Bulent Akgun

Structure of [C4mpyr][NTf2] Room-Temperature Ionic Liquid at Charged Gold Interfaces

The structure of 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([C(4)mpyr][NTf(2)]) room-temperature ionic liquid at an electrified gold interface was studied using neutron reflectometry, cyclic voltammetry, and differential capacitance measurements. Subtle differences were observed between the reflectivity data collected on a gold electrode at three different applied potentials. Detailed analysis of the fitted reflectivity data reveals an excess of [C(4)mpyr](+) at the interface, with the amount decreasing at increasingly positive potentials. A cation rich interface was found even at a positively charged electrode, which indicates a nonelectrostatic (specific) adsorption of [C(4)mpyr](+) onto the gold electrode.

Surface-initiated polymerization

1 D.E. Bergbreiter, A.M. Kippenberger: Hyperbranched Surface Graft Polymerizations.- 2 R.R. Bhat, M.R. Tomlinson, T. Wu, J. Genzer: Surface-Grafted Polymer Gradients: Formation, Characterization and Applications.- 3 W.J. Brittain, S.G. Boyes, A.M. Granville, M. Baum, B.K. Mirous, B. Akgun, B. Zhao, C. Blickle, M.D. Foster: Surface Rearrangement of Diblock Copolymer Brushes - Stimuli Responsive Films.- 4 A. Naji, C. Seidel, R.R. Netz: Theoretical Approaches to Neutral and Charged Polymer Brushes.-

Conformational Transition of Membrane-Associated Terminally Acylated HIV-1 Nef

Many proteins are posttranslationally modified by acylation targeting them to lipid membranes. While methods such as X-ray crystallography and nuclear magnetic resonance are available to determine the structure of folded proteins in solution, the precise position of folded domains relative to a membrane remains largely unknown. We used neutron and X-ray reflection methods to measure the displacement of the core domain of HIV Nef from lipid membranes upon insertion of the N-terminal myristate group. Nef is one of several HIV-1 accessory proteins and an essential factor in AIDS progression. 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 Å from the lipid headgroups. This work rules out previous speculation that the Nef core remains closely associated with the membrane to optimize interactions with the cytoplasmic domain of MHC-1.

Insertion mechanism of cell-penetrating peptides into supported phospholipid membranes revealed by X-ray and neutron reflection

X-Ray and neutron reflectivity measurements on systems composed of a 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) bilayer and transcription-activating-factor derived peptides (TDPs) have allowed us to determine the mechanism of membrane translocation. By monitoring the structural changes of the bilayers caused by the binding of TDPs while systemically varying temperature and TDP concentration, our results revealed the detailed molecular structures of the stepwise interactions that occurred during the translocation of TDP across the lipid bilayers. While little indication of membrane perturbation was observed at low TDP concentrations, we found that the TDP movement across the membrane induced defect formations in the membrane at higher TDP concentrations.

Loading and distribution of a model small molecule drug in poly(N-isopropylacrylamide) brushes: a neutron reflectometry and AFM study.

The structure of a hydrated poly(N-isopropylacrylamide) brush loaded with 5 vol % Isoniazid is studied as a function of temperature using neutron reflectometry (NR) and atomic force microscopy (AFM). NR measurements show that Isoniazid increases the thickness of the brush before, during and after the polymer collapse, and it is retained inside the brush at all measured temperatures. The Isoniazid concentration in the expanded brush is ~14% higher than in the bulk solution, and the concentration nearly doubles in the collapsed polymer, suggesting stronger binding between Isoniazid and the polymer compared to water, even at temperatures below the lower critical solution temperature (LCST) where the polymer is hydrophilic. Typically, additives that bind strongly to the polymer backbone and increase the hydrophilicity of the polymer will delay the onset of the LCST, which is suggested by AFM and NR measurements. The extent of small-molecule loading and distribution throughout a thermo-responsive polymer brush, such as pNIPAAm, will have important consequences for applications such as drug delivery and gating.

