Alioscka Sousa

Nanoscale Composition of Biphasic Polymer Nanocolloids in Aqueous Suspension

The molecular distribution in nanocolloids of poly(dimethyl siloxane) (PDMS) and an organic copolymer (methyl acrylate co-methyl methacrylate co-vinyl acetate) preserved in a frozen aqueous solution was investigated using cryovalence electron energy-loss spectroscopy (EELS) coupled with a scanning transmission electron microscope. Low energy-loss spectra depend upon valence electron structure, and we show that they are substantially different for the PDMS, the copolymer, and the vitrified water studied here. Combining a high efficiency detection system and the use of high-signal low-loss spectra in EELS, we achieved a spatial resolution of 8 nm without serious beam-induced specimen damage in this radiation-sensitive soft-materials system. To obtain quantitative phase maps of silicone and copolymer composition within individual nanoparticles, spectrum datasets were processed via multiple least squares fitting. Quantitative line profiles from the resulting compositional maps indicate that the PDMS lobe of biphasic nanoparticles contained a significant amount of the copolymer and a diffuse interface was formed. Since the nanoparticle synthesis involves polymerization of acrylate monomer dissolved in PDMS nanoparticle precursors, these results suggest that the evolution of the nanocolloid morphology during synthesis is kinetically frozen as the acrylate copolymer achieves some critical molecular weight.

Selective adsorption of surface-modified ferritin on a phase-separated polymer blend

We compare the adsorption behavior of wild and surface-modified ferritin on a homopolymer blend of poly(desaminotyrosyl tyrosine dodecyl ester carbonate) (PDTD) and poly(epsilon-caprolactone) (PCL). Wild ferritin was alkylated by zero length cross-linking to surface carboxylate groups activated by water soluble carbodiimide (EDC). Modification was confirmed by anion exchange chromatography. All the adsorption experiments were done using deionized water (pH 5.8) and ultramicrotomed thin polymer films of thickness approximately 100 nm. Transmission electron microscopy shows a clear selectivity of alkylated ferritin adsorption onto the PCL phase while wild ferritin predominantly adsorbs onto the PDTD phase. We attribute these differences to the nature of the electrostatic interaction between the two types of ferritin and the polymer surface.

Spectrum Imaging and Scatter Diagrams for the Nanoscale Morphology Characterization of Hydrated Polymers

Spectrum imaging using electron energy-loss spectroscopy (EELS) in the cryo-STEM is emerging as an important technique for spatially resolved compositional measurements in hydrated soft materials [1,2,3]. Here we study the early nanoscale morphological changes that occur in a PDTEPEG bioresorbable copolymer [4] during hydrolytic degradation and erosion in water. PDTE is a tyrosine-derived hydrophobic polycarbonate. The length scales characterizing the early morphological changes can be very small, and thus they challenge the limits of detection and spatial resolution of the spectrum-imaging approach. Previous works have reported a spatial resolution in the study of hydrated soft materials in the range of 80-200 nm. Trying to further increase resolution by decreasing the incident probe diameter, however, must be accompanied by a decrease in the number of electrons that hit the specimen to keep the electron dose below the threshold for damage in this sensitive class of materials. As a result, spectrum images can be significantly noisy, and their interpretation is not always possible from a simple visual inspection. Here we use scatter diagrams together with noise simulations to interpret water and PEG compositional maps that we collect with a spatial resolution of 15 nm, a factor of five or more better than previously achieved.