The Application Of Two-Dimensional Correlation Spectroscopy On The Study Of Epoxy Resins

Generalized two-dimensional(2D) correlation spectroscopy,proposed by Noda in 1993,is a powerful spectral technique.Compared with conventional analytical methods developed in the field of spectroscopy,2D correlation spectroscopy can not only provide better resolution,but also distinguish the motion sequential order of different functional groups of materials.Therefore,it has been widely applied into the researches related to the structures,conformations and configurations of polymers. This dissertation focuses on the application of 2D correlation spectroscopy on the study of water diffusion in epoxy networks and curing process of epoxy resins,which helps to reveal the mechanism.Water sorption behaviors of six different amine-cured epoxy resins were monitored at five temperatures ranging from 35 to 75℃.The sorption process was analyzed in detail by the combination of gravimetric measurements,positron annihilation lifetime spectroscopy(PALS),attenuated total reflection infrared spectroscopy(ATR-IR),near infrared spectroscopy(near-IR) spectroscopy and generalized 2D correlation spectroscopy.The experimental results suggested that the equilibrium water content was essentially governed by the polarity of the system, while diffusion was controlled by local chain reorientation and dehydration.In addition,four types of water molecules,termed as S₀,S₁,S_2L and S_2T,were detected, which refers to water molecules without hydrogen bond,with one hydrogen bond, with two loose hydrogen bonds and with two strong hydrogen bonds,respectively. Among these different types of water,S_2L molecules diffused first,which suggested that the forming of hydrogen bonds was the key factor during diffusion.Based on the above results,the mechanism of sorption was therefore proposed,and was verified by experimental results.In addition,the behaviors of water sorption in other epoxy resins were also presented as comparison,which proves the mechanism in other epoxy networks.Moreover,water diffusion in esterified novoloc epoxy resins with perfluorinated side chains was monitored by ATR-IR.The results indicated that the diffusion coefficient first increased and then decreased slightly with the increasing fluorine content,which could be attributed to the two opposite effects(enhanced hydrophobicity and the increased free volume) induced by perfluorinated side chain. Furthermore,three different kinds of water molecules,in this case,were found.It was interesting that in the sorption process,water molecules that interact with epoxy network changed sooner than those without interaction with epoxy network;while in the desorption process,the sequence changed to the opposite.As to the curing process,the isothermal curing process of diglycidyl ether of bisphenol A(DGEBA) cured with 4,4'-diaminodiphenylmethane(DDM) was in-situ monitored by infrared spectroscopy(IR) and near-IR spectroscopy.With the help of 2D correlation spectroscopy,we obtained the results that,during curing,the change of primary amine groups and epoxy groups were simultaneous,which took place prior to that of hydroxyl groups,followed by the change of aliphatic CH₂/CH groups resulted from the ring-opening reaction of epoxy groups.In addition,2D MIR×NIR hetero-spectral correlation spectroscopy was also employed,by means of which direct evidence of curing mechanism could be obtained,and obscure near-IR band assignments in the overlapped CH combination region could be clarified.At last,the cationic polymerization of DGEBA initiated by phosphotungstic acid was investigated by near-IR spectroscopy,which proved the 2D correlation analysis to be an effective tool to differentiate various curing mechanisms.Also,the cationic polymerization of DGEBA with tetrahydrofuran initiated by phosphotungstic acid was studied by near-IR spectroscopy.It was found that the conversion of epoxy groups increased linearly with reaction time in systems with low epoxy concentrations.The relationship between the epoxy consumption rate and the epoxy concentration was discussed based on the results of 2D correlation spectroscopy,which demonstrated the copolymerization between DGEBA and tetrahydrofurarmolecules.