Nanoparticle-Driven Orientation Transition and Soft-Shear Alignment in Diblock Copolymer Films via Dynamic Thermal Gradient Field

Sharp dynamic thermal gradient (∇T ≈ 45 °C mm(-1)) field-driven assembly of cylinder-forming block copolymer (c-BCP) films filled with PS-coated gold nanoparticles (AuNPs; dNP ≈ 3.6 nm, φNP ≈ 0-0.1) is studied. The influence of increasing AuNP loading fraction on dispersion and assembly of AuNPs within c-BCP (PS-PMMA) films is investigated via both static and dynamic thermal gradient fields. With φNP increasing, a sharp transition from vertical to random in-plane horizontal cylinder orientation is observed due to enrichment of AuNPs at the substrate side and favorable interaction of PMMA chains with gold cores. Furthermore, a detachable capping elastomer layer can self-align these random oriented PMMA microdomains into unidirectional hybrid AuNP/c-BCP nanolines, quantified with an alignment order parameter, S.

Temperature-dependent nanostructure of an end-tethered octadecane brush in tetradecane and nanoparticle phase behavior.

The phase behavior of a molecular brush-C(18) grafted to the surface of both a silicon wafer and SiO(2) nanoparticles was investigated as a function of temperature using neutron reflectometry (NR) and small-angle neutron scattering (SANS), respectively. The experiments demonstrate a phase change in the brush layer characterized by a straightening of the molecular configuration, increase in shell thickness, and increase in solvent concentration with decreasing temperature that corresponds to gelation in the nanoparticle dispersion.

Temperature-triggered micellization of block copolymers on an ionic liquid surface.

In situ neutron reflectivity was used to study thermally induced structural changes of the lamellae-forming polystyrene-block-poly(2-vinylpyridine) (PS-b-P2VP) block copolymer thin films floating on the surface of an ionic liquid (IL). The IL, 1-butyl-3-methylimidazolium trifluoromethanesulfonate, is a nonsolvent for PS and a temperature-tunable solvent for P2VP, and, as such, micellization can be induced at the air-IL interface by changing the temperature. Transmission electron microscopy and scanning force microscopy were used to investigate the resultant morphologies of the micellar films. It was found that highly ordered nanostructures consisting of spherical micelles with a PS core surrounded by a P2VP corona were produced. In addition, bilayer films of PS homopolymer on top of a PS-b-P2VP layer also underwent micellization with increasing temperature but the micellization was strongly dependent on the thickness of the PS and PS-b-P2VP layers.

Slip and depletion in a Newtonian liquid

Despite the fact that the magnitude of surface slip, which may occur for a liquid flowing past a solid wall, was quantified by different experimental techniques, the microscopic origin of this effect remains unclear. In the present article we present a neutron reflectivity study for a Newtonian liquid, hexadecane, in contact with solid walls at rest and under shear. In this system slip is not explained by a depleted liquid layer.

Control of interface nanoscale structure created by plasma-enhanced chemical vapor deposition.

Tailoring the structure of films deposited by plasma-enhanced chemical vapor deposition (PECVD) to specific applications requires a depth-resolved understanding of how the interface structures in such films are impacted by variations in deposition parameters such as feed position and plasma power. Analysis of complementary X-ray and neutron reflectivity (XR, NR) data provide a rich picture of changes in structure with feed position and plasma power, with those changes resolved on the nanoscale. For plasma-polymerized octafluorocyclobutane (PP-OFCB) films, a region of distinct chemical composition and lower cross-link density is found at the substrate interface for the range of processing conditions studied and a surface layer of lower cross-link density also appears when plasma power exceeds 40 W. Varying the distance of the feed from the plasma impacts the degree of cross-linking in the film center, thickness of the surface layer, and thickness of the transition region at the substrate. Deposition at the highest power, 65 W, both enhances cross-linking and creates loose fragments with fluorine content higher than the average. The thickness of the low cross-link density region at the air interface plays an important role in determining the width of the interface built with a layer subsequently deposited atop the first.