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Polyethylene oxide (PEO) has been widely used in biomedical fields. The antibiofouling property of the PEO-modified surface has been extensively investigated but is far from being fully understood. A series of PEOs with narrowly distributed molecular weight (Mw), synthesized with the technique of high vacuum anionic polymerization, have been successfully grafted onto the surface of silicon wafers. The power-law relationship between the thickness of the monolayer versus the Mw of the grafted PEO shows a scaling of 0.3, indicating compact condensing of the chains. The static contact angles show higher hydrophobicity for the layer of PEO with higher Mw, which can be attributed to the closely packed conformation of the chains with high density. The frequency shift of the contact resonance indicates that the Young's modulus decreases and the loss factor increases with the increase in the Mw of PEO and the thickness of the PEO layers. Dielectric spectroscopy of bare or PEO-grafted wafers in the aqueous solutions reveals an interfacial polarization, which results from compositional and structural changes in the interface layer and depends on temperatures and salt concentrations. At a given grafting density, the PEO chains are swollen in pure water, demonstrating hydrophilic behavior, whereas they collapse in salt solutions, showing hydrophobic characteristics. [ABSTRACT FROM AUTHOR]

Dynamic arrest transitions of colloidal suspensions containing nonspherical particles are of interest for the design and processing of various particle technologies. To better understand the effects of particle shape anisotropy and attraction strength on gel and glass formation, we present a colloidal model system of octadecyl-coated silica rods, termed as adhesive hard rods (AHR), which enables control of rod aspect ratio and temperature-dependent interactions. The aspect ratios of silica rods were controlled by varying the initial TEOS concentration following the work of Kuijk et al. (J. Am. Chem. Soc., 2011, 133, 2346-2349) and temperature-dependent attractions were introduced by coating the calcined silica rods with an octadecyl-brush and suspending in tetradecane. The rod length and aspect ratio were found to increase with TEOS concentration as expected, while other properties such as the rod diameter, coating coverage, density, and surface roughness were nearly independent of the aspect ratio. Ultrasmall angle X-ray scattering measurements revealed temperature-dependent attractions between octadecyl-coated silica rods in tetradecane, as characterized by a low-q upturn in the scattered intensity upon thermal quenching. Lastly, the rheology of a concentrated AHR suspension in tetradecane demonstrated thermoreversible gelation behavior, displaying a nearly 5 orders of magnitude change in the dynamic moduli as the temperature was cycled between 15 and 40 °C. The adhesive hard rod model system serves as a tunable platform to explore the combined influence of particle shape anisotropy and attraction strength on the dynamic arrest transitions in colloidal suspensions with thermoreversible, short-range attractions. [ABSTRACT FROM AUTHOR]

An accurate determination of the structure characteristics of protonated generation 5 polyamidoamine dendrimers in aqueous solution has been conducted by analyzing the small angle neutron scattering databased on a statistical mechanics model. In our investigation, the primary focus is to elucidate the effect of counterion valence on the counterion association and its impact on the intramolecular density profile within a dendrimer. In the range of our study for molecular protonation, a strong dependence of the structural properties of charged dendrimers on counterion valence is revealed. Our findings indicate that the association of a large amount of divalent counterions significantly reduces the effective charge of a dendrimer molecule. Surprisingly, no discernible transition of the density distribution profile is observed for the dendrimer charged by D2SO4, as opposed to our previous observation of a pronounced transition in intramolecular density profile for the dendrimer charged by DCl. These findings may be understood from the thermodynamic processes of counterions. [ABSTRACT FROM AUTHOR]

Poly(2-isopropenyl-2-oxazoline) (PIPOx) has drawn significant attention for numerous applications. However, the successful living anionic polymerization of 2-isopropenyl-2-oxazoline has not been reported previously. Herein, we describe how well-defined PIPOx with quantitative yields, controlled molecular weights from 6800 to over 100,000 g/mol and low polydispersity indices (PDI ≤ 1.17) were synthesized successfully via living anionic polymerization using diphenylmethylpotassium/diethylzinc (DPM-K/Et2Zn) in tetrahydrofuran (THF) at 0 °C. In particular, we report the precise synthesis of well-defined PIPOx with the highest molecular weight ever reported (over 100,000 g/mol) and low PDI of 1.17. The resulting polymers were characterized by ¹H and 13C nuclear magnetic resonance spectroscopy (NMR) along with size exclusion chromatography (SEC). Additionally, the reactivity of living PIPOx was investigated by crossover block copolymerization with styrene (St), 2-vinylpyridine (2VP), and methyl methacrylate (MMA). It was found that the nucleophilicity of living PIPOx is of this order: living PS > living P2VP > living PMMA > living PIPOx. The self-assembly behavior in bulk of PIPOx-b-PS-b-PIPOx triblock copolymers having different block ratios of 10:80:10 and 25:50:25 was studied using transmission electron microscopy (TEM). The formation of spherical and lamellar nanostructures, respectively, was observed. [ABSTRACT FROM AUTHOR]

