Nanophase-separated Morphology Research Articles

Synthesis and properties of hexafluoroisopropylidene-containing sulfonated poly(arylene thioether phosphine oxide)s for proton exchange membranes

A series of novel sulfonated poly(arylene thioether phosphine oxide)s with hexafluoroisopropylidene moieties (sPTPOF) were prepared by polycondensation of sulfonated bis(4-fluorophenyl)phenyl phosphine oxide and bis(4-fluorophenyl)phenyl phosphine oxide with 4,4′-(hexafluoroisopropylidene) diphenthiol. The incorporation of hexafluoroisopropylidene moieties to the resulting polymers is effective to increase the hydrophobicity of non-sulfonated segments and to decrease the swelling while maintaining high proton conductivity. For instance, sPTPOF-100 showed a proton conductivity of 0.090 S/cm as well as a swelling of 5.3% at 80 °C. In addition, the sPTPOF polymers exhibited excellent thermal properties and oxidative stability. AFM phase images illustrated that the sPTPOF membranes show a special nanophase-separated morphology, namely, the connectivity of ionic channels increased obviously but their width only slightly increased with increasing sulfonation degree. This special microstructure is favorable for promoting proton transport and restraining the swelling. The sPTPOF polymers are a promising material for proton exchange membranes.

Modeling the Effect of Annealing and Regioregularity on Electron and Hole Transport Characteristics of Bulk Heterojunction Organic Photovoltaic Devices

AbstractIn this study, the dark current-voltage characteristics of electron-only and hole-only poly(3-hexyl thiophene) (P3HT):[6,6]-phenyl C61-butyric acid methyl ester (PCBM) as a function of regioregularity (RR) and annealing time is investigated using the mobility edge (ME) model. This model is used to analyze the degradation of electron and hole mobilities as a function of annealing time for 93%-RR and 98%-RR P3HT:PCBM devices. The hole mobility is almost unchanged by the RR nature of P3HT and thermal annealing. The electron mobility, however, behaves differently after annealing. The electron mobility of 98%-RR devices, which is initially higher than that of the 93%-RR devices, experiences a steep decline with annealing. Based on ME analysis, this is due to an increase in trap states in the exponential tail caused by phase segregation of solid state blends of 98%-RR polymer and PCBM. The electron mobility of 93%-RR devices increases with annealing due to an optimization of nano-phase separated morphology.

Synthesis and characterization of multiblock copolymers based on hydrophilic disulfonated poly(arylene ether sulfone) and hydrophobic partially fluorinated poly(arylene ether ketone) for fuel cell applications

AbstractSulfonated fluorinated multiblock copolymers based on high performance polymers were synthesized and evaluated for use as proton exchange membranes (PEMs). The multiblock copolymers consist of fully disulfonated poly(arylene ether sulfone) and partially fluorinated poly(arylene ether ketone) as hydrophilic and hydrophobic segments, respectively. Synthesis of the multiblock copolymers was achieved by a condensation coupling reaction between controlled molecular weight hydrophilic and hydrophobic oligomers. The coupling reaction could be conducted at relatively low temperatures (e.g., 105 °C) by utilizing highly reactive hexafluorobenzene (HFB) as a linkage group. The low coupling reaction temperature could prevent a possible trans‐etherification, which can randomize the hydrophilic‐hydrophobic sequences. Tough ductile membranes were prepared by solution casting and their membrane properties were evaluated. With similar ion exchange capacities (IECs), proton conductivity and water uptake were strongly influenced by the hydrophilic and hydrophobic block sequence lengths. Conductivity and water uptake increased with increasing block length by developing nanophase separated morphologies. Atomic force microscopy (AFM) and transmission electron microscopy (TEM) experiments revealed that the connectivity of the hydrophilic segments was enhanced by increasing the block length. The systematic synthesis and characterization of the copolymers are reported. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 214–222, 2010

Photocrosslinkable Polythiophenes for Efficient, Thermally Stable, Organic Photovoltaics

