1. Spectroscopic and Microscopic Characterization of Immiscibly Blended Polyurethane Thermosets 1 Nicholas W. Heller, 1 Clive R. Clayton, 2 Spencer L. Giles 2 James H. Wynne, 3 Mark E. Walker, 3 Mark J. Wytiaz 1 Stony Brook University, Stony Brook, NY 11794-2275 2 Chemistry Division, US Naval Research Laboratory, Code 6124, 4555 Overlook Ave., SW, Washington, DC 20375 3 The Sherwin-Williams Company, 101 W. Prospect Ave., Cleveland, OH 44115 Abstract: The phase components of unique, low-reflectance resin-blended powder coatings from Sherwin-Williams were identified using several techniques, including a novel Raman spectroscopy (RS) mapping of the selective infiltration of styrene monomer. Blends of incompatible acrylic polyols, with low and high hydroxyl contents (OH), combined with matting agents and pigments, were crosslinked to produce unique low reflectance thermosets. The low reflectance resulted from a synergistic effect that originated from phase separation and the incorporation of the matting agents and pigments. RS conclusively identified the domains within the blended film using polyester embedded cross-section samples by selective infiltration of styrene monomer into the polymer matrix and low OH homopolymer. The domains consisted of the high OH polyurethane due to their higher crosslink density. Transmission electron microscopy (TEM) samples of the films were prepared and stained with uranium and lead salts for enhanced contrast; domains became more visible under TEM. TEM analysis of pigmented films revealed that nearly all pigments (e.g., magnetite) segregated into the domains and produced large aggregates that could be key to coating performance. The causes of pigment segregation are likely due to a surface energy gradient and are under investigation by photoelectron spectroscopy. These selective staining protocols may be used for optimizing the coating properties, and could be applied toward other immiscible blends. Abstract: The phase components of unique, low-reflectance resin-blended powder coatings from Sherwin-Williams were identified using several techniques, including a novel Raman spectroscopy (RS) mapping of the selective infiltration of styrene monomer. Blends of incompatible acrylic polyols, with low and high hydroxyl contents (OH), combined with matting agents and pigments, were crosslinked to produce unique low reflectance thermosets. The low reflectance resulted from a synergistic effect that originated from phase separation and the incorporation of the matting agents and pigments. RS conclusively identified the domains within the blended film using polyester embedded cross-section samples by selective infiltration of styrene monomer into the polymer matrix and low OH homopolymer. The domains consisted of the high OH polyurethane due to their higher crosslink density. Transmission electron microscopy (TEM) samples of the films were prepared and stained with uranium and lead salts for enhanced contrast; domains became more visible under TEM. TEM analysis of pigmented films revealed that nearly all pigments (e.g., magnetite) segregated into the domains and produced large aggregates that could be key to coating performance. The causes of pigment segregation are likely due to a surface energy gradient and are under investigation by photoelectron spectroscopy. These selective staining protocols may be used for optimizing the coating properties, and could be applied toward other immiscible blends. Future work: All films shown here were directly sprayed on tin substrates. We are currently observing the changes in bulk morphology of films sprayed on substrates with primer. Another endeavor is to observe how phase separation develops during the cure via TEM analysis of blended polymeric powders and quenched clear films of the blend. We are also using angle-resolved x-ray photoelectron spectroscopy (ARXPS) to examine pigments before and after their incorporation into the polymeric binder. The different incident angles for x-rays are useful for probing how the chemistry changes from the bulk to the surface. This may lead to insight on how the polymers bond to pigments. We are also staining bulk samples with dissolved uranyl acetate as an attempt to observe domains clearly under SEM. Acknowledgments : Funding for this project was provided by SERDP under WP-2207. Research facilities and instrumentation provided by Stony Brook University and the Center for Functional Nanomaterials, Brookhaven National Laboratory. TEM Analysis of Stained Pigmented Films Shows Pigments to Segregate into the Domains: Nearly all pigments exclusively reside in the domains. Large pigments also change the domain geometry from spherical to amorphous. If there are only small pigments (e.g., Fe 3 O 4 for black, TiO 2 for tan), domains stay spherical. The large holes are the results of pigment pullout from the sectioning, which is difficult to avoid for TEM preparation. Encapsulated Domain Continuous Polymer Matrix Raman Spectral Analysis of The Blended Film: Cross-section samples were prepared by embedding the sample specimens within a polyester embedding resin and polishing to a one micron grit finish prior to Raman analysis. Individual Raman spectra were then collected within the observed domain and the continuous polymer matrix for comparative analysis. Chemical mapping of the peak height ratio of the peaks at 1630 cm -1 :1600 cm -1 resulted in a chemical map clearly differentiating the domain from the continuous polymer matrix. This selective staining can be achieved with styrene vapor on the film surface. High OH Resin Low OH Resin Raman Analysis of the Homopolymer Films: Low High Comparative Raman spectral analysis and chemical mapping of the homopolymer films were consistent with the observations from the blended film. The low OH resin displayed a high intensity ratio for the 1630 cm -1 :1600 cm -1 peak height analysis that was consistent with the continuous polymer matrix of the blended film. The high OH resin displayed a low intensity ratio that was consistent with the encapsulated domain. Pigment flocculation is readily apparent, which becomes more pronounced as the pigment concentration increases; the tan film has the highest pigment concentration. The flocculation is caused by the pigments segregating into the domains, which can now be delineated by small pigments that are heavily concentrated up to the domain boundaries. References: 1. V.V. Gite, P.P. Mahulikar, D.G. Hundiwale, Progress in Organic Coatings 2010 68 307-312. 2.V.V. Verkholantsev,, Progress in Organic Coatings 1995 26 31-52. 3.S. Prati, G. Sciutto, E. Catelli, A. Ashashina, R. Mazzeo, Analytical and Bioanalytical Chemistry 2013 405 895-905. Characterization Techniques: Thermo Nicolet Almega Raman Spectrometer FEI Bio Twin G2 Transmission Electron Microscope JEOL 7600F Scanning Electron Microscope 4 µm 10 µm No Stain 10 µm U/Pb Stained Transmission Electron Micrographs of the Blended Clear Film: Staining microtomed sections of the embedded film with uranyl acetate and lead citrate enhanced the contrast between phases by darkening the domains, whereas the styrene stained the polymer matrix for Raman mapping. The small white holes are from outgassing of volatile byproducts. Si nanoparticles are found only in the domains (ID confirmed by XPS; left). Two types of domains were observed: Spherical (100 nm ≤ diameter ≤ 100 µm) Flattened (10 µm ≤ length ≤ 300 µm) SEM is Useful for Observing Pigments in their Natural State: Tan Black 10 µm Black Surface 2 µm Tan 10 µm Low High 10 µm 30 µm Low OH Resin 30 µm High OH Resin