Showing papers in "Macromolecules in 2009"

Polymeric Modification of Graphene through Esterification of Graphite Oxide and Poly(vinyl alcohol)

Abstract: Soluble graphene covalently functionalized with poly(vinyl alcohol) (PVA) has been synthesized by simple esterification reaction of carboxylic groups in graphite oxide using two different synthetic strategies. Graphene has emerged to be a promising material due to its novel electrical transport properties. However, the use of graphene is limited by the lack of an effective method for large-scale production. Several chemical methods have been explored to obtain soluble graphene including the reduction of graphite oxide (GO) in a stabilization medium and covalent modification by amidation of the carboxylic groups, nucleophilic substitution to epoxy groups, and the diazonuim salts coupling. These modified graphenes provide opportunities for researchers to employ them in designing new materials, such as polymer nanocomposites. In such materials, controlling the interfacial interaction between the filler and the polymer is crucial to control the properties. Although graphene-based polymeric nanocomposites have been developed, polymer chains have not been used to covalently modify graphene. Nevertheless, polymer-bounded carbon nanotubes (CNT) and fullerene (C60) 8 have been widely studied. PVA-functionalized CNTs have been obtained by esterification reaction and used as fillers in nanocomposites. In that sense, the presence of COOH in the edge planes of GO can be exploited to anchorage PVA by using a similar way. In this Communication, we report the covalent functionalization of graphene sheets with PVA (Scheme 1). We employed two synthetic strategies. The first one involves the direct esterification of GO, while the second one goes through its acyl chloride derivative (GOCl) (Supporting Information). The obtained products, named as GO-es-PVA and GOCl-es-PVA, are soluble in DMSO and water with the aid of heat, similar to PVA and PVACNT. In the present research a special interest has been paid to the isolation and characterization of the PVA-functionalizedGO. First, an appropriate elimination of the no reacted GO was achieved by centrifugation, which completely removes the GO and leaves the PVA-functionalized GO (see Supporting Information). A comprehensive spectroscopic analysis was carried out to ensure that the esterification reactions were successfully completed. The HNMR spectrum of the soluble GO-es-PVA andGOCles-PVA in DMSO-d6 are compared with that of neat PVA (Figure S2). The latter exhibits the polymer backbone signals at ∼3.82 ppm (methine) and ∼1.38 ppm (methylene) and the hydroxyl signals at 5-4 ppm, from which the stereoregularity of PVA is estimated as isotactic (mm):hetero (mr): syndiotactic (rr) of roughly 2:5:3. Upon the attachment to GO the PVA proton signals become a littlewider butmaintain similar chemical shifts (Figure S2). The signals are less broadened than in PVACNT suggesting that, in principle, a lower degree of functionalization was achieved in our case. It is interesting to pay special attention to the hydroxyl protons resonances which are resolvable in terms of configurational sequences (Figure 1). Several authors have concluded that the hydrogen-bonding tendency, which is dominant for hydroxyl units in a meso configuration, predominantly determines the hydroxyl proton shielding. Since hydrogen bonding leads to downfield shifts, the chemical shifts of hydroxyl sequences resonances increase when passing from iso (4.7 ppm) to hetero (4.5 ppm) and fromhetero to syndio (4.2 ppm) triad.On the other hand, it has been demonstrated that the reactivity of substituent groups along the polymer chain can be influenced by tactic sequence where the substituent atmm sequence exhibited higher reactivity than those at the rr counterpart. By a mere inspection of the evolution of the H spectrum, we can easily observe an increase in the rr signal in detriment of the mm one for both GO-es-PVA and GOCl-es-PVA which suggests that esterification reaction occur at isotactic configuration. Surprisingly, as shown in Figure 1, a new signal at 4.2 ppm upfield, very closed to the rr triads of unmodified PVA, is clearly observed in spite of the low degree of esterification. This signal can be related to hydroxyl protons next to acetate groups as have been reported for esterification of PVA. By integration of this H NMR signal we can evaluate, within the experimental uncertainties, the degree of functionalization, obtaining a modification of around 1.8%. Interestingly, this value is reasonably low due to the huge volume of graphitic laminates (GLs) and is in agreement with the decrease ofmm triad content, as consequence of being less sterically hindered internally than those at syndiotactic counterpart, specifically in the incorporation of the GLs. The FTIR spectra of GO-es-PVA and GOCl-es-PVA retained most of the bands of PVA, although some of them changed in intensity or even disappeared due to the modification, and show new bands (Figure 1). The development of the band at 1715 cm suggests the presence of new carbonyl species. This band is most often related to the CdO stretching motions of COOHgroups situated at the edges of theGO lamellae and has low intensity due to high aspect ratio of GO which makes the relative amount of edge sites very small. Similarly, in our case the band at 1715 cm can be attributed to CdO stretching of ester groups. Indeed, this band was also observed in the esterification reaction of PVA and glycerol, suggesting that esterification is also taking place in our system. This is confirmed by the marked increase in the band around 1640 cm, which appears weakly in PVA and has been assigned to adsorbedwater. However, this is a strong band centered at 1628 cm in the spectrum of the GO (Figure S1), and although may also be due to adsorbed water, it contains a significant contribution from the skeletal vibration of nonoxidized graphitic domains. In addition, some interesting changes in the relative intensities of the characteristics bands of PVA in the 1200-1000 cm region can be observed. These bands are attributed to the C-O of doubly H-bonded OH in crystalline regions (1144 cm) and C-O unbonded in amorphous zones (1096 cm). The intensity ratio of these bands (I1144/I1096) diminishes markedly for the esterified products, suggesting a large decrease in the degree of crystallinity of the modified polymer. Raman data further confirm the presence of GLs covalently bonded to PVA chains (Figure S3). The Raman spectra for *Corresponding author: Fax þ34 915 644 853; Tel þ34 915 622 900; e-mail horacio@ictp.csic.es. D ow nl oa de d by C SI C M L O R A T A M A Y O Q U IM O R G o n Se pt em be r 1, 2 00 9 | h ttp :// pu bs .a cs .o rg

