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Uniform onion micelles formed from up to ten nano-structured polymer layers were produced by the aqueous self-assembly of highly-branched copolymers. Highly-branched poly(alkyl methacrylate)s were chain extended with poly(acrylic acid) in a two-step reversible addition–fragmentation chain transfer-self-condensing vinyl polymerization (RAFT-SCVP) in solution. The resulting polymers were dispersed into water from oxolane (THF) using a self-organized precipitation-like method and the self-assembled particles were studied by phase-analysis light scattering, small-angle neutron scattering, and electron microscopy techniques. The relative hydrophobicity of the blocks was varied by changing the alkyl methacrylate (methyl, butyl, or lauryl) and this was found to affect the morphology of the particles. Only the poly(butyl methacrylate)-containing macromolecule formed an onion micelle structure. The formation of this morphology was observed to depend on: the evaporation of the good solvent (THF) during the self-assembly process causing kinetic trapping of structures; the pH of the aqueous phase; and also on the ratio of hydrophobic to hydrophilic segments within the copolymer. The lamellar structure could be removed by annealing the dispersion above the glass transition temperature of the poly(butyl methacrylate). To exemplify how these onion micelles can be used to encapsulate and release an active compound, a dye, rhodamine B (Rh B), was encapsulated and released. The release behaviour was dependent on the morphology of the particles. Particles formed containing the poly(methyl methacrylate) or poly(lauryl methacrylate) core did not form onions and although these materials absorbed Rh B, it was continuously released at room temperature. On the other hand, the lamellar structure formed from branch-poly(butyl methacrylate)-[poly(butyl methacrylate)-block-poly(acrylic acid)] allowed for encapsulation of approximately 45% of the dye, without release, until heating disrupted the lamellar structure.

Friction force microscopy has been used to demonstrate that biocompatible, lubricious poly(2-(methacryloyloxy)ethylphosphorylcholine) (PMPC) brushes exhibit different frictional properties depending on the medium (methanol, ethanol, 2-propanol, and water; the latter also with different quantities of added salt). The chemical functionalization of the probe (amine-, carboxylic acid-, and methyl-terminated probes were used) is not as important as the medium in determining the contact mechanics. For solvents such as methanol, where the adhesion between AFM probe and PMPC brushes is negligible, a linear friction-load relationship is observed. In contrast, the friction-load plot is nonlinear in ethanol or water, media in which stronger adhesion is measured. For ethanol, the data indicate Johnson-Kendall-Roberts (JKR) mechanics, whereas the Derjaguin-Muller-Toporov (DMT) model provided a good fit for the data acquired in water. Contact mechanics on zwitterionic PMPC brushes immersed in aqueous solutions of varying ionic strength followed the same trend, with high adhesion energies being correlated with a nonlinear friction-load relationship. These results can be rationalized by treating the friction force as the sum of a load-dependent term, attributed to molecular plowing, and an area-dependent shear term. In a good solvent for PMPC such as methanol, the shear term is negligible and the sliding interaction is dominated by molecular plowing. However, the adhesion energy is significantly larger in water and ethanol and the shear term is no longer negligible.

We have used neutron reflectometry to characterize the swelling behaviour of brushes of poly[2-(diethyl amino)ethyl methacrylate], a polybase, as a function of pH. The brushes, synthesized by the "grafting from" method of atom transfer radical polymerization, were observed to approximately double their thickness in low pH solutions, although the pKa is shifted to a lower pH than in dilute solution. The composition-depth profile obtained from the reflectometry experiments for the swollen brushes reveals a region depleted in polymer between the substrate and the extended part of the brush. © The Royal Society of Chemistry.

