Anal. Chem. 2005, 77, 4290
Correspondence
Comment on “Vibrational Spectroscopic Analysis of Silicones: A Fourier Transform-Raman and Inelastic Neutron Scattering Investigation” One of the major conclusions of this paper1 is a purported spectra-structure correlation relating specific Raman bands to chain length in poly(dimethylsiloxane) (PDMS) polymers up to nearly 1000 monomer units in length. The bands of interest, which are claimed to diminish in size as chain length increases, are at 998 and 3052 cm-1. A good deal of the discussion and results of this paper are dependent upon this correlation. Unfortunately, a major error has been made which invalidates this spectra-structure correlation: PDMS has no bands at or near 998 and 3052 cm-1. An IR and Raman study2 of several small methylsilicone species and PDMS, along with their deuterated analogues, indicates no Raman bands at these positions. No Raman spectra are shown in this publication, but the spectra of hexamethyldisiloxane and PDMS from another publication3 show no bands at 998 and 3052 cm-1. The molecular weight of the PDMS in this publication is not specified, but after reviewing the original Raman spectrum (measured around 1970 Dow Corning), it is possible to identify the spectrum as being 350 cs PDMS, which has a nominal molecular weight of 13 650. This would place it between spectra C and D of Figure 3 of the paper in question. The 998 and 3052 cm-1 bands should be weak but apparent in this 350 cs PDMS spectrum, but they are absent. To further illustrate the absence of 998 and 3052 cm-1 bands in PDMS, Figure 1shows five spectra of PDMS (spectra A-E in the figure) of varying molecular weight. These spectra are plotted in the same format as Figure 3 of the paper in question for easy comparison and cover about the same span of molecular weights. The PDMS samples were hexamethyldisiloxane and Dow Corning 200 Fluids having viscosities of 10, 100, 1000, and 10 000 cs. These viscosities correspond to nominal molecular weights of 1250, 5970, 28 000, and 62 000. The 998 and 3052 cm-1 bands are not present in any of the PDMS spectra of Figure 1, let alone being major features at low molecular weights such as in Figure 3 of the paper in question. (1) Jayes, L.; Hard, A. P.; Se´ne´, C.; Parker, S. F.; Jayasooriya, U. A. Anal. Chem. 2003, 75, 742-746. (2) Smith, A. L.; Anderson, D. R. Appl. Spectrosc. 1984, 38, 822-834. (3) Anderson, D. R. In Analysis of Silicones; Smith, A. L., Ed.; Wiley: New York, 1974; pp 247-286, especially Figures 10.3b and 10.4b.
4290 Analytical Chemistry, Vol. 77, No. 13, July 1, 2005
Figure 1. Raman spectra of different MW PDMS samples and some common monosubstituted aromatic materials: (A) hexamethyldisiloxane, MW 162; (B) PDMS, MW 1250; (C) PDMS, MW 5970; (D) PDMS, MW 28 000; (E) PDMS, MW 62 700; (F) toluene; (G) polystyrene; (H) MePh silicone fluid.
The band positions 998 and 3052 cm-1, along with 1600 cm-1 (also seen in the spectra of Figure 3 of the paper in question), are within ranges that are characteristic for certain aromatic species, including many common monosubstituted aromatics. Spectra F-H in Figure 1 show some common monosubstituted aromatics (toluene, polystyrene, and a MePh silicone fluid), and the characteristic aromatic bands mentioned above are apparent in all these spectra. A comparison of the spectra of these common aromatic materials with the spectra of Figure 3 of the paper in question clearly shows that the bands used for the chain length correlation at 998 and 3052 cm-1 are actually of aromatic origin and unrelated to the chain length of the PDMS. We do not have a plausible explanation for the source of the contamination, nor why the level of contamination gives some correlation with molecular weight. Raman spectroscopy, based on our experience at Dow Corning, has not been found to be effective at distinguishing chain length in PDMS polymers except for a few very low MW oligomers.
Elmer D. Lipp
Analytical Sciences Department, Dow Corning Corp., Midland, Michigan Upali A. Jayasooriya
School of Chemical Sciences and Pharmacy, University of East Anglia, Norwich NR4 TTJ, U.K. Received for review December 13, 2004. Accepted March 22, 2005. AC048163J 10.1021/ac048163j CCC: $30.25
© 2005 American Chemical Society Published on Web 05/26/2005
