| As known to all, LCST-type polymers have been widely applied in the field of "smart" material, and their properties are strongly dependent on polymer structures. This thesis mainly investigated the relationship between macroscopic properties and molecular structures in LCST-type polymer/water systems by means of different characterizations like fourier transform infrared spectroscopy (FTIR), differnetial scanning calorimetry (DSC), density functional theory (DFT) et al. There are seven chapters involving five research topics in the thesis, where we compared peculiar properties of different LCST-type polymers and figured out the chain conformations and dynamic mechanism under the perturbation at the molecular level.At first, the definition and category of LCST-type polymer were described in the first chapter, and some research progresses including polymer synthesis and application were also discussed. In addition, we also talked about several methods which were used to investigate the micro-dynamic mechanism of LCST-type polymer in water system, and mainly introduced basic and general knowledge of FTIR spectroscopy and2Dcos analysis.In second chapter, we compared the properties of poly(N-sopropylacrylamide)(PNIPAM) aqueous solutions with different concentrations using electron microscope, rheology and DSC measurement. These results showed that hydrogel was formed in the concentration of50wt%PNIPAM/H2O while it was still fluid in10wt%PNIPAM/H2O system even above LCST. Moreover, DSC data reflected that there was a lower LCST, boarder transition range and larger hysteresis of enthalpy change in the hydrogel sample (50wt%PNIPAM/H2O). Thus, compared with10wt%PNIPAM/H2O system, temperature-variable FTIR spectroscopy and two-dimensional correlation (2Dcos) analysis were applied to reveal the reasons of property changes in the self-confinement structures. At low temperature, PNIPAM hydrogel was constructed by some non-chemical bonding interactions like amide hydrogen bonds and hydrophobic interaction between PNIPAM chains, which decreased the transition rate in the confinement environment. Besides,2Dcos analysis also showed that dehydration sensitivity of C=O and CH3groups became lower due to confinement structure, especially in side group of PNIPAM chain. As a result, hydration abilities were restricted in cooling process.In third chapter, we mainly studied the core-shell structure of PNIPAM chain collapsed conformation which was an intermediate conformation formed in the heating process in dilute aqueous solution. But the relationship between intermediate structures and thermodynamic properties was still not clear. Herein, the behavior of the bimodal transition that two exothermic peaks took place in DSC cooling curves has been investigated through different effects of polymer molecular weight, polymer concentration and thermal history. The results showed that the higher-temperature peak (T2) was attributed to the hydration of the loose shell while the lower-temperature peak (Ti) was ascribed to the further hydration of the compact core. However, the bimodal transition in cooling was observed clearly only when PNIPAM concentration was less than its overlap concentration (C*). Moreover, it was found that the ratio of T2peak can be enhanced either with higher heating rate or less isothermal time at high temperature, since more structures of the loose periphery could be preserved in these cases. For the measurements in cooling-heating cycle, in addition, the result also revealed that the assembled behavior of the core-shell structure was reversible.In fourth chapter, the dynamic transfer of homopolymer PNIPAM from water to hydrophobic IL (1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide,[C2MIM][NTf2]) phase was investigated by in-situ React infrared (IR) spectroscopy to reveal the detailed transfer process and structural changes of PNIPAM, water and [C2MIM][NTf2]. Compared with the corresponding binary systems, this chapter summarized the four-step transfer behavior and their interactions among ternary components according to IR spectroscopy and multivariate curve resolution (MCR). At the initial stage, only some water diffused into the hydrophobic IL in the form of a symmetric1:2-type hydrogen bond (anion...H20...anion) while most of the PNIPAM molecules were preserved in the water layer in the form of aggregation. Afterwards, the water molecules bonded with PNIPAM in aggregated state and started to transfer together into IL layer in the second stage where PNIPAM was changed into unfolding structures with complex interactions like H2O...PNIPAM...anion, and meanwhile water structures of anion...(H2O)n...anion were ready to form due to further diffusion of water. After these stages, water molecules continued to dissolve in IL although the polymer transfer was completed, and they were inclined to form more and more water clusters (H2O)n deduced from the increase of OH region (3500-3300cm-1). Finally, all of the components (PNIPAM, water and IL) coexisted in the homogeneous IL phase with the help of their harmonious and complex hydrogen bonds and electric charge interactions.In fifth chapter, the thermoresponsive mechanism of poly(3-ethyl-N-vinyl-2-pyrrolidone)(C2PVP) during heating-cooling cycle has been investigated by means of temperature-dependent FTIR in combination with2Dcos technique and DFT. Compared with the secondary-amide thermoresponsive polymer such as PNIPAM, the unusual phenomenon of an asymmetrical-to-symmetrical transition of C=O peak shape was observed in the conventional IR spectra of tertiary-amide polymer C2PVP.2Dcos results confirmed that two species of C=O groups (C=O...2D2O and C=O...D2O) mainly coexisted at the initial heating, and they changed to the other two C=O groups involving the free C=O and C=O...DOD... O=C after heating with the latter being predominant above LCST. A slight hysteresis was observed in the cooling process attributed to the existence of weak "crosslinking" point originated from the structure of C=O...DOD...O=C. Based on2Dcos results, the dehydration process of different groups could be described in the following order: ethyl groups> C=O groups> CH2groups in ring (">" means prior to), and the reversible sequence was observed during hydration process. In PNIPAM system, CH2groups usually changed prior to C=O groups, while for C2PVP the dehydrated rate of CH2groups (in ring) laged behind that of C=O groups because of the steric hindrance of pyrrolidone structures. Besides, compared with other LCST-type polymers, we also investigated the effects of C2PVP polymer weight, concentration and additives (salts and methanol) on its phase behavior.In sixth chapter, we mainly discussed an amphiphilic polymer with hydrophobic CH groups and two different hydrophilic groups (C=O and C-O)--poly(3-(2-methoxyethyl)-N-vinyl-2-pyrrolidone)(MeOE-PVP)which was easy to absorb water vapor from humid environment. Herein, the hydration capabilities and structures of different chemical groups in MeOE-PVP film during water vapor sorption and desorption process were investigated by in situ FTIR spectroscopy, two-dimensional (2D) correlation analysis and density functional theory (DFT) calculations. When the film was tested in a stable humid environment, the changes of C=O group were more sensitive to water vapor than C-O groups which was further testified by2DIR maps and much more hydrated structures of C=O group can be formed after moisture absorption. Compared with MeOE-PVP in aqueous solution, it was believed that C-O group was a relatively stable hydrophilic group which was difficult to lose water molecules once it is at the hydrated state. In addition, combined with DFT calculations, it could be found that C=O and C-O groups of MeOE-PVP were inclined to interact with water molecules in the form of CO...H2O and CO...H2O...H2O, not the structure of CO...(H2O)2probably due to limited absorption concentration and contact area in MeOE-PVP film.Generally, in this thesis, we have investigated the hydrated structures of LCST-type polymers, polymer conformation and micro-dynamic mechanism of phase transition using several methods. Furthermore, we revealed the relationship between those properties of LCST-type polymers and polymer structures at a molecular level, which are benefit to new designs of other LCST-type polymers and some applications of "smart" materials in the future. |
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