Rheo-Optical Study of Polymers by Using Time-Resolved Soft-Pulse Compression Attenuated Total Reflection Step-Scan Fourier Transform Infrared Spectroscopy YUJI NISHIKAWA,* TATSUHIKO NAKANO, and ISAO NODA Material Analysis Division, Material Technology R&D Laboratories, Konica Minolta Technology Center Inc., 1 Sakura-machi, Hino-shi, Tokyo, 191-8511, Japan (Y.N.); Thermo Fisher Scientific K.K., C-2F, 3–9 Moriya-cho, Kanagawa-ku, Yokohama, 221-0022, Japan (T.N.); and The Procter & Gamble Company, 8611 Beckett Road, West Chester, Ohio 45069 (I.N.) A time-resolved soft-pulse dynamic compression attenuated total reflec- tion (ATR) step-scan Fourier transform rheo-optical system has been developed. This system was used to observe different viscoelastic properties of polyethyleneterephthalate (PET) and poly(3-hydroxybuty- rate-co-3-hydroxyhexanoate) (PHBHx). Resonance features were ob- served in the dynamic compression ATR spectrum of PHBHx with 625 Hz soft-pulse frequency. In contrast, the dynamic compression ATR spectrum of PET showed no resonance features. The resonance feature of PHBHx was found at 1723 cm 1 , which corresponds to the structural or morphological reorganization of a less ordered (Type II) crystalline form under compressive perturbation. The time-resolved evolution of infrared (IR) spectra was effectively analyzed by conventional generalized two- dimensional (2D) correlation analysis. The 2D-IR results indicate that the dynamic response of the well-ordered Type I crystalline state (1289 and 1261 cm 1 ) is faster than that of the Type II (1723, 1277, and 1228 cm 1 ). The present method shows promise for characterizing a wide variety of viscoelastic materials, including polymer alloys, blends, composites, and copolymers, and semicrystalline polymers. Index Headings: Step-scan FT-IR spectroscopy; Fourier transform infrared spectroscopy; Soft-pulse compression spectroscopy; Spectro- rheology; Rheo-optics; Time-resolved spectroscopy; Polyethylenetere- phthalate; PET; Polarized ATR; Attenuated total reflection; Internal reflection; Poly(3-hydroxybutyrate-co-3-hydroxyhexanoate); Two-dimen- sional infrared spectroscopy; 2D-IR spectroscopy. INTRODUCTION We have recently developed a dynamic sinusoidal compres- sion modulation attenuated total reflection (ATR) step-scan Fourier transform infrared spectroscopic method. 1–5 The combination of ATR and the dynamic compression infrared linear dichroism (ATR-DC-IRLD) technique was found to provide useful information about molecular responses. With ATR mode detection, even molecular groups with very strong IR absorption characteristics, which often suffer from spectral saturation in a conventional transmission mode DIRLD experiment, such as C ¼ O, COO, and imide groups, can be studied. We have further developed an impulse-induced (by using a square wave pulse) dynamic compression ATR step- scan time-resolved Fourier transform rheo-optical system. 6 The system was used to observe different viscoelastic properties of polyethyleneterephthalate (PET), poly(3-hydroxybutyrate-co- 3-hydroxyhexanoate) (PHBHx), and carbon black filled polyester–polyamide blend. The method showed promise for characterizing viscoelastic materials that have relatively long relaxation tails of about a few milliseconds, including ductile polymer alloys, blends, composites, copolymers, and semi- crystalline polymers. The result also showed that, by moving away from a fixed frequency sinusoidal experiment, we can effectively multiplex the time domain information of the complex viscoelastic responses of the system. However, detailed analysis of relatively hard polymers, such as PET, turned out to be much more difficult, because no long relaxation tail extending the duration of a few milliseconds could be seen in the dynamic ATR spectrum of such a system. In order to deal with this problem, a much faster response impulse-induced system seems to be needed. In such an accelerated mechanical perturbation system, however, some serious experimental problems will be encountered, as described later. To overcome this limitation, we have explored the possibility of employing a soft mechanical pulse by applying a short duration of fixed frequency mechanical compression, which is very similar to radio frequency (RF) pulses used in nuclear magnetic resonance (NMR) experi- ments. Such a soft pulse will carry the perturbation with a finite frequency range of multiplexing centered around the fixed frequency. In the present paper, we report the development of a novel time-resolved soft-pulse dynamic compression ATR step-scan rheo-optical system with microsecond time resolution. This system was used to observe different viscoelastic properties of PET and PHBHx. Most notably, an interesting resonance feature with specific soft-pulse frequency was observed in the case of PHBHx. LIMITATIONS OF HARD-PULSE EXPERIMENTS In the previous paper, 6 we developed a compression ATR rheo-optics system that employs a short square-wave pulse (so called hard-pulse). Specific functional groups in some ductile polymers (e.g., PHBHx) showed characteristic long relaxation tails typically lasting about 2.7 ms as the result of visco-elastic dynamic responses. However, in the case of hard polymers, such as PET, no long (millisecond level) relaxation tail could be observed. In these polymers, viscoelastic time responses may occur within less than several microseconds. To detect the rapid responses, a much faster dynamic compression system will be needed. Figure 1 shows the response time (settling time) feature of a fast-response piezoelectric multilayer actuator. While it is possible to achieve a response time of less than 10 ls, undesirable overshooting and ringing can be clearly seen. We found that it was no longer possible to effectively remove the overshooting and ringing in such a fast- response system, even by inserting electrical resistances. As described above, it is very difficult to apply a so-called Received 8 May 2009; accepted 4 August 2009. * Author to whom correspondence should be sent. E-mail: yuji. nishikawa@konicaminolta.jp. 1204 Volume 63, Number 11, 2009 APPLIED SPECTROSCOPY 0003-7028/09/6311-1204$2.00/0 Ó 2009 Society for Applied Spectroscopy