For environmentally-switchable adhesive systems to be reused repeatedly, the adhesive strength must not deteriorate after each adhesion cycle. An important criterion to achieve this goal is that the integrity of the interface must be retained after each adhesion cycle. Furthermore, in order to have practical benefits, reversing the adhesion must be a relatively rapid process. Here, a double-network hydrogel of poly(methacrylic acid) and poly[oligo(ethylene glycol)methyl ether methacrylate] is shown to undergo adhesive failure during pH-switchable adhesion with a grafted (brush) layer of polycationic poly[2-(diethyl amino)ethyl methacrylate], and can be reused at least seven times. The surfaces are attached at pH 6 and detached at pH 1. A single-network hydrogel of poly(methacrylic acid), also exhibits pH-switchable adhesion with poly[2-(diethyl amino)ethyl methacrylate] but cohesive failure leads to an accumulation of the hydrogel on the brush surface and the hydrogel can only be reused at different parts of that surface. Even without an environmental stimulus (i.e. attaching and detaching at pH 6), the double-network hydrogel can be used up to three times at the same point on the brush surface. The single-network hydrogel cannot be reused under such circumstances. Finally, the time taken for the reuse of the double-network hydrogel is relatively rapid, taking no more than an hour to reverse the adhesion.

We describe an investigation of fluorescence correlation spectroscopy into the diffusion of fluorescein-tagged dextran (FDEX) in a poly(methacrylic acid) (PMAA) hydrogel. The temperature dependence of FDEX diffusion is shown to follow Zimm behavior in pure water, and the decrease in the diffusion coefficient when in the PMAA hydrogel has been modeled. The addition of acid and alkali (HCl and NaOH, respectively) not only control the swelling and collapse of the hydrogel but also reveal a strong pH dependence of the dextran diffusion coefficient, which shows a (nonmonatonic) increase with pH. The addition of NaCl and CaCl2 salts similarly showed evidence of network swelling, most notably at low salt concentration, but also that the diffusion coefficient within the gel at these low concentrations is larger than that in the equivalent solution without the hydrogel, indicating that the combination of hydrogel and salt works to increase the diffusion coefficient above that in pure water. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012

The frictional properties of poly(2-(methacryloyloxy)ethylphosphorylcholine) (PMPC) brushes grown from planar silicon surfaces by atom transfer radical polymerization (ATRP) have been characterized using in situ friction force microscopy (FFM). The dry thicknesses of the PMPC brushes ranged from 20 to 421 nm. For brush layers with dry thicknesses greater than ca. 100 nm, the coefficient of friction decreased with increasing film thickness. For shorter brushes, the coefficient of friction varied little with brush thickness. We hypothesize that the amount of bound solvent increases as the brush length increases, causing the osmotic pressure to increase and yielding a reduced tendency for the brush layer to deform under applied load. A comparison of the force-displacement plots acquired for various PMPC brushes under water supports this hypothesis, since a greater repulsive force is measured for thicker brushes. FFM was also used to investigate the well-known co-nonsolvency behavior exhibited by PMPC chains. For a PMPC brush layer of 307 nm dry thickness, the friction force was determined as a function of the volume fraction of alcohol in alcohol/water mixtures. Unlike a previous macroscopic study, a significant increase in the coefficient of friction was observed for ethanol/water mixtures at a volume fraction of 90%. This is attributed to brush collapse due to co-nonsolvency, leading to loss of hydration of the brush chains and hence substantially reduced lubrication. Force measurements normal to the surface indicate much greater hysteresis between approaching and retraction curves under co-nonsolvency conditions. However, no such effect was observed for 2-propanol/water and methanol/water mixtures over a wide range of volume fractions, in agreement with recent ellipsometric studies of PMPC brushes.

Angle resolved XPS (ARXPS) and scanning force microscopy (SFM) are used to study polystyrene/poly(vinyl ethyl ether) 50/50 wt% blend thin films spin cast from toluene solution, as a function of polystyrene molecular weight and film thickness. ARXPS is used to investigate the composition depth profile (CDP) of the blend thin films and SFM to study their surface morphology and miscibility. The CDPs are modelled by an empirical hyperbolic tangent function with three floating parameters. These are determined by non-linear least squares regression, their uncertainties estimated and the curve fit residuals analysed to demonstrate that the hyperbolic tangent CDP is a satisfactory fit to the ARXPS data. Conclusions are drawn regarding the behaviour of the blend thin films as the thickness and polystyrene molecular weight are varied. Flory-Huggins interaction parameters (chi) for the mixtures are calculated based upon the segregation data, and suggest a value of chi = 0.05 to be appropriate for this system. (c) 2009 Elsevier B.V. All rights reserved.

