In-situ Infrared And Generalized Two-dimensional Correlation Spectroscopic Studies On The Melting And Crystallization Behaviors Of Crystalline Polymers

Generalized Two-dimensional Infrared Correlation Spectroscopy (2D-IR) is a new analytical technology. However, little attention has been paid on the principle of its two major advantages (band resolution enhancement, the determination of sequential order of events). In particular, more problems may arise when the concepts and principles of the mechanical perturbation-based2D-IR were directly transferred into the generalized2D-IR. So it is urgent to clarify the sources of its problem. Combined with in-situ FTIR spectroscopy, the generalized two-dimensional correlation spectroscopy was employed to explore the crystallization and melting behaviors of some typical crystalline polymers focusing on the problems and possible solutions. This study will not only promote the theoretical and methodological improvements of this analytical technology, but also have important scientific significance on understanding the crystallization and melting behaviors of crystalline polymers at the molecular level.The crystallization behavior of poly (vinylidene fluoride)(PVDF) while gelation in cyclohexanone was first studied in this thesis. New band features have been resolved by generalized2D IR correlation spectroscopy but they are not real bands. Slightly different experimental data may convert the signs of the cross peaks in the asynchronous plots. The sequential order results obtained from the generalized2D IR correlation spectroscopy are not consistent with the experimental observations. It was found that the PVDF crystals were formed before the formation of gel network, and the PVDF chains started to order even in the induction period of crystallization. The molecular interactions between PVDF and cyclohexanone include the weak C-H---O hydrogen bond between the CH2groups of PVDF and the C=O groups of cyclohexanone and the C-F---H hydrogen bond between the CF2groups of PVDF and CH2groups of cyclohexanone, as well as the dipole-dipole interactions between the CF2groups of PVDF and the C=O groups of cyclohexanone.For the melt crystallization of isotactic polystyrene (iPS), the in situ IR results revealed that no local sequential order was found for the helical conformational bands (1081,1051,921cm-1) with different monomer units. However, the generalized two-dimensional infrared correlation spectroscopy gave inconsistent sequential orders for films with different thickness. The crystallization kinetics results show that the Avrami exponent value may be affected by the thickness of the film. In situ infrared analysis indicated that the decrease of crystalline band intensities did not lead to the simultaneous increase of the amorphous band intensities during the melting process of high-density polyethylene (HDPE). Two-dimensional correlation analysis revealed the sequential order of1339>1363>1351cm-1for the non-planar conformational defect band intensity changes which, however, is inconsistent with the in situ infrared analysis conclusion.