Characterization of multiphase polymer morphology with electron energy-loss spectroscopy in the scanning transmission electron microscope.

摘要

The traditional methods for studying polymer microstructure in the transmission electron microscope largely hinge on the use of differential heavy-element staining to induce amplitude contrast. However, adequate staining agents are not available for all polymer systems. Furthermore, nonlinearities, in the distribution of stain, particularly at interfaces, degrade the achievable resolution. Spatially-resolved electron energy-loss spectroscopy (EELS) provides a new opportunity to study polymer morphology with its ability to detect a broad range of rich spectroscopic features at high spatial resolution. This thesis demonstrates the applicability of spatially-resolved EELS to the study of polymer morphology using homopolymer blends of nylon/HDPE, PS/PE, PS/PVP, and PPS/PET. Optimal use of this technique is governed by three main factors: (i) the availability of spectral fingerprints distinguishing various polymers; (ii) the limits of achievable resolution; and (iii) the ultimate constraints imposed by electron irradiation. These issues are addressed and quantified in both the low-loss and core-loss regimes using spatially resolved studies of polystyrene homopolymer and polystyrene/poly(vinylpyridine) (PS/PVP) blends, respectively. Valence EELS studies of polystyrene show that fast secondary electrons (FSE) play a major role in polystyrene radiation chemistry. FSE are energetically favorable to induce changes in polystyrene aromaticity via C-K shell electron ionization because their energy ranges from ∼2 keV to E0, where E0 is the incident electron energy (200 keV). The C-K shell electron ionization-cross-section for lower energy FSE can be 2 orders of magnitude higher than that for FSE with energies of order tens of keV or more. Furthermore, those FSE that possess lower energy (∼2--10 keV) propagate with a trajectory that is almost orthogonal to the incident electron beam direction. Therefore, FSE can degrade polystyrene aromaticity well beyond the area that is probed by the incident electrons. These two characteristics of FSE ultimately define the spatial resolution limit and signal quality available from spatially-resolved valence EELS of polystyrene. The ultimate spatial resolution attainable from core-loss spectroscopy is limited by the accuracy with which one can model the collected spectroscopic data. Data processing and signal extraction become critical in the core-loss region due to the small inelastic scattering cross-section and hence relatively low signal-to-background ratio is expected in this energy range. These issues are discussed in the context of nanometer-scale profiling of the PS/PVP homopolymer interface. The background-subtracted integrated nitrogen signal was used to spatially map the distribution of PVP across this interface. After deconvolving the broadening effects of the incident electron intensity distribution, the PS/PVP interface was found to have a hyperbolic tangent type functional form in agreement with theoretical treatments. The interface width was determined to equal 4.5 nm and is in good agreement with independent measurements by neutron reflectivity in lamellar PS-PVP diblock copolymers.