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.

Molecular mechanisms of the chain diffusion between crystalline and amorphous fractions in polyethylene

“…The presence of gauche conformations and some sequences of gauche and trans conformations (for example, gg and gtg) in PE crystallites is confirmed by infrared spectroscopic measurements [32,33], and they are abundant there at high temperatures (see [33] and the comparison between 13 C NMR-and X-ray-determined degree of crystallinity of a PE sample at elevated temperatures ( [34], p. 4335e4336)). However, this fact cannot be regarded as a direct proof for the existence of the above-mentioned conformational defects of a definite type (the simplest one is tg þ tg À tg þ t).…”

Electron Transfer Reactivity Patterns at Chemically Modified Electrodes: Fundamentals and Application to the Optimization of Redox Recycling Amplification Systems

“…Infrared spectroscopic studies of alkanes and related systems have revealed much regarding the details of conformation and packing arrangements [153][154][155][156][157][158][159][160][161][162][163][164][165][166]. Research focused on the application of this powerful characterization tool to study adsorbed molecules on metal surfaces was being developed by the late 1950s [167].…”

Electron transfer reactivity patterns at chemically modified electrodes: fundamentals and application to the optimization of redox recycling amplification systems

“…Vibrational spectroscopy has long been used for chemical structure analysis. Infrared spectroscopic studies of alkanes and related systems have revealed much regarding the details of conformation and packing arrangements [153][154][155][156][157][158][159][160][161][162][163][164][165][166]. Research focused on the application of this powerful characterization tool to study adsorbed molecules on metal surfaces was being developed by the late 1950s [167].…”