FTIR analysis of the phase content in low-density polyethylene

Abstract

The phase content of a low-density polyethylene was studied by analysis of the CH2 rocking vibrations in non-oriented films prepared from the press (P-films) or from solution (S-films). Spectral simulations of the transmission spectra give the mass fractions of the orthorhombic phase αortho and of two noncrystalline phases (monoclinic-like and amorphous). The values of αortho (IR) are compared to αortho (i) where (i) stands for the X-ray diffraction, density, and DSC techniques. New results are obtained concerning the orthorhombic order and the change of phase content with aging. A two-phase analysis is justified for non-aged films containing a small amount of the monoclinic-like phase. The values of αortho (IR) are larger than αortho (i), the difference ranging between 0.12 and 0.43. The difference is a measure of the short-range order. αortho (IR) can reach 0.73 for the S-films. The stability of the short-range order phase is investigated. The sample is also analyzed using the trace of slow calorimetry. The difference between αortho by DSC and by slow calorimetry is a measure of strainable order. During aging, the variation in the phase content is large for the noncrystalline phases (in content and frequency) and small for the orthorhombic. The increase of the monoclinic-like phase during aging suggests that it is a precursor of the more stable orthorhombic organization. The quantification of two noncrystalline phases on fresh and aged films clarifies some ambiguity found in the literature about the monoclinic-like phase and the localization of bands in the rocking region for sample characterization. Analysis of other regions of the spectrum is needed to confirm the present results. Key words: low-density PE, phase content, FTIR, network phase, slow calorimetry.