Combining Neutron Reflectivity Research Articles

Effect of Trapped Solvent on the Interface between PS-b-PMMA Thin Films and P(S-r-MMA) Brush Layers

The orientation of block copolymer (BCP) features in thin films can be obtained by spin-coating a BCP solution on a substrate surface functionalized by a polymer brush layer of the appropriate random copolymer (RCP). Although this approach is well established, little work reporting the amount and distribution of residual solvent in the polymer film after the spin-coating process is available. Moreover, no information can be found on the effect of trapped solvent on the interface between the BCP film and RCP brush. In this work, systems consisting of poly(styrene)-b-poly(methyl methacrylate) thin films deposited on poly(styrene-r-methyl methacrylate) brush layers are investigated by combining neutron reflectivity (NR) experiments with simulation techniques. An increase in the amount of trapped solvent is observed by NR as the BCP film thickness increases accompanied by a significant decrease of the interpenetration length between the BCP and RCP, thus suggesting that the interpenetration between grafted chains and block copolymer chains is hampered by the solvent. Hybrid particle-field molecular dynamics simulations of the analyzed system confirm the experimental observations and demonstrate a clear correlation between the interpenetration length and the amount of trapped solvent.

Not just a fluidifying effect: omega-3 phospholipids induce formation of non-lamellar structures in biomembranes.

Polyunsaturated omega-3 fatty acid docosahexaenoic acid (DHA) is found in very high concentrations in a few peculiar tissues, suggesting that it must have a specialized role. DHA was proposed to affect the function of the cell membrane and related proteins through an indirect mechanism of action, based on the DHA-phospholipid effects on the lipid bilayer structure. In this respect, most studies have focused on its influence on lipid-rafts, somehow neglecting the analysis of effects on liquid disordered phases that constitute most of the cell membranes, by reporting in these cases only a general fluidifying effect. In this study, by combining neutron reflectivity, cryo-transmission electron microscopy, small angle neutron scattering, dynamic light scattering and electron paramagnetic resonance spectroscopy, we characterize liquid disordered bilayers formed by the naturally abundant 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine and different contents of a di-DHA glycero-phosphocholine, 22:6-22:6PC, from both a molecular/microscopic and supramolecular/mesoscopic viewpoint. We show that, below a threshold concentration of about 40% molar percent, incorporation of 22:6-22:6PC in the membrane increases the lipid dynamics slightly but sufficiently to promote the membrane deformation and increase of multilamellarity. Notably, beyond this threshold, 22:6-22:6PC disfavours the formation of lamellar phases, leading to a phase separation consisting mostly of small spherical particles that coexist with a minority portion of a lipid blob with water-filled cavities. Concurrently, from a molecular viewpoint, the polyunsaturated acyl chains tend to fold and expose the termini to the aqueous medium. We propose that this peculiar tendency is a key feature of the DHA-phospholipids making them able to modulate the local morphology of biomembranes.

Surface properties of the ethanol/water mixture: Thickness and composition

Ethanol is a common amphiphilic solvent often used in conjunction with water. However, despite its widespread use, key questions regarding the thickness and composition of molecules at the ethanol/water/air surface remain unclear. Recent thermodynamic analyses, Bagheri and co-authors (2016) and Santos and Reis (2018), indicated that the interfacial thickness is not constant. However, the interfacial thickness from these two analyses follows opposite trends. This study aims to provide a detailed description of the thickness and composition of the interfacial layer by combining neutron reflectivity (NR) experiments with rigorous molecular simulation. The interfacial composition was determined from molecular simulations and, in conjunction with the Gibbs excess concentration, used to calculate the interfacial thickness. It was found that the thickness decreased exponentially and reached a plateau of ~ 8.2 Å. The results confirm the trends obtained thermodynamically from surface tension. The study also provides a new theoretical framework to describe the interfacial layer of water/alcohol mixtures.

Duplex nanoporous alumina and polyelectrolyte adsorption: more insights from a combined neutron reflectivity and electron microscopy study.

