MICRO-RAMAN IMAGING OF HETEROGENEOUS POLYMER SYSTEMS - GENERAL APPLICATIONS AND LIMITATIONS

This article assesses the use of micro-Raman imaging with respect to p olymer science. This relatively novel technique allows, at high spatia l resolution, the acquisition of chemical and morphological informatio n over an area of a sample. Using Raman imaging by confocal laser line scanning, a wide range of problems in polymer analysis has been studi ed to outline the capabilities and limitations of the technique. Three ternary polymer blends consisting of polypropene/polyethene/ethene-pr opene copolymer, polybutyleneterephthalate/polycarbonate/very low dens ity polyethene, and rene-co-maleicanhydrate/poly-2,6-dimethylphenylene oxide were studied with regard to compositional and morphological het erogeneities. In a binary polymer blend consisting of two different ac rylate monomers, the refractive index profile established after artifi cially induced diffusion of the main components was determined from th e concentration gradients. The distribution of unreacted free melamine in a cured melamine-formaldehyde resin was analyzed. Furthermore, the general structure of a composite sample consisting of polyethene fibe rs in an epoxide matrix was studied. Raman imaging proved suitable for the characterization of heterogeneities in composition and morphology on a size scale equal to or larger than 1 mu m. In this sense, the te chnique helps to close the gap between infrared microscopy, with its c omparatively poor spatial resolution, on the one hand, and transmissio n electron microscopy, with its limited chemical information, on the o ther hand. For heterogeneities on a submicron scale, the value of the technique is limited to the determination of average information. When combined with curve fitting, Raman imaging permitted us to determine the composition of the polypropene/polyethene/ethene-propene copolymer blend with an accuracy of 5-10%. The main limitations to micro-Raman imaging of polymer systems based on the confocal laser line scanning t echnique have been identified as the destruction of the samples due to insufficient heat dissipation of the high-incident laser power, inter ferences due to fluorescence, and the stability of the instrumentation during long collection times required for good signal-to-noise ratio spectra of weak Raman scatterers. (C) 1996 John Wiley & Sons, Inc.