Bandgap and Molecular Level Control of the Low-Bandgap Polymers Based on 3,6-Dithiophen-2-yl-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione toward Highly Efficient Polymer Solar Cells

Abstract: A series of low-bandgap polymers based on a soluble chromophore of 3,6-dithiophen-2-yl-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione (DPP) unit were synthesized by introducing of different electron-rich building blocks copolymerized with DPP unit. Four new DPP-based polymers, PDPP-DTS, PDPP-F, PDPP-BDT, and PDPP-BDP, were characterized by GPC, TGA, NMR, UV−vis absorption, and electrochemical cyclic voltammetry. The results indicate that their bandgaps as well as their molecular energy levels are readily tuned by copolymerizing with different conjugated electron-donating units. In order to investigate their photovoltaic properties, polymer solar cell (PSC) devices based on PDPP-DTS, PDPP-F, PDPP-BDT, and PDPP-BDP were fabricated with a structure of ITO/PEDOT:PSS/polymers:PC70BM(1:2,w/w)/Ca/Al under the illumination of AM 1.5G, 100 mW/cm2. The power conversion efficiencies (PCE) of the four DPP-based PSC devices were measured and shown in this paper. The best performance of the PSC device was obtained by using...

Design of Antibacterial Surfaces and Interfaces: Polyelectrolyte Multilayers as a Multifunctional Platform

Abstract: The adhesion and proliferation of bacteria on abiotic surfaces pose challenges related to human infection, including subsequent formation of antibiotic-resistant biofilms in both healthcare and industrial applications. Although the design of antibacterial materials is a longstanding effort, the surface properties that modulate adhesion of viable bacteria—the critical first step in biofilm formation—have been difficult to decouple. This partial and limited success is due chiefly to two factors. First, bacteria cells exhibit multiple, complex adhesion mechanisms that vary with bacteria strain, rapid genetic mutations within a given strain, and mutable environmental stimuli such as nutrient levels and fluid velocities. Second, there exist only a limited number of studies that systematically characterize or vary the physical, chemical, and mechanical properties of potential antimicrobial materials. Here, we briefly review the dominant strategies for antimicrobial material surface design, including the advanta...