Poly(methacrylic acid) brushes have been prepared utilizing the ‘‘grafting from’’ technique and a livingradical synthesis route using a two stage process. Firstly a poly(1-ethoxyethyl methacrylate) brush wassynthesized by atom transfer radical polymerization and then thermally decomposed to poly-(methacrylic acid). The swelling behaviour of the weak polyacid brush was investigated as a function ofpH and salt concentration in aqueous solutions using atomic force microscopy. Force pulling measure-ments were used to establish the molecular weight and the grafted chain density. The swelling transitionwas found to be at pH 9; which is significantly different to the pK a (5.5) of untethered poly(methacrylicacid). We attribute this large shift in pK a to the high grafting density of these brushes. This can beexplained as a resultof the Coulombic repulsion of neighbouring charges. High salt concentrations (0.3 MNa þ ) also collapse the brush layer. Conversely low salt concentrations cause an increase in the thicknessof the brush, a behaviour expected for osmotic brushes. 2008 Elsevier Ltd. All rights reserved.

Poly(methacrylic acid) (PMAA) undergoes a conformational transition between pH 4 and 6 from a hypercoiled structure to a water-swollen state. There has been much speculation as to the exact nature and driving force of the transition. In this paper, we present a comprehensive investigation of the conformational switch of PMAA using techniques which report on various length scales: fluorescence energy transfer experiments provide unique information on the nanometer length scale while dynamic light scattering (DLS) offers an insight into longer range interactions involved in the transition. Fluorescence energy transfer measurements demonstrate that PMAA undergoes subtle molecular rearrangements between pH 2 and 5 as short-range hydrophobic interactions between methyl groups are broken down by increasing concentrations of mutually repulsive carboxylate anions. Although such rearrangements have been proposed to account for the pH behavior of PMAA, we reveal them experimentally using techniques sensitive to na...

The preferential segregation of one polymer component to a surface from a binary blend is due to the different surface energies of the polymers. Polymer surface segregation plays an important role in materials engineering because the size of the polymer chain means that surfaces can have properties significantly different from the bulk of the film. The utility and limitations of mean-field theory in describing surface segregation in miscible polymer systems is presented. As the miscibility of the polymer mixture decreases, the size of the segregated layer increases. The approach to the critical point (critical adsorption) requires a different treatment. For partially miscible polymers, there may be a transition between a finite surface segregated layer and complete wetting. The wetting transition can be understood using a mean-field construction. Factors involved in the kinetics of polymer surface segregation are discussed, before concluding with a discussion of the role of chain architecture, revealing the role of entropy in driving surface segregation.

The molar mass and dispersity of a polycation, poly[2-(dimethyl amino)ethyl methacrylate)] (PDMAEMA) grafted from a poly(methyl methacrylate) (PMMA) backbone, was measured by single-molecule force spectroscopy (SMFS) and shown to be consistent with results from gel permeation chromatography for the same comb polymer in aqueous solution. Comparison was then made between the comb polymer and PDMAEMA brushes that were grown from the substrate, as a function of the pH and ionic strength of the surrounding medium, and the limits of reliable characterization of the polymers are determined. A large discrepancy was observed between the responses of the comb and brush layer at low pH when the PDMAEMA molecules are extended from the supporting substrate. Here it is believed that the atomic force microscope (AFM) tip can penetrate the comb layer and selectively desorb side-chains of the comb. In the case of the well solvated PDMAEMA brushes at high pH, the tip preferentially selects larger chains, resulting in an over-estimate of the brush molar mass. The addition of salt also influenced the molar mass obtained by this technique. It is believed that salted brushes did not adhere well to the AFM tip, with subsequent desorption resulting in an underestimate of the molar mass. However, SMFS was shown to be capable of demonstrating the effect of salt on brush conformation, with greater swelling after the addition of a small amount of NaCl, but a significant decrease when 100 mM is added.

