2D reflectometry for the investigation of polymer interfaces: off-specular neutron scattering.

Specular and off-specular neutron reflectometry have been used in a combined approach to study thin polymer films. Our goal in this work is to illustrate the power of the off-specular scattering technique to probe the properties of the buried interface of immiscible polymer bilayers of deuterated polystyrene and protonated poly(methyl methacrylate) (h-PMMA). The diffuse scattering stemming from these systems is discussed in relation to thermal fluctuations at the polymer/polymer interface, providing a means to extract in-plane correlation lengths from buried interfaces. In addition the onset of hole formation in the top layer is evidenced by the diffuse scattering, not easily detectable by specular reflection alone.

Taming the Strength of Interfacial Interactions via Nanoconfinement.

The interaction between two immiscible materials is related to the number of contacts per unit area formed by the two materials. For practical reasons, this information is often parametrized by the interfacial free energy, which is commonly derived via rather cumbersome approaches, where properties of the interface are described by combining surface parameters of the single materials. These combining rules, however, neglect any effect that geometry might have on the strength of the interfacial interaction. In this Article, we demonstrate that the number of contacts at the interface between a thin polymer coating and its supporting substrate is altered upon confinement at the nanoscale level. We show that explicitly considering the effect of nanoconfinement on the interfacial potential allows a quantitative prediction of how sample geometry affects the number of contacts formed at the interface between two materials.

Substrate-Induced Phase of a Benzothiophene Derivative Detected by Mid-Infrared and Lattice Phonon Raman Spectroscopy.

The presence of a substrate-induced polymorph of 2,7-dioctyloxy[1]benzothieno[3,2-b]benzothiophene is probed in microscopic crystals and in thin films. Two experimental techniques are used: lattice phonon Raman and IR spectroscopy. The bulk crystal and substrate-induced phase have an entirely different molecular packing, and therefore, their Raman spectra are characteristic fingerprints of the respective polymorphs. These spectra can be unambiguously assigned to the individual polymorphs. Drop-cast and spin-coated thin films on solid substrates are investigated in the as-prepared state and after solvent-vapor annealing. Because Raman spectroscopy is less sensitive with decreasing film thickness, IR spectroscopy is shown to be a more feasible tool for phase detection. The surface-induced phase is mainly present in the as-prepared thin films, whereas the bulk phase is present after solvent-vapor annealing. This result suggests that the surface-induced phase is a metastable polymorph.

Unexpected impact of irreversible adsorption on thermal expansion: Adsorbed layers are not that dead.

We investigated the impact of irreversible adsorption on the mechanisms of thermal expansion of 1D confined polymer layers. For spincoated films (polystyrene on aluminum) of constant thickness, the thermal expansion coefficient of the melt drops upon annealing following the kinetics of irreversible adsorption of the chains onto the supporting substrate, while the thermal expansion of the glass is annealing invariant. These perturbations are explained in terms of the reduction in free volume content, upon immobilization of monomers onto the substrate. To shed more light on this phenomenon, we performed an extensive investigation of the thermal expansion of irreversibly adsorbed layers of polystyrene on silicon oxide. We verified that, contrarily to recent speculations, these films cannot be modeled as dead layers - immobilized slabs lacking of segmental relaxation. On the contrary, thin adsorbed layers show an increase in thermal expansion with respect to the bulk, due to packing frustration. Immobilization plays a role only when the thickness of the adsorbed layers overcomes ∼10 nm. Finally, we show that for adsorbed layers the difference in thermal expansion between the melt and the glass is sufficiently high to investigate the glass transition down to 3 nm. Owing to this unique feature, not shared by spincoated films, adsorbed layers are the perfect candidate to study the properties of extremely thin polymer films.

How irreversible adsorption affects interfacial properties of polymers.

