Mark D. Foster

Optical properties and enhancement factors of the tips for apertureless near-field optics

Resonant excitation of surface plasmons of a metal or metal-coated tip is crucial for achieving high enhancement of an optical signal with apertureless near-field optics. However, it remains a challenge to measure the optical spectrum of a tip with sub-wavelength dimensions. We present a technique based on total internal reflection microscopy to measure the optical properties of tips. A dependence of the optical resonance on the metal deposited is shown for silver-coated and gold-coated tips. These tips were also used to measure the tip-enhanced Raman spectra of silicon and a polymer blend of poly(3,4-ethylenedioxythiophene) and poly(styrenesulfonate) (PEDOT/PSS) at 514.5 and 647 nm incident wavelengths. Qualitative agreement was observed between the tip-enhanced Raman spectra and the optical resonance of the tip measured with the technique developed.

Structure of symmetric polyolefin block copolymer thin films

The microstructure of thin films of nearly symmetric poly(ethylene–propylene)–poly(ethylethylene) (PEP‐PEE) diblock copolymers (f=0.55, where f is the volume fraction of PEP) was characterized by neutron reflectometry (NR). A symmetric film structure in which the PEE block segregates preferentially to both interfaces is observed above and below the bulk order–disorder transition (ODT). Measurements at room temperature for several chain lengths, N, provide a real‐space picture of the change in interdomain interfacial profiles associated with the crossover between the strong and weak segregation limits. The polymer/air and substrate/polymer interfaces are observed to induce an ordered microstructure even when the center of the film is disordered. The characteristic dimension of this near surface microstructure is larger than the corresponding bulk value for values of χN lying between those of the bulk Gaussian‐to‐stretched‐coil and order–disorder transitions, where χ is the segment–segment interaction param...

X-Ray Scattering Methods for the Study of Polymer Interfaces

Abstract A variety of characterization methods based on X-ray scattering may be used to sensitively probe the structure of polymer surfaces and interfaces. Among these are small angle scattering in transmission, specular reflectivity, grazing incidence off-specular scattering, and grazing incidence diffraction. X-ray measurements provide information on electron density differences across interfaces, total interfacial area and volume fraction of a dispersed phase, microroughness of sharp interfaces, interface width, parameters of near-interface electron density profiles, and in-plane ordering in thin films and near surfaces. This review focuses on experimental aspects of the methods as analytical techniques, considering the relative merits of different approaches, and reviewing some recent measurements not yet reviewed elsewhere. Particular attention is given to the rapidly growing fields of specular reflectivity and off-specular scattering. After discussing general theoretical concepts applicable to all t...

X-ray reflectometer for study of polymer thin films and interfaces

Abstract An X-ray reflectometer for studying polymer thin films and interfaces is described. Reflectivities may be measured over seven of magnitude with good resolution in both reflection and scattering geometries. The apparatus may be used to determine film thickness, average film density or density profile perpendicular to an interface, and microroughnesses at the corresponding interfaces. Results are presented for the characterization of a thin poly (styrene) film and the determination of interface width between thin films of poly (p-bromostyrene) and poly(styrene).

Differentiation of Linear and Cyclic Polymer Architectures by MALDI Tandem Mass Spectrometry (MALDI-MS2)

Abstract[M + Ag]+ ions from cyclic and linear polystyrenes and polybutadienes, formed by matrix-assisted laser desorption ionization (MALDI), give rise to significantly different fragmentation patterns in tandem mass spectrometry (MS2) experiments. In both cases, fragmentation starts with homolytic cleavage at the weakest bond, usually a C–C bond, to generate two radicals. From linear structures, the separated radicals depolymerize extensively by monomer losses and backbiting rearrangements, leading to low-mass radical ions and much less abundant medium- and high-mass closed-shell fragments that contain one of the original end groups, along with internal fragments. With cyclic structures, depolymerization is less efficient, as it can readily be terminated by intramolecular H-atom transfer between the still interconnected radical sites (disproportionation). These differences in fragmentation reactivity result in substantially different fragment ion distributions in the MS2 spectra. Simple inspection of the relative intensities of low- versus high-mass fragments permits conclusive determination of the macromolecular architecture, while full spectral interpretation reveals the individual end groups of linear polymers or the identity of the linker used to form the cyclic polymer. FigureMacrocyclic and linear polystyrene and polydiene architectures are conclusively distinguished by the MS2 fragmentation patterns of Ag+-cationized oligomers.

