Influencing polymer properties by regiospecific complexation

ABA block-copolymers in which the A segments are capable of forming complexes and B is a non-complexing segment, have been used to prepare polymer materials with properties that can be changed by adding a complexing agent. The complex forming segments were poly(ethylene oxide) (PEO), linear polyethylenimine (LPEI) and poly(N-tert-butylethylenimine) (PTBEI). Commercially available liquid ABA block-copolymers, in which A is PEO and B is poly(propylene oxide), were investigated with high molar mass poly(acrylic acid) (PAA) as the complexing agent for PEO. It was found that the mixtures containing 3 to 7 wt.-% of PAA, showed a marked shear-thickening behavior leading eventually to gelation. This was attributed to the transformation of intramolecular polymer complexes, at low shear rates, to intermolecular complexes, at high shear rates, due to the chain stretching of PAA. ABA copolymers in which A is LPEI or PTBEI and B polytetra-hydrofuran (PTHF), were prepared. Complexation of these copolymers with low molecular weight poly-acids or PAA in polar and non-polar solvents as well as in bulk have been investigated. ABA copolymers in which A is PEO and B a PTHF segment were prepared. These block-copolymers show two melting points: one at appr. 55°C, due to the PEO segments, and one at appr. 30°C due to the PTHF. Upon addition of alkali metal salts such as sodium iodide or sodium thiocyanate, complexes with PEO are formed and as a consequence, the melting point of the PEO segments shifts to appr. 160°C. The complexed materials behave as thermoplastic elastomers up to that temperature.     

Azidocryptands-synthesis, structure, and complexation properties

Cryptands bearing an intraannular azido substituent have been synthesized and characterized spectroscopically. Their complexation properties were investigated by picrate extraction analysis. The oxygen-containing cryptands were found to be good ligands for alkali cations, with a preference for Li(+) and Na(+). The molecular structure of the complex with KBr was determined by X-ray crystallography. In this, the first structurally characterized complex of an aryl azide bound to a metal cation, the potassium cation was found to show ninefold coordination to four oxygen atoms and two nitrogen atoms of the crown ether moiety, to the bromide anion and to N1 of the azido group, as well as C1 of the benzene ring.     

Fluoropolyether coatings: Relationships of electrochemical impedance spectroscopy measurements,barrier properties,and polymer structure

Ten model coatings, selected and obtained from a family of fluorinated resins synthesized by the reaction of perfluoroether oligomeric diols of different molecular weights with polyisocyanurates of hexamethylenediisocyanate (HDI) and isophoronediisocyanate (IPDI), were characterized with differential scanning calorimetry, mechanical testing, and electrochemical impedance spectroscopy measurements. The electrochemical and chemico‐physical measurements show that the glass‐transition temperature of the starting isocyanate trimers greatly influences the properties of the final urethane coatings; the IPDI trimer gives harder coatings with lower water permeabilities than the corresponding HDI‐based materials. Moreover, for each class of materials (from IPDI or HDI), the fluorine content plays a relevant role: the higher the fluorine percentage, the lower the water absorption into the coatings. Furthermore, the chain length of the polyols used for the synthesis of the prepolymers is a variable that exhibits great influence on the coating properties: coatings containing shorter perfluoropolyether segments show better barrier properties. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 40: 52–64, 2002     

On the complexation of proteins and polyelectrolytes

Both natural and synthetic polyelectrolytes form strong complexes with a variety of proteins. One peculiar phenomenon is that association can take place even when the protein and the polyelectrolyte carry the same charge. This has been interpreted as if the ion-dipole interaction can overcome the repulsive ion-ion interaction. On the basis of Monte Carlo simulations and perturbation theory, we propose a different explanation for the association, namely, charge regulation. We have investigated three different protein-polymer complexes and found that the induced ionization of amino acid residues due to the polyelectrolyte leads to a surprisingly strong attractive interaction between the protein and the polymer. The extra attraction from this charge-induced charge interaction can be several kT and is for the three cases studied here, lysozyme, alpha-lactalbumin, and beta-lactoglobulin, of the same magnitude or stronger than the ion-dipole interaction. The magnitude of the induced charge is governed by a response function, the protein charge capacitance Z2-Z2. This fluctuation term can easily be calculated in a simulation or measured in a titration experiment.     