Growing experimental evidence shows that the behavior of polymer chains confined at the nanoscale level strongly depends on the degree of adsorption correlated to the number density of monomers pinned onto the supporting substrate. In this contribution, after introducing the physics behind the mechanisms of irreversible adsorption, we review recent experimental observations on how adsorption affects properties of polymer melts confined in 1D, focusing on those related to the thermal glass transition, maximum water uptake, viscosity and crystallization. These findings strongly support a new physical framework of confined soft matter, not trivially limited to finite size effects and interfacial interactions, but also enriched by non-equilibrium phenomena.

Irreversible Adsorption Erases the Free Surface Effect on the Tg of Supported Films of Poly(4-tert-butylstyrene).

When cooled at constant rate, a 25 nm thin film of poly(4-tert-butylstyrene) vitrifies 50 K lower than in bulk. This record sets the largest depression in thermal glass transition temperature (Tg) ever observed upon confinement at the nanoscale level. Same as for other supported polymer layers, this reduction in Tg has been attributed to the presence of a free surface, the ensemble of molecules at the interface with air remaining in the liquid state also at temperatures well below bulk Tg. Here, we verify that such tremendous shifts can be erased upon prolonged annealing in the liquid state, hinting at a metastable nature of confinement effects. We demonstrate that the recovery of bulk behavior and the manifestation of the free surface are enslaved to the kinetics of irreversible adsorption of chains on the supporting substrate.

Mechanisms of Polymer Adsorption onto Solid Substrates.

Controlling polymer/substrate interfaces without modifying chemistry is nowadays possible by finely tuning the formation of adsorbed layers. The complex processes leading to irreversible attachment of chains onto solid substrates are governed by two mechanisms: molecular rearrangement and potential-driven adsorption. Here we introduce an analytical method to differentiate these two mechanisms. By analyzing experiments and simulations, we investigate how changes in thermal energy and interaction potential affect equilibrium and nonequilibrium components of the adsorption kinetics. We find that the adsorption process is thermally activated, with activation energy comparable to that of local noncooperative processes. On the other hand, the final adsorbed amount depends on the interface interaction only (i.e., it is temperature independent in experiments). We identify a universal linear relation between the growth rates at short and long adsorption times, suggesting that the monomer pinning mechanism is independent of surface coverage, while the progressive limitation of free sites significantly limits the adsorption rate.

Decoupling of Dynamic and Thermal Glass Transition in Thin Films of a PVME/PS Blend.

The discussions on the nanoconfinement effect on the glass transition and glassy dynamics phenomena have yielded many open questions. Here, the thickness dependence of the thermal glass transition temperature Tgtherm of thin films of a PVME/PS blend is investigated by ellipsometry. Its thickness dependence was compared to that of the dynamic glass transition (measured by specific heat spectroscopy) and the deduced Vogel temperature (T0). While Tgtherm and T0 showed a monotonous increase, with decreasing film thickness, the dynamic glass transition temperature (Tgdyn) measured at a finite frequency showed a nonmonotonous dependence that peaks at 30 nm. This was discussed by assuming different cooperativity length scales at these temperatures, which have different sensitivities to composition and thickness. This nonmonotonous thickness dependence of Tgdyn disappears for frequencies characteristic for T0. Further analysis of the fragility parameter showed a change in the glassy dynamics from strong to fragile, with decreasing film thickness.

Structural Evolution of an Organic Semiconducting Molecule onto a Soft Substrate.

The structural organization and evolution of the organic semiconducting molecule 2,7-dioctyloxy[1]benzothieno[3,2-b]-benzothiophene on a soft matrix is studied. Thin films of a blend formed from polystyrene and the molecule were prepared by spin-coating onto silicon substrates, which were subsequently studied by using a combination of microscopy and scattering techniques. The organic semiconducting molecule segregated to the surface and developed a phase with a different structure to the bulk, as in the case of a substrate induced phase observed previously. Under a solvent vapor annealing procedure, the growth of micrometer-sized tetragonal crystals onto the polymer surface was observed, which was not evidenced for the silicon substrates.