Thermally Self-Healing Polymeric Materials : The Next Step to Recycling Thermoset Polymers?

Abstract: We developed thermally self-healing polymeric materials on the basis of furan-functionalized, alternating thermosetting polyketones (PK-furan) and bis-maleimide by using the Diels−Alder (DA) and Re...

Cutinase-Catalyzed Hydrolysis of Poly(ethylene terephthalate)

Abstract: A detailed study and comparison was made on the catalytic activities of cutinases from Humilica insolens (HiC), Pseudomonas mendocina (PmC), and Fusarium solani (FsC) using low-crystallinity (lc) and biaxially oriented (bo) poly(ethylene terephthalate) (PET) films as model substrates. Cutinase activity for PET hydrolysis was assayed using a pH-stat to measure NaOH consumption versus time, where initial activity was expressed as units of micromoles of NaOH added per hour and per milliliter of reaction volume. HiC was found to have good thermostability with maximum initial activity from 70 to 80 °C, whereas PmC and FsC performed best at 50 °C. Assays by pH-stat showed that the cutinases had about 10-fold higher activity for the lcPET (7% crystallinity) than for the boPET (35% crystallinity). Under optimal reaction conditions, initial activities of cutinases were successfully fit by a heterogeneous kinetic model. The hydrolysis rate constant k2 was 7-fold higher for HiC at 70 °C (0.62 μmol/cm2/h) relative to...

Synthesis and Characterization of Poly(phenylene)-Based Anion Exchange Membranes for Alkaline Fuel Cells

Abstract: Cationic polymer membranes that conduct free anions comprise an enabling area of research for alkaline membrane fuel cells and other solid-state electrochemical devices that operate at high pH. The synthesis of anion exchange membranes based on a poly(phenylene) backbone prepared by a Diels−Alder reaction is demonstrated. The poly(phenylene)s have benzylic methyl groups that are converted to bromomethyl groups by a radical reaction. Cationic polymers result from conversion of the bromomethyl groups to ionic moieties by quaternization with trimethylamine in the solid state. The conversion to benzyltrimethylammonium groups is incomplete as evidenced by the differences between the IEC values measured by titration and the theoretical IECs based on 1H NMR measurements. The anion exchange membranes formed from these polymers have hydroxide ion conductivities as high as 50 mS/cm in liquid water, and they are stable under highly basic conditions at elevated temperatures.

Block Copolymers for Organic Optoelectronics

Abstract: While polymers hold significant potential as low cost, mechanically flexible, lightweight large area photovoltaics and light emitting devices (OLEDs), their performance relies crucially on understanding and controlling the morphology on the nanometer scale. The ca. 10 nm length scale of exciton diffusion sets the patterning length scale necessary to affect charge separation and overall efficiency in photovoltaics. Moreover, the imbalance of electron and hole mobilities in most organic materials necessitates the use of multiple components in many device architectures. These requirements for 10 nm length scale patterning in large area, solution processed devices suggest that block copolymer strategies previously employed for more classical, insulating polymer systems may be very useful in organic electronics. This Perspective seeks to describe both the synthesis and self-assembly of block copolymers for organic optoelectronics. Device characterization of these inherently complex active layers remains a sign...

Thermal Expansion of Graphene Composites

Abstract: Isolated graphene sheets were achieved by graphite intercalation and charge-induced exfoliation. The resultant graphene oxide sheets were incorporated into polymer composites and thermal expansion was investigated by a thermo-mechanical analyzer. The test results indicated that inclusion of graphene into composites resulted in low coefficients of thermal expansion (CTEs), and increasing graphene fraction reduced CTEs more significantly. The 5 wt % graphene oxide-based composite shows 31.7% reduction below the glass transition temperature. Preliminary measurement of thermal conductivity also indicated that graphene composites significantly improved the thermal conductivity of polymer matrix. Thermal conductivity of 5% graphene composites showed about 4-fold increment in comparison to the polymer matrix. This finding will provide a solid foundation for graphene-enabled thermal management in microelectronics.

Thiol-Yne Photopolymerizations: Novel Mechanism, Kinetics, and Step-Growth Formation of Highly Cross-Linked Networks.