AbstractPhotocrosslinkable bromine‐functionalized poly(3‐hexylthiophene) (P3HT‐Br) copolymers designed for application in solution‐processed organic photovoltaics are prepared by copolymerization of 2‐bromo‐3‐(6‐bromohexyl) thiophene and 2‐bromo‐3‐hexylthiophene. The monomer ratio is carefully controlled to achieve a UV photocrosslinkable layer while retaining the π–π stacking feature of the conjugated polymers. The new materials are used as electron donors in both bulk heterojunction (BHJ) and bilayer type photovoltaic devices. Unlike devices prepared from either P3HT:PCBM blend or P3HT‐Br:PCBM blend without UV treatment, photocrosslinked P3HT‐Br:PCBM devices are stable even when annealed for two days at the elevated temperature of 150 °C as the nanophase separated morphology of the bulk heterojunction is stabilized as confirmed by optical microscopy and grazing incidence wide angle X‐ray scattering (GIWAXS). When applied to solution‐processed bilayer devices, the photocrosslinkable materials show high power conversion efficiencies (∼2%) and excellent thermal stability (3 days at 150 °C). Such performance, one of the highest obtained for a bilayer device fabricated by solution processing, is achieved as crosslinking does not disturb the π–π stacking of the polymer as confirmed by GIWAXS measurements. These novel photocrosslinkable materials provide ready access to efficient bilayer devices thus enabling the fundamental study of photophysical characteristics, charge generation, and transport across a well‐defined interface.

Nanostructured Proton Conducting Polystyrene−Poly(vinylphosphonic acid) Block Copolymers Prepared via Sequential Anionic Polymerizations

Poly(styrene-b-vinylphosphonic acid) diblock copolymers have been prepared via sequential anionic polymerization and evaluated as nanostructured polymer electrolytes. The ionic block copolymers were synthesized by first initiating the polymerization of styrene using n-butyllithium in tetrahydrofuran at −78 °C. 1,1-Diphenylethylene was then added to the living polystyryl anions before charging diethyl vinylphosphonate to polymerize the second block. The poly(diethyl phosphonate) block was subsequently completely hydrolyzed to obtain the poly(vinylphosphonic acid) block. Analysis by calorimetry showed two distinct glass transitions of the acidic copolymers, indicating phase separation between the two blocks. The glass transition temperature of the densely phosphonated blocks was strongly influenced by the formation of anhydride links through reversible self-condensation reactions at elevated temperatures. Studies of thin copolymer films by tapping mode atomic force microscopy revealed nanophase-separated morphologies with continuous phosphonated domains. In addition, the acidic block copolymers were found to self-assemble into spherical micellar nanoparticles which, in turn, formed branched arrays of supramolecular “necklace-like” chain structures. Block copolymers equilibrated at 25 °C and 98% relative humidity reached proton conductivities in the order of 30 mS/cm at 130 °C. (Less)

Synthesis and characterization of itaconic anhydride and stearyl methacrylate copolymers

The free-radical copolymerization and the properties of comb-like copolymers derived from renewable resources, itaconic anhydride (ITA) and stearyl methacrylate (SM), are described. The ITA–SM copolymers were nearly random with a slight alternating tendency. The copolymers exhibited a nanophase-separated morphology, with the stearate side-chains forming a bilayer, semi-crystalline structure. The melting point (Tm) of the side-chains and the crystallinity decreased with increasing ITA concentration. The crystalline side-chains suppressed molecular motion of the main chain, so that a glass transition temperature (Tg) was not resolved unless the ITA concentration was sufficiently high so that Tg>Tm. The softening point and modulus of the copolymers increased with the increasing ITA concentration, but the thermal stability decreased.

Bipolar Copolymers as Host for Electroluminescent Devices: Effects of Molecular Structure on Film Morphology and Device Performance

Bipolar transport polymers have been developed as host materials for electroluminescent devices by incorporating both electron-transporting and hole-transporting functionalities into copolymers. Two different copolymers having the same molecular weight (Mn ∼ 30 kg/mol) and the same fraction of electron-transporting monomers (fOXA = 0.50) have been synthesized in the form of random and diblock copolymers, respectively. The effect of molecular structure and film morphology of these bipolar polymers on device performance has been studied. For the diblock copolymers, pronounced phase segregation forming different nanomorphologies has been observed by modern microscopic techniques, which is not observed for the random counterparts under the same thin film preparation conditions. The results of single-layer polymer light emitting diodes (PLEDs) show that the nanophase separation morphology of diblock copolymers has a significant effect on device performance: lowering charge transport and facilitating the hole−...

