Investigation of the Variation of Unit Cell Dimensions of Crystalline Polymers with High‐Resolution FT‐IR Spectroscopy

Abstract

Summary: High‐resolution FT‐IR spectroscopy has been used for the first time to characterize the variation of the unit cell dimensions of high‐density polyethylene (HDPE). In combination with the unit cell parameters of HDPE measured at different temperatures by Swan using wide‐angle X‐ray diffraction, the relationship between the rocking band shift (730 cm−1) and the change of the unit cell volume of HDPE has been established.High‐resolution variable‐temperature FT‐IR spectra of HDPE rocking bands with decreasing temperature.magnified imageHigh‐resolution variable‐temperature FT‐IR spectra of HDPE rocking bands with decreasing temperature.

Measurement of the variation of crystal lattice structures of ethylene copolymers with high‐resolution cryogenic FTIR spectroscopy

“…Combining Swan's equations1 with the band shift data of the crystalline region measured in this study, a relationship between the IR band shifts of the crystalline region and the variation of unit cell parameters was established 20. Moreover, we have proved the following linear equations, which can quantitatively measure the variation of the unit cell parameters and the specific unit cell volume of HDPE.…”

“…These factors are responsible for the different expansion coefficients of a set of unit cell parameters ( a , b , and c ) of orthorhombic crystal. Therefore, the deformation of unit cell may bring in the change of the dihedral angle of orthorhombic crystal; bigger dihedral angle corresponds to higher A 720 /A 730 ratio 19, 20. In the process of investigating the deformation of orthorhombic crystal, the changes of the dihedral angle of orthorhombic crystal can be expressed by the ratio of A 720 /A 730 .…”

“…FTIR spectroscopy, which is considered as a powerful tool for the characterization of crystal lattice vibration of polyolefins, can provide abundant structural information at molecular level and has received wide attention 12–28. Previous investigation showed that there are two molecular chains in each orthorhombic unit cell of polyethylene, and the methylene rocking bands split into doublet peaks at 720–730 cm −1 .The deformation of orthorhombic crystal lattice of polyethylene can be characterized quantitatively by the variation of peak positions, half‐widths, as well as the ratio of integrated area at 720 and 730 cm −1 bands.…”

“…Su et al20 investigated the variation of the unit cell parameters of high‐density polyethylene (HDPE) with high‐resolution FTIR spectroscopy, and finally established a quantitative correlation between the band shifts of the crystalline region and the variation of the unit cell parameters. It is well known that the crystalline phase of ethylene copolymers is mainly composed of ethylene segments, and their crystallization behavior is similar to that of polyethylene.…”

A Criterion for Flow‐Induced Oriented Crystals in Isotactic Polypropylene under Pressure

“…Prior to accessing the criterion for flow‐induced oriented crystals in iPP crystallized at pressure, we need to suggest the method used to measure the formation of oriented crystals. In the current work, FTIR spectroscopy, which is chemically specific and quite susceptible to chain conformation, was applied to characterize the formation of oriented crystals. Figure a shows the polarized FTIR spectrums of iPP sample crystallized at 50 MPa after imposing a shear flow (duration 20 s).…”