Raman imaging of polymer gels and elastomer blends

Abstract

The incorporation of high-resolution optics in a Raman spectrometer allows sampling from areas less than one micron in diameter. The addition of a confocal microscope improves the axial resolution to a couple of microns. The fast data collection combined with high lateral and vertical resolutions makes possible scanning experiments in which the specimen is advanced in micron size steps. Analysis of the spectra provides information on the spatial composition of the sample. For macroporous N-isopropylacrylamide (NIPA) gel the temperature induced evolution of the pore structures is characterized. This model is used to explain surface roughness of the gels and characteristics of a NIPA-acrylamide (PAAM) interface. At room temperature, the average sizes of the pores and the width of polymer-rich areas are 75 ?m and 20 ?m, respectively. At higher temperatures polymer chains bunch together and this process accelerates rapidly near the volume phase transition temperature (34¿C). The porous structure of the NIPA extends to the gel's boundary causing surface roughness, which, like the bulk material, is temperature dependent. A shrinking process results in a dense shell on the surface. Also, the surface becomes smoother due to hydrophobic interactions between isopropyl groups in the NIPA gel. For a polymer-polymer interface we showed that different drying and diffusion times affect the topography of the interfacial region. Phase separation in binary mixtures of two polymers, polyisobutadiene (BR) and brominated poly(isobuthylene-co-para-methylstyrene) (BIMS), is studied for different compositions of the blends. Binary blends of BIMS and BR do mix better in the presence of precipitated silica and domain sizes decrease from approximate 5 ?m to less than 1?m. Blend components with polar groups like BIMS, silica, and zinc stearate can be found in close proximity to each other. The blend morphology of the uncured samples is dependent upon temperature. Increased temperature decreases domain sizes, but the effect is reversed for temperatures exceeding 150¿C. Since the blends studied were prepared without using an antioxidant, cis-polybutadene degrades at a rate dependent upon temperature, wavelength of the incident light and its fluence, amount of zinc stearate, and sample thickness.