Novel polyelectrolytes with regular structure synthesis,properties and applications

Cationic polyelectrolytes and polymeric betaines with narrow molecular weight distribution as well as block copolymers containing charged and uncharged blocks of different hydrophilicity/hydrophobicity were synthesized by different routes of radical polymerization. The cationic polyelectrolytes were characterized with respect to solution properties and electrolyte behaviour. The block copolymers serve as powerful stabilizers in precipitation and emulsion polymerization processes.     

Silica nanoparticles at interfaces modulated by amphiphilic polymer and surfactant

The interfacial behavior of silica nanoparticles in the presence of an amphiphilic polymer poly( N-isopropylacrylamide) (PNIPAM) and an anionic surfactant sodium dodecyl sulfate (SDS) is studied using neutron reflectivity. While the nanoparticles do not show any attraction to hydrophilic and hydrophobic surfaces in pure water, presence of the amphiphilic polymer induces significant adsorption of the nanoparticles at the hydrophobic surface. This interfacial behavior is activated due to interaction of the nanoparticles with PNIPAM, the amphiphilic nature of which leads to strong adsorption at a hydrophobic surface but only weak interaction with a hydrophilic surface. The presence of SDS competes with nanoparticle-PNIPAM interaction and in turn modulates the interfacial properties of the nanoparticles. These adsorption results are discussed in relation to nanoparticle organization templated by dewetting of charged polymer solutions on a solid substrate. Our previous studies showed that nanoparticle assembly can be induced to form complex morphologies produced by dewetting of the polymer solutions, such as a polygonal network and long-chain structures. This approach, however, works on a hydrophilic substrate but not on a hydrophobic substrate. These observations can be explained in part by particle-substrate interactions revealed in the present study.     

Interfacial water structure at polymer gel/quartz interfaces investigated by sum frequency generation spectroscopy

Interfacial structures of water at polyvinyl alcohol (PVA) and poly(2-acrylamido-2-methypropane) sulfonic acid sodium salt (PNaAMPS)/quartz interfaces were investigated by sum frequency generation (SFG) spectroscopy. Two broad peaks were observed in OH stretching region at 3200 and 3400 cm(-1), corresponding to the symmetric OH stretching of tetrahedrally coordinated, i.e., strongly hydrogen bonded "ice-like" water, and the asymmetric OH stretching of water in a more random arrangement, i.e., weakly hydrogen bonded "liquid-like" water, respectively, in both cases. The "liquid-like" water became dominant when the PVA gel was pressed against the quartz surface. The relative intensity of the SFG signal due to the "liquid-like" water to that due to the "ice-like water" at the quartz surface modified with a self-assembled monolayer of aminopropyltrimethoxysilane (APS) became higher when the negatively charged PNaMPS gel was contacted to the APS modified quartz surface in a solution of pH = 12, where the surface was negatively charged and electrostatic repulsive interaction and low friction were present between the PNaMPS gel and the APS modified surface. It, however, did not change in a solution of pH = 2, where the surface was positively charged and electrostatic attractive interaction and very high friction were present between the PNaMPS gel and the APS modified surface. These results suggest the important role of water structure for small friction at the polymer gel/solid interface.     

Azomacrocyclic derivatives of imidazole: synthesis, structure, and metal ion complexation properties

New azocrown ethers comprising imidazoles in the macrocycle have been synthesized. Imidazole, 2-methyl-, 4-methyl-, and 4-phenylimidazole were incorporated to form macrocyclic units by coupling with the appropriate bis-diazonium salts. The syntheses were performed under high dilution conditions. The X-ray structure of a water adduct of the 21-membered crown ether derivative of 4-methylimidazole 8 has been solved. Metal cation binding was investigated with the use of UV-vis spectroscopy in acetonitrile, methanol, and methanol-water mixtures. The obtained chromoionophores were tested as ion-carriers in ion-selective membrane electrodes.     

