Mapping the Structure of a Hydrated Polymer Blend Using Energy-Loss Spectroscopy in the Cryo-STEM
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Mapping the Structure of a Hydrated Polymer Blend Using Energy-Loss Spectroscopy in the Cryo-STEM Alioscka Sousa, Abdelaziz Aitouchen1 and Matthew Libera Department of Chemical, Biomedical, and Materials Engineering, Stevens Institute of Technology, Hoboken, NJ 07030, USA 1 FEI Company, Tempe, AZ 85282, USA ABSTRACT We use electron energy-loss spectroscopy (EELS) in the cryo-STEM to determine the spatial distribution of water in a model frozen-hydrated two-phase polymer blend composed of hydrophilic poly(vinylpyrrolidone) (PVP) and hydrophobic poly(styrene) (PS). We demonstrate that it is possible to directly correlate the water spatial distribution with variations in the underlying polymer morphology. HAADF-STEM imaging of both dry and frozen-hydrated specimens shows weak contrast between the polymer phases but gives no information regarding the composition of these phases and no indication of where water might be localized. Spatiallyresolved EELS spectra collected at 100 nm pixel size show that this system is composed of discrete PVP-rich domains dispersed in a continuous PS-rich matrix. The PVP-rich domains were found to be hydrated up to a level of ~ 23 wt%. We have made our compositional maps fully quantitative, given as mass-fraction maps, by measuring the total inelastic scattering crosssections per unit mass of water, PVP and PS. This is an important quantity which we have determined for an incident beam energy of 200 keV. Hydrated PVP gives rise to hydrogen evolution when irradiated above an electron dose of 1500 e/nm2 as evidenced from changes in the 13 eV region, and this effect gives rise to dose-limited resolution in these experiments. INTRODUCTION Dissolved solvents play a key role in determining many important polymer properties. Biological response to a polymer surface, for example, is often influenced by the local level of hydration at length scales associated with cells and adhesion-mediating proteins. Likewise, molecular permeability is often determined by the length scale and degree of percolation associated with solvent-rich paths in polymer membranes. Despite its importance, however, there has not yet been much experimental work performed to correlate solvent distribution at micro and nano length scales with variations in underlying polymer morphology due to phase separation or crystallinity. Experimental techniques like weight gain and NMR, for example, can determine the degree of water uptake and nature of water-polymer binding with great precision. However, these measurements are typically averaged over the entire specimen volume. In contrast, we are using electron energy-loss spectroscopy (EELS) in the scanning transmission electron microscope (STEM) to map the spatial distribution of water and other solvents in polymers because of the combined spectral sensitivity and spatial resolution that these techniques bring. This approach builds on the established cryo-electron microscopy technique originally developed in the biological community [1] and earlier applications of spatially-res
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