Hydrogel brushes are materials composed of a water-swollen network, which contains polymer chains that are grafted with another polymer. Using a thermally responsive polymer, poly( N -isopropyl acrylamide) (polyNIPAM), as the graft component we are able to maintain the critical solution temperature ( T crit ), independent of the overall composition of the material, at approximately 32°C. The change in swelling at T crit is a function of the amount of polyNIPAM in the system. However, there is a much smaller change in the surface contact angles at T crit . PolyNIPAM-based materials have generated considerable interest, as ‘smart’ substrates for the culture of cells and here, we show the utility of hydrogel brushes in cell culture. Chondrocytes attached to the hydrogel brushes and yielded viable cell cultures. Moreover, the chondrocytes could be released from the hydrogel brushes without the use of proteases by reducing the temperature of the cultures to below T crit to induce a change in the conformation of the polyNIPAM chain at T crit . The importance of the crosslink hydrogel component is illustrated by significant changes in cell attachment/cell viability as the crosslink density is changed.

Specular neutron reflectivity and nuclear reaction analysis (NRA) are used to measure the interfacial width between poly(9,9-dioctylfluorene) (F8) and poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT) in a 500 nm bilayer. Annealing at temperatures up to 280 °C allows us to vary this width over a large range, from ∼1 nm to greater than 30 nm. Approximate calculations using the predictions of self-consistent field theory (SCFT) for both Gaussian and semiflexible chains suggest that in the liquid−liquid regime the Gaussian results of SCFT are valid for this system. We also demonstrate the ability to control the interfacial width in 100 nm bilayers, on both silicon and indium tin oxide (ITO). The procedure of “preannealing” the F8 and F8BT layers, before bringing them together to make a bilayer, allows us to independently control the interfacial width and the properties of the bulk of these 100 nm films.

Progress in the development of actuating molecular devices based on responsive polymers is reviewed. The synthesis and characterization of "grafted from brushes and triblock copolymers is reported. The responsive nature of polyelectrolyte brushes, grown by surface initiated atomic transfer radical polymerization (ATRP), has been characterized by scanning force microscopy, neutron reflectometry, and single molecule force measurements. The molecular response is measured directly for the brushes in terms of both the brush height and composition and the force generated by a single molecule. Triblock copolymers, based on hydrophobic end blocks and polyacid midblock, have been used to produce polymer gels where the deformation of the molecules can be followed directly by small angle Xray scattering (SAXS), and a correlation between molecular shape change and macroscopic deformation has been established. A Landolt pHoscillator, based on bromate/sulfite/ferrocyanide, with a room temperature period of 20 min and a range of 3.1

In-situ grazing incidence X-ray diffraction has been used to investigate the kinetics of crystallization at the surface of thin films of poly(ethylene terephthalate) (PET). By varying the angle of incidence of the X-ray beam around the critical angle of the film, the penetration depth can be tuned to allow a direct comparison of molecular ordering in the surface and bulk of the film. The results show that ordering occurs significantly faster at the surface in the temperature range 90−100 °C (close to the bulk glass transition temperature of 75 °C). The (0 1 1) and (0 1 0) peaks narrow more rapidly and achieve a lower width, indicating that the crystallization progresses more rapidly in these crystallographic directions. This enhanced ordering is attributed to a combination of surface energy effects, which promote localized packing, and to an enhanced segmental mobility near the free surface.

We have used helium-3 nuclear reaction analysis to study the morphology of spin-cast blends of poly(9,-dioctylfluorene) (F8) and poly(9,-dioctylfluorene-alt-benzothiadiazole) as a function of composition, casting solvent, and initial solvent concentration. In blends cast from toluene, a surface segregated layer is observed for a broad range of F8 compositions. The surface excess has a maximum of ?nm at an F8 volume fraction = 0.5, dropping down to ?2?nm for = 0.2 and?0.8. The existence of such surface segregated layers could play an important role in charge transport in optoelectronic devices made from blends of conjugated polymers. Films cast from chloroform show negligible surface segregation. Scanning probe microscopy experiments show that films cast from toluene exhibit significant in-plane structure, in contrast to those cast from chloroform, which show minimal lateral structure.

