The Use of PA-IR for Dynamic Measurements:
Dynamic Infrared Linear Dischroism (DIRLD) of Polymeric Films
When infrared measurements are coupled with dynamic mechanical deformation, it results in a technique that can probe polymeric structure and orientation during elastic deformation. Dynamic Infrared Linear Dichroism (DIRLD) was originated by Marcott and Noda1 in the 1980s and has been shown to have the potential to connect microscopic structural properties such as orientation and conformation with macroscopic properties such as toughness and elasticity. The basis of the experiment is shown in Figure 1. A DMA device is inserted into the sample chamber of an infrared absorption instrument; the sample is perturbed with an oscillatory strain in the elastic domain, and infrared spectra are recorded during the strain cycle. The actual changes induced by a small amplitude strain (<0.3%) are quite small, so a very high sensitivity measurement is needed. The original measurements were made with single channel dispersive infrared instruments. This allowed one to focus on a very small wavelength range and achieve the required sensitivity. However, the measurement times were significant, often in excess of an hour. In the 1990s the technique was extended to FT-IR with a very complicated set of demodulation electronics 2. Measurement times were reduced to less than an hour, but sensitivity and spectral artifacts were still a problem. Using PA-IR, this experiment can produce high sensitivity DIRLD spectra in very short measurement times 3. Figure 2 shows the PA-IR spectrum of isotactic polypropylene. Also shown are the DIRLD in phase spectra obtained with a step-scan FT-IR and a PA-IR instrument. The PA-IR spectrum has been inverted to allow a reasonable comparison with the FT-IR results. The acquisition time for the FT-IR experiment was 48 minutes. The acquisition time for the PA-IR experiment was 48 seconds. The agreement between the two spectra is excellent.
The linearity of the measurement is shown in figure 3 where the applied strain is increased from 0.02% to 0.14%. The peak amplitudes show the high degree of linearity with applied strain.
Figure 4 shows that useable DIRLD data can even be obtained on the millisecond time scale.
The high sensitivity and rapid temporal response of the PA-IR measurement have been combined to make DIRLD measurements simple, fast, and sensitive.
1) Marcott and Noda, In Handbook of Vibrational Spectroscopy; Griffiths and Chalmers, Eds.; John Wiley & Sons Inc.: Chichester, 2002; Vol. 4, pp 2576-2592
2) "Step-Scan Fourier Transform Infrared Study on the Effect of Dynamic Strain on Isotactic Polypropylene", B. O. Budevska, C. J. Manning, P. R. Griffiths, R. T. Roginski, Applied Spectroscopy, 1993, 47, 1843
3) "A Faster Approach to Infrared Rheo-Optics Using a Planar Array Infrared Spectrograph", C. Pellerin, C. M. Snively, J. F. Rabolt and D. B. Chase, Applied. Spectroscopy., 2004, 58, 799.