Micro-Raman imaging of heterogeneous polymer systems: General applications and limitations

Abstract

SYNOPSISThis article assesses the use of micro-Raman imaging with respect to polymer science. This relatively novel technique allows, at high spatial resolution, the acquisition of chemical and morphological information over an area of a sample. Using Raman imaging by confocal laser line scanning, a wide range of problems in polymer analysis has been studied to outline the capabilities and limitations of the technique. Three ternary polymer blends consisting of polypropene/polyethene/ethene-propene copolymer, polybutyleneterephthalate/polycarbonate/very low density polyethene, and styrene-co-acrylonitrile/styreneco-maleicanhydrate/poly-2,6-dimethylphenylene oxide were studied with regard to compositional and morphological heterogeneities. In a binary polymer blend consisting of two different acrylate monomers, the refractive index profile established after artificially induced diffusion of the main components was determined from the 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 fibers 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 Fm. In this sense, the technique helps to close the gap between infrared microscopy, with its comparatively poor spatial resolution, on the one hand, and transmission electron microscopy, with its limited chemical information, on the other 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 technique have been identified as the destruction of the samples due to insufficient heat dissipation of the high-incident laser power, interferences 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. 0

Raman Imaging Instrumentation

“…While not as widely used as its point scanning predecessor, line scanning Raman imaging systems are commercially available and have been utilized in numerous applications [9][10][11][12]. A relatively new development in line scanning technology is Renishaw's StreamLineÔ Plus imaging that utilizes a time-interleaved sampling approach for reducing data acquisition times.…”

“…11 shows an example Raman image of a pharmaceutical tablet acquired using a Raman imaging fiberscope. The figure shows a bright-field image(Figure 2.11a), grayscale Raman image(Figure 2.11b) revealing aspirin domains (bright regions), and a plot of the imaging spectrometer Raman spectra(Figure 2.11c) from region 1 (localized aspirin) and region 2 (excipient).…”

Practical considerations in the study of main-chain thermotropic liquid-crystalline polymers by vibrational microscopy

“…Although it can be considered in many laboratories as a routine analytical method, the combination of the advantages and disadvantages of microscopic analysis with those of each spectroscopic method give rise to a series of conditioning factors which can have important repercussions depending on the type of material under study. The study of polymeric materials by vibrational microscopy is well documented in the literature [32][33][34][35][36][37][38], however, it is convenient to highlight some of the more important features with respect to TLCPs.…”

“…In the study of mesophase distribution, it is convenient that the minimum domain size should fall above the spatial resolution limit defined by the spectroscopic technique. However, even though this is not the case, it is still possible to carry out mapping experiments similar to those used for the determination of compositional or structural heterogeneity polymer blends [37,[48][49][50], generating a structural representation over specified areas of the sample as a function of the aperture used. Although structural details smaller than the diffraction limit can be observed in many birefringent textures, by careful sample preparation it is sometimes possible to generate regions of the sample with highly homogeneous mesomorphic structures, which can permit their study by vibrational microscopy.…”

Analysis of car shredder polymer waste with Raman mapping and chemometrics

“…A large variety of questions can be answered using Raman chemical imaging, by utilizing the spatial information as well as the spectral signals. The miscibility of different binary polypropylene/polyurethane [8], polyethylene/polypropylene [9][10][11], polyamide/polytetrafluoroethylene [12] and ternary [13] polymer blends and their spatial structure can be studied using chemical imaging methods. Different types of heterogeneities (compositional, structural and morphological) can be defined and separately analysed [14].…”