Archie P. Smith

NEXAFS spectromicroscopy of polymers: overview and quantitative analysis of polyurethane polymers

Abstract The successful application of X-ray spectromicroscopy to chemical analysis of polymers is reviewed and a detailed application to quantitative analysis of polyurethanes is presented. Near Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy is the basis of chemical sensitive X-ray imaging, as well as qualitative and quantitative micro-spectroscopy. These capabilities are demonstrated by a review of recent work, and by presentation of new results outlining a methodology for quantitative speciation of polyurethane polymers. C 1s inner-shell excitation spectra of a series of molecular and polymeric model compounds, recorded by gas phase inelastic electron scattering (ISEELS) and solid phase NEXAFS techniques, are used to understand the spectroscopic basis for chemical analysis of polyurethanes. These model species contain the aromatic urea, aromatic urethane (carbamate) and aliphatic ether functionalities that are the main constituents of polyurethane polymers. Ab initio calculations of several of the model molecular compounds are used to support spectral assignments and give insight into the origin and relative intensities of characteristic spectral features. The model polymer spectra provide reference standards for qualitative identification and quantitative analysis of polyurethane polymers. The chemical compositions of three polyurethane test polymers with systematic variation in urea/urethane content are measured using the spectra of model toluene diisocyanate (TDI) urea, TDI-carbamate, and poly(propylene oxide) polymers as reference standards.

X-ray spectromicroscopy of polymers and tribological surfaces at beamline X1A at the NSLS

Abstract We provide a general overview of microspectroscopy and spectromicroscopy for materials characterization at beamline X1A at the national synchrotron light source (NSLS). Two instruments, the scanning transmission X-ray microscope (STXM) and scanning photoemission microscope (SPEM), are available. The STXM has been able to provide a spatial resolution of better than 50 nm for several years and near edge X-ray absorption fine structure (NEXAFS) spectroscopy is performed in transmission from thin samples at an energy resolution of typically 0.3 eV at the carbon K-edge. Numerous applications in polymer science and biology have been performed to date. We restrict our review to polymer science applications and present new results of several polymer systems. The SPEM has a spatial resolution of about 250 nm in routine operation and was recently upgraded with a hemispherical sector analyzer to improve the data throughput. We present the latest SPEM results, which were generated from a tribological sample.

High-energy mechanical milling of poly(methyl methacrylate), polyisoprene and poly(ethylene-alt-propylene)

High-energy mechanical milling has been performed on poly(methyl methacrylate) (PMMA) at ambient and cryogenic temperatures, as well as on polyisoprene (PI) and poly(ethylene-alt-propylene) (PEP) at cryogenic conditions only. Milling conducted at ambient temperature has a substantially greater impact on the molecular characteristics of PMMA than milling at cryogenic temperatures. An increase in the milling time is accompanied by substantial reductions in PMMA molecular weight and, hence, glass transition temperature and impact strength under both sets of experimental conditions. An unexpected trend identified here is that the PMMA molecular weight distribution initially broadens and subsequently narrows with increasing milling time. Solid-state mechanical milling promotes comparable decreases in molecular weight and glass transition temperature in PEP (at a slower rate relative to PMMA), but induces chemical crosslinking in PI, as confirmed by FTIR spectroscopy. Charlesby‐Pinner analysis yields not only the degree of PI crosslinking, but also the relative crosslinking and scission rates of PI, during cryogenic milling. q 2000 Elsevier Science Ltd. All rights reserved.

X-ray microscopy in polymer science: prospects of a ‘new’ imaging technique☆

Abstract A relatively non-invasive imaging technique, which employs highly focused, tunable X-rays, is described. This technique—scanning transmission X-ray microscopy—can be used to investigate the bulk characteristics of polymeric materials with chemical sensitivity at a spatial resolution of about 50nm. We present examples ranging from unoriented multiplase polymers to highly oriented Kevlar fibres. In the case of oriented samples, a dichroism technique is used to determine the orientation of specific chemical bonds. Extension of the technique to investigate surfaces of bulk samples is discussed.