Changes of supermolecular structure of polymer materials at various processings

The DSC method is found convenient for the routine use in the control of changes of the supermolecular structure induced in polyethylene by treatment under various thermo-mechanical conditions. Heating and stretching of the samples act complementarily causing rearrangement of the structure, measurable even at low deformation (λ = 1.5), and below melting temperature.     

Optoelectronic and charge transport properties at organic-organic semiconductor interfaces: comparison between polyfluorene-based polymer blend and copolymer

We report detailed studies of optoelectronic and charge transport properties at the organic-organic semiconductor interfaces formed between polymer chains (interchain) and within a polymer chain (intrachain). These interfaces are fabricated using poly(9,9-di-n-octylfluorene-alt-N-(4-butylphenyl)diphenylamine) (TFB [f8-tfb]) (electron-donor) and poly(9,9-di-n-octylfluorene-alt-benzothiadiazole) (F8BT [f8-bt]) (electron-acceptor) conjugated polymers, by blending them together or by covalently attaching them via a main polymer backbone (copolymer). For optoelectronic properties, when a bulky and twisted tfb molecule is incorporated into a rigid F8BT conjugated backbone, it disturbs the conjugation of F8BT polymer, leading to a blue-shift in the lowest absorption transition. However, by acting as an effective electron donor, it assists the formation of an intrachain singlet exciton that has a strong charge-transfer character, leading to a red-shifted and longer-lived emission than that of F8BT. An extremely efficient and fast energy transfer from tfb donor to bt acceptor is observed in the copolymer (<1 ps) compared to transfer from TFB to F8BT in the blend (tens of ps). This efficient energy transfer in the copolymer is found to be associated with its low fluorescence efficiency (40-45% vs 60-65% for blend) because of the migration of radiative singlet excitons to low-energy states such as triplet and exciplex states that are nonemissive or weakly emissive. The presence of molecular-scale tfb-f8-bt interfaces in the copolymer, however, does not hinder an efficient transport of charge carriers at high drive voltages. Instead, it provides a better balance of charge carriers inside the device, which leads to slower decay of the device efficiency and thus more stable light-emitting diodes with increasing voltage than the blend devices. These distinctive optoelectronic and charge transport properties observed at different organic-organic semiconductor interfaces will provide useful input for the design rules of conjugated polymers required for improved molecular electronics.     

Formation,structure and properties of polymer networks

The systematic study of intramolecular reaction and gel points together with measurements of moduli of trifunctional and tetrafunctional end-linked networks show that gel points are always delayed and network imperfections always result from the intramolecular reaction which must occur when such networks are formed. The quantitative interpretation and prediction of gel points and moduli directly from reactant structures and reaction conditions are discussed.     

Structure,conformation and dynamics of polymer chains at solid melt interfaces

We have performed molecular dynamics, and lattice Monte Carlo simulations of polymeric melts in the vicinity of solid surfaces. The structural features of the solid-melt interface were very simple. The interfacial width was comparable to the segment size. Inside this narrow interface the segment density profile was oscillatory. The density oscillations were much less pronounced than those present at solid-atomic liquid interfaces. On a scale much larger than the segment size, chain conformations were found to be identical with those of ideal chains next to a reflective barrier. In particular, the number of surface-segment contacts scaled like the square root of the molecular weight. Extensive molecular dynamics simulations showed that chain desorption times increase with molecular weight but at a rate much slower than the longest relaxation time of Rouse chains. Therefore, sufficiently long chains desorbed almost freely from the surface despite the presence of attractive surface-segment interactions. A study of chain relaxation dynamics confirmed that the Rouse modes constitute an appropriate set of normal coordinates for chains in the melt interacting with a solid surface. The effect of the surface on mode relaxation was significant. All relaxation processes of chains located within a couple of radii of gyration from the surface were slowed down considerably. This effect, however was approximately the same for fast and slow modes and independent of molecular weight for sufficiently long chains.     