Living anionic polymerization of acrylates is challenging due to intrinsic side reactions including backbiting reactions of propagating enolate anions and aggregation of active chain ends. In this study, the controlled synthesis of poly(1-adamatyl acrylate) (PAdA) was performed successfully for the first time via living anionic polymerization through investigation of the initiation systems of sec-butyllithium/diphenylethylene/lithium chloride (sec-BuLi/DPE/LiCl), diphenylmethylpotassium/diethylzinc (DPMK/Et2Zn), and sodium naphthalenide/dipenylethylene/diethylzinc (Na-Naph/DPE/Et2Zn) in tetrahydrofuran at -78 °C using custom glass-blowing and high-vacuum techniques. PAdA synthesized via anionic polymerization using DPMK with a large excess (more than 40-fold to DPMK) of Et2Zn as the ligand exhibited predicted molecular weights from 4.3 to 71.8 kg/mol and polydispersity indices of around 1.10. In addition, the produced PAdAs exhibit a low level of isotactic content (mm triads of 2.1%). The block copolymers of AdA and methyl methacrylate (MMA) were obtained by sequential anionic polymerization, and the distinct living property of PAdA over other acrylates was demonstrated based on the observation that the resulting PAdA-b-PMMA block copolymers were formed with no residual PAdA homopolymer. The PAdA homopolymers exhibit a very high glass transition temperature (133 °C) and outstanding thermal stability (Td: 376 °C) as compared to other acrylic polymers such as poly(tert-butyl acrylate) and poly(methyl acrylate). These merits make PAdA a promising candidate for acrylic-based thermoplastic elastomers with high upper service temperature and enhanced mechanical strength. [ABSTRACT FROM AUTHOR]

A three-dimensional triply interpenetrated mixed metal-organic framework, Zn2(BBA)2(CuPyen)·Gx (M'MOF-20; BBA = biphenyl-4,4′-dicarboxylate; G = guest solvent molecules), of primitive cubic net was obtained through the solvothermal reaction of Zn(NO3)2, biphenyl-4,4′-dicarboxylic acid, and the salen precursor Cu(PyenH2)(NO3)2 by a metallo-ligand approach. The triple framework interpenetration has stabilized the framework in which the activated M'MOF-20a displays type-I N2 gas sorption behavior with a Langmuir surface area of 62 m2 g-1. The narrow pores of about 3.9 Å and the open metal sites on the pore surfaces within M'MOF-20a collaboratively induce its highly selective C2H2/CH4 and CO2/CH4 gas separation at ambient temperature. [ABSTRACT FROM AUTHOR]

Polyoxometalates (POMs) using {Mo72V30} as an example, dissolved in water, can interact with amine-terminated polydimethylsiloxane (PDMS-NH2) dissolved in toluene at the water/toluene interface to form POM-surfactants that significantly lower the interfacial tension and can be used to stabilize liquids via interfacial elasticity. The jamming of the POM-surfactants at the water/oil interface with consequent wrinkling occurs with a decrease in the interfacial area. The packing density of the POM-surfactants at the interface can be tuned by varying the strength of screening with the addition of cations with differing hydrated radii. [ABSTRACT FROM AUTHOR]

