Preparation Of Graphene/Thermo-responsive Polymer Composites And Two-dimensional FTIR Spectroscopic Study Of Their Phase Transition Behaviors

This dissertation mainly focuses on preparation and self-assembly behavior of graphene/thermo-responsive polymer composites, which can be mainly divided into four parts:1) preparation of graphene/thermotropic liquid crystalline polymer composites;2) application of two-dimensional correlation spectroscopy (2DCOS) in investigating the mechanism of liquid crystalline phase transition3of graphene/thermotropic liquid crystalline polymer system;3) preparation of graphene/LCST-type cellulose ethers filtration membrane;4) application of2DCOS in investigating the mechanism of LCST phase transition of hydroxypropyl cellulose aqueous solution.As a unique2D nanocarbon material, graphene has stimulated great interest due to its extraordinary electronic, thermal and mechanical properties. Recently, reports about the research of graphene concentrate maily on new-energy materials and graphene nanocomposites. Thermotropic liquid crystalline polymers are the mostly investigated intelligent or smart polymers up to now, which would self-assemble into long-range ordered structures under temperature stimulus. LCST-type polymers are a kind of important polymeric material. They can dissolve in water or other solvents at low temperature, but form micelles or gels with elevating temperature due to intramolecular and intermolecular interactions. Upon cooling, they can restore to the original state. Some natural polymers such as some cellulose ethers (e.g. methylcellulose, hydroxypropyl cellulose) possess LCST phase transition phenomenon with increasing temperature in their aqueous solutions, and the solutions turn from homogeneous systems to heterogeneous systems.2D correlation spectroscopy (2DCOS) is a new strategy for interpreting various physical-chemical phenomena on molecular level. By spreading peaks along a second dimension,2DCOS can sort out complex or overlapped spectral features and get an enhanced spectral resolution. Due to the different response of different species to external variable, additional useful information about molecular motions or conformational changes can be extracted which cannot be obtained straight from conventional1D spectra.This dissertation is divided into four chapters.Chapter Ⅰ is the introduction of our entire research work, where we give a brief introduction in five fields:1) the fabrication methods and research progress of graphene and graphene-based nanocomposites;2) the concept, history, and research progress in self-assembly mechanism of liquid crystalline polymers, especially mesogen-jacketed liquid crystalline polymers;3) the concept, classification, application and research progress of LCST-type polymers, especially LCST-type cellulose ethers;4) the rise, principle, spectral reading and application of two-dimensional correlation spectroscopy (2DCOS) and perturbation correlation moving window (PCMW);5) research objectives of this dissertation.In chapter Ⅱ, we will discuss the preparation of chemically modified graphene oxide/mesogen-jacketed liquid crystalline polymer (poly{2,5-bis[(4-methoxyphenyl)oxycarbonyl]styrene}(PMPCS)) nanocomposites and2DCOS study of the phase transition mechanism of the nanocomposite system. In section Ⅰ of chapter Ⅱ, we prepare chemically modified graphene oxide withthermotropic liquid crystalline polymer (PMPCS) via atom transfer radical polymerization (ATRP) and click chemistry. Two different molecular-weight PMPCS are prepared via ATRP and grafted onto graphene oxide sheets via click,.chemistry. Raman and XPS results show that there exist π-π stacking interactions and charge transfer effect between graphene oxide planes and the grafted PMPCS chains. And more profound interactions are observed with PMPCS chains going through liquid crystalline (LC) phase transition. Moreover, better compatibility of PMPCS-grafted graphene oxide sheets is found in PMPCS matrix compared with pristine graphene oxide. By post-treating steps of solution blending, precipitating, vacuum drying and compression molding, chemically modified graphene oxide reinforced PMPCS nanocomposites are obtained. Research shows that the nanofiller has outstanding improving effect on storage modulus of PMPCS with only2.0wt%filler content. In section Ⅱ of chapter Ⅱ, we investigate the phase transition mechanism of theabove nanofiller/PMPCS system on molecular level using2DCOS. Research shows that graphene oxide has strong interactions with PMPCS chains. Before LC phase transition, PMPCS chains would first adsorb onto graphene oxide sheets. With the rise of temperature, PMPCS goes through LC phase transition, and the existence of graphene oxide greatly hinders the LC phase transition, resulting in space confinement effect. This work has great theoretical significance in studying the phase transition behavior of thermotropic liquid crystalline polymers in confined space.Chapter Ⅲ concentrates on the preparation of graphene/LCST-type cellulose ethers filtration membrane and study of the mechanism of the LCST phase transition behavior of hydroxypropyl cellulose aqueous solution using2DCOS.behavior Ⅰ of chapter Ⅲ, we introduce the preparation of graphene/cellulose ethers filtration membrane. In the preparation of graphene/cellulose ethers membranes, we first prepare three kinds of aqueous solutions of graphene oxide/cellulose ethers (methylcellulose MC, hydroxypropyl cellulose HPC, carboxy methyl cellulose sodium salt SCMC) with a mass ratio of10:90. The above three systems are reduced at90℃using NaBH4as reductant. After one-month standing, only SCMC system remains stable, and large-scale aggregation and precipitation occur in the other two systems. If the above three systems are reduced at40℃, all systems remain stable after one-month standing. This is because the collapse and aggregation of molecular chains at90℃induce the precipitation of graphene sheets. Graphene/cellulose ethers composite membranes are fabricated by vaccum filtration of the membrane solutions prepared under40℃. The as-prepared membranes are round in shape and multilayered with compact structure and flexibility. Compared with the much more fragile membrane made from pure graphene, the composite membranes possess much better mechanical properties. The study in this section not only discovers the facilitating effect of cellulose ethers in the dispersion of graphene, but also discovers that the binding effect of cellulose ethers on graphene sheets in the filtration membranes could improve the mechanical properties of the composite membranes.In section Ⅱ of chapter Ⅲ, we analyze the mechanism of the LCST phase transition behavior of hydroxypropyl cellulose aqueous solution.20wt%HPC aqueous solution will turn from clear to turbid at45℃(coil-to-globule transition), and the system still possesses mobility. When the temperature is raised to60℃, the system turns from solution to gel, and loses its mobility (sol-gel transition). This phenomenon is different from previous findings that HPC only precipitates in its aqueous solution above45℃. The mechanism of this interesting phenomenon is illustrated in this section using2DCOS on molecular level to reveal the essence of this phenomenon. Both mid-infrared and near infrared analysis are combined. The former is used to study the motions of C-H groups in HPC and the latter reveals the motions of O-H groups and the role of water in the LCST phase transition process. Research shows that in HPC/water system, there are hydrogen bonding interactions and hydrophobic interactions between HPC molecules. The transition of the system from clear to turbid at45℃is mainly due to hydrophobic interactions. And the transition of the system from solution to gel at60℃is mainly due to hydrogen bonding interactions.Chapter IV is the summary of our research work. The main body of the dissertation emphasizes on graphene/thermo-responsive polymers, and focuses on the preparation of graphene/thermo-responsive polymer nanocomposites. Besides, the mechanisms of the phase transition behavior of the thermo-responsive polymers are studied using2DCOS. Our researches refer both to new materials synthesis and design and to structure-property relationship, which may be useful for other researchers who want to deepen their understanding of the self-assembly mechanism of thermo-responsive materials and guide rational design of experiments.