Synthesis, Characterizations And Self-Assembly Of Hyperbranched Polyethers

Hyperbranched polymers (HPs) have received considerable attention in the resent decade due to their unique molecular structures as well as their special physical and chemical properties. Compared with the traditional linear polymers, HPs possess some traits such as low solution and melt viscosity, good solubility, a large amount of terminal groups, and so on. In addition, HPs can be easily prepared through a one-step polymerization procedure. These advantages make HPs promising in the fields of surface modification, polymer processing, biomedicine, and coating etc. Recently, the researches on HPs have been focusing on two important directions. One is to explore more effective methods to control the degree of branching (DB) of HPs, and the other is to seek more convenient and practical approaches of preparing HPs and finally industrialize the synthesis of HPs. More recently, HPs have been freshly applied in the area of supramolecular self-assembly. The self-assembly of HPs, reflecting the principle of from irregularity to regularity in nature, has attracted people's great interests. In this dissertation, we mainly focus on the researches of the synthesis, characterizations and self-assembly of HPs. The dissertation includes two primary parts of investigation contents. The first part, including Chapter 2 and 3, describes the synthesis and characterizations of a series of hyperbranched poly[3-ethyl-3-(hydroxymethyl)oxetane]s (PEHOs) with a similar molecular weight and different DBs. The second part, including Chapter 4, 5, and 6, elucidates the preparation and self-assembly of amphiphilic hyperbranched multiarm copolyethers.In Chapter 2, PEHOs were prepared by the cationic ring-opening polymerization. In the experiments, we found two key factors that may influence the DB of PEHO. One is the feed ratio of monomer to initiator, and the lower the ratio, the higher the DB of the obtained PEHO. The other is the reaction temperature. In the temperature range of -50 oC～+30 oC, the DB of the prepared PEHO increases with increasing reaction temperature and inclines to be of no change when the temperature is higher than 10 oC. In terms of the conclusions, we prepared PEHO samples with a similar molecular weight and different DBs.In Chapter 3, we systematically investigated the influence of the DB on the physical properties of PEHO including crystallinity, thermodynamic properties, and free volume. X-ray diffraction (XRD) and Differential Scanning Calorimetry (DSC) measurements indicate that PEHOs with a small DB are semicrystalline polymers, the crystallinity of PEHO decreases with increasing DB, and PEHO will become amorphous when DB is higher than 40%. The data of glass transition temperature (Tg) obtained by DSC show that the Tg of PEHO gradually reduces with the increase of DB. Thermal Gravimetric Analysis (TGA) indicates that DB does not evidently affect the temperature of decomposition (Td) of PEHO. Furthermore, we carried out the positron annihilation lifetime spectrum (PALS) measurement to study the effect of DB on the nanostructures of PEHO. The results show that the effects of DB mainly focus on the concentration of the free volume rather than on the size of the free volume of PEHO, and the higher the DB, the bigger the concentration of the free volume holes of PEHO, which leads to the decrease of Tg and crystallinity of PEHO. The PALS outcomes explain the influence of DB on the macroscopic physical properties of PEHO from a microscopic point of view.In Chapter 4, a series of amphiphilic hyperbranched multiarm copolyethers of PEHO-star-PPO with different hydrophile-lipophile ratios (RA/C) were synthesized by a"one-pot two-step"cationic ring-opening polymerization method. The results of Nuclear Magnetic Resonance (NMR) and Size Exclusion Chromatography (SEC) prove that PPO arms have been covalently attached to PEHO cores. DSC and TGA results show that both Tg and Td of PEHO-star-PPO copolymers decrease with increasing RA/C. The self-assembly behaviors of PEHO-star-PPO copolyethers were investigated by Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM), Dynamic Light Scattering (DLS), and so on. The results indicate that the ill-defined PEHO-star-PPO molecules can aggregate into large regular spherical micelles with average diameters of 100 to 300 nanometers, and the average sizes of the spherical micelles will decrease as RA/C increases. The self-assembly mechanism was explored by temperature-variable FTIR, NMR, TEM, etc. According to the obtained results, we suggest a possible self-assembly mechanism named as multi-micelle aggregate (MMA) to clarify the formation of the large micelles.In Chapter 5, a crew-cut amphiphilic hyperbranched multiarm copolyether, PEHO-star-PEO, was prepared by the"one-pot two-step"cationic ring-opening polymerization. The results of NMR and SEC confirm that PEO arms have been covalently grafted to PEHO cores. The crew-cut PEHO-star-PEO molecules can self-assemble into large compound vesicles (LCVs) with an average diameter of 46.9±17.8μm. Such big sizes provide us a unique advantage to study the three-dimensional structure as well as the dynamic behaviors of the LCVs by real-time observations with an optical microscope. Through real-time observations, we found that the formation of the LCVs is an interesting hierarchical self-assembly process. The crew-cut PEHO-star-PEO molecules first self-assemble into vesicles, then the sticky vesicles interconnect together as a result of the successive hydration and membrane fusion to form the special intermediates named as three-dimensional vesicle stack (TDVS), and finally the TDVS will transform into the giant LCVs after it is broken by external disturbances. The strong cohesion of the vesicles formed by the crew-cut PEHO-star-PEOs plays a significant role in the hierarchical self-assembly of the LCVs. The stability of the LCVs was also investigated. It was found that a key factor affecting the stability of the LCVs is the vesicle fusion. The stability of an LCV will enhance with increasing the number of the vesicles inside the LCV. If undisturbed, the LCVs containing a large number of vesicles can keep stable until the solvent volatilizes completely.In Chapter 6, we prepared fluorescence-labeled PEHO-star-PEO, DNS-PEHO-star-PEO. NMR, UV/Vis spectrometry, fluorescence spectrometry, and SEC measurements prove that the dansyl groups (DNS) and PEO arms have been covalently linked to PEHO cores, indicating the hyperbranched multiarm molecular structure of DNS-PEHO-star-PEO. Moreover, we studied the self-assembly behaviors of DNS-PEHO-star-PEO in THF/H2O solvent with increasing water content. The results obtained by fluorescence analysis, fluorescence microscopy, TEM, and DLS measurements indicate that DNS-PEHO-star-PEO molecules can aggregate into small micelles in THF/H2O solvent with a small amount of water, the small micelles will evolve into multi-micelle aggregates with increasing water content, and the multi-micelle aggregates will finally transform into giant vesicles with micron sizes. The outcomes reflect that the morphology transitions of polymer aggregates can be tracked by the fluorescence spectra of the DNS groups linked to the polymers. The green fluorescence emitted by the giant vesicles under a fluorescence microscope displays the distinct vesicle structure. Finally, the lowest critical solution temperature (LCST) of the giant vesicles, being about 20.6 oC, was measured by a temperature-variable UV/Vis spectrometer.