Electrical contact to low-dimensional (low-D) materials is a key to their electronic applications. Traditional metal contacts to low-D semiconductors typically create gap states that can pin the Fermi level (EF). However, low-D metals possessing a limited density of states at EF can enable gate-tunable work functions and contact barriers. Moreover, a seamless contact with native bonds at the interface, without localized interfacial states, can serve as an optimal electrode. To realize such a seamless contact, one needs to develop atomically precise heterojunctions from the atom up. Here, we demonstrate an all-carbon staircase contact to ultranarrow armchair graphene nanoribbons (aGNRs). The coherent heterostructures of width-variable aGNRs, consisting of 7, 14, 21, and up to 56 carbon atoms across the width, are synthesized by a surface-assisted self-assembly process with a single molecular precursor. The aGNRs exhibit characteristic vibrational modes in Raman spectroscopy. A combined scanning tunneling microscopy and density functional theory study reveals the native covalent-bond nature and quasi-metallic contact characteristics of the interfaces. Our electronic measurements of such seamless GNR staircase constitute a promising first step toward making low resistance contacts. [ABSTRACT FROM AUTHOR]

Individual carbon nanotubes (CNTs) exhibit exceptional mechanical properties. However, difficulties remain in fully realizing these properties in CNT macro-assemblies, because the weak inter-tube forces result in the CNTs sliding past one another. Herein, a simple solid-state reaction is presented that enhances the mechanical properties of carbon nanotube fibers (CNTFs) through simultaneous covalent functionalization and crosslinking. This is the first chemical crosslinking proposed without the involvement of a catalyst or byproducts. The specific tensile strength of CNTFs obtained from the treatment employing a benzocyclobutene-based polymer is improved by 40%. Such improvement can be attributed to a reduced number of voids, impregnation of the polymer, and the formation of covalent crosslinks. This methodology is confirmed using both multiwalled nanotube (MWNT) powders and CNTFs. Thermogravimetric analysis, differential scanning calorimetry, x-ray photoelectron spectroscopy, and transmission electron microscopy of the treated MWNT powders confirm the covalent functionalization and formation of inter-tube crosslinks. This simple one-step reaction can be applied to industrial-scale production of high-strength CNTFs. [ABSTRACT FROM AUTHOR]

Understanding the nature of dopant dynamics in the solid state is critical for improving the longevity and stability of organic electronic devices and for optimizing the doping-induced solubility control (DISC) patterning method. In this work, we use quasi-elastic neutron scattering (QENS) and fluorescence quenching techniques to develop a comprehensive picture of both the microscopic and macroscopic dynamics of the soluble p-type molecular dopant tetrafluoromethyloxycarbonyltricyanoquinodimethane (F4MCTCNQ) in the conductive polymer poly(3-hexylthiophene-2,5-diyl) (P3HT). Specifically, fast dynamics (ps-ns) of the dopant, such as the methyl and the methoxycarbonyl group rotations, are observed in QENS experiments. From confocal fluorescence microscope experiments, longer-range/slower dopant diffusion (ms-days) is captured. However, in order to fit these data, it is necessary to incorporate a Langmuir isotherm equilibrium between the neutral and ionized dopant molecules. Ionized F4MCTCNQ is strongly favored by the equilibrium, but it diffuses 3 orders of magnitude slower than neutral species. Moreover, the macroscopic diffusion is found to depend mostly on the minority concentration of neutral dopant molecules in the film. Finally, the global diffusion coefficient of the monoester-substituted dopant F4MCTCNQ is shown to be more than an order of magnitude smaller than that of the widely used dopant F4TCNQ. [ABSTRACT FROM AUTHOR]

Whereas poly(3-alkyl-2,5-thiophene)s (P3AT), with many potential applications, have been extensively investigated, their ortho-connected isomers, poly(5-alkyl-2,3-thiophene)s (P5AT), have never been reported because of the difficulty in their syntheses. We herein present the first synthesis of regioregular P5AT via controlled Suzuki cross-coupling polymerization with PEPPSI-IPr as catalyst, affording the polymers with tunable molecular weight, narrow polydispersity (PDI), and well-defined functional end groups at the gram scale. The helical geometry of P5AT was studied by a combination of NMR, small-angle X-ray scattering (SAXS), and scanning tunneling microscopy (STM). Particularly, the single polymer chain of poly(5-butyl-2,3-thiophene) (P5BT) on highly oriented pyrolytic graphite (HOPG) substrates with either M or P helical conformation was directly observed by STM. The comparison of UV-vis absorption between poly(5-hexyl-2,3-thiophene) (P5HT) (λ = 345 nm) and poly(3-hexyl-2,5-thiophene) (P3HT) (λ = 450 nm) indicated that the degree of conjugation of the backbone in P5HT is less than in P3HT, which may be a consequence of the helical geometry of the former compared to the more planar geometry of the latter. Moreover, we found that P5HT can emit green fluorescence under UV (λ = 360 nm) irradiation. [ABSTRACT FROM AUTHOR]