Synthesis and Nanophase-Separated Morphology of a Diblock Copolymer of Polypentafluorostyrene-b-Poly(methyl methacrylate) by Atom Transfer Radical Polymerization

We synthesized a symmetric, fluorine-containing diblock copolymer of polypentafluorostyrene-b-poly(methyl methacrylate) (PPFS-b-PMMA) by atom transfer radical polymerization (ATRP). The PPFS macroinitiator was also synthesized by ATRP by using 1-phenylethyl bromide initiator. The chemical structure of the polymer was confirmed by GPC, 1H-NMR and DSC measurements. Due to the chemical dissimilarity between the PPFS and PMMA blocks, the microphase-separation was occurred during annealing at high temperature, displaying a lamellar morphology with a thickness of approximately 35 nm.

New Nanophase Separated Intelligent Amphiphilic Conetworks and Gels

AbstractAmphiphilic conetworks (APCN) are new materials composed of covalently bonded otherwise immiscible hydrophilic and hydrophobic polymer chains. The amphiphilic nature of these new crosslinked polymers is indicated by their swelling ability in both hydrophilic and hydrophobic solvents. Special synthetic techniques have been developed for the preparation of these new unique materials, such as poly(2‐hydroxyethyl methacrylate)‐l‐polyisobutylene (PHEMA‐l‐PIB), poly(methacrylic acid)‐l‐polyisobutylene (PMAA‐l‐PIB) and poly(N,N‐dimethylaminoethyl methacrylate)‐l‐polyisobutylene (PDMAEMA‐l‐PIB) (‐l‐ stands for linked by). Due to their unique architecture, macrophase separation of the immiscible components is prevented by the chemical bonding in the conetworks. As a results, phase separation leads to nanodomains with usually 2‐20 nm domain sizes as shown by AFM measurements. The nanophase separated morphology may also lead to smart temperature responsive gels with high mechanical stability, such as in the case of poly(N,N‐dimethylaminoethyl methacrylate)‐l‐polyisobutylene APCNs as discovered during these studies. In another approach, poly(2‐hydroxyethyl methacrylate)‐l‐polyisobutylene and poly(methacrylic acid)‐l‐polyisobutylene APCNs were prepared by a special two‐step process. The new PMAA‐l‐PIB polyelectrolyte APCNs possess smart (intelligent) reversible pH‐responsive properties in aqueous media. These unique conetwork structures and properties of these new emerging materials may lead to numerous new potential applications, such as smart materialk products, sustained drug release matrices, biomaterials, nanohybrids, nanotemplates etc.

Surface morphology control of immiscible polymer-blend thin films

The effects of the molecular weights (molecular weight of polystyrene, M w,PS, varying from 2.9 to 129 k) on the surface morphologies of spin-coated and annealed polystyrene/poly (methyl methacrylate) (PS/PMMA=50/50, w/w) blend films were investigated by atomic force microscopy and X-ray photoelectron spectroscopy. For the spin-coated films, when the M w,PS varied from 2.9 to 129 k, three different kinds of surface morphologies (a nanophase-separated morphology, a PMMA cellular or network-like morphology whose meshes filled with PS, a sea-island like morphology) were observed and their formation mechanisms are discussed, respectively. Upon annealing, two different morphology-evolution processes were observed. It is found that a upper PS-rich phase layer is formed when M w, PS <4 k, and this behavior is mainly attributed to the low interfacial tension between PS and PMMA component. When M w, PS >4 k, the PS-rich phase forms droplets on top of the PMMA-rich phase layer which wets the SiO x substrate. These results indicate that the surface morphology of the polymer blend films can be controlled by the polymer molecular weight and annealing conditions.