Abstract: Radical-mediated thiol-yne step-growth photopolymerizations are utilized to form highly cross-linked polymer networks. This reaction mechanism is shown to be analogous to the thiol-ene photopolymerization; however, each alkyne functional group is capable of consecutive reaction with two thiol functional groups. The thiol-yne reaction involves the sequential propagation of a thiyl radical with either an alkyne or a vinyl functional group followed by chain transfer of the radical to another thiol. The rate of thiyl radical addition to the alkyne was determined to be approximately one-third of that to the vinyl. Chain-growth polymerization of alkyne and vinyl functionalities was only observed for reactions in which the alkyne was originally in excess. Analysis of initial polymerization rates demonstrated a near first-order dependence on thiol concentration, indicating that chain transfer is the rate-determining step. Further analysis revealed that the polymerization rate scaled with the initiation rate to an exponent of 0.65, deviating from classical square root dependence predicted for termination occurring exclusively by bimolecular reactions. A tetrafunctional thiol was photopolymerized with a difunctional alkyne, forming an inherently higher cross-link density than an analogous thiol-ene resin, displaying a higher glass transition temperature (48.9 vs -22.3 degrees C) and rubbery modulus (80 vs 13 MPa). Additionally, the versatile nature of this chemistry facilitates postpolymerization modification of residual reactive groups to produce materials with unique physical and chemical properties.

Functionalized Conjugated Microporous Polymers

Abstract: A range of conjugated microporous polymer networks has been prepared using Sonogashira−Hagihara cross-coupling of 1,3,5-triethynylbenzene with a number of functionalized dibromobenzenes. Porous poly(arylene ethynylene) networks with surface areas up to 900 m2/g were produced. The surface chemistry of the networks was varied by monomer selection, thus allowing control over physical properties such as hydrophobicity. Additionally, it was shown that the dye sorption behavior of the networks can be controlled by varying the hydrophobicity. This expands significantly on the utility of this approach, allowing high surface area networks to be prepared with properties that can be tailored for applications such as catalysis and separations.

Conjugated Microporous Polymer Networks via Yamamoto Polymerization

Abstract: The Yamamoto polymerization has been used for the synthesis of conjugated, microporous polymer networks. The polymerization is advantageous as only a single, halogen-functionalized monomer can be used to form polymer networks. Polymer networks are presented based on spirobifluorene and benzene units, which reveal high surface areas of up to 1275 m2/g. Furthermore, copolymerization can be carried out using additionally linear linkers during the polymerization. The nearly regular incorporation of such units into the networks enables to tailor the luminescence properties of the materials.

Effect of Molecular Weight and Salt Concentration on Conductivity of Block Copolymer Electrolytes

Abstract: The ionic conductivity, σ, of mixtures of nearly symmetric polystyrene-block-poly(ethylene oxide) copolymers and Li[N(SO2CF3)2] (LiTFSI) salt was measured as a function of molecular weight, salt concentration, and temperature The molecular weight of the poly(ethylene oxide) block, MPEO, was varied from 7 to 98 kg/mol The molar ratio of lithium to ethylene oxide, r, was varied from 002 to 010 In general, σ increases with increasing MPEO for all values of r The data can be summarized by plots of normalized conductivity, σn, versus MPEO, where σn = σ/(fϕPEOσPEO), ϕPEO is the PEO volume fraction in the copolymer, σPEO is the conductivity of PEO homopolymer, and f is a morphology-dependent factor set equal to 2/3 for our lamellar samples The temperature-dependent conductivity data at a given salt concentration collapse onto a single curve when plotted in this format At r = 0085, σn values reach a plateau in the vicinity of unity in the high MPEO limit At other values of r, σn continues to increase wi

Supramolecular Polymerizations and Main-Chain Supramolecular Polymers

Abstract: The past few decades have seen a significant growth in the field of supramolecular polymerizations, in which (reversible) noncovalent interactions (e.g., hydrogen bonding, metal−ligand coordination, π−π stacking, etc.) between (macro)monomeric units are utilized to build polymeric assemblies. These polymeric aggregates can exist in an equilibrium state between low and high molecular weight species, which in turn opens the door to a new matrix of properties. For example, such supramolecular polymers are potentially an interesting class of stimuli- or environmentally responsive, “smart” materials. Additionally, the establishment of an equilibrium during the assembly process imparts on the system the ability to “proofread” assemblies and, depending on the nature of the (macro)molecules, can allow efficient access to controlled, well-defined nanostructures. This Perspective will focus on the basic concepts of such supramolecular polymerizations, such as mechanism of assembly growth, and the effects of growth ...

