Thermally induced conformational changes in polyethylene studied by two-dimensional near-infrared–infrared hetero-spectral correlation spectroscopy

The amount of nonplanar gauche bonds was monitored as a function of increasing temperature in three different polyethylene (PE) samples by means of mid-infrared (MIR) and near-infrared (NIR) spectroscopy. The hetero-spectral two-dimensional (2D) correlation analysis was carried out between the NIR spectral region of 4365–4235 cm−1 and the well-established MIR spectral region of 1375–1265 cm−1, where bands due to nonplanar conformer are detected. This approach allowed us to identify the NIR band at 4265 cm−1, which behaves in a way similar to MIR bands originating from conformational-defect sequences. By combining the result of our current study and that of our previous report obtained on different types of PE, it is suggested that the NIR band originates from conformational-defect sequences in PE. This finding opens up a unique and useful way to study the state of conformational disorder in PE crystal by NIR spectroscopy, monitoring the intensity of the NIR band at 4265 cm−1. The use of NIR spectroscopy allows researchers to directly probe the degree in the formation of conformational-defect sequences in thick, real-world PE samples that cannot be studied by conventional MIR spectroscopy. The 2D correlation spectroscopy analysis among the MIR CH2 wagging conformational-defect-mode bands on linear low-density PE (LLDPE) and low-density PE (LDPE) revealed the formation of nonplanar conformer represented by the band at 1368 cm−1 proceeds prior to those by other band at 1308 cm−1. This result agrees well with our previous finding on high-density PE (HDPE). We therefore propose with strong confidence that the bands at 1368 and 1308 cm−1 arise from different conformational-defect sequences, even though both of the bands have been proposed to arise from the same conformer of gtg′ (kink) + gtg sequence.