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
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).