Thermally induced conformational and structural disordering in polyethylene crystal studied by near-infrared spectroscopy

Thermally induced structural and conformational changes in polyethylene (PE) samples were explored by using near-infrared (NIR) spectroscopy. The differences in the temperature-dependent structural disordering process among six PE samples were depicted by monitoring the intensities of NIR bands characteristic of orthorhombic crystalline phase. The temperature dependency of bands in the NIR region that have been considered to be due to orthorhombic crystalline lattice was compared to that of a band at 1378 cm−1 due to the methyl symmetric bending mode. The intensity decrease of the band in the mid-infrared (MIR) region seems to sensitively reflect the overall disordering of orthorhombic crystalline structure. As a result of this study, the intensity decrease of the bands in the NIR spectral region was found to proceed at lower temperature than that of the band at 1378 cm−1. This finding suggests the status of orthorhombic crystalline structure probed by the intensity of the band at 1378 cm−1 and that by the “crystalline” bands in the NIR spectral region may not be identical. The NIR spectra were further analyzed by two-dimensional (2D) correlation spectroscopy to provide the in-depth analysis of NIR bands. The 2D correlation spectroscopy has detected the presence of two NIR bands at 4342 and 4290 cm−1 due to orthorhombic crystalline phase and those at 5840 and 5640 cm−1 due to amorphous phase. The hetero-spectral 2D correlation analysis was carried out between the NIR spectral region of 4365-4240 cm−1 and the well-established MIR spectral region for CH2 wagging deformation region of 1390-1240 cm−1, where bands due to nonplanar conformer are detected. This approach allowed us to determine NIR bands, which behave in a way similar to MIR bands originating from conformational defect sequences that exist in the orthorhombic crystalline lattice, the amorphous domain and the chain fold regions. As a result of the hetero-spectral 2D NIR-MIR correlation spectroscopic studies on the development of conformational defect sequence in three types of PE samples, it was concluded that the intensity of a band at 4265 cm−1 changes in the same manner as the MIR bands at 1368, 1353 and 1308 cm−1 assignable to gtg, gg and gtg′ (kink) conformations. This finding means that the state of conformational disorder in PE crystal can be studied by monitoring the intensity of the NIR band at 4265 cm−1. The use of NIR spectroscopy makes it possible to directly probe the degree in the formation of conformational defect sequences in thick PE products typically produced in industry, which cannot be studied by MIR spectroscopy. This paper thus provides in-depth fundamental understandings on NIR spectra of PE as well as the results of our study regarding structural and conformational changes in PE crystals probed by NIR spectroscopy.