Synthesis And Phase Transition Behavior Of Thermo-responsive Polymers With Special Structure

Thermo-responsive polymers are the most widely investigated intelligent or smart polymers up to now. LCST-type polymers experiences a reversible coil-to-globule phase transition during heating and cooling cycles, which is similar to the denaturation of proteins, thus may have the potential in the application in drug release, diagnosis and tissue engineering. Two-dimensional correlation spectroscopy (2DCOS) is a new method to interpret various physical-chemical phenomena at molecular level.2DCOS could not only enhance the spectral resolution and sort out complex or overlapped spectral features by spreading peaks along a second dimension, but also provide the response order of different species to external variable. Our paper mainly focus on the synthesis and phase transition behavior of thermo-responsive polymers with special structure.The thesis is divided into five parts.Chapter Ⅰ is the introduction of our entire research work, which we briefly introduce the basic concept and relevant researches involving in the paper. It contains three parts that are introduction of thermo-responsive polymers,2D correlation spectroscopy and research goals and mentality of the graduate thesis. The first part is the concept, classification, application and research progress of LCST-type polymers, especially representative polymers such as poly(N-isopropylacrylamide) (PNIPAM) and poly(oligo(ethylene glycol) methacrylate) (POEGMA). The second part is the rise, principle, spectral reading method and application of 2DCOS and PCMW (perturbation correlation moving window).In chapter Ⅱ, we mainly focus on the synthesis and unusual phase transition behavior of poly(N-isopropylacrylamide)-poly(2-hydroxyethyl methacrylate) (PNIPAM-PHEMA) interpenetrating polymer network (IPN) microgel. IPN microgel was prepared by two-step synthesis method. In the synthesis process, the collapsed particles from the first polymerization serve as the nuclei for the second polymerization. FT-IR, AFM, DLS were used to help propose the mechanism of the formation of IPN microgel. Due to the hydrogen bonds between NIPAM and HEMA monomers, polymerization of HEMA takes place earlier inside the PNIPAM microgel than on the outside. We further convinced our conclusions by control experiment without PNIPAM as the nuclear. Considering the effects of reaction time and reaction temperature on the formation of IPN microgel, we give detailed discussion and propose relevant mechanism. However, during our research on VPTT, we find an interest phenomenon:the VPTT of IPN microgel have an obvious decrease in contrast with PNIPAM microgel, which is different general IPN system. The unusual change of VPTT could be attributed to the hydrogen bonds between HEMA and NIPAM and the hydrophobic property of PHEMA, which lead to the hydrophobic environment of the IPN micrgel. IPN-PHEMA core-shell microgel is also obtained in our research and the VPTT of the microgel is only replied on the IPN core.In chapter Ⅲ, we present our research of the thermodynamics of poly [2-(2-methoxyethoxy) ethyl methacrylate-co-ethylene glycol) methyl ether methacrylate)] [P(MEO2MA-co-PEGMA2080)] during phase transition. We obtain a set of FT-IR spectra during heating process through in-situ tracing by FT-IR measurements. By curve fitting, three different types of carbonyl groups could be found in amide I region and the complex changes and different absorption coefficients between different groups could be responsible for the complex phase transition behavior. According to 1D FT-IR spectra, the heating process could be divided into three regions and each region is analyzed by 2DCOS. During the heating process, the side chain has an early response than the main chain and the driving force of the phase transition is the hydration change of side chain. Different response order could be found in different temperature regions and the amphiphilic property of PEGMA2080 plays an important role in the formation of the micelle and the conformation of segment adjustment. It is concluded that during the phase transition P(MEO2MA-co-PEGMA2080) chains successively experience "unimers-clusters-micelles-aggregates" four consecutive conformation changes.In chapter IV, we successfully synthesize a series of graft polymer with POEGMA as the core and PNIPAM as the brush. Due to the decoupling effect of long flexible ethylene glycol spacers and the large molar ratio between PNIPAM and POEGMA, core-shell morphology with coiled POEMGA main chains was observed in the copolymers. Double endothermic peaks were identified in the DSC curves and the shoulder peak at higher temperature region "grow" more obvious with increasing the molecular weight of PNIPAM. By temperature-variable 1H NMR, turbidity and DLS measurements, we propose that the graft density in different regions and the hydrophilic POEGMA core could be responsible for the unusual phase transition. A reversible "sol-gel" transition in the aqueous solution with the concentration of more than 5 wt% was observed and the gel phenomenon was explained by the classical physical cross-linking mechanism.In chapter V, we provide a new method to prepare graphene/polymer nanocomposites. By crosslinking of the polymer matrix, poly(vinyl alcohol), we aimed to increase the distribution and stability of graphene, in order to increase the related property of the nanocomposites. We have also detailed characterized the structure, the mechanical property and thermal property of the nanocomposites. We discusses the impact of crosslinking and graphene on the nanocomposites and proposed relevant mechanism.Chapter Ⅵ is the summary of the thesis. Our research mainly focus on thermo-responsive polymers, including the synthesis of synthesis and phase transition behavior of thermo-responsive polymers with special structure by 2DCOS. By preparing thermo-responsive polymers with special structure, the application range could be widened and the depth analyze of the response mechanism might be helpful for other researchers who want to deep their understanding of thermo-responsive materials.