Joseph A. Gardella

π* ← π Shakeup Satellites for the Analysis of Structure and Bonding in Aromatic Polymers by X-Ray Photoelectron Spectroscopy

The applications of ESCA to polymer surface analysis include the use of the secondary final-state effects which lead to satellite structure near the core-level photoemission (PE) lines. Specifically, unsaturated and aromatic functionalities in organic compounds and polymers lead to π* ← π shakeup peaks of less than 10 eV lower kinetic energy (higher binding energy). In the surface analysis of polymers, these features can be utilized for qualitative analysis, identification of the presence and structure of aromatic bonding, and quantitative analysis in determining the amount of a particular block or the aromatic containing function in the near-surface region. Carbon Is shakeups are most often used, but the present study includes detailed qualitative and quantitative analysis of shakeup structures from PE lines from each type of atom in hydrocarbon-, siloxane-, and sulfur-containing polymers. These results show the importance of including the shakeup intensity in quantitative peak area calculations and in peak fitting of complex PE envelopes. These studies prove in a variety of systems that the effects of third-row atoms on the final state lead to the presence of shakeup features in atoms with orbitals which do not participate in the aromatic orbital initial state, thus complicating interpretation of structure from the presence of these features. Results from the siloxane and sulfone polymers indicate that previously held assumptions about the nature of the initial-state molecular orbital may overlook the contribution of empty 3d orbitals or increased charge density on the Si or S atom which would spread the pi orbitals to the oxygen in the aromatic siloxane or sulfone systems. Finally, analysis of these features can provide quantitative analysis of polymeric surface structure by monitoring the relative intensity of the feature to the main PE line.

Spatial control of neuronal cell attachment and differentiation on covalently patterned laminin oligopeptide substrates

The spatial control of neuronal cell attachment and differentiation via specific receptor mediated interactions, may provide an effective means for the in vitro reconstruction of neuronal cell architecture. In this study, receptor‐specific oligopeptide sequences derived from the extracellular matrix (ECM) molecule laminin, a potent neural cell attachment and differentiation promoter were covalently bound on fluorinated ethylene propylene (FEP) films. The degree of receptor‐specific cell attachment and the ability to spatially control neurite outgrowth by covalently patterning the oligopeptide sequences on the FEP film surface were assessed.

Patterned neuronal attachment and outgrowth on surface modified, electrically charged fluoropolymer substrates

Fluorinated ethylenepropylene copolymer (FEP) and polyvinylidene fluoride (PVDF) can generate static and transient electrical charges, respectively, after bulk molecular rearrangements induced by electrical charging techniques. Neurons cultured on electrically active FEP and PVDF show increased levels of nerve fiber outgrowth compared to electrically neutral material. The purpose of the present study was to determine if the addition of charged surface groups to the surfaces of FEP and PVDF would modify the influence of bulk electrical charges on cultured neurons. Mouse neuroblastoma (Nb2a) cells were cultured on electrically charged and uncharged FEP and PVDF substrates with covalently modified surfaces containing hydroxyl (OH) and amine (NH2) groups. Surface chemical modification was performed on the entire surface or in discrete striped regions. Nb2a cells cultured on electrically active FEP and PVDF showed greater levels of differentiation than cells on electrically neutral substrates. The presence of NH2 groups attenuated these responses in serum-containing media. Cells attached to NH2 rich surfaces generally displayed a flatter morphology and tended to remain attached for longer time periods. Cells cultured on stripe-modified substrates in serum-containing media showed a strong preferential attachment to modified regions, especially on NH2 stripes. In summary, bulk electrical charges are more important than surface charges in stimulating Nb2a cell differentiation. Surface groups serve to modulate neuronal morphology and confer specific attachment promoting properties in serum-containing media. The development of an optimal neuronal regeneration template may require the incorporation of specific bulk and surface properties.

Adhesive Electroless Metallization of Fluoropolymeric Substrates

A process for producing patterned metal deposits on fluoropolymeric substrates is described. A metal ion—chelating organosilane is chemisorbed by self-assembly onto a fluoropolymer surface after radio-frequency glow discharge plasma surface hydroxylation. Positional modulation of the surface hydrophobicity is illustrated by wetting. The silane covalently binds an aqueous palladium catalyst and subsequent electroless deposition yields homogeneous or patterned metal deposits that exhibit excellent adhesion to the fluoropolymer.

Tools to Rapidly Produce and Screen Biodegradable Polymer and Sol-Gel-Derived Xerogel Formulations:

A new method to rapidly produce and screen biodegradable polymer- and xerogel-based formulations is described. The approach is based on a high-speed pin printer and imaging with an epi-fluorescence microscope/charge-coupled device detector. By using this approach we can produce and screen over 600 formulations/h and rapidly identify lead formulations and/or compositions that are the most useful for the development of biodegradable devices or (bio)sensors.

Quantitative analysis of technical polymer mixtures by matrix assisted laser desorption/ionization time of flight mass spectrometry.