The adsorption of block copolymers from a homopolymer matrix is studied as a function of nonadsorbing block length and adsorbing block type. The copolymers are poly(deuterated styrene-block-methyl methacrylate), dPS-b-PMMA, and poly(deuterated styrene-block-(dimethylamino)ethyl methacrylate), dPS-b-PDMAEMA, where the PMMA and PDMAEMA blocks adsorb to silicon oxide, SiOx, and the dPS block extends into a polystyrene, PS, matrix. Using neutron reflectivity and low-energy forward recoil spectrometry, the volume fraction profile, φ(z), copolymer interfacial excess, z*, interfacial width, w, and adsorbed layer thickness, h, are investigated as a function of the degree of polymerization of the dPS block, NdPS. For NdPS 200, D is less than Rg. Thus, a crossover from a collapsed to a stretched conformation occurs near the NdPS for entanglements in PS. Correspondingly, z*, w, and h strongly i...

We discuss some recent findings relating to the structure of interfaces in semiconducting polymer devices. The structure of three different types of interface is characterized via neutron reflectivity and scanning force microscopy. In the first example we find that enrichment of dopant occurs at the surface of a doped polymeric conductor, and that this enriched layer penetrates several nanometres into the material. Secondly, we find that the interface between a semiconducting polymer and an insulating polymer is not molecularly sharp, but is rather broad with a width typically of the order of 1 nm. Finally we present some initial neutron reflectivity measurements on the interface between two different semiconducting polymers (one of which is deuterated). Again we find that this interface is diffuse, with a typical width of the order of a few nanometres.

We have measured the equilibrium swelling of polystyrene networks by linear deuterated polystyrene using the ion beam technique of helium-3 nuclear reaction analysis. These measurements enable an analysis of the swelling of networks at low degrees of swelling, and our results showed more swelling than predicted by using the usual forms of Flory−Rehner theory. The swelling of the networks by linear polymers can be explained if we consider the network as connected heterogeneous clusters which unfold during the swelling process. Reasonable results can also be obtained in terms of the Flory−Rehner model if a value of the Flory−Huggins interaction parameter is used that is dependent on the density of cross-links in the network. However, the resultant interaction parameter is negative, in contrast with the general result for isotopic mixtures. We were also able to swell the networks with linear polystyrene, which was initially mixed with high molecular weight polystyrene (which could not penetrate the network),...

We have used neutron reflectometry and carbon and neon forward recoil spectrometry to measure the surface segregation of deuterated polystyrene from a hydrogenous polystyrene network. We find that when the linear polymer is of a high molecular weight (~600 000), the surface segregated profile can be predicted by mean field theory. In these systems the segregation is a rather slow function of time, reflecting the large number of entanglements in such crosslinked mixtures. When the deuterated polystyrene is of a lower molecular weight (~100 000), the surface segregated layer does not evolve monotonically with time but the shape of the profile can be predicted by mean-field theory. However, when the network is significantly crosslinked, the linear polymer is expelled from the network.

We present the results of a neutron reflectometry study of the kinetics of penetration of a grafted polymer layer into a permanently cross-linked, chemically identical network. The polymer used was polystyrene. Initially the grafted chains are completely excluded from the network, and the interface between them is sharp. The segment density profile was measured after a series of annealing times to follow the kinetics of penetration. The range of annealing times was extended over 9 orders of magnitude by using time−temperature superposition. We observe a slow approach to equilibrium, in good agreement with theory, which predicts this to be logarithmic in time. The kinetics of penetration is slower for more densely cross-linked networks and for greater brush grafting densities. We also note a grafting density dependence of the equilibrium profile but, somewhat unexpectedly, no dependence on the density of cross-links.

