Laurence A. Belfiore

Enhanced efficiency of polymer solar cells by incorporated Ag–SiO2 core–shell nanoparticles in the active layer

In this article, we creatively incorporated Ag–SiO2 core–shell nanoparticles (Ag–SiO2-NPs) into photo-/electro-active layers consisting of poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) in polymer solar cells (PSCs). By this way, the photovoltaic performances of PSCs have largely been enhanced. The results demonstrate a 13.50% enhancement of short-circuit photocurrent density (Jsc) and a 15.11% enhancement of power conversion efficiency (PCE) as the weight percent of doped Ag–SiO2-NPs is 1.5 wt% in the active layer of corresponding PSCs. We attribute the enhancement to the localized surface plasmon resonance (LSPR) effect of Ag–SiO2-NPs, by which the incident light harvesting is enlarged. Whereas, the incorporated bare Ag nanoparticles (Ag-NPs) in the active layer of PSCs decreases the PCE, which is ascribed to the quenching of excitons at the surface of Ag-NPs and the poor dispersion of Ag-NPs in the active layer. Importantly, this work provides a new approach to enhance the performance of PSCs via the LSPR effect of Ag–SiO2-NPs other than via non-circular nanometals.

Macromolecule–metal complexes: ligand field stabilization and thermophysical property modification

Abstract When transition metal cations coordinate to ligands in the sidegroup of a polymer and modify the thermal response of a macromolecular complex, the enhancement in T g can be explained by focusing on ligand field stabilization of the metal d-electrons. The methodology to identify attractive coordination complexes and predict relative increases in T g is described in terms of the local symmetry of the complex, the molecular orbital pattern, and the d-electron configuration. Interelectronic repulsion is considered for pseudo-octahedral d 6 and d 7 complexes in the glassy state when there is ambiguity in the order in which the d-orbitals are populated. Ligand field stabilization energies are calculated for simple octahedral geometries, as well as 5-coordinate complexes with reduced symmetry, such as square pyramidal, trigonal bipyramidal, and pentagonal planar. If the transition metal cation bridges two different macromolecules in the glassy state via coordination crosslinks, then 5-coordinate complexes with one surviving metal–polymer bond above T g represent reasonable geometries in the molten state. This model of thermochemical synergy in macromolecule–metal complexes with no adjustable parameters considers the glass transition as an endothermic process in which sufficient thermal energy must be supplied to dissociate intermolecular bridges or coordination crosslinks and produce coordinatively unsaturated molten state complexes. The enhancement in T g correlates well with the difference between ligand field stabilization energies in the glassy and molten states for Ru 2+ (d 6 ), Co 2+ (d 7 ), and Ni 2+ (d 8 ) complexes with either poly(4-vinylpyridine), or poly( l -histidine). Larger increases in T g are measured in complexes with the synthetic poly(α-amino acid) relative to those with poly(4-vinylpyridine), but the universality of the model is not sufficient to predict relative T g enhancements in complexes with different polymers.

Quantitative analysis of protein adsorption via atomic force microscopy and surface plasmon resonance.

Surface properties have a significant influence on the performance of biomedical devices. The influence of surface chemistry on the amount and distribution of adsorbed proteins has been evaluated by a combination of atomic force microscopy (AFM) and surface plasmon resonance (SPR). Adsorption of albumin, fibrinogen, and fibronectin was analyzed under static and dynamic conditions, employing self-assembled monolayers (SAMs) as model surfaces. AFM was performed in tapping mode with antibody-modified tips. Phase-contrast images showed protein distribution on SAMs and phase-shift entity provided information on protein conformation. SPR analysis revealed substrate-specific dynamics in each system investigated. When multi-protein solutions and diluted human plasma interacted with SAMs, SPR data suggested that surface chemistry governs the equilibrium composition of the protein layer.

Transition metal compatibilization of poly(vinylamine) and poly(ethylene imine)

Poly(vinylamine), PVA, complexes with cobalt chloride hexahydrate exhibit a 45 °C enhancement in the glass-transition temperature per mol % of the d-block metal cation. Poly(ethylene imine), PEI, complexes with CoCl2(H2O)6 exhibit a 20 °C enhancement in Tg per mol % Co2+. Since the basicities of primary and secondary amines are comparable (i.e., pKb,PVA ≈ 3.34 vs. pKb,PEI ≈ 3.27) and the rates at which each polymeric ligand displaces waters of hydration in the coordination sphere of Co2+ are similar, transition metal compatibilization is operative in blends of both polymers with CoCl2(H2O)6. These two polymers are immiscible in the absence of the inorganic component. Infrared spectroscopy suggests that nitrogen lone pairs in PVA and PEI coordinate to Co2+. The stress–strain response of a 75/25 blend of PVA and PEI with 2 mol % Co2+ reveals a decrease in elastic modulus from 4.4 × 109 N/m2 to 5.7 × 107 N/m2, a decrease in fracture stress from 3.7 × 107 N/m2 to 2.0 × 106 N/m2, and an increase in ultimate strain from 1.3 to 12% relative to the 75/25 immiscible polymer–polymer blend. A plausible explanation for this effect is based on the fact that cobalt chloride hexahydrate compatibilizes both polymers by forming a coordination bridge between nitrogen lone pairs in dissimilar chains. Hence, poly(ethylene imine), which is very weak with a Tg near −40 °C, is integrated into a homogeneous structure with poly(vinylamine) and the mechanical properties of the individual polymers are averaged in the compatibilized ternary complex.

