Generalized 2D IR Correlation Spectroscopic Studies On The Melting And Crystallization Behaviors Of Some Biodegradable Polymers

Melting and crystallization behaviors of polymers have long been an important and fundamental issue in polymer science because the properties and structure of polymers are closely related. The mechanical properties, thermal processing performance and biodegradability are largely controlled by the crystallinity and crystal structures of biodegradable polymers. The application of generalized two-dimensional infrared (2D IR) correlation spectroscopy in the melting and crystallization behaviors of polymers has been paid much attention in recent years. The wide application of this technology in various fields due to its two major features. One is the enhanced spectral resolution by spreading peaks along the second dimension. The other is the determination of sequential order of band intensity changes using the so-called "sequential order" rules. However, our theoretical analysis and experimental studies show that conclusions derived from the "sequential order" rules of generalized 2D correlation spectroscopy may lack physical significance. In addition, many new bands resolved by 2D IR are artifacts. In this work, the crystallization or melting behaviors of some biodegradable polymer such as PLLA, PHB and its copolymers P(HB-co-HHx) were studied, by analyzing the C=O stretching regions with a series of data processing methods, including calculation of difference spectrum, second derivative spectrum, fourier self-deconvolution, and curve-fitting, combined with 2D correlation analysis. The objective of this work is to investigate and better understand the melting and crystallization behavior of these polymers and, at the same time, to test the reliability of the information provided by 2D IR correlation spectroscopy. The main contents are as follows:l.The crystallization behavior of biosynthesized PHB film cast from 1,1,2,2-tetrachloromethane(TCM) was studied by in situ FTIR spectroscopy and 2D correlation analysis. In situ spectral analysis results reveal that there are probably five component bands under the C=O band profile, and the crystalline and amorphous band intensities change simultaneously during the crystallization process, with no local sequential order. Further analysis on the relative area percentage changes of the five component bands suggests that the crystalline component only changes in a fully cooperative manner with part of the amorphous component at the initial crystallization period.2.The crystallization behavior of solution-cast P (HB-co-HHx) film and the non-isothermal melt crystallization process of P (HB-co-HHx) were studied by in situ FTIR spectroscopy and 2D correlation analysis. There are probably five component bands under the overlapped C=O band profile of P (HB-co-HHx), as PHB. During the crystallization of solution-cast film, the following local sequential order has been obtained:1722cm-1≈1736cm-1>1712cm-1≈1729cm-1>1746cm-1. Further analysis on the relative area percentage changes of the five component bands also suggests that the crystalline component only changes in a fully cooperative manner with part of the amorphous component. In the non-isothermal melt crystallization process, no significant (local) sequential order was found for the intensity variation of each component band. That's to say, all the sub-bands change in a fully cooperative manner.3.The isothermal cold-crystallization process of PLLA was studied by in situ FTIR and 2D correlation analysis. The individual component bands in the C=O stretching region are attributed to the different molecular conformations. The local sequential order obtained from the in situ spectral analysis results is:1760cm-1(gt)≈1768cm-1(tg)>1753cm-1(tt)>1778cm-1(gg)4.The information provided by the generalized 2D IR correlation spectroscopy disaccords with the in situ spectral analysis results.In summary, in situ FTIR spectroscopy and 2D correlation analysis are applied to the study the crystallization and melting behaviors of a few typical biodegradable polymers and some interesting results have been obtained. In addition, the applicability of generalized 2D IR correlation spectroscopy is tested, and the results from our experiments indicate that the information provided by the generalized 2D IR correlation spectroscopy may lack physical significance.