Infrared dichroic spectra

Infrared dichroic spectra were measured for a uniaxially stretched film of poly(3-hexylthiophene)... 

A dynamic linear infrared dichroism study was carried out as a function of temperature on isotropic and cold drawn HDPE using a rapid-scan setup. The dynamic in-phase dichroic spectrum and its components (parallel and perpendicular polarised spectra) were analysed. 15 refs. 

The external reflection of infrared radiation can be used to characterize the thickness and orientation of adsorbates on metal surfaces. Buontempo and Rice [153-155] have recently extended this technique to molecules at dielectric surfaces, including Langmuir monolayers at the air-water interface. Analysis of the dichroic ratio, the ratio of reflectivity parallel to the plane of incidence (p-polarization) to that perpendicular to it (.r-polarization) allows evaluation of the molecular orientation in terms of a tilt angle and rotation around the backbone [153]. An example of the p-polarized reflection spectrum for stearyl alcohol is shown in Fig. IV-13. Unfortunately, quantitative analysis of the experimental measurements of the antisymmetric CH2 stretch for heneicosanol [153,155] stearly alcohol [154] and tetracosanoic [156] monolayers is made difflcult by the scatter in the IR peak heights. 

These devices are based on the anisotropic absorption of light. Usually molecular crystals exhibit this property and tourmaline is the classical example for this. For practical purposes, however, micro crystals are oriented in polymer sheets. Polymers containing chromophors become after stretching dichroic polarizers. The devices produced in this manner are called polawids. They have found a broad application in many technologies. Their application in spectroscopy is limited to the near ultraviolet and to the visible and near infrared range of the spectrum. In vibrational spectroscopy polaroids are employed as analyzers only for Raman spectroscopy. 

Many investigators employ xenon lamps because they have a continuous spectrum and cover the UV, VIS, and short infrared (IR) bands similar to solar irradiance. The use of an IR dichroic mirror in combination with a UV filter for fhe UV band with these lamps produces a very close simulation of the daylight irradiance (Fig. 1). 

Polarized FTIR spectroscopy has been used extensively to study polymer orientation (i.e., the dichroic ratio and dichroic difference are normally obtained from spectra recorded sequentially with the infrared radiation polarized parallel and perpendicular to a reference direction). To improve the sensitivity of this technique, and to follow accurately the dynamics of orientation, FTIR spectroscopy has been coupled with a polarization modulation (PM) technique whereby the dichroic difference spectrum is recorded directly, thus minimizing instrumental and sample fluctuations (this is discussed later in the chapter). 

As is usual in polymers, the infrared spectrum in polarized light of stretch-oriented samples shows dichroism [8]. For PA [38], an anomalous dichroic behavior has been observed and interpreted. 

---