Nanophase-Separated Blends of Acceptor and Donor Conjugated Polymers. Efficient Electroluminescence from Binary Polyquinoline/Poly(2-methoxy-5-(2‘-ethylhexyloxy)-1,4-phenylenevinylene) and Polyquinoline/Poly(3-octylthiophene) Blends

Binary blends of the acceptor conjugated polymer poly(2,2‘-(3,3‘-dioctyl-2,2‘-bithienylene)-6,6‘-bis(4-phenylenequinoline)) (POBTPQ) with donor conjugated polymer poly(2-methoxy-5-(2‘-ethylhexyloxy)-1,4-phenylenevinylene) (MEH−PPV) or poly(3-octylthiophene) (POT) were nanophase-separated and observed to exhibit efficient electroluminescence in light-emitting-diodes. The 90−120 nm phase-separated morphology of MEH−PPV-containing blends was characteristic of dimixing by spinodal decomposition whereas that of POT-containing blends consisted of nucleation and growth type spherical domains (50−400 nm) dispersed in a matrix. Efficient Förster energy transfer was observed in both blend systems. Voltage-tunable orange-red ↔ yellow ↔ green electroluminescence was observed in the POBTPQ:MEH−PPV blend diodes at a composition of 10−80 wt % MEH−PPV. Only red emission characteristic of POT was observed from POBTPQ:POT blend devices due to efficient energy transfer from POBTPQ. Large enhancements in performance of the bipolar blend light-emitting diodes were observed compared to that of the homopolymer diodes. The compositional dependence of luminance and external quantum efficiency of the blend devices was very different for the MEH−PPV and POT blends, reflecting the difference in morphology. Electric-field-induced photoluminescence quenching confirmed the bipolar charge transport in the blends and associated improved electron−hole recombination and device efficiencies. These results demonstrate that efficient electroluminescence can be achieved in bipolar blends of conjugated polymers and that nanophase-separated morphology is essential to voltage-tunable multicolor light emission in blend devices.

Studies of the aging of nafion®/silicate nanocomposites using 29Si solid state NMR spectroscopy

AbstractSol‐gel reactions of tetraethylorthosilicate were conducted within the nanophase‐separated morphology of Nafion® films to yield Nafion®/silicate nanocomposites. The chemical aging of silicate phase molecular structure when the nanocomposites were exposed to (1) ambient air, (2) applied heat, (3) liquid water, (4) methanol reflux, and (5) heated sulfuric acid was explored by means of 29Si solid state NMR spectroscopy. In particular, the characteristic chemical shifts and relative Intensities of peaks that identify Qn = (HO)4‐n Si(OSi)n molecular subunits in silicate structures were determined for samples exposed to these aging conditions. Shifts in the Q peak distribution reflected the evolution of degree of Si atom coordination about SiO4 units. The coordination states are always Q4 and Q3 although a small Q2 population is occasionally present. Thus, a significant degree of silicate phase coordination can develop despite constraints posed by the Nafion® medium. Percent Q4 increases in all cases but then reaches a maximum after which it decreases, the greatest de‐polymerization being for samples aged in water. The greatest increase in Q4 relative to the un‐aged control occurred for samples heated at 100°C. Exposure to ambient air produced changes in Q4 that were small, but not insignificant. While the initial increase in Si atom substitution around SiO4 units can be rationalized in terms of gradual condensation reactions between residual SiOH groups, accompanied by the liberation of water molecules and their removal from the site of the reaction, the eventual decrease in coordination is not currently understood.

