Raman spectroscopy has been used for polymer characterization for quite a while, but mainly for research purposes instead of routine analysis. The recent advances in Raman instrumentation now enable rapid acquisition of spectra on equipment, which is much more user friendly and affordable when compared to the past. This article discusses the application of Raman spectroscopy in the analysis of polymers and the effects that can be monitored on a regular basis for industrial analytical purposes.
Figure 1. Fingerprint spectra.
Raman spectra can be applied for identification purposes by cataloguing a fingerprint spectra collection. These spectra can be utilized for:
- Monitoring production of a product
- Identifying contaminants in a production process
- Confirming incoming product (QC)
By analyzing the attached fingerprint spectra (Figure 1), an unknown polymer can be easily identified from its Raman spectrum.
Polymerization of a monomer to create a polymer almost always results in the loss of a double bond. Hence, a polymerization process can be monitored by monitoring the disappearance of the band of the carbon double bond. The spectra of a monomer starting material and a partially reacted batch of polymer are shown in Figure 2.
Figure 2. Spectra of CR-39 monomer: A - infrared and B -Raman. and of partly polymerized monomer (ca. 80% consumption of C=C bonds): C - infrared and D - Raman. (This figure was reproduced from O’Donnell and O’Sullivan, A Kinetic Study of Crosslinking Vinyl Polymerization by Laser Raman Spectroscopy. Polymer Bull 5, 103-110, 1981.).
The Raman spectrum not only allows observing the degree of polymerization, but also shows higher sensitivity to the presence of the unpolymerized >C=C< band, compared to IR spectroscopy.
Orientation and Crystallization
Polymers are prone to orientation when extruded, indicating the aligning of the molecular axis along the extrusion direction. By orienting a polymer sample in the instrument coordinate system and exploring the polarization of the Raman light, information on the orientation of the polymer in the part can be deduced.
Like molecular crystals and inorganic materials, polymers are also available in a crystalline form. The sample’s thermal and stress history helps determining the degree of crystallinity not more than 50%. The crystalline form of polymers can be monitored by observing some details in the spectra.
Raman Versus IR Spectroscopy
Although routine polymer analysis is more widely performed by means of FTIR, IR spectroscopy cannot analyze certain things. The Raman bands of double and triple bonds seem to be much stronger when compared to those of the IR. Moreover, these bonds may be completely inactive in the IR in certain cases. Similarly, other vibrational features are more amenable to analysis by Raman when compared to IR.
It is difficult to perform IR analysis on aqueous solutions due to the tendency of the solvent to be so opaque. Since, water’s Raman spectrum is very weak, it provides an ideal solvent to monitor chemistry in a solution. Hence, monitoring emulsion polymerization can be more easily performed on Raman than on IR.
Spectra of a polyacrylic acid acquired at various degrees of neutralization are illustrated in Figure 3. Differences in the spectra show that chemical differences can be identified easily. The identification of water solvent is also easy at around 1640cm-1. Hence, it is possible to use the water solvent as an internal intensity standard to facilitate concentration calibrations. Nevertheless, dispersive Raman with a red laser can be used to study a large proportion of these materials.
Figure 3. Raman spectra of poly (acrylic acid) at different degrees of neutralization. Degree of neutralixation, α: (a) 0, (b) 0.2, (c) 0.4, (d) 0.8, (e) 1.0. 25% aqueous solution. The dashed lines indicate the backgrounds. (These spectra are reproduced from Koda. et.al, Raman Spectroscopic Studies on the Interaction Between Counterion and Polyion, Boiphysical Chem., 15, 65-72, 1982).
Dispersive Versus FT Raman Spectroscopy
Traditionally, Raman has been analyzed with dispersive spectrometers, as was IR absorption spectroscopy in the early years. Over two decades ago, it was demonstrated that a Michelson interferometer could generate IR spectra by carrying out a ‘Fourier Transform’ on the interferometric data. Considering various instrumental reasons, interferometric acquisition of spectral data has been performed mostly in the mid IR, where vibrational frequencies take place.
About 6 to 8 years ago, it was shown that an interferometer operating in the NIR region could record Raman spectra by exciting the spectra with a YAG laser. FT Raman (FTR) is ideally suited for analyzing many fluorescing materials that are virtually impossible to be analyzed with visible lasers. However, FTR cannot be used in the analysis of emulsion polymerization, carbons and catalysts, whereas dispersive Raman is able to deliver very useful spectra.
Recent advances in Raman instrumentation make the technique more affordable, compact and user friendly. As a result, it is now possible to exploit all of the demonstrated capabilities of the spectroscopy for industrial applications, including its use in conjunction with statistical techniques for concentration calibrations.
This information has been sourced, reviewed and adapted from materials provided by HORIBA Scientific.
For more information on this source, please visit HORIBA Scientific.