(RSC Advances) Two-Dimensional Correlation Infrared Spectroscopy Reveals the Detailed Molecular Movements during the Crystallization of Poly(ethylene-co-vinyl alcohol)
作者：Gehong Su, Tao Zhou,* Xifei Liu, Jihai Zhang, Jianjun Bao, and Aiming Zhang
關(guān)鍵字：EVOH, crystallization, two-dimensional correlation infrared spectroscopy
論文來源：期刊
具體來源：RSC Advances, 2015, 5, 84729-84745
發(fā)表時間：2016年
In this paper, the quantitative spectroscopic evidence for the leading role of hydrogen bonds during the cooling crystallization of poly(ethylene-co-vinyl alcohol) (EVOH) was successfully obtained. Furthermore, the detailed molecular movements during the crystallization of EVOH were revealed via two-dimensional correlation infrared spectroscopy. Two cooling crystallization processes were detected at 159 °C and around 101-105 °C via the combination of DSC and the newly proposed scaling-MW2D correlation FTIR spectroscopy. These two crystallization processes were defined as the primary crystallization (region I) and secondary perfection process (region II) of EVOH. The methods of calculating the enthalpies of the pair-bonded hydrogen bonding (ΔHh), vinyl alcohol (VA) repeating unit crystallization (ΔHC-VA), VA repeating unit diffusing into the crystal lattice (ΔHl-VA), and ethylene (ET) repeating unit crystallization (ΔHC-ET) were established via Van’t Hoff plots. For region I and II, the contributions of the pair-bonded hydrogen bonding of VA repeating units to entire EVOH crystallization were 42.8% (ΔHh=-90.4±5.2 kJ/mol) and 64.6% (ΔHh=-75.8±3.1 kJ/mol). However, the contributions of the ET crystallization to EVOH crystallization were 19.4% (ΔHC-ET=-41.0±2.0 kJ/mol) and 32.7% (ΔHC-ET=-38.3±0.8 kJ/mol), which were only a half of the contribution of pair-bonded hydrogen bonding. The 2D correlation analysis was used to investigate the sequential order of groups’ movement involved in the crystallization. It was found that Region I had 3-steps. The first step is the formation of hydrogen bonds of VA repeating unit, and the second step is VA repeating unit diffusing into the crystal lattice, resulting in the primary crystallization. The third step is the ET repeating unit crystallization accompanied by the movement of VA repeating unit without hydrogen bonding in the amorphous. Region II had 4-steps. The first step is also the generation of hydrogen bonds of VA repeating unit. The second step is a local rearrangement of the lattice in the imperfect crystalline of VA repeating unit, called as the secondary perfection process. The third step is the movement of VA repeating unit without hydrogen bonding in the amorphous. The fourth step is the weak crystallization of ET repeating unit at a low temperature.