Measurements of infrared frequency shifts in stressed polymers

A difference-spectrum method is used to measure vibrational frequency shifts in mechanically stressed polymers. It is shown that the peak-to-peak height of the difference-spectrum intensity profile is a linear function of small frequency shifts with a slope inversely proportional to the band halfwidth for Lorentzian or Gaussian bands. The method is applied to measure frequency shifts in uniaxially stressed ultraoriented isotactic polypropylene films, using a double beam infrared grating dispersion spectrometer with a nonstressed sample in the reference beam. It is shown that frequency shifts can be measured with an accuracy of better than 0.01 cm^?1 using the difference-spectrum method.