Two-Dimensional (2D) Polarized Near-Infrared (NIR) Correlation Spectroscopy for Characterizing Reorientation of Low-Density Polyethylene (LDPE)

A polarized near-infrared (NIR) study for characterizing the orientation distribution of polymer chains is described. A low-density polyethylene (LDPE) sheet prepared with a hot rolling press is gradually rotated while being probed with a high-speed polarized NIR spectrometer based on an acousto-optic tunable filter (AOTF) to obtain a series of NIR spectra. During the change in the rotation angle, the spectral feature varies depending on the direction of the molecular vibration within the sample. The fine features of the spectral changes are readily analyzed with the two-trace two-dimensional (2T2D) correlation method by using the spectrum measured at 0° as a sample spectrum. The asynchronous correlation intensities between the crystalline and amorphous bands of the LDPE exhibited a sinusoidal variation depending on the selected reference spectrum (i.e., polarization angle). In particular, the intensities provide local maximum points at 90° and 270° where the direction of the polarized NIR light becomes parallel to the hot rolling direction. Consequently, the cyclic pattern of the asynchronous correlation intensity suggests that the hot rolling treatment substantially induces the reorientation of the polymer chains and subsequent additional crystallization, which makes the LDPE hard and brittle.