The mechanical properties of polymer films such as tensile strength and resistance to tearing depend strongly on the orientation of the polymer chains. Fourier transform infrared (FT-IR) spectroscopy can be used to measure the degree of orientation both within the plane of the film and normal to it. Tilting the film allows dipole changes normal to the plane to be measured so that three-dimensional orientation can be investigated. Attenuated total reflectance (ATR) measurements are sensitive to orientation both in the plane and normal to it, but measure only surface regions. Transmission spectra of thin films often contain interference fringes (channel spectra) that need to be minimized. This article illustrates the different measurements and also considers some of the practical issues.
Synthetic polymers are semicrystalline and contain regions with some order in the arrangement of the polymer chains and disordered amorphous regions. The orientation of the polymer chains within thin films is often anisotropic as a result of the manufacturing process. For example, stretching a film in one direction will tend to align the polymer chains in that direction. Polymer chain orientation has a big effect on mechanical properties such as tensile strength and tear resistance. In general, tensile strength is greater in a direction parallel to the polymer chains than in directions normal to them and the same applies to tear resistance.
Orientation in the plane of films produced by extrusion is defined relative to the machine direction and the transverse direction. Orientation can be investigated in a normal transmission measurement of the absorbance for radiation polarized in either direction. However, because the electric field is in the plane of the film this approach cannot measure any absorption with a dipole moment change normal to the plane. That information can be obtained by transmission at non-normal incidence or, for surface layers, by attenuated total reflectance (ATR).