Donor−Acceptor Poly(thiophene-block-perylene diimide) Copolymers: Synthesis and Solar Cell Fabrication

Abstract: Donor−acceptor diblock copolymers, composed of regioregular poly(3-hexylthiophene) (rrP3HT) as the electron donor block and poly(perylene diimide acrylate) (PPDA) as the electron acceptor block, were prepared and used to fabricate a solar cell. Vinyl-terminated rrP3HT was synthesized by Grignard metathesis polymerization and converted to an alkoxyamine macroinitiator. This polythiophene macroinitiator was used in controlled free radical polymerization of a perylene diimide-containing acrylate. The targeted donor−acceptor diblock copolymers were prepared with low-to-moderate polydispersity indices (1.2−1.4) and molecular weights sufficiently high for producing films by spin-coating. These copolymers showed efficient photoluminescence quenching in the solid state, indicative of charge separation, and were used to produce a solar cell with power conversion efficiency of 0.49%.

Efficient Synthesis of Narrowly Dispersed Brush Polymers via Living Ring-Opening Metathesis Polymerization of Macromonomers

Abstract: Various macromonomers (MMs) were efficiently synthesized through the copper-catalyzed “click” coupling of a norbornene moiety to the chain end of poly(methylacrylate), poly(t-butylacrylate), and polystyrene that were prepared using atom transfer radical polymerization. Ring-opening metathesis polymerization (ROMP) of these MMs was carried out using the highly active, fast-initiating ruthenium catalyst (H2IMes)(pyr)2(Cl)2RuCHPh in THF at room temperature. ROMP of MMs was found to be living with almost quantitative conversions (>90%) of MMs, producing brush polymers with very low polydispersity indices of 1.01−1.07 and high Mn’s of 200−2600 kDa. The efficient ROMP of such MMs provides facile access to a variety of brush polymers and overcomes previous difficulties in the controlled polymerization of MMs. Atomic force microscopy of the brush polymer products revealed extended, wormlike shapes as a result of significant steric repulsion of densely grafted side chains.

Formation Mechanism of β-Phase in PVDF/CNT Composite Prepared by the Sonication Method

Abstract: Two poly(vinylidene fluoride)(PVDF)/carbon nanotube (CNT) composites are prepared by solution sonication and mechanical mixture approaches. It is found that α-phase coexists with β-phase in the composite prepared by sonicating the PVDF/CNT mixture solution, while no β-phase can be observed in the composite prepared from the mechanical mixture route. With the help of the density functional theory calculations, it is explained that a large amount of energy is required for transforming trans−gauche−trans−gauche′ (TGTG′) into trans−trans (TT) conformations and the TT molecular chain can be bound on the CNT surface tightly. The emergence of β-phases is independent of zigzag carbon atoms on the CNT surface. The formation mechanism of β-phase is proposed based on the theoretical calculations and experimental results.

Toward Tailorable Porous Organic Polymer Networks: A High-Temperature Dynamic Polymerization Scheme Based on Aromatic Nitriles

Abstract: Triazine-based polymer scaffolds with controllable porosity were prepared through a dynamic polymerization scheme based on a broad variety of aromatic nitriles, upon polytrimerization of the latter from ZnCl2 salt melts at high temperatures. In the present study, the reaction parameters such as the temperature and monomer concentration as well as the geometry and functionality of the monomers were systematically varied. A comprehensive overview of the influences of these parameters on the outcome of the polymerization reaction in terms of chemical nature and porous properties of the materials is proposed. Rather than the variation of the monomer size and/or geometry, the reaction parameters were found to play a crucial role for tuning the porosity of the materials and especially the reaction temperature. Finally, the use of functional aromatic bridging units allowed the formation of functional polymer scaffolds with the ability to coordinate metal salts, introducing the possibility to design new metallo-o...