Interfacial Morphology and Effects on Device Performance of Organic Bilayer Heterojunction Solar Cells.

The effects of interface roughness between donor and acceptor in a bilayer heterojunction solar cell were investigated on a polymer-polymer system based on poly(3-hexylthiophene) (P3HT) and poly(dioctylfluorene-alt-benzothiadiazole) (F8BT). Both polymers are known to reorganize into semicrystalline structures when heated above their glass-transition temperature. Here, the bilayers were thermally annealed below glass transition of the bulk polymers (≈140 °C) at temperatures of 90, 100, and 110 °C for time periods from 2 min up to 250 min. No change of crystallinity could be observed at those temperatures. However, X-ray reflectivity and device characteristics reveal a coherent trend upon heat treatment. In X-ray reflectivity investigations, an increasing interface roughness between the two polymers is observed as a function of temperature and annealing time, up to a value of 1 nm. Simultaneously, according bilayer devices show an up to 80% increase of power conversion efficiency (PCE) for short annealing periods at any of the mentioned temperatures. Together, this is in agreement with the expectations for enlargement of the interfacial area. However, for longer annealing times, a decrease of PCE is observed, despite the ongoing increase of interface roughness. The onset of decreasing PCE shifts to shorter durations the higher the annealing temperature. Both, X-ray reflectivity and device characteristics display a significant change at temperatures below the glass transition temperatures of P3HT and F8BT.

Lipid polyunsaturation determines the extent of membrane structural changes induced by Amphotericin B in Pichia pastoris yeast.

The activity of the potent but highly toxic antifungal drug Amphotericin B (AmB), used intravenously to treat systemic fungal and parasitic infections, is widely accepted to result from its specific interaction with the fungal sterol ergosterol. While the effect of sterols on AmB activity has been intensely investigated, the role of membrane phospholipid composition has largely been ignored, and structural studies of native membranes have been hampered by their complex and disordered nature. We show for the first time that the structure of fungal membranes derived from Pichia pastoris yeast depends on the degree of lipid polyunsaturation, which has an impact on the structural consequences of AmB activity. AmB inserts in yeast membranes even in the absence of ergosterol, and forms an extra-membraneous layer whose thickness is resolved to be 4-5 nm. In ergosterol-containing membranes, AmB insertion is accompanied by ergosterol extraction into this layer. The AmB-sponge mediated depletion of ergosterol from P. pastoris membranes gives rise to a significant membrane thinning effect that depends on the degree of lipid polyunsaturation. The resulting hydrophobic mismatch is likely to interfere with a much broader range of membrane protein functions than those directly involving ergosterol, and suggests that polyunsaturated lipids could boost the efficiency of AmB. Furthermore, a low degree of lipid polyunsaturation leads to least AmB insertion and may protect host cells against the toxic effects of AmB. These results provide a new framework based on lipid composition and membrane structure through which we can understand its antifungal action and develop better treatments.

Are polymers glassier upon confinement?

Glass forming systems are characterized by a stability against crystallization upon heating and by the easiness with which their liquid phase can be transformed into a solid lacking of long-range order upon cooling (glass forming ability). Here, we report the thickness dependence of the thermal phase transition temperatures of poly(l-lactide acid) thin films supported onto solid substrates. The determination of the glass transition, cold crystallization and melting temperatures down to a thickness of 6 nm, permitted us to build up parameters describing glass stability and glass forming ability. We observed a strong influence of the film thickness on the latter, while the former is not affected by 1D confinement. Further experiments permitted us to highlight key structural morphology features giving insights to our ellipsometric results via a physical picture based on the changes in the free volume content in proximity of the supporting interfaces.

Thermal stability and molecular ordering of organic semiconductor monolayers: effect of an anchor group.