A typical type of core-shell polyoxometalates can be obtained through the Keggin-type polyoxometalate-templated growth of a layer of spherical shell structure of {Mo72Fe30}. Small-angle X-ray scattering is used to study the structural features and stability of the core-shell structures in aqueous solutions. Time-resolved small-angle X-ray scattering is applied to monitor the synthetic reactions, and a three-stage formation mechanism is proposed to describe the synthesis of the core-shell polyoxometalates based on the monitoring results. New protocols have been developed by fitting the X-ray data with custom physical models, which provide more convincing, objective, and completed data interpretation. Quasi-elastic and inelastic neutron scattering are used to probe the dynamics of water molecules in the core-shell structures, and two different types of water molecules, the confined and structured water, are observed. These water molecules play an important role in bridging core and shell structures and stabilizing the cluster structures. [ABSTRACT FROM AUTHOR]

The dynamics of water in polyethylene oxide (PEO)/LiCl solution has been studied with quasielastic neutron scattering experiments and molecular dynamics (MD) simulations. Two different time scales of water diffusion representing interfacial water and bulk water dynamics have been identified. The measured diffusion coefficient of interfacial water remained 5-10 times smaller than that of bulk water, but both were slowed by approximately 50% in the presence of Li+. Detailed analysis of MD trajectories suggests that Li+ is favorably found at the surface of the hydration layer, and the probability to find the caged Li+ configuration formed by the PEO is lower than for the noncaged Li+-PEO configuration. In both configurations, however, the slowing down of water molecules is driven by reorienting water molecules and creating water-Li+ hydration complexes. Performing the MD simulation with different ions (Na+ and K+) revealed that smaller ionic radius of the ions is a key factor in disrupting the formation of PEO cages by allowing spaces for water molecules to come in between the ion and PEO. [ABSTRACT FROM AUTHOR]

ControlledPd(0)/t-Bu3P-catalyzed Suzuki cross-couplingpolymerizations of AB-type monomers via the chain-growth mechanismwith a series of ArPd(t-Bu3P)X (X = I,Br, Cl) complexes as initiators were described. Both isolated PhPd(t-Bu3P)X (X = I, Br) complexes and ArPd(t-Bu3P)X (X = I, Br, Cl) complexes insitugenerated from Pd2(dba)3/t-Bu3P/ArX (X = I, Br, Cl) were employed as initiatorsfor the controlled Pd(0)/t-Bu3P-catalyzedSuzuki cross-coupling polymerization. The in situgenerated ArPd(t-Bu3P)X complexes werefound to be better initiators in general. Among them, the combinationsof p-BrC6H4I, p-HOCH2C6H4Br and p-PhCOC6H4Br with Pd2(dba)3/t-Bu3P were identified as highly robustinitiator systems, resulted in polymers with narrow PDIs (1.13–1.20).In addition, the Pd2(dba)3/t-Bu3P/p-HOCH2C6H4Br and Pd2(dba)3/t-Bu3P/p-PhCOC6H4Br initiator systems also offered heterobifunctional chain ends withhigh fidelity. Our study showed that an additional amount of t-Bu3P in the initiator system helped to achievethe narrow PDIs, likely by stabilizing Pd(0) species in the initiatorsystem via coordination to form more stable Pd(t-Bu3P)n(n≥2) complexes. Our study opens a new avenue toward well-defined conjugatedpolymers. [ABSTRACT FROM AUTHOR]

Replacing traditional liquid electrolytes by polymers will significantly improve electrical energy storage technologies. Despite significant advantages for applications in electrochemical devices, the use of solid polymer electrolytes is strongly limited by their poor ionic conductivity. The classical theory predicts that the ionic transport is dictated by the segmental motion of the polymer matrix. As a result, the low mobility of polymer segments is often regarded as the limiting factor for development of polymers with sufficiently high ionic conductivity. Here, we show that the ionic conductivity in many polymers can be strongly decoupled from their segmental dynamics, in terms of both temperature dependence and relative transport rate. Based on this principle, we developed several polymers with "superionic" conductivity. The observed fast ion transport suggests a fundamental difference between the ionic transport mechanisms in polymers and small molecules and provides a new paradigm for design of highly conductive polymer electrolytes. [ABSTRACT FROM AUTHOR]

