Self-assembly And Phase Transition Of Stimuli-responsive Multiarm Copolymers Based On Hyperbranched Polymers

Hyperbranched polymers are a novel kind of three dimensional torispherical irregular macromolecules possessing highly branched architectures, many inner cavities and a large amount of terminal functional groups. Due to their unique molecular structures and properties, hyperbranched polymers have become the hot topics in many research fields. Up to now, great progresses have been made in the synthesis, characterization, modification, and application of hyperbranched polymers. However, many problems still need to be resolved, such as the exploration of the unknown features, the explanation of new phenomena and the spread of the application fields. In this dissertation, based on the summarization of previous research works of hyperbranched polymers, some creative investigations are conducted in the modification and self-assembly of hyperbranched polymers. The novel kinds of functional star copolymers based on hyperbranched polyethers with stimuli responsibilities were synthesized successfully, their stimuli responsibilities and self-assembly behaviors in solution were investigated in details, and their potential uses in drug delivery, preparation of inorganic hollow nanoparticles were also explored. The main results are shown as follows.1. Facile preparation of novel peptosomes through complex self-assembly of hyperbranched polyester and polypeptide;The work reported for the first time a facile template-free method to prepare polypeptide-based vesicles (peptosomes) through one-step complex self-assembly of carboxyl-terminated hyperbranched polyester H40 and cationic poly(L-lysine) (PLL). The preparation of such the peptosomes, named as complex peptosomes (CPs) here, is very simple just by mixing two kinds of the polymer aqueous solution together. The CP size can be readily controlled from nanosize to microsize through the adjustment of polymer concentration. The results of TEM prove that the fusion between nanosized CPs leads to large CPs. We also observed the inside-out inversion of CPs'structure by adjusting the weight ratio of PLL to polyester (Rw). The inner layer is PLL and the out layer is the polyester at Rw less than 0.5 while the inner is polyester and the out layer is PLL at Rw more than 0.5. The resulting nanosized CPs show unexpected size stability independent of a broad solution pH range, long-term storage stability and almost no cytotoxicity. An electrostatic complex self-assembly mechanism was proposed based on the results of TEM, SEM, LCSM, DLS/SLS and Zeta potentials characterizations. We also explored the application of CPs in drug delivery and the preparation of inorganic hollow nanoparticles.2. The controllable synthesis and supramolecular self-assembly of hyperbranched multiarm copolymers HBPO-star-PNIPAM;A novel temperature-responsive hyperbranched multiarm copolymer with a hydrophobic hyperbranched poly(3-ethyl-3-(hydroxymethyl)oxetane) (HBPO) core and thermosensitive poly(N-isopropylacrylamide) (PNIPAM) arms was synthesized via the atom transfer radical polymerization (ATRP) of NIPAM monomers from a hyperbranched HBPO macroinitiator. It was found that HBPO-star-PNIPAM self-assembled into multimolecular micelles (around 60 nm) in water at room temperature according to pyrene probe fluorescence spectrometry, 1H NMR, TEM, and DLS measurements. The micelle solution showed a reversible thermosensitive phase transition at a lower critical solution temperature (LCST) (around 32°C) observed by variable temperature optical absorbance measurements. Variable temperature NMR and DLS analyses proved that the LCST transition was originated from the secondary aggregation of the micelles driven by increasing hydrophobic interaction due to the dehydration of PNIPAM shells on heating. The drug loading and release properties of HBPO-star-PNIPAM micelles were also investigated by using prednisone acetate as a model drug. The micelles showed a much improved drug encapsulation efficiency and temperature-dependent sustainable release behavior due to the special micellar structure. In addition, the micelles exhibited no apparent cytotoxicity against human HeLa cells.3. The controllable synthesis and thermosensitive supramolecular self-assembly of hyperbranched multiarm copolymers PEHO-star-PMMA;We here describe a novel strategy to impart thermo-responsive property to a hyperbranched multiarm copolymer which possesses a hyperbranched polyether (PEHO) as core and linear poly(methylacrylic acid) (PMMA) as arms. A series of PEHO-star-PMMA copolymers with different branched degree of PEHO cores and different length of PMAA arms were synthesized. The hyperbranched polymers are highly soluble in pure aqueous solution at all temperatures and exhibits no thermoresponsive property. However, after adding micro-amount tetrahydrofuran (THF) into the polymer solution, reversible LCST phase transition was observed at elevated temperatures. Moreover, the LCST of PEHO-star-PMMA can be readily tuned by adding different amounts of THF. In addition, the LCST also varied as a function of the branched degree of PEHO core as well as the length of PMAA arms. A novel interface-recognization induced thermosensitive phase transition mechanism was proposed and proved. The novel strategy to render hyperbranched polymer thermosensitive might open up the possibility to utilizing them for the applications such as micro-analysis and intelligent sensors.4. Supramolecular self-assembly of pH sensitive hyperbranched polyether and the simulation of self-assembly processes by using ABEEM/MM/MD method;A pH sensitive hyperbranched polyether (HBPO-SA) was facilely prepared by one-step modification of the hydroxyl groups into carboxyl groups by using succinic anhydride. It is found that HBPO-SA exhibits pH sensitive self-assembly behaviors. It self-assembles into vesicles at pH less than 5 and large compound vesicles (LCV) at pH less than 1. By adjusting the pH value of the solution, the size of obtained vesicles can be easily controlled. The lower the pH is, the bigger the vesicles are. Previously, we found that the same hyperbranched polyether modified by maleic anhydride (HBPO-MA) self-assemble into micelles although it also shows pH sensitive self-assembly behaviors. By comparing HBPO-SA with HBPO-MA, we found a very interesting phenomenon that a slight change between terminal groups of HBPO cause a vast difference in morphologies of the self-assemblies. We carried out the dynamics simulation of self-assembly processes of HBPO-SA and HBPO-MA by using ABEEM/MM/MD method, respectively. The simulation results are well consistent with the experiment phenomena.5. Characterization and analysis of thermosensitive phase transition of vesicles based on hyperbranched multiarm copolymer HBPO-star-PEO by using steady and dynamic fluorescence spectrum;Solvation dynamics of Courmarin 480 in hyperbranched polymer vesicles (BP vesicles) in water was studied by using steady and dynamic fluorescence spectrum. The steady-state studies indicate that in the BP vesicles the C-480 molecules reside in the vicinity of the PEO arms of BP vesicles. The dynamic studies show that in BP vesicles C-480 exhibits wavelength dependent fluorescence decays. Based on the results of steady and dynamic fluorescence measurements, a time-resolved fluorescence spectrum was constructed. The time-resolved Stokes shift indicates that the salvation dynamics of water in the vicinity of the PEO arms of BP vesicles is bimodal with two components of 2.42 and 58.56 ns at 20 oC below the LCST of BP vesicles. The long component is attributed to the water which hydrogen bonded to the PEO arms of BP vesicles while the short component arises from the water close to that. We also studied salvation dynamics at 34 oC above the LCST. It is found that the long components dramatically decreased when the temperature is raised above the LCST, which indicate the hydrogen bonds between the water and PEO arms is destroyed. The results from salvation dynamics support the molecular mechanism of thermosensitive phase transition of BP vesicles we proposed previously.