The thermal stability and molecular order in monolayers of two organic semiconductors, PBI-PA and PBI-alkyl, based on perylene derivatives with an identical molecular structure except for an anchor group for attachment to the substrate in PBI-PA, are reported. In situ X-ray reflectivity measurements are used to follow the stability of these monolayers in terms of order and thickness as temperature is increased. Films have thicknesses corresponding approximately to the length of one molecule; molecules stand upright on the substrate with a defined structure. PBI-PA monolayers have a high degree of order at room temperature and a stable film exists up to 250 °C, but decomposes rapidly above 300 °C. In contrast, stable physisorbed PBI-alkyl monolayers only exist up to 100 °C. Above the bulk melting point at 200 °C no more order exists. The results encourage using anchor groups in monolayers for various applications as it allows enhanced stability at the interface with the substrate.

Substrate-induced phase of a [1]benzothieno[3,2-b]benzothiophene derivative and phase evolution by aging and solvent vapor annealing.

Substrate-induced phases (SIPs) are polymorphic phases that are found in thin films of a material and are different from the single crystal or "bulk" structure of a material. In this work, we investigate the presence of a SIP in the family of [1]benzothieno[3,2-b]benzothiophene (BTBT) organic semiconductors and the effect of aging and solvent vapor annealing on the film structure. Through extensive X-ray structural investigations of spin coated films, we find a SIP with a significantly different structure to that found in single crystals of the same material forms; the SIP has a herringbone motif while single crystals display layered π-π stacking. Over time, the structure of the film is found to slowly convert to the single crystal structure. Solvent vapor annealing initiates the same structural evolution process but at a greatly increased rate, and near complete conversion can be achieved in a short period of time. As properties such as charge transport capability are determined by the molecular structure, this work highlights the importance of understanding and controlling the structure of organic semiconductor films and presents a simple method to control the film structure by solvent vapor annealing.

Polymorphism of dioctyl-terthiophene within thin films: The role of the first monolayer.

The origins of specific polymorphic phases within thin films are still not well understood. The polymorphism of the molecule dioctyl-terthiophene is investigated during the presence of a silicon-oxide surface during the crystallisation process. It is found that a monolayer of molecules forms two-dimensional crystals on the surface. In the case of thicker films crystalline islands are formed, a comparison of the three polymorphic phases observed within thin films and the thermodynamically more stable single crystal phases reveals distinct differences which can be related to an adaption of the molecular packing with the flat surface of the substrate.

X-ray structural investigation of nonsymmetrically and symmetrically alkylated [1]benzothieno[3,2-b]benzothiophene derivatives in bulk and thin films.

A detailed structural study of the bulk and thin film phases observed for two potential high-performance organic semiconductors has been carried out. The molecules are based on [1]benzothieno[3,2-b]benzothiophene (BTBT) as conjugated core and octyl side groups, which are anchored either symmetrically at both sides of the BTBT core (C8-BTBT-C8) or nonsymmetrically at one side only (C8-BTBT). Thin films of different thickness (8-85 nm) have been prepared by spin-coating for both systems and analyzed by combining specular and grazing incidence X-ray diffraction. In the case of C8-BTBT-C8, the known crystal structure obtained from single-crystal investigations is observed within all thin films, down to a film thickness of 9 nm. In the case of C8-BTBT, the crystal structure of the bulk phase has been determined from X-ray powder diffraction data with a consistent matching of experimental and calculated X-ray diffraction patterns (Rwp = 5.8%). The packing arrangement of C8-BTBT is similar to that of C8-BTBT-C8, that is, consisting of a lamellar structure with molecules arranged in a "herringbone" fashion, yet with lamellae composed of two head-to-head (or tail-to-tail as the structure is periodic) superimposed molecules instead of only one molecule for C8-BTBT-C8. As for C8-BTBT-C8, we demonstrate that the same phase is observed in bulk and thin films for C8-BTBT whatever the film thickness investigated.

Production and analysis of perdeuterated lipids from Pichia pastoris cells.