Quantitative orientational analysis of a polymeric material (Kevlar® fibers) with x‐ray microspectroscopy

It has previously been shown that x‐ray linear dichroism microscopy can be utilized to image and determine orientation in a polymeric material at high spatial resolution. We have now expanded on this technique and extracted quantitative information about the orientation of specific functional groups in a polymeric system from submicron areas. This is accomplished by acquiring and analyzing spectral data sets rather than just images at specific energies. It has allowed us to compare the relative lateral orientation of various grades of Kevlar® fibers.

High-Energy Cryogenic Blending and Compatibilizing of Immiscible Polymers**

ciency Z in solid films of PA-PPV and MEH-PPV has been determined by xerographic discharge experiments over two orders of magnitude in field and for different illumination wavelengths. Onsagers theory with a Gaussian distribution of electron±hole radii describes the electric field dependences of Z well. The large distribution parameter a is a characteristic of a small electron±hole pair binding energy of approximately 0.1±0.2 eV. For both polymers, high efficiencies of up to 20 % at a field of 100 V/mm were measured. Within the polymer absorption, Z was found to be nearly independent of wavelength. This is in agreement with the well established picture of a fast vibronic and energetic relaxation following photoexcitation. Addition of C60 to PA-PPV increases the primary quantum yield to unity and photogeneration efficiencies of up to 50 % are measured.

Bulk and surface characterization of a dewetting thin film polymer bilayer

We have monitored the progression of the dewetting of a partially brominated polystyrene (PBrS) thin film on top of a polystyrene (PS) thin film with scanning transmission x-ray microscopy (STXM) as well as photoemission electron microscopy (PEEM). We mapped the projected thickness of each constituent polymer species and the total thickness of the film with STXM, while we determined the surface composition with PEEM. Our data show that the PBrS top layer becomes encapsulated during the later stages of dewetting and that atomic force microscopy topographs cannot be utilized to determine the contact angle between PBrS and PS.

X-ray microscopy of novel thermoplastic/liquid crystalline polymer blends by mechanical alloying

SUMMARY: Incorporation of liquid crystalline polymers (LCPs) into commodity polymers remains a challenge in the design of high-performance, low-cost polymeric blends. Blends of a thermoplastic polymer and a nematic LCP are produced here by mechanical alloying. Functionality sensitive X-ray microscopy reveals LCP dispersions as small as 100 nm in diameter. Intimate mixing remains upon subsequent melt processing, indicating that mechanical alloying is suited for applications such as recycling.

Optimization of scanning transmission X-ray microscopy for the identification and quantitation of reinforcing particles in polyurethanes

The morphology, size distributions, spatial distributions, and quantitative chemical compositions of co-polymer polyol-reinforcing particles in a polyurethane have been investigated with scanning transmission X-ray microscopy (STXM). A detailed discussion of microscope operating procedures is presented and ways to avoid potential artifacts are discussed. Images at selected photon energies in the C 1s, N 1s and O 1s regions allow unambiguous identification of styrene-acrylonitrile-based (SAN) copolymer and polyisocyanate polyaddition product-based (PIPA) reinforcing particles down to particle sizes at the limit of the spatial resolution (50 nm). Quantitative analysis of the chemical composition of individual reinforcing particles is achieved by fitting C 1s spectra to linear combinations of reference spectra. Regression analyses of sequences of images recorded through the chemically sensitive ranges of the C 1s, N 1s and O 1s spectra are used to generate quantitative compositional maps, which provide a fast and effective means of investigating compositional distributions over a large number of reinforcing particles. The size distribution of all particles determined by STXM is shown to be similar to that determined by TEM. The size distributions of each type of reinforcing particle, which differ considerably, were obtained by analysis of STXM images at chemically selective energies.

Cryogenic mechanical alloying as an alternative strategy for the recycling of tires

Abstract Cryogenic mechanical alloying (CMA) is investigated as a viable strategy by which to produce highly dispersed blends composed of thermoplastics and tire, thereby providing a potentially new route by which to recycle discarded tires. Morphological characterization of these blends by near-edge X-ray absorption fine structure (NEXAFS) microscopy demonstrates that, upon CMA, ground tire is highly dispersed within poly(methyl methacrylate) (PMMA) and poly(ethylene terephthalate) (PET) matrices at sub-micron size scales. Incorpo-ration of polyisoprene (PI) homopolymer into the blends to improve dispersion efficacy is also examined. Neither PI nor the tire is found to interact chemically with PMMA or PET under the milling conditions employed here.