Atomistic molecular dynamics (MD) simulations were carried out to investigate the local dynamics of polyelectrolyte dendrimers dissolved in deuterium oxide (D2O) and its dependence on molecular charge. Enhanced segmental dynamics upon increase in molecular charge is observed, consistent with quasielastic neutron scattering (QENS) measurements. A strong coupling between the intradendrimer local hydration level and segmental dynamics is also revealed. Compelling evidence shows these findings originate from the electrostatic interaction between the hydrocarbon components of a dendrimer and the invasive water. This combined study provides fundamental insight into the dynamics of charged polyelectrolytes and the solvating water molecules. [ABSTRACT FROM AUTHOR]

Lithium-ion-conducting solid electrolytes hold promise for enabling high-energy battery chemistries and circumventing safety issues of conventional lithium batteries. Achieving the combination of high ionic conductivity and a broad electrochemical window in solid electrolytes is a grand challenge for the synthesis of battery materials. Herein we show an enhancement of the room-temperature lithium-ion conductivity by 3 orders of magnitude through the creation of nanostructured Li3PS4. This material has a wide electrochemical window (5 V) and superior chemical stability against lithium metal. The nanoporous structure of Li3PS4 reconciles two vital effects that enhance the ionic conductivity: (1) the reduction of the dimensions to a nanometer-sized framework stabilizes the high-conduction β phase that occurs at elevated temperatures, and (2) the high surface-to-bulk ratio of nanoporous β-Li3PS4 promotes surface conduction. Manipulating the ionic conductivity of solid electrolytes has far-reaching implications for materials design and synthesis in a broad range of applications, including batteries, fuel cells, sensors, photovoltaic systems, and so forth. [ABSTRACT FROM AUTHOR]

Four porous isostructural mixed-metal-organic frameworks (M'MOFs) have been synthesized and structurally characterized. The pores within these M'MOFs are systematically tuned by the interplay of both the metalloligands and organic ligands which have enabled us not only to direct their highly selective separation of chiral alcohols 1-phenylethanol (PEA), 2-butanol (BUT), and 2-pentanol (2-PEN) with the highest ee up to 82.4% but also to lead highly selective separation of achiral C2H2/C2H4 separation. The potential application of these M'MOFs for the fixed bed pressure swing adsorption (PSA) separation of C2H2/C2H4 has been further examined and compared by the transient breakthrough simulations in which the purity requirement of 40 ppm in the outlet gas can be readily fulfilled by the fixed bed M'MOF-4a adsorber at ambient conditions. [ABSTRACT FROM AUTHOR]

We present detailed studies of the relationship between ionic conductivity and segmental relaxation in polymer electrolytes. The analysis shows that the ionic conductivity can be decoupled from segmental dynamics and the strength of the decoupling correlates with the fragility but not with the glass transition temperature. These results call for a revision of the current picture of ionic transport in polymer electrolytes. We relate the observed decoupling phenomenon to frustration in packing of rigid polymers, where the loose local structure is also responsible for the increase in their fragility. [ABSTRACT FROM AUTHOR]

PTB7 semiconducting copolymer comprising thieno[3,4-b]thiophene and benzodithiophene alternating repeat units set a historic record of solar energy conversion efficiency (7.4%) in polymer/fullerene bulk heterojunction solar cells. To further improve solar cell performance, a thorough understanding of structure–property relationships associated with PTB7/fullerene and related organic photovoltaic (OPV) devices is crucial. Traditionally, OPV active layers are viewed as an interpenetrating network of pure polymers and fullerenes with discrete interfaces. Here we show that the active layer of PTB7/fullerene OPV devices in fact involves hierarchical nanomorphologies ranging from several nanometers of crystallites to tens of nanometers of nanocrystallite aggregates in PTB7-rich and fullerene-rich domains, themselves hundreds of nanometers in size. These hierarchical nanomorphologies are coupled to significantly enhanced exciton dissociation, which consequently contribute to photocurrent, indicating that the nanostructural characteristics at multiple length scales is one of the key factors determining the performance of PTB7 copolymer, and likely most polymer/fullerene systems, in OPV devices. [ABSTRACT FROM AUTHOR]