Effectiveχ and surface segregation in blends of star and linear polystyrene

The existence in blends of linear and star branched polystyrenes (PS) of a bulk thermodynamic interaction due to differences in macromolecular architecture has been demonstrated with small angle neutron scattering measurements. It has also been demonstrated that a regularly star-branched polystyrene material segregates preferentially to the surface of a blend of star and linear molecules.

Morphology and ordering behavior of a poly(styrene)-b-poly(ferrocenyldimethylsilane) diblock copolymer

Abstract The thermal behavior and morphology of a diblock copolymer of polystyrene and poly(ferrocenyldimethylsilane) (PFS) ( M n =36 000, φ PS =0.308) have been surveyed using small angle X-ray scattering and transmission electron microscopy. As cast films are partly crystalline. The crystal structure is seriously perturbed by the propensity of the molecules to microphase segregation. After annealing above the glass transition temperature of polystyrene both crystallization and definition of the microphase separated domains are improved. The system forms an ordered microphase segregated structure once annealed above the melting temperature of the PFS block. Even then both cylindrical and spherical morphologies are observed and annealing times of a few days do not force the system to develop the long range order generally associated with carefully annealed polystyrene- b -polydiene polymers of comparable molecular weight.

Neutron and X-ray reflectivity studies of human serum albumin adsorption onto functionalized surfaces of self-assembled monolayers

Neutron and X‐ray reflectivity (NR and XR) have been widely used for the investigation of the structure of thin organic films. Here we demonstrate how these sensitive techniques may be applied to the study of protein adsorption to well‐characterized self‐assembled monolayers (SAMs) with different chemical functionalities. NR can be used for in situ study, while XR provides complementary information on the initial surfaces and dried layers measured in air after protein has been adsorbed. In situ measurements of adsorption of human serum albumin onto a hydrophilic NH2‐terminated monolayer clearly show the presence of a thin layer of adsorbed protein next to the SAM. Adsorption of albumin onto a hydrophobic, deuterated, CD3‐terminated SAM causes even bigger changes in the NR. Upon replacing the protein solution with protein‐free buffer solution, the reflectivities from both kinds of monolayers do not change, demonstrating that the albumin adsorption is irreversible after several hours of contact with the protein solution. X‐ray reflectivity measurements of dried substrates performed ex situ in air provide a lower bound estimate of the amount of protein which must be at the interface in situ. This combination of techniques provides a uniquely sensitive approach for studying changes that occur upon protein adsorption at an interface.

SAXS investigation of model carbon pore structure and its change with gasification

Abstract The evolution of the internal structure of a model process carbon, Carbosieve-S, during gasification with CO2 is studied by small angle x-ray scattering (SAXS). The SAXS data are interpreted using a random pore structure model based on a three-dimensional Voronoi tessellation. This model contains voids of random shape and various size. By fitting the predicted scattering to observed scattering intensities versus scattering vector for partially converted samples, pore structure parameters for the micro and mesoporosity as well as the surface area are determined as a function of gasification conversion. The utility of the model and SAXS in studying changing pore structures is discussed.

Competitive adsorption of human serum albumin and gamma-globulin from a binary protein mixture onto hexadecyltrichlorosilane coated glass.

The kinetics of competitive adsorption of proteins onto hexadecyltrichlorosilane coated glass (HTS-glass) from model solutions containing fluorescein isothiocyanate (FITC)-labeled human serum albumin (HSA-FITC) and gamma-globulin (HGG-FITC) were studied by total internal reflection fluorescence (TIRF) spectroscopy. The processes of displacement of HSA-FITC by HGG are independent of the conformational state of HSA adsorbed onto glass. On HTS-glass, displacement of protein is hindered by the presence of large numbers of CH3-terminated alkyl tails which induce conformational (reorientational) changes in HSA-FITC and HGG-FITC adsorbed from simple solutions. In contrast to HSA, adsorption of HGG onto HTS-glass from a simple solution is characterized by the absence of irreversible adsorption in the initial portion of the kinetic curve. Competition between HSA and HGG-FITC induces replacement of end-on adsorbed HGG-FITC on HTS-glass surface with subsequent desorption of the HGG-FITC into solution. Upon further increase in the HSA concentration in solution the competition of HSA for adsorption sites prevails, which leads to a decrease in the amount of adsorbed HGG-FITC and, consequently, to a decrease in the rate of its displacement.