conductivities. The ionic clustering of ionomers bearing sulfobenzoyl side chains was shown to be promoted by ionomers with flexible polymer backbones, which subsequently resulted in higher proton conductivity, but with the drawback of having lower thermal stability. The water uptake characteristics in the ionomers bearing sulfobenzoyl side chains was efficiently controlled by incorporating non-sulfonated comonomers

Copoly(arylene ether nitrile) and Copoly(arylene ether sulfone) lonomers with Pendant Sulfobenzoyl Groups for Proton Conducting Fuel Cell Membranes

Three series of fully aromatic ionomers with naphthalene moieties and pendant sulfobenzoyl side chains were prepared via K2CO3 mediated nucleophilic aromatic substitution reactions. The first series consisted of poly(arylene ether)s prepared by polycondensations of 2,6-difluoro-2'-sulfobenzophenone (DFSBP) and 2,6-dihydroxynaphthalene or 2,7-dihydroxynaphthalene (2,7-DHN). In the second series, copoly(arylene ether nitrile)s with different ion-exchange capacities (IECs) were prepared by polycondensations of DFSBP, 2,6-difluorobenzonitrile (DFBN), and 2,7-DHN. In the third series, bis(4-fluorophenyl)sulfone was used instead of DFBN to prepare copoly(arylene ether sulfone)s. Thus, all the ionomers had sulfonic acid units placed in stable positions close to the electron withdrawing ketone link of the side chains. Mechanically strong proton-exchange membranes with IECs between 1.1 and 2.3 meq g(-1) were cast from dimethylsulfoxide solutions. High thermal stability was indicted by high degradation temperatures between 266 and 287 degrees C (1 degrees C min(-1) under air) and high glass transition temperatures between 245 and 306 degrees C, depending on the IEC. The copolymer membranes reached proton conductivities of 0.3 S cm(-1) under fully humidified conditions. At IECs above similar to 1.6 meg g(-1), the copolymer membranes reached higher proton conductivities than Nafion (R) in the range between -20 and 120 degrees C. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 49: 734-745, 201

Locating sulfonic acid groups on various side chains to poly(arylene ether sulfone)s: effects on the ionic clustering and properties of proton-exchange membranes

Poly(arylene ether sulfone)s carrying various aromatic mono-, di-, and trisulfonated side´chains have been investigated with respect to their nanoscale structure and key membrane properties. Sulfobenzoyl, sulfonaphthoxybenzoyl, disulfonaphthoxybenzoyl, and trisulfopyrenoxybenzoyl side chains were attached to the poly(arylene ether sulfone) main chain by employing different combinations of metallation and nucleophilic aromatic substitution reactions. The nature of the sulfonated side chains was found to either promote or suppress the formation of ionic clusters, in relation to the ionic clustering occurring in corresponding polymers carrying sulfonic acid groups directly on the main chain. Analysis by small angle X-ray scattering (SAXS) of solvent cast membranes showed that the ionic clustering was promoted by placing the sulfonic acid groups on relatively long sulfonated naphthoxybenzoyl or pyrenoxybenzoyl side chains. This resulted in SAXS profiles that indicated larger characteristic separation lengths and narrower ionomer peaks, as compared with corresponding main-chain sulfonated polymers. On the other hand, the ionic clustering was almost completely suppressed in membranes based on polymers functionalized with short 2-sulfobenzoyl side chains. Proton conductivity measurements at low or moderate water contents showed a trend of increasing conductivities with the length and the sulfonic acid functionality of the side chain. The structure of the side chain also influenced the thermal stability and glass transition temperature of the membranes

Influence of the polymer backbone structure on the properties of aromatic ionomers with pendant sulfobenzoyl side chains for use as proton-exchange membranes

Six different ionomers having various aromatic polymer backbones with pendant 2-sulfobenzoyl side chains were prepared by nucleophilic aromatic substitution reactions of lithium 2,6-difluoro-2′-sulfobenzophenone with 4,4-biphenol, 2,7-dihydroxynaphthalene, 4,4-isopropylidenediphenol, 4,4-dihydroxydiphenyl ether, 4,4′-thiodiphenol, and 4,4′-thiobisbenzenethiol, respectively, to produce four poly(arylene ether)s, one poly(arylene ether sulfide), and one poly(arylene sulfide). Mechanically tough proton-exchange membranes with ion-exchange capacities in the narrow range from 1.9 to 2.3 mequiv/g were cast from the high-molecular-weight ionomers, and subsequently investigated with respect to their structure−property relationships. Glass transitions were only detected for ionomers in the sodium salt form, and increasing glass-transition temperatures (Tg) were found to give higher thermal decomposition temperatures. Analysis by small-angle X-ray scattering indicated that the ionic clustering was promoted for ionomers with flexible polymer backbones and low Tg values. The proton conductivity of the membranes at 80 °C under fully humidified conditions was found between 0.02 and 0.2 S/cm and appeared to depend primarily on the Tg

Facile synthesis and polymerization of 2,6-difluoro-2’-sulfobenzophenone for aromatic proton conducting ionomers with pendant sulfobenzoyl groups

The lithium salt of 2,6-difluoro-2′-sulfobenzophenone was conveniently synthesized in one-pot by reacting 2,6-difluorophenyllithium with 2-sulfobenzoic acid cyclic anhydride in THF at −70 °C whereafter the product crystallized out of solution. A poly(arylene ether) and a poly(arylene sulfide) were prepared by polycondensation reactions to demonstrate the reactivity and efficacy of this new monomer to produce sulfonated high-molecular weight aromatic polymers for fuel cell proton-exchange membranes. This work demonstrated that organolithium chemistry may offer versatile and straightforward pathways to new functional monomers with fluorine atoms activated for nucleophilic aromatic substitution reactions

Sulfonated poly(arylene ether sulfone) ionomers containing di- and tetrasulfonated arylene sulfone segments

Poly(arylene ether sulfone) (PSU) ionomers containing disulfonated aryl-SO2-aryl and tetrasulfonated aryl-SO2-aryl-aryl-SO2-aryl segments, respectively, were synthesized and studied to establish their structure–property relationships as proton-exchange membranes. High molecular weight PSUs with different distributions of sulfone bridges in the backbone were prepared by nucleophilic aromatic substitution reactions involving 4,4′-dichlorodiphenyl sulfone (DCDPS), 4,4′-bis[(4-chlorophenyl)sulfonyl]-1,1′-biphenyl (BCPSB), 4,4′-isopropylidenediphenol (bisphenol A), and 4,4′-(1,4-phenylenediisopropylidene)bisphenol (bisphenol P). The polymers were sulfonated via metallation and reaction with sulfur dioxide, followed by oxidation of the resulting sulfinates. This procedure allowed the introduction of two sulfonic acid units on electron-deficient aryl rings in ortho positions to each sulfone bridge of the PSUs. Analysis by small angle X-ray scattering of solvent cast membranes showed that ionic clustering was promoted in ionomers containing sulfonated BCPSB residues and flexible bisphenol P residues. The fully sulfonated PSUs had ion-exchange capacities (IECs) of 3.3–4.1 meq g−1 and were water soluble. However, partly sulfonated polymers with IECs of approx. 1.7 meq g−1 showed high proton conductivity at moderate water uptake and decomposed only above 240 °C during heating 1 °C min−1 under air. This work demonstrated that BCPSB residues can be conveniently and fully tetrasulfonated, which opens possibilities to prepare various aromatic copolymers and membranes with locally very high densities of hydrolytically stable sulfonic acid groups