The Application Of Two-Dimensional Infrared Correlation Spectroscopy On The Study Of Polymer Systems

The generalized two-dimensional correlation infrared spectroscopy based on time-resolved IR signals, has been applied to analyze the inter- and intra-molecular interaction. 2D IR spectra can enhance apparent spectral resolution efficiently and provide information about the specific order of the spectral intensity changes from different functional groups during the measurement from the analysis of the synchronous and corresponding asynchronous spectra. 2D IR correlation spectroscopy has been applied to study the structure and conformation of the mono-component polymer system but also the multi-component polymer system. Accordingly, we investigate the changes of the structure and conformation of some multi-component polymer system during the phase transition processIn the study of the hydration and phase transition behavior of PVME aqueous solution, PVME aqueous solution separates into two phases above the lower critical solution temperature (LCST), which induced the destruction of the H-bond between the water and ether in the chain of PVME and the hydrophobic of CH in the chain of PVME. 1D IR spectra and 2D IR correlation spectra were made study the hydration and phase transition behavior of Poly (vinyl methyl ether) (PVME) aqueous solutions.In the transmittance MIR spectra, the C-H and C-C band display weak red shifts gradually with increasing concentration below 43-wt%, and C-O shows no shift. Above 43-wt%, the C-H and C-C bands display red shifts obviously whereas C-O shows blue shift. Below 43%, there are three kinds of water molecules: an average of 2.7 water molecules on each repeat unit as a cooperative polymer-water complex through HB to the ether oxygen, a second average of 2.3 additional water molecules forming a cluster around the hydrophobic sites of each repeat unit as the iceberg structure, and the rest of the water beyond 5 molecules per repeat unit being free in the system. Above 43%, no free water molecules exist in the PVME aqueous solutions and the chain of polymer dehydrate gradually with increasing concentration. Using 2D IR correlation analysis, two different states of polymer chains were found in the solution system below LCST: hydrated C-H and dehydrated C-H partly.In the transmittance NIR spectra, the overtone of OH stretching band appears at 6886 cm^-1 during 10-40 wt% PVME aqueous solution, which is induced by the bulk water of system. A shoulder peak appears 6434 cm^-1 in the 50 wt% PVME aqueoussolution, and four peaks appear at 7056, 6779, 6434 cm^-1 in 60-90 wt% PVME aqueous solution, which are induced by three kinds of water molecules.The phase transition behavior of PVME in aqueous solution during heating was investigated by Fourier transform infrared (FTIR) spectroscopy with attenuated total reflection (ATR) accessory. 5% PVME aqueous solution show phase separation behavior above 39℃ and equilibrate at 46℃. The 15% PVME aqueous solution show phase separation behavior above 37℃ and equilibrate at 42℃. When KCl was added to PVME aqueous solutions, the phase transition temperature was reduced and the phase separation process was widened. After phase separation, there are some water molecules exist in the polymer phase. Upon phase transition, the intensity of PVME bands increased dramatically and the positions of bands show a remarkable shift which result from the hydration of polymer chains. 2D IR analysis results indicate that hydrated group change prior to dehydrated group with increasing temperature on the phase transition. It may suggest polymer chains interact first and aggregate in the bottom of ATR crystal, and then polymer chains dehydrated. The C-H group and C-C group dehydrate prior to the C-O group.In the system of PVME-salt aqueous solution, the separation temperature decreased linearly with increasing the concentration of inorganic salt, the radium of alkaline cation and electronegativity of inorganic ions. On the other hand, the apparent features of 2D IR correlation spectra did not change compared to those for PVME aqueous solution in the absence of KCl. The result indicates that the underlying phase transition mechanism itself was not altered by the presence of KCl, although the transition temperature is shifted.In the study of Diffusion Process of Water molecules in Poly(ε-caprolactone), the diffusion process has been investigated using in-situ FTIR-ATR spectroscopy. In this system, the original broad water OH bands in 1D IR spectra can be effectively differentiated into four bands, located at 3641, 3593, 3410, 3203 cm^-1, respectively, using 2D correlation analysis. The bands at 3641 cm^-1 (antisymmetric) and 3593 cm^-1 (symmetric) are assigned to the OH stretching vibration of water partially hydrogen-bonded with hydrophilic carbonyl group (C=O) of PCL, while the other band pair at 3410 cm^-1 (antisymmetric) and 3203 cm^-1 (symmetric) could be attributed to the stretching vibration of water fully hydrogen-bonded with another water molecules. According to the result of the asynchronous correlation, it was concludedthat the water molecules at first diffuse into free volume (microvoids) or are molecularly dispersed into the PCL matrix and then form hydrogen bond with the C=O group of the polymer during the process of water diffusion. In addition, the diffusion coefficient was measured through nonlinear curve fitting of OH band areas in the range of 3800-3000 cm^-1.Two-dimensional infrared (2D IR) correlation spectroscopy was also applied to study the conformation change during poly (vinylidene fluoride) (PVDF) melting process. The absorption spectral changes are observed with increasing temperature. These observed changes in IR spectra might be attributed to four kinds of PVDF components: α-PVDF, β-PVDF, γ-PVDF, and amorphous component.2D IR correlation analysis indicates that during melting process four kinds of PVDF component change asynchronously: above all, and then the β-PVDF transfers to amorphous component, the α-PVDF and γ-PVDF transfers to amorphous component, finally the amorphous component relaxes its molecule chains.As for the system of gelation behavior of Poly(vinylidene fluoride) (PVDF) in Cyclohexanone solution, PVDF converts easily into a thermo- reversible gel through crystallization by standing at room temperature in cyclohexanone. In this study, the FTIR spectra were measured continuously at room temperature until the solution converted to a gel. In the time of 25-45 min, the IR band of PVDF increased dramatically, which induced polymer chains convert from random statistical coils to the TGT(G|-) conformation (a form). In the time of 45-90 min, the IR band of PVDF still increased slowly, which induced the ordered conformation convert gel physically.Using 2D-IR analysis, the C=O stretching band changed during the gelation process. During conformation ordering process, the band (1707 cm^-1) of cyclohexanone dimer decreased while the band (1718 cm^-1)of monomer increased, furthermore, The new band at 1695 cm^-1 increased, which could be assigned to the C=O group interacted the CF₂ group in the polymer chain. The bands of PVDF crystalline changed prior to the bands of cyclohexanone during the conformation ordering. However, the bands of PVDF crystalline changed synchronously with the bands of cyclohexanone during the gelation process.As for the bands of PVDF chains, the band at 762 cm^-1 varied prior to the bands at 873 cm^-1 and 796 cm^-1 during conformation ordering process. Actually, 762 cm^-1 isassigned to the CF₂ group of PVDF, 873 cm^-1 is involved the C-C group of PVDF and 796 cm^-1 is attribute to the CH₂ group of PVDF, which it indicate the CF₂ change fast than the C-C and CH₂ group. However, the correlation crosspeaks disappeared between 762 cm^-1 and 873, 96 cm^-1 during the later state of gelation process. At the same time, the bands of PVDF and solvent still varied which suggest that it is the physical interaction process between PVDF chain and solvent.