Poly(vinylamine) complexes with f‐block salts from the lanthanide series that exhibit significant glass‐transition temperature enhancement

Six f-block salts from the lanthanide series form complexes with poly(vinyl amine) and increase the glass-transition temperature of the polymer. Results for poly(vinylamine) complexes with EuCl3(H2O)6 and TbCl3(H2O)6 surpass those for d7 cobalt complexes that were studied previously. The glass-transition temperature increases by 49 °C per mol % Eu3+ and 50 °C per mol % Tb3+, up to 2 mol % of the f-block cations. At 5 mol % Eu3+, Tg is slightly higher than 250 °C with no visual evidence of thermal degradation of either component in the complex. This corresponds to a Tg enhancement of almost 200 °C with respect to the undiluted polymer. The increases in Tg for these lanthanide complexes with poly(vinylamine) obey the following trend: up to 2 mol % of the f-block cation. With the exception of Gd(CH3COO)3, which contains different anionic ligands than all of the other trichlorides, this trend correlates inversely with the highest dehydration/dehydrochlorination temperature of each undiluted lanthanide salt, as measured via calorimetry above the melting point and verified by thermogravimetry. Waters of hydration and amino sidegroups undergo ligand substitution in the coordination sphere of the lanthanides. Since lanthanide cations are classified as hard acids, it is not unreasonable that they form complexes with the nitrogen lone pair in the amino sidegroup of the polymer, which is classified as a hard base. Micro-clustering of several amino side groups reduces chain mobility significantly in the vicinity of each metal center, produces coordination crosslinks, and increases Tg. Complementary solution studies reveal that hydrogels form with swelling ratios between 20 and 50 at Eu3+ mole fractions between 0.01 and 0.05 with respect to poly(vinylamine). Infrared spectroscopic observations suggest that the amino nitrogen lone pair in poly(vinylamine) interacts with these lanthanide metal centers.

Miscibility studies in polymer-diluent blends and segmented block copolymers via high-resolution carbon-13 solid-state nuclear magnetic resonance spectroscopy

Abstract Miscibility in blends and short-segmented block copolymers has been studied at the molecular level with the aid of high-resolution solid-state nuclear magnetic resonance (n.m.r.) spectroscopy. The spectroscopic results are in agreement with those from differential scanning calorimetry (d.s.c.). The blend of poly(methyl methacrylate) and 2,2′-dinitrobiphenyl is completely miscible. This is a consequence of near-neighbour interactions between diluent molecules and the pendant groups of the macromolecular chain. The polyether-polyester block copolymers (duPonts Hytrel copolymers) are incompletely phase-separated. Supporting evidence is derived from the observation of n.m.r. signals due to polyester segments dissolved in the polyether-rich mobile domains. As the overall polyester content of the copolymer is increased, molecular mobility in both the crystalline and amorphous domains becomes more restricted. This conclusion is based upon the effect of composition on the proton spin-lattice relaxation times in the rotating frame of reference. Finally, proton dipolar communication via spin diffusion within the soft segment becomes more efficient as the fraction of uncrystallized polyester segments in the polyether-rich domains increases.

Effect of photocurrent enhancement in porphyrin–graphene covalent hybrids

Graphene oxide (GO) sheets were covalently functionalized with 5-p-aminophenyl-10,15,20-triphenylporphyrin (NH2TPP) by an amidation reaction between the amino group in NH2TPP and carboxyl groups in GO. The Fourier transform infrared spectroscopy, nuclear magnetic resonance, scanning and transmission electron microscopies reveal that NH2TPP covalent bonds form on the double surface of graphene oxide sheets, generating a unique nano-framework, i.e., NH2TPP-graphene-NH2TPP. Its UV-visible spectroscopy reveals that the absorption spectrum is not a linear superposition of the spectra of NH2TPP and graphene oxide, because a 59nm red shift of the strong graphene oxide absorption is observed from 238 to 297nm, with significant spectral broadening between 300 and 700nm. Fluorescence emission spectroscopy indicates efficient quenching of NH2TPP photoluminescence in this hybrid material, suggesting that photo-induced electron transfer occurs at the interface between NH2TPP and GO. A reversible on/off photo-current density of 47mA/cm(2) is observed when NH2TPP-graphene-NH2TPP hybrid sandwiches are subjected to pulsed white-light illumination. Covalently-bound porphyrins decrease the optical HOMO/LUMO band gap of graphene oxide by ≈1eV, according to UV-visible spectroscopy. Cyclic voltammetry predicts a small HOMO/LUMO band gap of 0.84eV for NH2TPP-graphene-NH2TPP hybrid sandwiches, which is consistent with efficient electron transfer and fluorescence quenching.