Ambient-curable polysiloxane coatings: structure and mechanical properties

Ambient-curable polysiloxane coatings were prepared by hydrolysis and condensation of 3-methacryloxypropylmethyldimethoxysilane (MPDS) and methyltriethoxysilane (MTES) and subsequently mixing with 3-aminopropyltriethoxysilane (APS). The structures of the as-obtained polysiloxane oligomers as well as the dried polysiloxane coatings on tinplate substrates were analyzed by FTIR and ²⁹Si NMR. The mechanical properties of the coatings were thoroughly examined at both macro-level and micro-level using a pendulum hardness rocker, an impact tester, and a nanoindentation/nanoscratch instrument. Effects of the molar ratio of MPDS/MTES, the dosage of aqueous ammonia solution, and the catalytic condition on the structure of polysiloxane oligomers as well as the structure and mechanical properties of the polysiloxane coatings were investigated. The dried coatings with thickness of 15–26 μm are highly elastic. The hardness (Koenig hardness and microhardness), impact resistance and scratch resistance are mainly dependent on the condensation degree of polysiloxane coatings rather than on the organic component of the coatings. A proper pre-hydrolysis process or more APS is benefit for enhancing the mechanical strength of the polysiloxane coatings. Polysiloxane coatings with high hardness and excellent scratch resistance can be prepared preferentially at low molar ratio of MPDS/MTES.     

Molecular structure and dynamics at the interfaces within bulk heterojunction materials for solar cells

The molecular structures within the interfaces of the bulk heterojunction material comprising regioregular-poly(3-hexylthiophene-2,5-diyl), rrP3HT, and C(60) or its soluble derivative, [6,6]-phenyl-C(61)butyric acid methyl ester, PCBM, have been studied by one- and two-dimensional nuclear magnetic resonance (NMR). The local structure within the interface was inferred from chemical shift (CS) data obtained from composite films (CFs) fabricated at room temperature (PCBMCF-RT and C(60)CF-RT) and from CFs that had been subsequently annealed at 150 degrees C for 30 min (PCBMCF-A150 and C(60)CF-150A). In PCBMCF-RT, the alkyl side chains of rrP3HT are close to the C(60) ball; C(60) is essentially 'wrapped' by the alkyl side chains. In PCBMCF-A150, the alkyl side chains self-assemble such that rrP3HT and PCBM are separated. The observation of well-defined splittings in the CS spectrum of the (13)C of C(60) in C(60)CF-A150 indicates a distortion from spherical symmetry. Measurements of the spin-lattice relaxation rate, 1/T(1), of C(60) imply local magnetic field fluctuations that arise from the dynamics of the C(60) distortion.     

Molecular basis of healing and fracture at polymer interfaces

The interdiffusion of polymer chains across a polymer–polymer interface, and subsequent fracture to re-create the interface is reviewed. In particular, films formed via latex coalescence provide a very large surface area. Of course, latex film formation is a very important practical problem. Healing of the interface by interdiffusion is treated using the de Gennes reptation theory and the Wool minor chain reptation model. The self-diffusion coefficients of polystyrene and the polymethacrylates obtained by small-angle neutron scattering, SANS, direct non-radiative energy transfer, DET, and other techniques are compared. Reduced to 150,000 g/mol and 135°C, both polystyrene and poly(methyl methacrylate) have diffusion coefficients of the order of 10^?16?10^?17 cm²/sec. Variations in the diffusion coefficient values are attributed to the experimental approaches, theoretical treatments and molecular weight distribution differences. An activation energy of 55 kcal/mol was calculated from an Arrhenius plot of all polystyrene data reduced to a number-average molecular weight of 150,000 g/mol, using an inverse square molecular weight conversion method. Interestingly, this is in between the activation energies for the α and β relaxation processes in polystyrene, 84 and 35 kcal/mol, respectively. Fracture of polystyrene was considered in terms of chain scission and chain pull-out. A dental burr apparatus was used to fracture the films. For low molecular weights, chain pull-out dominates, but for high molecular weights, chain scission dominates. At 150,000 g/mol, the energy to fracture is divided approximately equally between the two mechanisms. Above a certain number average molecular weight (about 400,000 g/mol), the number of chain scissions remains constant at about 10²⁴ scissions/m³. Energy balance calculations for film formation and film fracture processes indicate that the two processes are partly reversible, but have important components of irreversibility. From the interdiffusion SANS data, the diffusion rate is calculated to be about 1 Å/min, which is nine orders of magnitude slower than the dental burr pull-out velocity of about 0.8 cm/sec.     

