This work had three objectives: to improve the reported assignments of the vibrational frequencies in several organosilicon monomers and oligomers; to assign the vibrational frequencies of polydimethylsiloxane (PDMS); and to relate the vibrational spectra of PDMS to its other physical properties, especially at sub-ambient temperatures, Infrared and Raman spectra were obtained on the title compounds, and five sets of low-temperature IR spectra were obtained on PDMS. Some low-temperature spectra were also taken on the cyclosiloxanes. Assignment of all frequencies was accomplished, although in some cases, e.g., the methyl rocking frequencies at 650-870 cm<sup>−1</sup> and the skeletal deformations below 400 cm<sup>−1</sup>, our confidence in their correctness is less than complete. The low-temperature study showed striking changes in the IR spectrum of PDMS below its glass transition temperature <i>T</i><sub><i>g</i></sub>, Bands became sharper and in some cases showed splitting. This behavior is consistent with the concept of polymer motion being frozen below <i>T</i><sub><i>g</i></sub>, Except for the appearance of a strong, sharp band at 662 cm<sup>−1</sup>, not much change was seen in the spectrum at the "cold crystallization" point. Possibly the expected spectral changes were swamped by absorption from the more plentiful amorphous portion of the polymer matrix.
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