The differing glass transitions between deuterated polystyrene and poly(α-methylstyrene) are shown to have a profound effect on their interdiffusion coefficient. We have measured the interdiffusion coefficient as a function of concentration and temperature for two blends. The results can be interpreted in terms of the `fast' theory of interdiffusion with the temperature dependence obtained from a Williams–Landel–Ferry (WLF) treatment. The intradiffusion coefficients of the two polymers have also been deduced by this analysis. It is found that the deuterated polystyrene has a much more concentration-dependent intradiffusion coefficient than the poly(α-methylstyrene). Thermodynamic slowing down is also observed in one blend.

The tracer diffusion of deuterated polystyrene (dPS) into natural polystyrene–poly( α -methyl styrene) (hPS–P α MS) blends has been studied as well as the diffusion of dPS–P α MS blends into hPS–P α MS blends. Nuclear reaction analysis (NRA) as a depth profiling technique is shown to be useful to study the different systems. The diffusion constants have been determined as a function of temperature and as a function of composition of the blend of the tracer and the matrix. Blends as a tracer show a strong enhancement in diffusion in comparison to the pure dPS tracer, indicating that the intradiffusion coefficient measured in a sample where there is no chemical gradient is much bigger than the tracer diffusion measured from a thin film of pure dPS. The glass transition temperature T g has a considerable influence on the diffusion.

We have used neutron reflectometry to study the kinetics of the formation of a surface-enriched layer of deuterated polystyrene from a film of a blend with normal (hydrogenated) polystyrene. Three blends are considered: one using a completely miscible blend and two approaching the coexistence curve from the one-phase region. A simple theory based upon the assumption that the surface-enriched layer is in local equilibrium with a depletion layer provides qualitative agreement with the data. The formation of a wetting layer "depinned" from the surface layer is not observed in the times studied in the experiment.

International audience; We report the increased crystallization of poly(3-hexylthiophene) (P3HT) in the donor-acceptor mixture of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) with P3HT by the addition of a block copolymer, P3HT-b-PI, where PI refers to polyisoprene. The photovoltaic performance of devices created using this blend is markedly improved by the addition of the diblock copolymer. We have characterized the structure of thin films of the P3HT-b-PI containing mixtures using optical microscopy, scanning force microscopy, UV-vis absorption spectroscopy, neutron reflectometry, and grazing incidence X-ray diffraction (GIXD). The GIXD data provide the information on the crystallinity of the films, the absorption data were used to confirm that the addition of the diblock was responsible for the increase in crystallization, neutron reflectometry data reveal a PCBM-rich region near the hole injection layer, and the two microscopy techniques revealed the structural effect of the crystallization at the surface of the films.

Interfaces between a poly(3-hexylthiophene) [P3HT] and an end-grafted (brush) layer of poly(methyl methacrylate) [PMMA] are shown using neutron reflectometry to be dependent on heat treatment. Annealing the samples allows part of the brush layer to cross into the P3HT layer creating a very asymmetric interface. We suggest that the P3HT rearrangement occurs, creating space for movement of the brush into the film. This interpenetration was observed with two different molecular weight (17.5 and 28 kg mol−1) P3HT films. Output characteristics of devices made from P3HT layers on PMMA brushes show that different amounts of heat treatment do not significantly change the device performance. Saturated hole mobilities are dependent on heat treatment, with devices made from a smaller molecular weight P3HT (22 kg mol−1) demonstrating larger mobilities than devices created using 48 kg mol−1 P3HT, but only after heat treatment.

Neutron reflectivity was used to study the interface between the semiconducting polymer poly(9,9-dioctylfluorene) (PFO) and the insulating polymer poly(methyl methacrylate) (PMMA). The PFO/PMMA interfacial width was measured in the nematic and crystalline phases of the PFO, both with the PMMA on top of the PFO and vice versa. These interfaces are broad compared to atomic length scales, with measured interfacial widths in the range from 10 to 20 A. We found that the interfacial width was independent of both the chosen geometry and the thermal processing history. The equilibrium interfacial width only depended on temperature, with the width in the nematic phase of the PFO being broader than in the crystalline regime.

