Influence of defects on the infrared spectra of polymers

Abstract

Abstract For an infinitely repeated regular polymer chain structure the only vibrations which are optically active are those in which the phase of the motion is the same in each unit (the factor‐group modes). Frequencies for which the phase difference is nonzero are optically inactive but can become activated by the presence of defects in the chain. Such defects would normally be chemical impurities or conformational irregularities in the chain. A simple theory is developed which shows that for a dilute system of defects the major characteristics determining possible activation of the nonfactor‐group modes are: ( 1 ) the strength of the coupling between the defect vibration and the vibrations of the neighboring chain, and ( 2 ) whether or not the natural frequency of the defect vibration lies inside a lattice band of the regular chain. An analysis of the low‐ and high‐frequency regions of the spectrum of low‐density polyethylene, based on the above considerations, indicates that several features of the spectrum can be associated with defect‐induced absorption. A similar explanation can account for certain intensity changes in the CCl stretching region of syndiotactic poly(vinyl chloroide) when this polymer is submitted to mechanical treatment.