Solid state 13C n.m.r. detection of molecular mixing in polymer blends that exhibit multiple eutectic phase transformations

Abstract Blends of poly(ethylene oxide) with various isomers and derivatives of dihydroxybenzene generate rich macroscopic phase behaviour characterized by solid-solid-liquid (eutectic) transformations. These phase transitions, which occur above ambient temperature, are measured directly by d.s.c., and a thermal analysis summary is contained in the temperature-composition projections. The work described here focuses on site-specific interactions at the molecular level that are responsible for this unique phase behaviour. Solid state n.m.r. spectroscopy is useful as a diagnostic probe of strong interactions and crystal structure modifications. The isotropic 13C chemical shift detects phase coexistence at temperatures well below the eutectic and liquidus transitions where the d.s.c. thermograms are featureless. I.r.-detectable hydrogen bonds between the ether oxygen of poly(ethylene oxide) and the hydroxyl protons in either hydroquinone, 2-methylresorcinol or 5-methylresorcinol distort electron density within the π-orbitals of the aromatic ring. Consequently, the aromatic carbon n.m.r. signals of the resorcinol-like small molecules are sensitive to hydrogen bonding and the formation of molecular complexes. Multiple signals are observed for chemically equivalent carbon sites in the small molecules when phase coexistence is favoured. Crystal structure considerations and molecular packing within the unit cell also generate 13C chemical shift multiplicities, illustrated here for undiluted hydroquinone. The correlation between phase behaviour and 13C n.m.r. diagnostics provides a phenomenological interpretation of the spectroscopic results, particularly for trieutectic blends of poly(ethylene oxide) with 2-methylresorcinol. The combination of n.m.r. and thermal analysis allows high-temperature d.s.c.-measured phase boundaries to be extrapolated to lower temperatures dictated by the n.m.r. experiment.

Immobilization and controlled release of β-galactosidase from chitosan-grafted hydrogels

Chitosan-grafted hydrogels were employed for immobilization and controlled released of β-galactosidase. These hydrogels containing immobilized enzymes were employed to simulate the production of lactose-free food and controlled release of β-galactosidase into lactose-intolerant individuals. The degree of swelling, efficiency of immobilization (i.e., fractional uptake of enzyme), and controlled release were studied as a function of pH and temperature. The degrees of swelling decreased in acidic media: 49.4 g absorbed water per g hydrogel at pH 7.0, and 8.4 g absorbed water per g hydrogel at pH 3.5. The immobilization efficiency was 19%, indicating that chitosan-grafted hydrogels are promising matrices for enzyme adsorption and immobilization. Cyclic experiments reveal that chitosan-grafted hydrogels containing immobilized enzymes can be reused several times without introducing additional enzyme prior to each cycle. There is no significant decrease in the activity of the immobilized enzyme during reutilization studies. All results were conducted in triplicate by considering t-tests at a 95% significance level. Analysis of β-galactosidase activity and controlled release reveals that chitosan-grafted hydrogels containing immobilized enzymes are useful for the production of lactose-free food and controlled enzyme release with high performance.

Solid-state complexes of poly(L-histidine) with metal chlorides from the first row of the d-block

Solid-state characterization of poly(L-histidine) was obtained via differential scanning calorimetry, thermogravimetric analysis, optical microscopy, and infrared spectroscopy. The glass transition temperature of poly(L-histidine) is 169°C. This thermal transition has not been reported previously. Poly(L-histidine)s Tg increases when complexes are produced with the following divalent transition metal chlorides: cobalt chloride hexahydrate, nickel chloride hexahydrate, copper chloride dihydrate, and anhydrous zinc chloride. At 10 mol % salt, nickel chloride increases Tg by 69°C. The enhancement in poly(L-histidine)s Tg correlates well with ligand field stabilization energies for pseudo-octahedral dn complexes (n = 7, 8, and 10) from the first row of the d-block. However, d9 copper(II) complexes do not conform to this empirical correlation. Infrared spectroscopic evidence indicates that these metal chlorides form complexes with the imidazole ring in the histidine side group and the amide group in the main chain of the polymer.