Physics Letters A 321 (2004) 141–146 www.elsevier.com/locate/pla Brillouin scattering study on the polypropylene glycol by using a nonscanning Fabry–Perot interferometer Jae-Hyeon Ko ∗ , Seiji Kojima Institute of Materials Science, University of Tsukuba, Tsukuba, Ibaraki 305-8573, Japan Received 9 September 2003; accepted 8 December 2003 Communicated by J. Flouquet Abstract A nonscanning Fabry–Perot interferometer was used to examine the molecular weight dependence of the Brillouin spectra of glass-forming polypropylene glycol (PPG) and polypropylene glycol monomethyl ether (PPGME) along with their monomers. PPG series showed very similar acoustic properties except for the PG monomer, while those of PPGME were very sensitive to the molecular weight. These results correlate well with the temperature dependences of the dielectric relaxation frequency and the glass transition temperatures of these molecules obtained from dielectric and thermal measurements. All these results signified that the length scale of hydrogen bonded network of PPG may be so long compared to the length of one chain that the Brillouin scattering reflects the density fluctuations of only local segmental motions.  2003 Elsevier B.V. All rights reserved. PACS: 64.70.Pf; 78.35.+c; 62.60.+v Keywords: Brillouin scattering; Nonscanning Fabry–Perot interferometer; Polypropylene glycol; Glass transition; Segmental motions Polypropylene glycols (PPG) with different mole- cular weights have been known to show very similar dielectric [1–4], thermal [3,5,6] and acoustic proper- ties [7–9]. Characteristic relaxation parameters of the main α relaxation, the glass transition temperature and acoustic behaviors differ slightly upon the change of the molecular weight above 400 g/mol. This insen- sitivity to the chain length of molecules has been at- tributed to some segmental motions, which are related * Corresponding author. Present address: Samsung Corning R & D Center, Paldal-gu, Sin-dong 472, 442-390 Suwon, Gyeonggi Province, South Korea. E-mail addresses: jaehyeonko@hanmail.net (J.-H. Ko), kojima@bukko.tsukuba.ac.jp (S. Kojima). to the slowing down of the glass transition dynamics of this compound. The situation changes drastically when one of the two hydroxyl groups is replaced by a methyl group [6,9], when all oxygens are replaced by sulfurs [10], or when PPG molecules are diluted with a solvent like methylcyclohexane [11]. All these results indicate that the role of the hydrogen bond between the monomer units and the kind of the end-group are very important in understanding the glass transition dynam- ics of PPG. However, most studies have been carried out on molecules of relatively large molecular weights. A sys- tematic investigation on the low-molecular-weight PPG molecules including propylene glycol (PG) mo- nomer is thus necessary for better understanding of 0375-9601/$ – see front matter  2003 Elsevier B.V. All rights reserved. doi:10.1016/j.physleta.2003.12.016