Neutron reflectivity (NR) is a powerful technique to investigate the incorporation of nanomaterials (polymers, nanoparticles, etc) into multilayer porous systems. Here we propose an experimental approach combining NR and scanning electron microscopy (SEM) to successfully characterize duplex nanoporous anodic aluminum oxides (nAAO) and to extract quantitative information about the entering and adsorption of polyelectrolytes (PEs) in nanopores. Duplex nAAO are promising systems to study the influence of geometrical constriction, i.e. the reduction of pore diameters along the pore channel, on the confinement of condensed matters.

Gravimetric and density profiling using the combination of surface acoustic waves and neutron reflectivity

A new approach is described herein, where neutron reflectivity measurements that probe changes in the density profile of thin films as they absorb material from the gas phase have been combined with a Love wave based gravimetric assay that measures the mass of absorbed material. This combination of techniques not only determines the spatial distribution of absorbed molecules, but also reveals the amount of void space within the thin film (a quantity that can be difficult to assess using neutron reflectivity measurements alone). The uptake of organic solvent vapours into spun cast films of polystyrene has been used as a model system with a view to this method having the potential for extension to the study of other systems. These could include, for example, humidity sensors, hydrogel swelling, biomolecule adsorption or transformations of electroactive and chemically reactive thin films. This is the first ever demonstration of combined neutron reflectivity and Love wave-based gravimetry and the experimental caveats, limitations and scope of the method are explored and discussed in detail.

Combining Neutron Reflectivity and Hydrogen Deuterium Exchange Mass Spectrometry to Resolve Structural details of Membrane Associated Proteins

While X-ray crystallography and NMR are available to determine the structure of folded proteins in solution, few methods can resolve structural details for membrane-associated proteins. Recently neutron reflection (NR) has emerged as a useful method to derive structural information for membrane-associate proteins. However, while NR provides the overall residue distribution normal to the membrane, information pertaining to specific residues is not available from this method. Toward addressing this deficiency we combined NR with hydrogen-deuterium exchange mass spectrometry (HDX-MS). HDX-MS has proven useful in studying protein complexes and binding of drug molecules to proteins in solution. We combined these methods to study the structure of membrane-associated HIV Nef. Nef is an HIV-1 accessory proteins and an essential factor in AIDS progression. Nef exists in both membrane-associated and cytosolic fractions. Membrane-association is achieved by an N-terminal myristoylation essential for the virus in vivo as well as a cluster of basic residues within the N-terminal arm. NR revealed that upon insertion of the myristate and residues from the N-terminal arm, Nef transitions from a closed to open conformation that positions the core domain 70 A from the lipid headgroups.1 Deuterium exchange of HIV-1 Nef was analyzed in solution and also when membrane-associated. Significant differences in deuterium uptake between the solution form and lipid-associated state reveal the disposition of a flexible loop and the oligomeric state upon binding. Hundreds of proteins are known to be lipidated and many are potential targets for therapeutic intervention. The present approach will be useful to resolve the membrane-bound conformations of these proteins, and to inform on the effects of protein-protein interaction at the membrane and disruption of said interactions with pharmacological agents.Akgun, B. et al, Structure 21, 1822, 2013.

Cholesterol modulates the fusogenic activity of a membranotropic domain of the FIV glycoprotein gp36