New Understanding in Tuning Toughness of β-Polypropylene: The Role of β-Nucleated Crystalline Morphology

Abstract: It is widely believed that the trigonal β-form is favorable and effective for toughening isotactic polypropylene (iPP). Therefore, β-form content should be achieved as high as possible to realize excellent toughness in iPP. However, in this study, we demonstrate that the connection between crystallites might mainly determine the toughness of iPP instead of the β-crystal content. A new rare earth nucleator (WBG) was used to generate the rich β-crystalline structure in the compression-molded bars that were fabricated upon different molten temperatures (Tf). Interestingly, the increase in tensile elongation can be as large as 8 times for increased Tf. The polymorphic composition and overall crystallinity of β-nucleated iPP are almost independent of Tf. Nevertheless, the β-nucleated crystalline morphology has completely changed. Three types of β-crystalline morphology, namely, β-spherulite, β-transcrystalline entity, and “flower”-like agglomerate of β-crystallites, are sequentially obtained with increasing Tf...

Synthesis of Polystyrene−Polylactide Bottlebrush Block Copolymers and Their Melt Self-Assembly into Large Domain Nanostructures

Abstract: High molecular weight polystyrene−polylactide (PS−PLA) bottlebrush block copolymers have been shown to self-assemble into highly ordered lamellae structures with domain spacings as large as 163 nm, as identified by ultrasmall-angle X-ray scattering. Bottlebrush block copolymers were synthesized by a combination of living radical and ring-opening polymerizations. The backbone was prepared by RAFT block copolymerization of solketal methacrylate (SM) and 2-(bromoisobutyryl)ethyl methacrylate (BIEM). Polystyrene branches were grafted by ATRP from poly(BIEM) block, and PLA branches were grafted from the poly(SM) block after the removal of ketal groups. The investigation into the self-assembly of PS−PLA bottlebrush block copolymers with varying lengths of branches and backbones revealed a number of unusual trends, which were attributed to their dynamic, three-dimensional structure. The results suggest that in phase-separated melts the bottlebrush block copolymer backbone, while extended, still possesses a certa...

Model Transient Networks from Strongly Hydrogen-Bonded Polymers

Abstract: Random copolymers consisting of n-butyl acrylate backbones with quadruple hydrogen-bonding side chains based on 2-ureido-4[1H]-pyrimidinone (UPy) have been synthesized via controlled radical polymerization and postpolymerization functionalization. Through this synthetic strategy high UPy monomer content (15 mol %) can be reached while maintaining low polydispersity and excellent control over molecular weight, providing model reversible networks with well-defined molecular architecture. Despite low Tgs and a lack of entanglements or crystallinity, these materials behave as thermoplastic elastomers through the strong but reversible association of UPy groups. Bulk properties such as the plateau modulus, tensile modulus, and relaxation time scale are primarily determined by the average distance between UPy’s along the chain. Starting from a difunctional initiator, triblock copolymers can also be synthesized containing a homopolymer midblock and random copolymer end blocks, effectively concentrating the hydrog...

Establishing a New Mechanical Nonlinear Coefficient Q from FT-Rheology: First Investigation of Entangled Linear and Comb Polymer Model Systems

Abstract: The nonlinear response of monodisperse linear and comb polymer melts has been investigated under oscillatory shear with Fourier-transform rheology (FT-rheology). The relative intensity of the third harmonics (I3/1), which quantify nonlinearity, was found to depend on the strain amplitude (γ0) at small and medium strain amplitudes quadratically regardless of the excitation frequency, temperature, and polymer topology. From these results, for the first time, we proposed new nonlinear coefficient Q, which is defined as Q ≡ I3/1/γ02, and we also defined zero-strain nonlinearity, Q0, as a constant value at relatively small strain amplitude (limγ0→0 Q ≡ Q0; e.g., the zero shear viscosity). In the case of the linear polymer melt, the Q value displays a constant value at small and medium strain amplitude. At large strain amplitude, we detect that the value of Q is finally reduced (Q(γ0) is decreasing). The investigated comb polymer shows very different behavior; the value of Q displays an overshoot (Q(γ0) is incr...