The deuterated homopolymers and their corresponding polystyrene-block-polybutadiene and polystyrene-block-polyisoprene copolymers require lower cloud pressures than their hydrogenous analogues to dissolve in a compressible alkane solvent, such as propane. For symmetric diblocks, deuteration reduces the micellization pressure. By contrast, for asymmetric diblocks with a long diene block relative to the styrene block, deuteration can increase the micellization pressure. All in all, however, the deuteration effects, while measurable, do not qualitatively change the principal diblock properties in compressible propane solutions, such as pressure-induced micelle decomposition, micelle formation and micelle size, and their temperature dependence. Therefore, isotope labeling should be a useful approach to neutron-scattering characterization for styrene-diene block copolymers in compressible alkane systems. [ABSTRACT FROM AUTHOR]

High-pressure solutions of polystyrene-block-polybutadiene and polystyrene-block-polyisoprene in compressible propane or propylene exhibit a robust micellar region that grows in pressure−temperature coordinates with increasing copolymer concentration, molecular weight, and styrene/diene block ratio. This happens because, while the micellization pressure strongly increases with increasing copolymer concentration, molecular weight, and styrene/diene block ratio, the micellar cloud pressure (the pressure at which the micelles aggregate and precipitate from solution) is largely insensitive to these variables. In other words, neither the block size nor the block ratio nor the copolymer concentration seems to affect much the copolymer separation from solution in the micellar region. [ABSTRACT FROM AUTHOR]

Micellar solutions of polystyrene-block-polybutadiene and polystyrene-block-polyisoprene in propane are found to exhibit significantly lower cloud pressures than the corresponding hypothetical nonmicellar solutions. Such a cloud-pressure reduction indicates the extent to which micelle formation enhances the apparent diblock solubility in near-critical and hence compressible propane. Concentration-dependent pressure−temperature points beyond which no micelles can be formed, referred to as the micellization end points, are found to depend on the block type, size, and ratio. The cloud-pressure reduction and the micellization end point measured for styrene−diene diblocks in propane should be characteristic of all amphiphilic diblock copolymer solutions that form micelles in compressible solvents. [ABSTRACT FROM AUTHOR]

We investigate a series of poly(amidoamine) starburst dendrimers (PAMAM) of different generations in acidic, aqueous solutions using small-angle neutron scattering (SANS). While the overall molecular size is found to be practically unaffected by a pD change, a strong generational dependence of counterion association is revealed. Upon increasing the dendrimer generation, the effective charge obtained from our SANS experiments only shows a small increase in contrast to the nearly exponential increase predicted by a recent atomic simulation. We also find that with the same degree of molecular protonation the specific counterion association, which is defined as the ratio of bound chloride anions to positively charged amines in solutions, is larger for higher-generation PAMAM dendrimer. The associated counterion density also increases upon increasing generation number. [ABSTRACT FROM AUTHOR]

The structures and association properties of thermosensitive block copolymers of poly(methoxyoligo(ethylene glycol) norbornenyl esters) in D 2O were investigated by small angle neutron scattering (SANS). Each block is a comblike polymer with a polynorbornene (PNB) backbone and oligo ethylene glycol (OEG) side chains (one side chain per NB repeat unit). The chemical formula of the block copolymer is (OEG 3NB) 79−(OEG 6.6NB) 67, where subscripts represent the degree of polymerization (DP) of OEG and NB in each block. The polymer concentration was fixed at 2.0 wt % and the structural changes were investigated over a temperature range between 25 and 68 °C. It was found that at room temperature polymers associate to form micelles with a spherical core formed by the block (OEG 3NB) 79and corona formed by the block (OEG 6.6NB) 67and that the shape of the polymer in the corona could be described by the form factor of rigid cylinders. At elevated temperatures, the aggregation number increased and the micelles became more compact. At temperatures around the cloud point temperature (CPT) T= 60 °C a correlation peak started to appear and became pronounced at 68 °C due to the formation of a partially ordered structure with a correlation length ∼349 Å. [ABSTRACT FROM AUTHOR]