Organic-inorganic hybrid materials: perfluorinated ionomers as sol-gel polymerization templates for inorganic alkoxides

Mauritz et al. exploited the polar/nonpolar nanophase-separated morphologies of Nafion® perfluorinated ionomer membranes, as well as a Nafion® ionomeric precursor film, as interactive templates that are capable of directing the condensation polymerizations of inorganic alkoxides and organoalkoxysilanes as well as the ultimate geometrical distribution of the inorganic oxide, or organically modified silicon oxide phases that result upon drying. This paper is a review of our extensive studies involving the in situ sol-gel reactions of the alkoxides of silicon, titanium, aluminum, zirconium and organoalkoxysilanes, as well as their mixtures and two step reactions involving these monomers. Throughout this presentation, we demonstrate how various spectroscopic (FTIR, 29Si solid state NMR, dielectric relaxation, pyrene (Py) fluorescence probe emission + UV absorption) microscopic, X-ray scattering, thermal (DSC, TGA, TGA-FTIR, DMA), mechanical tensile, and gas permeation tools were employed to interrogate the structures and properties of these heterogeneous materials over a range of dimensional scales ranging from the molecular to the macroscopic. Importantly, we established that these organic/inorganic materials are indeed structured on the scale of nanometers. Methods of tailoring the energetic environment, or polarity, within the cluster domains by the insertion of inorganic oxide or organically modified silicate nanostructures are presented. Finally, we discuss the potential for these nanocomposite membranes within a number of areas including gas and liquid separations technology as well as proton exchange membranes for fuel cell technology.

Novel Nafion�/ORMOSIL hybrids via in situ sol-gel reactions: 2. Probe of ORMOSIL phase nanostructure by 29Si solid state NMR spectroscopy

Perfluorosulfonic Acid (PFSA)]/[Organically-Modified Silicon Oxide (ORMOSIL)] hybrids were formulated via sol-gel reactions of mixtures of tetraethoxysilane (TEOS) and diethoxydimethylsilane (DEDMS) in the nanophase-separated morphology of a PFSA membrane (Nafion®). The molecular structures of the ORMOSIL phases were investigated by 29Si solid state NMR spectroscopy and the spectra compared with those of corresponding bulk ORMOSILs. PFSA-in situ ORMOSIL composition can be tailored by manipulating TEOS/DEDMS source solution composition and quantified by 29Si solid state NMR spectroscopy. Copolymerization of TEOS and DEDMS monomers, rather than block formation, occurs within the PFSA as well as in the bulk. As DEDMS feed concentration increases, ORMOSIL nanostructures are more hydrophobic and more flexible. Dimethylsiloxane rings are dominant in PFSAs in which hydrolysis + condensation of pure DEDMS occurs.

Structure and morphology of sol-gel prepared polymer-ceramic composite thin films

Poly( p-phenylene vinylene) (PPV)-silica composite thin films were prepared via the sol-gel process incorporating a water-soluble PPV precursor. After solvent evaporation, the spin-coated or quiescently cast films were cured and the PPV precursor and the silica sol were completely converted into the PPV-silica composites. Quiescently cast films show an improved wave-guiding behaviour with a χ 3 value only slightly smaller than pure PPV films. Their structure consists of a nanophase-separated morphology composed of amorphous silica and paracrystalline PPV. Small-angle X-ray scattering and bright field transmission electron microscopy images indicate a random arrangement of the nanoscale domains with average domain size of approximately 4 nm. In contrast, spin-coated films show clusters and cluster-cluster aggregates approximately 100 nm in diameter. The clusters consist of particulate silica and pores, each about 3 nm in size, with the PPV contained in the pores and between the clusters. The observed morphologies are related to film preparation conditions in terms of sol-gel processing, taking the presence of the polymer into account.

Flagellenes: nanophase-separated, polymer-substituted fullerenes

This paper describes the synthesis and characterization of fusible and soluble polystyrene (PS) adducts of the spheroidal C{sub 60} fullerene and named flagellenes, C{sub 60}(PS){sub x`}, where x is variable (x {ge} 10). These materials spontaneously partition the fullerene moiety into domains in a nanophase-separated glass. Gel permeation chromatography and NMR confirm that mixtures of C{sub 60}(PS){sub x} adducts can be synthesized by adding living PS carbanions (number average molar mass = 2000) to C{sub 60}. Transmission electron microscopy shows nanophase-separated morphologies with fullerene-rich domains. The flagellenes could facilitate the processing and fabrication of fullerene-based materials and thereby contribute to the realization of the electronic and optical properties of C{sub 60} in technological applications.