The Investigation Of Two Dimensional Correlation Spectroscopy Features And Its Application In Complicated Intensity Variations

Due to its important features:spectral resolution enhancement and sequential order determination, two dimesional correlation spectroscopy (2DCOS), a powerful analysis technique, is widely used in many different fields, especially the study of polymer materials. Through the utilizations in lots of work, the application value of 2DCOS has been proved sufficiently, while some defects of this method in some certain intensity vatiations has also been revealed. In this paper, the simulation method based on experimental data has been used to investigate the characters and the application skills of 2DCOS to deepen the understanding of the useful method.Firstly, we performed a simulation in the region of OH stretching vibration of water to evaluate the contributions of spectral effects on a highly overlapped band and study the dynamic changes of water during diffusion process in Poly(s-caprolactone) (PCL) matrix. The time-dependent infrared spectra were collected by attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy. The comparison between experimental and simulated asynchronous 2D correlation spectra figures out that the artificial asynchronous cross peaks are induced by band overlapping. During the diffusion process, the water clusters disperse into PCL matrix where the moderate hydrogen-bonded ones diffuse faster than the strong ones. The hydrogen bonds between waters are weakened because of the limited free volume, and then the free and bound water bonded with PCL are formed simultaneously.Sencondly, water diffusion through low density polyethylene (LDPE) film was investigated by ATR-FTIR and 2DCOS over a temperature range of 25℃to 80℃. At different temperature, the diffusion mechanisms are simiar:water molecules disperse in LDPE, cluster water with moderate hydrogen bonds diffuse faster than strong ones. Additionally, the hydrogen bonds between water molecules may have been further weakened or even broken to form free water due to limited free volume. The diffusion coefficients have been calculated from overall, fast and slow diffusion process, respectively. They increase with temperature which raises the swelling of molecular chain and reduceds the size of the penetrants.Finally, a comprehensive study on application of 2DCOS in phase transion system, where the spectral intensity always changes in an "S" or "anti-S" shaped curve, has been operated by simulation methods. It is concluded that the whole process can be divided into three parts:head, middle and tail, with the help of the asynchronous perturbation-correlation moving-window two-dimensional (PCMW2D) correlation spectroscopy. The sequential order in head part indicates the band with an early start of the overall intensity vatiation changes before the other one; and in middle part, the band with a samller transition point changes earlier; while in tail part, the band changing prior to the other one ends intense intensity variation and trends to a stable state earlier. Whereas, the sequential order involved with an intermediate always misleads our understanding of a continuous reaction. The results of actual systems: temperature-dependent IR spectra of Poly[di(butyl)vinyl terephthalate] (PDBVT) and concentration-dependent ATR-FTIR spectra of methanol-acetonitrile mixture, are consistent with the simulation results.Through the systematical researches above, part of the feartures and the application skills of 2DCOS have been proposed, which can help us get a fluent master of this effective method and avoid the mistakes caused by inappropriate operations.