A rational strategy has been used to immobilize open metal sites in ultramicroporosity for stronger binding of multiple H2 molecules per unsaturated metal site for H2 storage applications. The synthesis and structure of a mixed zinc/copper metal-organic framework material Zn3(BDC)3[Cu(Pyen)] •(DMF)5(H2O)5 (H2BDC = 1,4 benzenedicarboxylic acid and PyenH2 = 5-methyl-4-oxo-1,4-dihydro-pyridine-3-carbaldehyde) is reported. Desolvation provides a bimodal porous structure Zn3(BDC)3[Cu(Pyen)] (M′MOF 1) with narrow porosity (<0.56 nm) and an array of pores in the bccrystallographic plane where the adsorbate-adsorbent interactions are maximized by both the presence of open copper centers and overlap of the potential energy fields from pore walls. The H2 and D2 adsorption isotherms for M′MOF 1 at 77.3 and 87.3 K were reversible with virtually no hysteresis. Methods for determination of the isosteric enthalpies of H2 and D2 adsorption were compared. A virial model gave the best agreement (average deviation <1 standard deviation) with the isotherm data. This was used in conjunction with the van't Hoff isochore giving isosteric enthalpies at zero surface coverage of 12.29 ± 0.53 and 12.44 ± 0.50 kJ mol-1 for H2 and D2 adsorption, respectively. This is the highest value so far observed for hydrogen adsorption on a porous material. The enthalpy of adsorption, decreases with increasing amount adsorbed to 9.5 kJ mol-1 at ~1 .9 mmol g-1 (2 H2 or D2 molecules per Cu corresponding to adsorption on both sides of planar Cu open centers) and is virtually unchanged in the range 1.9-3.6 mmol g-1. Virial analysis of isotherms at 87.3 K is also consistent with two H2 or D2 molecules being bound to each open Cu center. The adsorption kinetics follow a double exponential model, corresponding to diffusion along two types of pores, a slow component with high activation energy (13.35 ± 0.59 kJ mol-1) for the narrow pores and a faster component with low activation energy (8.56 ± 0.41 kJ mol-1). The D2 adsorption kinetic constants for both components were significantly faster than the corresponding H2 kinetics for specific pressure increments and had slightly lower activation energies than the corresponding values for H2 adsorption. The kD2/kH2 ratio for the slow component was 1.62 ± 0.07, while the fast component was 1.38 ± 0.04 at 77.3 K, and the corresponding ratios were smaller at 87.3 K. These observations of kinetic isotope quantum molecular sieving in porous materials are due to the larger zero-point energy for the lighter H2, resulting in slower adsorption kinetics compared with the heavier D2. The results show that a combination of open metal centers and confinement in ultramicroporosity leads to a high enthalpy for H2 adsorption over a wide range of surface coverage and quantum effects influence diffusion of H2 and D2 in pores in M′MOF 1. [ABSTRACT FROM AUTHOR]

Drawing an analogy to the paradigm of quasielastic neutron scattering, we present a general approach for quantitatively investigating the spatiotemporal dependence of structural anisotropy relaxation in deformed polymers by using small-angle neutron scattering. Experiments and nonequilibrium molecular dynamics simulations on polymer melts over a wide range of molecular weights reveal that their conformational relaxation at relatively high momentum transfer Q and short time can be described by a simple scaling law, with the relaxation rate proportional to Q. This peculiar scaling behavior, which cannot be derived from the classical Rouse and tube models, is indicative of a surprisingly weak direct influence of entanglement on the microscopic mechanism of single-chain anisotropy relaxation. [ABSTRACT FROM AUTHOR]

Hyperfine interaction (HFI), originating from the coupling between spins of charge carriers and nuclei, has been demonstrated to strongly influence the spin dynamics of localized charges in organic semiconductors. Nevertheless, the role of charge localization on the HFI strength in organic thin films has not yet been experimentally investigated. In this study, the statistical relation hypothesis that the effective HFI of holes in regioregular poly(3-hexylthiophene) (P3HT) is proportional to 1/N0.5 has been examined, where N is the number of the random nuclear spins within the envelope of the hole wave function. First, by studying magnetoconductance in hole-only devices made by isotope-labeled P3HT we verify that HFI is indeed the dominant spin interaction in P3HT. Second, assuming that holes delocalize fully over the P3HT polycrystalline domain, the strength of HFI is experimentally demonstrated to be proportional to 1/N0.52 in excellent agreement with the statistical relation. Third, the HFI of electrons in P3HT is about 3 times stronger than that of holes due to the stronger localization of the electrons. Finally, the effective HFI in organic light emitting diodes is found to be a superposition of effective electron and hole HFI. Such a statistical relation may be generally applied to other semiconducting polymers. This Letter may provide great benefits for organic optoelectronics, chemical reaction kinetics, and magnetoreception in biology. [ABSTRACT FROM AUTHOR]