Journal of the Korean Physical Society, Vol. 66, No. 7, April 2015, pp. 1120∼1124 Temperature and Pressure Dependences of Acoustic Anomalies of PET Films Studied by using Brillouin Spectroscopy Young Ho Ko ∗ and Kwang Joo Kim Agency for Defense Development, P.O. Box 35, Yuseong, Daejeon 305-600, Korea Byoung Wan Lee, Min-Seok Jeong and Jae-Hyeon Ko † Department of Physics, Hallym University, Chuncheon 200-702, Korea (Received 28 April 2014) The acoustic properties of biaxially-oriented polyethylene terephthalate (PET) were investigated as a function of either temperature or pressure by using Brillouin spectroscopy. The Brillouin frequency shift of the longitudinal acoustic mode of both biaxially-oriented and amorphous PET materials showed a change in the slope near 80 ◦ C, which was the approximate glass transition temperature. The acoustic damping of amorphous PET exhibited large values near the melting temperature compared to that of semicrystalline PET. This indicated stronger coupling between the acoustic waves and the structural relaxation process in the amorphous state. The pressure dependences of the sound velocities were investigated at pressures up to 8.5 GPa by using a diamond anvil cell. The pressure-density relationship could be obtained based on the Birch-Murnaghan equation of state. PACS numbers: 78.35.+c, 62.50.+p, 43.58.Dj, 64.30.Jk Keywords: High-pressure, Brillouin scattering, PET, Sound velocity, Diamond anvil cell DOI: 10.3938/jkps.66.1120 I. INTRODUCTION Polymer behaviors under high temperature and high pressure are important from the viewpoint of both fun- damental physics and practical applications [1,2]. Both thermodynamic variables have substantial effects on the interatomic forces, which modify various properties of polymers. In spite of the extensive usage of polymeric materials in various application fields, their properties under various thermodynamic conditions, in particular, under high pressure, have not been reported in very much detail. Another interesting aspect of polymer science is related to the understanding of the vitreous state and the microscopic nature of the glass transition of polymeric materials [3]. Several relaxation processes are usually ob- served during vitrification, and the correlation between the main relaxation and other secondary relaxation pro- cesses has attracted great attention recently [1, 4]. In this context, acoustic properties are one of the impor- tant characteristics of polymeric materials because the strain caused by acoustic waves can couple to various relaxation processes. This coupling induces anomalous changes in acoustic properties such as the sound veloci- ∗ E-mail: yhko@add.re.kr † E-mail: hwangko@hallym.ac.kr; Fax: +82-33-256-3421 ties and the acoustic damping factors [5]. Brillouin spec- troscopy has been a powerful tool in the investigation of the acoustic properties of polymers [6–9]. The shift in the wavelength of the light incident on condensed mat- ter is caused by the inelastic scattering between the in- cident photons and thermally-excited acoustic phonons. By adopting various temperature controllers and high- pressure equipment, one can investigate the elastic prop- erties over wide temperature and pressure ranges [10]. Polyethylene terephthalate (abbreviated as PET henceforth) is a well-known polymeric material used in a wide range of applications. The glass transition temper- ature of amorphous PET is known to be approximately 70 ◦ C [11]. In contrast to other polymers, the acous- tic properties of PET have not been reported in detail over wide temperature and pressure ranges. The acous- tic anisotropy and the related elastic constants of ori- ented PET films were studied by using Brillouin scatter- ing at room temperature [12,13]. The low-temperature acoustic properties of amorphous PET were investigated by using ultrasonic measurements [14]. However, there seems to be no report on the equation of state (EOS) of PET. This study aims at reporting for the first time the sound velocities of the longitudinal and the transverse acoustic waves of PET at pressures from ambient pres- sure up to 8.5 GPa. The pressure-density relationship will be reported based on a model for the equation of -1120-