John Ankner

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Creation of stable poly (ethylene oxide) surfaces on poly (methyl methacrylate) using blends of branched and linear polymers

…, 1997

Thin film miscible blends of poly (methyl methacrylate)(PMMA) and a branched random copolymer of ... more

Neutron Reflectivity Study of the Density Profile of a Model End-Grafted Polymer Brush: Influence of Solvent Quality

Physical Review Letters, 1994

... A. Karim 1 , SK Satija 1 , JF Douglas 1 , JF Ankner 2 , and LJ Fetters 3 1 National Institute... more

INTERNAL LAYERING OF SPIN-ASSISTED MULTILAYER LBL FILMS STUDIED WITH NEUTRON REFLECTIVITY

by Veronika Kozlovskaya and John Ankner

Spin-Assisted Layer-by-Layer Assembly: Variation of Stratification as Studied with Neutron Reflectivity †

by Veronika Kozlovskaya and John Ankner

Langmuir, 2009

We apply neutron reflectivity to probe the internal structure of spin-assisted layer-by-layer (SA... more We apply neutron reflectivity to probe the internal structure of spin-assisted layer-by-layer (SA-LbL) films composed of electrostatically assembled polyelectrolytes. We find that the level of stratification and the degree of layer intermixing can be controlled by varying the type and concentration of salt during SA-LbL assembly. We observe well-defined layer structure in SA-LbL films when deposited from salt-free solutions. These films feature 2-nm-thick bilayers, which are ∼3-fold thicker than those in conventional LbL films assembled under similar conditions. The addition of a 10 mM phosphate buffer promotes progressive layer interdiffusion with increasing distance from the substrate. However, adding 0.1 M NaCl to the phosphate buffer solution restores the layer stratification. We also find that SA-LbL films obtained from buffer solutions are more stratified as compared to the highly intermixed layers seen in conventional LbL films from buffer. Our results yield new insights into the mechanism of SA-LbL assembly and the final microstructure in comparison with traditional LbL assemblies.

format_quoteSA-LbL films show distinct layering when salt-free; salt concentration impacts layer intermixing significantly.format_quote

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Tailoring Architecture of Nanothin Hydrogels: Effect of Layering on pH-Triggered Swelling

by Veronika Kozlovskaya and John Ankner

ACS Macro Letters, 2013

ABSTRACT We have tailored the internal architecture of ultrathin poly(methacrylic acid) (PMAA) hy... more ABSTRACT We have tailored the internal architecture of ultrathin poly(methacrylic acid) (PMAA) hydrogels from well stratified to highly intermixed by controlling the internal structure in layer-by-layer templates used for hydrogel fabrication. We have found pH-triggered swelling properties of these hydrogels to be significantly affected by hydrogel architecture. Well-stratified hydrogels exhibited a dramatic 10-fold increase in thickness when transitioned between pH = 5 and 7.5, unlike the 2-fold swelling observed in less-organized hydrogels.

Hydrogen-Bonded Polymer Multilayers Probed by Neutron Reflectivity

by Veronika Kozlovskaya and John Ankner

Langmuir, 2008

We present a neutron reflectivity study of the internal structure of multilayers made of a weak p... more We present a neutron reflectivity study of the internal structure of multilayers made of a weak polyelectrolyte and a neutral component where interactions between adjacent layers are controlled by hydrogen-bonding. We found the degree of interpenetration of polymer layers expressed as the interlayer roughness to be strongly correlated with the strength of intermolecular interactions between the adjacent layers. In addition, polymer layers become more diffuse with a distance from the substrate. Our results demonstrate that hydrogen-bonded films exhibit a close correlation between their structure and properties, which is essential for various applications.

Anisotropic Diffusion of Polyelectrolyte Chains within Multilayer Films

by Veronika Kozlovskaya and John Ankner

ACS Macro Letters, 2012

We have found diffusion of polyelectrolyte chains within multilayer films to be highly anisotropi... more We have found diffusion of polyelectrolyte chains within multilayer films to be highly anisotropic, with the preferential chain motion parallel to the substrate. The degree of anisotropy was quantified by a combination of fluorescence recovery after photobleaching and neutron reflectometry, probing chain diffusion in directions parallel and perpendicular to the substrate, respectively. Chain mobility was controlled by ionic strength of annealing solutions and steric hindrance to ionic pairing of interacting polyelectrolytes.

format_quoteMulti-component polyelectrolyte multilayers (PEMs) can enhance photoluminescence and drug delivery through controlled internal stratification.format_quote

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Localized entrapment of green fluorescent protein within nanostructured polymer films

by Veronika Kozlovskaya and John Ankner

Soft Matter, 2011

... Veronika Kozlovskayaa, John F. Anknerb, Hugh O&amp;amp;amp;#x27;Neillb, Qiu Zhangb and Eu... more

Silk Layering As Studied with Neutron Reflectivity

by Veronika Kozlovskaya and John Ankner

Langmuir, 2012

Neutron reflectivity (NR) measurements of ultrathin surface films (below 30 nm) composed of Bomby... more Neutron reflectivity (NR) measurements of ultrathin surface films (below 30 nm) composed of Bombyx mori silk fibroin protein in combination with atomic force microscopy and ellipsometry were used to reveal the internal structural organization in both dry and swollen states. Reconstituted aqueous silk solution deposited on a silicon substrate using the spin-assisted layer-by-layer (SA-LbL) technique resulted in a monolayer silk film composed of random nanofibrils with constant scattering length density (SLD). However, a vertically segregated ordering with two different regions has been observed in dry, thicker, seven-layer SA-LbL silk films. The vertical segregation of silk multilayer films indicates the presence of a different secondary structure of silk in direct contact with the silicon oxide surface (first 6 nm). The layered structure can be attributed to interfacial β-sheet crystallization and the formation of well-developed nanofibrillar nanoporous morphology for the initially deposited silk surface layers with the preservation of less dense, random coil secondary structure for the layers that follow. This segregated structure of solid silk films defines their complex nonuniform behavior in the D(2)O environment with thicker silk films undergoing delamination during swelling. For a silk monolayer with an initial thickness of 6 nm, we observed the increase in the effective thickness by 60% combined with surprising decrease in density. Considering the nanoporous morphology of the hydrophobic silk layer, we suggested that the apparent increase in its thickness in liquid environment is caused by the air nanobubble trapping phenomenon at the liquid-solid interface.