Probing molecules using perdeuteration (i.e deuteration in which all hydrogen atoms are replaced by deuterium) is extremely useful in a wide range of biophysical techniques. In the case of lipids, the synthesis of the biologically relevant unsaturated perdeuterated lipids is challenging and not usually pursued. In this work, perdeuterated phospholipids and sterols from the yeast Pichia pastoris grown in deuterated medium are extracted and analyzed as derivatives by gas chromatography and mass spectrometry respectively. When yeast cells are grown in a deuterated environment, the phospholipid homeostasis is maintained but the fatty acid unsaturation level is modified while the ergosterol synthesis is not affected by the deuterated culture medium. Our results confirm that the production of well defined natural unsaturated perdeuterated lipids is possible and gives also new insights about the process of desaturase enzymes.

Neutron reflectometry elucidates density profiles of deuterated proteins adsorbed onto surfaces displaying poly(ethylene glycol) brushes: evidence for primary adsorption.

The concentration profile of deuterated myoglobin (Mb) adsorbed onto polystyrene substrates displaying poly(ethylene glycol) (PEG) brushes is characterized by neutron reflectometry (NR). The method allows to directly distinguish among primary adsorption at the grafting surface, ternary adsorption within the brush, and secondary adsorption at the brush outer edge. It complements depth-insensitive standard techniques, such as ellipsometry, radioactive labeling, and quartz crystal microbalance. The study explores the effect of the PEG polymerization degree, N, and the grafting density, σ, on Mb adsorption. In the studied systems there is no indication of secondary or ternary adsorption, but there is evidence of primary adsorption involving a dense inner layer at the polystyrene surface. For sparsely grafted brushes the primary adsorption involves an additional dilute outer protein layer on top of the inner layer. The amount of protein adsorbed in the inner layer is independent of N but varies with σ, while for the outer layer it is correlated to the amount of grafted PEG and is thus sensitive to both N and σ. The use of deuterated proteins enhances the sensitivity of NR and enables monitoring exchange between deuterated and hydrogenated species.

Dynamics of monolayer-island transitions in 2,7-dioctyl-benzothienobenzthiophene thin films.

The order in molecular monolayers is a crucial aspect for their technological application. However, the preparation of defined monolayers by spin-coating is a challenge, since the involved processes are far from thermodynamic equilibrium. In the work reported herein, the dynamic formation of dioctyl-benzothienobenzothiophene monolayers is explored as a function of temperature by using X-ray scattering techniques and atomic force microscopy. Starting with a disordered monolayer after the spin-coating process, post-deposition self-reassembly at room temperature transforms the initially amorphous layer into a well-ordered bilayer structure with a molecular herringbone packing, whereas at elevated temperature the formation of crystalline islands occurs. At the temperature of the liquid-crystalline crystal-smectic transition, rewetting of the surface follows resulting in a complete homogeneous monolayer. By subsequent controlled cooling to room temperature, cooling-rate-dependent kinetics is observed; at rapid cooling, a stable monolayer is preserved at room temperature, whereas slow cooling causes bilayer structures. Increasing the understanding and control of monolayer formation is of high relevance for achieving ordered functional monolayers with defined two-dimensional packing, for future applications in the field of organic electronics.

PEG polymer brushes via the Langmuir-Schaefer technique: the transfer ratio.

The Langmuir-Schaefer technique allows horizontal transfer of monolayer films of amphiphilic molecules from an air-water interface to a hydrophobic solid substrate. Spectroscopic ellipsometry has been used to estimate the transfer ratio of Polyethylene Glycol (PEG) polymer brushes prepared by this Langmuir-Schaefer method. We measured the transfer ratio for various degrees of polymerization and surface densities at the air-water interface. We do not observe a significant change of transfer ratio with the degree of polymerization. We also show that the transfer ratio is lower from an air-water monolayer film in the liquid phase than from a monolayer in the solid phase. Control over the transfer ratio is a crucial step in many experiments since the efficacy of the repression of proteins depends strongly on the assumption of the characteristics of the brushes.