Effects of Layer Thickness and Annealing of PEDOT:PSS Layers in Organic Photodetectors

Abstract: We have investigated the effects of thickness variation and thermal treatment of the electrode polymer poly(3,4-ethylenedioxythiophene):poly(styrenesulfonic acid) (PEDOT:PSS) in photovoltaic and photodetector devices using conjugated polymer blends as the photoactive material. By variation of the PEDOT:PSS layer thickness between 25 and 150 nm, we found optimum device performance, in particular low dark current and high external quantum efficiency (EQE) and open-circuit voltage (Voc), at around 70 nm. This has been observed for two different active layers. Annealing studies on the PEDOT:PSS films, with temperatures varied between 120 and 400 °C, showed an optimum device performance, in particular EQE and Voc at 250 °C. This optimum performance was found to be associated with loss of water from the PSS shell of the PEDOT:PSS grains. For annealing temperatures above 260 °C, device performance was dramatically reduced. This was associated with chemical decomposition leading to loss of sulfonic acid, although...

A Quantitative Study of PCBM Diffusion during Annealing of P3HT:PCBM Blend Films

Abstract: Scanning transmission X-ray microscopy has been used to quantitatively map the composition of P3HT:PCBM blend films in the vicinity of PCBM crystals formed during annealing at 140 °C The observed PCBM concentration profiles around these crystals have been fitted to Fick’s second law of diffusion and the diffusion constant found to be 25 × 10−14 m2 s−1 The PCBM concentration at the crystal boundary was found to be 19% (v/v) and is interpreted, together with the annealing temperature of 140 °C, as a point on the bimodal line of the composition−temperature phase diagram The diffusion of PCBM through P3HT is observed to be bulk-dominated, in contrast to the surface/interface-dominated diffusion observed in MDMO-PPV:PCBM blend films by Yang et al(1, 2)

Ordered Network Mesostructures in Block Polymer Materials

Abstract: Block polymers are formed by the covalent union of two or more chemically distinct homopolymers These composite macromolecules self-assemble into a variety of ordered morphologies with features on the nanometer length scale, a phenomenon that has interested researchers for roughly four decades The known ordered morphologies include numerous multiply continuous network mesostructures, the focus of this review Multiply continuous network morphologies contain two or more chemically distinct domains that continuously percolate through the specimen in all three dimensions They have captivated researchers because of their superior mechanical properties and could potentially find utility in technologies such as catalysis, photonic materials, solar cells, and separations This review summarizes experimental and theoretical investigations of the structures and properties of network morphologies in AB block copolymer and ABC block terpolymer systems and includes a discussion of some proposed technological appli

True Chemical Structure of Double Network Hydrogels

Abstract: We elucidate why the anomalous high strength of double network gels is obtained when the second network is polymerized without any cross-linkers. We have synthesized truly independent-DN gels (named “t-DN” gels), which do not have any covalent bonds between the first and the second networks, and showed that the t-DN gels cannot be toughened by the un-cross-linked second network. It means that the high strength of usual DN gels without the cross-linker of the second network is actually achieved by the interconnection between the two networks through covalent bonds (so usual DN gels were named “c-DN” gels). Further, we found that the t-DN gels become stronger than the c-DN gels when the second network is loosely cross-linked. As the t-DN gels have a more simple structure than the c-DN gels, we expect that the toughening mechanism of DN gels will be clarified by studying the t-DN gels in the future.