Synthesis,structure, and complexation properties of hydroxybenzyl analogs of diethylenetriaminepentaacetic acid

As part of continuous search for new chelating agents for transition metal ions, a practical method for the preparation of hydroxybenzyl derivatives of diethylenetriaminepentaacetic acid (DTPA) has been developed in this study. N,N″-bis(2-hydroxybenzyl)diethylenetriamine-N,N′,N″-triacetic acid (HBDTTA) and four other hydroxybenzyl derivatives of DTPA have been synthesized. The structures and chelating capacity of three ligands with Fe³⁺ have been predicted using density functional theory-based calculations at the BP86/TZVP level within a previously developed and tested computational procedure. The results show that like DTPA, HBDTTA can adopt several different six- and seven-coordinate complex structures. The good stability of Fe³⁺ complexes of two HBDTTA derivatives indicates that HBDTTA-type ligands can be modified by removing some of their functional groups without greatly affecting their metal-binding efficiency.     

Nanoparticle assembly at fluid interfaces: structure and dynamics

The self-assembly of nanoparticles at fluid interfaces, driven by the reduction in interfacial energy, was investigated. With spherical, tri-n-octyl-phosphine-oxide covered cadmium selenide (CdSe) nanoparticles (1-8 nm), thermal fluctuations compete with the interfacial segregation giving rise to a size-dependent self-assembly of the particles. The structure of the nanoparticle assembly was studied using electron microscopy, atomic force microscopy, and X-ray scattering in situ, which indicate that the particles form a densely packed monolayer. The energetics of the adsorption of nanoparticles onto the interface was revealed by time-dependent fluorescence studies on a mixture of two different sized nanoparticles at the interface. The dynamics of the nanoparticles at the fluid interface, probed using fluorescence photobleaching methods, suggests a liquid-like behavior. The results have implications in the design of hierarchical self-assemblies of nanoparticles for the one-step fabrication of devices on multiple length scales.     

Water structure at interfaces: the present situation

This paper discusses the structure of adsorbed water at interfaces. It begins with a review of the development of the research, then examines and compares some of the most important models in this field. The results of recent spectroscopic work, especially those of IR spectroscopy, are discussed and applied to the selection of the most probable model.     

Formation,structure and properties of polymer networks: theory and modelling

The application of a Monte-Carlo (MC) algorithm to account fully for loop formation in RA₂ + R′B₃ and RA₂ + R′B₄ polymerisations is described. The resulting interpretation of experimental elastic moduli of polyurethane networks prepared at different dilutions shows it is essential to account for elastic losses in loop structures of all sizes. An important parameter, x, is introduced, namely the average fractional loss of elasticity per larger loop structure relative to the loss per smallest loop structure. Values of x vary between 0.50 and 0.60, depending on junction point functionality, reactant or network chain stiffness and number of skeletal bonds per smallest loop structure. Application of the MC calculations to the formation and resulting structure of poly(dimethyl siloxane) networks again predicts significant reductions in modulus due to loop structures. However, comparison with experimental modulus data shows that the reductions in modulus due to loops are outweighed by increases due to chain entanglements.