We report quantitative MALDI-TOF measurements for polydimethylsiloxane (PDMS) of two different molecular weights using the relative ratio of the signal intensities of integrated oligomer distributions for these two molecular weight distributions. By reporting the ratio of intensities of the integrals of two oligomer distributions, we assume that the ionization and desorption efficiencies, crystallization conditions and other factors affecting intensity are similar. Poly(methyl methacrylate) (PMMA-33,000) was mixed with PDMS samples to show whether the presence of another material might affect the desorption efficiency. Quantitative values for the number-average molecular weight (Mn), weight-average molecular weight (Mw) and polydispersities (D) were calculated using the oligomer distributions. The results show a linear relationship between the analyte concentrations and the signal intensities in the range from 1,000 Da to 10,000 Da, and the desorption efficiency of these two PDMS materials was the same even in the presence of PMMA.

Spectroscopic and electrical calculation of band alignment between atomic layer deposited SiO2 and β-Ga2O3 ( 2¯01)

The energy band alignment between atomic layer deposited (ALD) SiO2 and β-Ga2O3 ( 2¯01) is calculated using x-ray photoelectron spectroscopy and electrical measurement of metal-oxide semiconductor capacitor structures. The valence band offset between SiO2 and Ga2O3 is found to be 0.43 eV. The bandgap of ALD SiO2 was determined to be 8.6 eV, which gives a large conduction band offset of 3.63 eV between SiO2 and Ga2O3. The large conduction band offset makes SiO2 an attractive gate dielectric for power devices.

Quantitative mass spectrometry of technical polymers: a comparison of several ionization methods

The development of soft ionization methods such as matrix-assisted laser desorption/ionization (MALDI), electrospray ionization (ESI) and secondary ion mass spectrometry (SIMS) has led to an increased use of mass spectrometry in characterizing technical (synthetic) polymers. In this paper, we compare the relative performance of these three ionization methods for characterizing the molecular weights, polydispersities and quantification of relative amounts of polymer components in mixtures. Two polymers used in biomaterials, poly(dimethylsiloxane) and poly(ethylene glycol), are employed as the model polymer systems for our survey because of their well-defined molecular weights and importance as surfactants in biomaterials and because many of their surface and solution-phase properties are well understood. Matrix-assisted laser desorption/ionization can be used to examine the surface and bulk composition of biomaterials, whereas secondary ion mass spectrometry is used for examining monolayer and submonolayer coverage of polymers on surfaces and electrospray ionization is suited for examination of extractables from biomaterials. Secondary ion mass spectrometry and electrospray show discriminate behavior against ionization of higher molecular weight oligomers, especially of poly(dimethylsiloxane). Matrix-assisted laser desorption/ionization appears to exhibit the best performance for reliable molecular weight determination at higher masses and polydispersity characterization as well as for quantification of components in polymer mixtures. The results are discussed within the context of the ionization mechanisms by which each soft ionization technique operates and by the attributes of the mass analyzers (time-of-flight and Fourier transform mass spectrometers) employed.

THE BINDING OF FREE OLIGOPEPTIDES TO CYCLODEXTRINS : THE ROLE OF THE TYROSINE GROUP

The formation ofα-cyclodextrin (α-CD) and,β-cyclodextrin (β-CD) inclusion complexes with free tyrosine and the tyrosine residues within two free oligopeptides were investigated using steady-state fluorescence spectroscopy. The oligopeptides consist of five amino acids (pentapeptide) and the tyrosine residues are located at then-termini. The two peptides used in this study have well-known biological functions and are known to bind selectively to specific cell receptors. Cyclodextrins were used to model this receptor-peptide (protein-ligand) interaction. Equilibrium binding constants and the enthalpy and entropy of binding were recovered. Molecular size of the tyrosine-containing species and pH (7.0 vs. 10.0) were found to have little affect onα-CD binding. However, tyrosine binding toβ-CD was dependent on the size (free tyrosine vs. peptide), structure, and pentapeptide conformation.

Quantitative ATR FT-IR Analysis of Surface Segregation of Polymer Blends of Polystyrene/Poly(Dimethylsiloxane)-co-polystyrene

The surface segregation of a series of two-component polymer blends of polystyrene/poly(dimethylsiloxane)-co-polystyrene has been studied by attenuated total reflection Fourier transform infrared (ATR FT-IR) spectroscopy. We examined the details of calibration for quantitative ATR FT-IR. The quantitation is based on calibration of Beers law by transmission FT-IR measurements. The effect of spectral distortion and penetration depth dependence of radiation wavelength due to ATR sampling is evaluated. ATR spectra are corrected for penetration depth dependence before being used for quantitation. The corrected ATR spectra were compared with the transmission spectra of the same samples. Peak area ratios using one peak from each component were measured for the determination of the concentration of surface poly(dimethylsiloxane). The correction and calibration procedure improves the accuracy of quantitative surface analysis in the range of micrometer sampling depth. Unlike other approaches for quantification of ATR data on polymers, this method can be used for systems with a complex, unknown concentration gradient near the surface such as the blend system reported here.