High-Performance Carboxylated Polymers of Intrinsic Microporosity (PIMs) with Tunable Gas Transport Properties†

Abstract: Carboxylated polymers of intrinsic microporosity (carboxylated PIMs) are reported as potential high-performance materials for membrane-based gas separation Carboxylated PIM membranes were prepared by in situ hydrolysis of the nitrile groups of PIM-1 films Structural characterization was performed by Fourier transform infrared spectroscopy (FTIR) and proton nuclear magnetic resonance (1H NMR) The degree of hydrolysis was determined by carbon elemental analysis The thermal properties were evaluated by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) Compared with PIM-1, carboxylated PIMs with different degrees of hydrolysis have similar thermal and mechanical properties but show higher selectivity for gas pairs such as O2/N2, CO2/N2, He/N2, and H2/N2 with a corresponding decrease in permeability Selectivity coupled to high permeability combines to exceed the Robeson upper-bound line for the O2/N2 gas pair This work demonstrates that significant improvements in gas separati

Morphology of Porous and Wrinkled Fibers of Polystyrene Electrospun from Dimethylformamide

Abstract: Submicron diameter fibers of polystyrene are electrospun from solutions in dimethylformamide (DMF). When electrospun in a high-humidity environment, the interior of these fibers was found to be highly porous rather than consolidated, despite the smooth and nonporous appearance of the fiber surfaces. The formation of interior porosity is attributed to the miscibility of water, a nonsolvent for the polymers in solution, with DMF. The resulting morphology is a consequence of the relatively rapid diffusion of water into the jet, leading to a liquid−liquid phase separation that precedes solidification due to evaporation of DMF from the jet. When electrospun in a low-humidity environment, the fibers exhibit a wrinkled morphology that can be explained by a buckling instability. Understanding which morphology forms under a given set of conditions is achieved through the comparison of three characteristic times: the drying time, the buckling time, and the phase separation time. The morphology has important consequ...

Synthesis, Characterization, and Gas Permeation Properties of a Novel Group of Polymers with Intrinsic Microporosity: PIM-Polyimides

Abstract: A range of polyimides with characteristics similar to a polymer of intrinsic microporosity (PIM) were prepared by reaction with various aromatic diamines of a bis(carboxylic anhydride) incorporating a spiro-center. The polymers exhibited high surface area, as determined by nitrogen adsorption, and high thermal stability. Membrane gas permeation experiments showed PIM-polyimides to be among the most permeable of all polyimides and to have selectivities close to the upper bound for several important gas pairs. A group contribution method was used to predict permeability coefficients and separation factors for further PIM-polyimide structures, revealing worthwhile targets for future synthetic efforts.

Well-Dispersed Fractal Aggregates as Filler in Polymer−Silica Nanocomposites: Long-Range Effects in Rheology

Abstract: We are presenting a new method of processing polystyrene−silica nanocomposites, which results in a very well-defined dispersion of small primary aggregates (assembly of 15 nanoparticles of 10 nm diameter) in the matrix. The process is based on the use of a high boiling point solvent, in which the nanoparticles are well dispersed, and a controlled evaporation procedure. The filler’s fine network structure is determined over a wide range of sizes, using a combination of small-angle neutron scattering (SANS) and transmission electronic microscopy (TEM) experiments. The mechanical response of the nanocomposite material has been investigated for both small (ARES oscillatory shear and dynamical mechanical analysis) and large deformations (uniaxial traction) as a function of the concentration of the particles in the matrix. Our findings show that with a simple tuning parameter, the silica filler volume fraction, we can investigate in the same way the structure−property correlations related to the two main reinfo...

Effect of LiClO4 on the Structure and Mobility of PEO-Based Solid Polymer Electrolytes

Abstract: The relationship between structure, PEO mobility, and ionic conductivity is investigated for the solid polymer electrolyte, PEO/LiClO4. Amorphous and semicrystalline samples with ether-oxygen-to-lithium ratios ranging from 4:1 to 100:1 are measured. Previous X-ray diffraction results show that three crystalline phases can form in this system depending on the LiClO4 concentration: (PEO)3:LiClO4, pure PEO, and (PEO)6:LiClO4. We use SANS to determine that the (PEO)3:LiClO4 phase forms cylinders with a radius of 125 A and a length of 700 A. We also measure the amount and size of pure PEO lamellae by exploiting the neutron scattering length density contrast that arises because of crystallization. The samples are thermally treated such that the (PEO)6:LiClO4 phase does not form. QENS is used to measure PEO mobility directly in amorphous and semicrystalline samples, and it reveals two processes. The first process at short times is attributed to the segmental mobility of PEO, and the second process at longer time...