Trans-gauche concentration in crystalline polyethylene measured by the intensity of rocking mode vibrations of deuterated methylene groups

Abstract

Polyethylene with a low concentration of deuterated methylene groups displays an infrared band in the frequency region 646–651 cm−1. This band is attributed to a rocking normal mode of a CD2 group with one of the adjacent dihedral angles approximately trans and the other approximately gauche (tg). This mode vibrates at 620 cm−1 when the dihedral angles adjacent to the CD2 group are both trans (tt). In polyethylene crystals tg sequences can occur only in defects where constraints cause some of the dihedral angles to be approximately trans or gauche. Calculations of the rocking mode vibrational frequencies of CD2 groups in model chains which incorporate some of the typical defects showed that the bands were broadened but not completely disrupted by the distorted dihedral angles. Measurements of the relative intensities of the CD2 rocking bands show an increase in the concentration of tg sequences consistent with the thermal generation of defects which may be involved in transport of the polymer chain through the crystal. Confidence in the particular values of the concentration ratio is limited by uncertainties in the determination of the base line for the infrared bands of interest. Quantitative measurements of the concentration ratio tg/(tg+tt), determined from the integrated band intensities, fall between limits set by reasonable independent estimates of the concentration of folds and point dislocations.