Chemical grafting of silane end-functionalized polymer on silicon surfaces

MRS Proceedings, 1993

... Rev. Lett., 69, 93 (1992), and references therein. [7] Jones, RAL; Norton, LJ; Shull, KR; Kra... more

Magnetic coupling of permalloy accross Fee Fe[001]

7th Joint MMM-Intermag Conference. Abstracts (Cat. No.98CH36275), 1998

Vortices in a Superconducting Thin Film Observed by Polarized Neutron Reflectivity

ABSTRACT

Entropic Routes to Hydrophilic/Biocompatible Surfaces

by John Ankner and Sushil Satija

ABSTRACT

Temperature and Field Dependence of Non-Collinear Magnetic Coupling of Permalloy across fcc Fe(001) Interlayers

ABSTRACT

Interfaces of High-Tc Superconductors Studied by X-ray and Neutron Reflectivity

High-resolution x-ray reflectivity and spin-polarized neutron reflectivity are used to study meta... more High-resolution x-ray reflectivity and spin-polarized neutron reflectivity are used to study metal interdiffusion and magnetic-field penetration at YBCO thin-film interfaces. For Au in-situ deposited directly on YBCO thin films, x-ray reflectivity reveals an abrupt interface of width 5ÅFor Pb ex-situ deposited directly on YBCO thin films, a substantially broader interface of width 50Åis found. Implications of these results to tunnel

A Neutron Reflectivity Study of Copolymer/Homopolymer Interfaces

The behavior of the interface between polystyrene (PS) and polymethylmethacrylate (PMMA) was stud... more The behavior of the interface between polystyrene (PS) and polymethylmethacrylate (PMMA) was studied using neutron reflectometry. A random copolymer, denoted P(S_f-r-MMA_1-f) where f is the styrene fraction, located at the interface, was manipulated to observe the effects that copolymer composition and layer thickness have on the interface. Neutron reflectivity measurements show that for random copolymer layers with deuterated styrene fractions at four different compositions (f=0.3,0.5,0.8,0.9), material from the random copolymer layer was taken up uniformly in the more soluble of the two adjacent homopolymer layers. They also show that for varying thicknesses of f=0.8 random copolymer, a ``pure'' copolymer layer is achieved only for thicknesses greater than 127 ÅIn addition, a strong composition- and interface-dependence of the effective \chi_S-MMA for the copolymer/homopolymer system was found. Specifically, interfacial widths between the random copolymer and the PS and PMMA homopolymers are asymmetric and much broader than predicted if \chi_S-MMA were assumed to be composition-independent and equal to the value determined from studies of PS/PMMA diblock copolymers.

Selective Solvent Effects in Block Copolymer Thin Films

ABSTRACT

The Structure of Selectively Swollen, Multilayer, Lamellar, Block Copolymer Films

We report structural changes induced in lamellar poly(styrene-b-2-vinylpyridine) (PS-PVP) diblock... more We report structural changes induced in lamellar poly(styrene-b-2-vinylpyridine) (PS-PVP) diblock and PS-PVP-PD triblock copolymer films by direct contact with a methanol reservoir. Films of d/2, 3d/2 and 5d/2 are examined, where d is the spacing of the lamellar periodicity. Particular attention is focused on 3d/2, for which several molecular weights and the two different architectures have been studied. The films have been characterized in both the dry and swollen states by neutron reflectivity. Exposure to methanol cyclically, with thorough drying in between, produces reversible changes in the structures of the films. Methanol swells the PVP domains by up to 600triblock films reduces the swelling significantly. Comparison of diblock and triblock swelling can be made in light of recent theories of mesogels.

Ordering of Block Copolymers in Confined Spaces

ABSTRACT

Structural, Field, and Temperature Dependence of Non-Collinear Magnetic Coupling in Fe/Cr(001) Superlattices

Utilizing the magneto-optic Kerr effect (MOKE) in conjunction with x-ray and neutron reflectometr... more Utilizing the magneto-optic Kerr effect (MOKE) in conjunction with x-ray and neutron reflectometry (NR) and diffraction, we have studied the magnetic coupling of Fe/Cr(001) superlattices grown at room temperature and 250 ^circC. Only the samples grown at elevated temperature exhibit non-collinear coupling of 5.0 nm Fe layers across 1.7 nm Cr interlayers. The non-collinear samples feature a less-disordered in-plane interfacial structure than those grown at room temperature. Using x-ray diffuse reflectivity, we have measured a length scale of 10 nm for these in-plane features. We have also observed, via NR, a remanent non-collinear coupling angle of 50^circ and, via MOKE and NR, a gradual approach to saturation at upwards of 7 kOe. These features can be explained qualitatively by the proximity magnetism model of Slonczewski. In addition, we have studied the temperature dependence of thin 1.7 nm and thick 8.8 nm Cr interlayer samples and find a suppression of coupling in the thick film below its T_N=240 K. The thin film remains coupled down to 40 K. Both films exhibit a loss of magnetic coherence above room temperature.

John Ankner

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