Lipid composition of viral envelopes is usually rich in sphingolipids and cholesterol (CHOL). These components have a stiffening effect on the membrane, thus enhancing the energetic barrier to be overcome for its fusion with the T-cell plasma membrane, a fundamental step of the infection process. In this work, we demonstrate that the octapeptide (C8) corresponding to the Trp770–Ile777 sequence of the Feline Immunodeficiency Virus gp36 is highly effective in inducing the fusion of palmitoyl oleoyl phosphatidylcholine (POPC)/sphingomyelin (SM)/CHOL membranes. We analyze the molecular mechanism of the C8–membrane interactions combining Neutron Reflectivity (NR) and Electron Spin Resonance (ESR) experiments, and molecular dynamics simulations. A strict interplay among the different lipids in the peptide-induced fusion mechanism is highlighted. Since CHOL preferentially locates close to SM, POPC molecules remain relatively free to interact with the peptide, driving its positioning at the membrane interface. Here, C8 comes in contact with CHOL-interacting SM molecules, causing a strong perturbation of acyl chain ordering, which is a necessary condition for membrane fusion. Our findings suggest that CHOL rules, by an indirect mechanism, the activity of viral fusion glycoproteins.

Hydrogen distribution in Nb/Ta superlattices

The distribution of hydrogen in Nb/Ta superlattices has been investigated by combined neutron reflectivity and x-ray scattering. We provide evidence to support that strain modulations determined with x-ray diffraction can be interpreted as modulations in hydrogen content. We show that the hydrogen concentration is modulated and favors Nb, in agreement with previous studies. We measure the concentration directly using neutron reflectivity and demonstrate no detectable change in the distribution of hydrogen with temperature, in stark contrast to previous studies.

Softening of PMMA Brushes upon Collapse/Swelling Transition. A Combined Neutron Reflectivity and Nanomechanical Cantilever Sensor Study

In this work, we establish a direct correlation between chain mechanics and structural properties of polymer brushes upon swelling. We present experimental results on poly(methyl methacrylate) (PMMA) brushes prepared via surface initiated atomic transfer radical polymerization. Neutron reflectivity studies gave insight into the brush thickness and volume fraction profiles of the brush, gradually swollen with solvent mixtures. Comparison of our experiments with scaling theory yielded specific polymer−solvent interaction parameters and gave insight into the desorption and adsorption behavior of bad and good solvents, respectively. Insight into the brush’s chain mechanics was obtained from surface stress investigations using the nanomechanical cantilever sensor bending technique. It was shown that polymer brush swelling leads to a decrease in surface stress due to chain disentanglements and the related softening of the polymer brush under Θ-solvent conditions.

Interface between a Polysulfone and Polyamide As Studied by Combined Neutron Reflectivity and Small-Angle Neutron Scattering Techniques

Interfaces between the two phases formed in immiscible polymer pairs were studied by means of neutron reflectivity (NR) combined with small-angle neutron scattering (SANS). The interfacial widths evaluated by the two methods were compared for two systems: a nonreactive system composed of polyamide (PA) and polysulfone (PSU) and a reactive system composed of PA and PSU with a phthalic anhydride reactive end group (PSU-R) in which PSU-b-PA block copolymers can be formed at the interface. SANS measurements were made on bulk mixtures, and the NR measurements were performed on thin film bilayer stacks of PA and PSU or PSU-R. The intrinsic interfacial widths, WI,diffuse, obtained from the two methods were compared for the samples that underwent similar annealing protocols and were quenched below the glass transition temperatures Tg's to room temperature. A consistent result for WI,diffuse can be obtained for the two methods only after taking into account the following important factors: (i) the SANS intensity should be corrected for the frozen density heterogeneities within each phase as well as the frozen thermal composition fluctuations; (ii) the contribution of capillary wave fluctuations to a net (or observed) interfacial width, WI,obs, is more significant for NR than SANS. Without the former correction, the SANS profiles at high scattering vector, q, showed asymptotic behavior of q-n (n < 4), giving an erroneous conclusion on WI,obs. The WI,obs values obtained for the reactive system were found to be larger than those for the nonreactive system for both methods, reflecting block copolymer formation at the interfaces. The interfacial area density, Σ, for the bulk mixture of the reactive system rapidly decreased with time and reached a constant value within ca. 2 min after annealing above the Tg's, while Σ for the nonreactive system kept decreasing with time. This elucidates that the growth of the phase-separating domains in the reactive system was pinned by the formation of block copolymers at the interfaces during early annealing times.