International audience; Herein we propose an approach toward the optimization of the photovoltaic performance of bulk heterojunctions by tuning the composition of the active layer with respect to the molecular weight of the semiconducting polymer. We used a poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) blend as a typical system and varied the molecular weight of the P3HT semiconducting polymer in order to determine its influence on the bulk heterojunction morphology as well as on the optoelectronic characteristics of the device. We have systematically mapped out the phase diagram for different molecular weight P3HTs blended with PCBM and observed the presence of a eutectic composition, which shifts to higher content of P3HT for lower molecular weight P3HTs. This shift inherent to the P3HT molecular weight is also apparent in the photovoltaic performance as the eutectic composition corresponds to the best of the photovoltaic properties. The P3HT molecular weight dependence of the eutectic composition is due to the molecular weight dependence of the P3HT crystallization behavior, which leads to dramatic morphological changes of the bulk heterojunction.

We introduce a copolymer with a comb topology that has been engineered to assemble in a brush configuration at an air-water interface. The molecule comprises a 6.1 kDa poly(methyl methacrylate) backbone with a statistical amount of poly[2-(dimethyl amino)ethyl methacrylate] polybase side chains averaging 2.43 per backbone.. Brush layers deposited with the hydrophobic PMMA backbone adsorbed to hydrophobized silicon are stable in water even when stored at pH values less than 2.0 for over 24 h. The use of a Langmuir trough allows a simple controlled deposition of the layers at a variety of grafting densities. Depth profiling of brush layers was performed using neutron reflectometry and reveals a significant shifting of the responsiveness of the layer upon changing the grafting density. The degree of swelling of the layers at a pH value of 4 (below the pK(b)) decreases as grafting density increases. Lowering the pH of the subphase during deposition causes the side chains to become charged and more hydrophilic extending to a brush-like configuration while at neutral pH the side chains lie in a "pancake" conformation at the interface. (C) 2009 Elsevier Ltd. All rights reserved.

The frictional behaviour of end-grafted poly[2-(dimethyl amino)ethyl methacrylate] films (brushes) has been shown by friction force microscopy to be a strong function of pH in aqueous solution. Data were acquired using bare silicon nitride and gold-coated tips, and gold coated probes that were functionalized by the deposition of self-assembled monolayers. At the extremes of pH (pH = 1, 2, and 12), the friction-load relationship was found to be linear, in agreement with Amontons' law of macroscopic friction. However, at intermediate pH values, the data were fitted by single asperity contact mechanics models; both Johnson-Kendall-Roberts (JKR) and Derjaguin-Muller-Toporov models were observed, with JKR behaviour fitting the data better at relatively neutral pH.

In good solvent 6,13-(triisopropylsilylethynyl) pentacene (TIPS-pentacene) was found to persist only as a unimer in solution at very low concentration, forming dimers and subsequently higher aggregates with increasing concentration well below the solubility limit. The extent of dimer and higher aggregate formation was found to be influenced by the addition of isotactic polystyrene through the formation of a supramolecular complex between TIPS-pentacene and isotactic polystyrene in a mutually good solvent through π–π stacking interaction. We believe this behaviour is a contributory factor in the previously reported improved organic thin film transistor device performance and stability of such blends mitigating the loss in performance often ascribed to solution processed small molecule organic semiconductors.

We show that films of poly(3,4-ethylene dioxythiophene)-poly(styrene sulfonate) crosslinked with glycerol function well as hole injection layers for light-emitting diodes even after immersion in water. Neutron reflectometry, X-ray photoelectron spectroscopy, and nuclear reaction analysis are used to characterise the structure of these layers. The glycerol at a moderate concentration of 5% (w/v) provides the best performance when used as a part of a light-emitting diode, but larger quantities impact adversely on device quality. Glycerol is shown to be present in considerable quantities in the film, at ∼13% by volume at the surface. No significant surface segregation of any component is observed in these films, although poly(3,4-ethylene dioxythiophene) is absent from the surface.