Small Molecule Additive-induced Morphological Transition Of Block Copolymers Based On PSSS

Amphiphilic diblock copolymer self-assembly has attracted much attention for many years as a convenient method for preparing various structured nano-objects such as spherical micelles,worm-like micelles and vesicles,which can find potential applications in various domains.Polymerization-Induced Self-Assembly of block copolymer nano-objects via RAFT aqueous dispersion polymerization can produce diblock copolymer nano-particles in the form of either spheres,worms(sometimes described as "fibers"),or vesicles,with the advantages of high solid content,simple operation and good repeatability.The aggregated morphology of block copolymers can be tuned by multiple factors such as the copolymer block lengths,the quality of the solvent or the presence of small molecular additives.In this thesis,linear poly(sodium-p-styrene sulfonate)-stabilized diblock copolymers were synthesized by polymerization-induced self-assembly via RAFT aqueous dispersion polymerization.Self-assembly behaviors and the aggregated morphological transformation of poly(sodium-p-styrene-sulfonate)-block-poly(N-isopropylacrylamide)(PS S S 50-b-PNIPAM x)and poly(sodium-p-styrene-sulfonate)-block-poly(diacetone-acrylamide)(PSSS50-b-PDAAMx)in the presence of small molecule additives were investigated by 1H NMR,dynamic light scattering(DLS)and transmission electron microscopy(TEM).First,thermo-responsiveness and self-assembly behaviors of poly(sodium-p-styrene sulfonate)-block-poly(N-isopropylacrylamide)(PSSS50-b-PNIPAM300)in pure water,water/methanol and water/acetone binary solvent were investigated by 1H NMR,DLS and TEM methods.It was found that PNIPAM chains in water/methanol and water/acetone binary solvent possess a lower LCST than in water,however,the temperature-induced collapse of PNIPAM in water/acetone binary solvent was obviously weaker compared to the collapse in pure water and water/methanol binary solvent.DLS and TEM studies demonstrate that the self-assembly process and morphology of PSSS50-b-PNIPAM300 in aqueous solution is successfully tuned by addition of small solvent molecules.Secondly,a series of PSSS50-PDAAMx diblock copolymers were synthesized in aqueous media targeting mean degrees of polymerization(x)of 100,250,350,500 and 600 for the core forming PDAAM block.The self-assembly morphological transition of PSSS50-PDAAMx induced by NaCl were investigated by 1H NMR,DLS and TEM.For PSSS50-b-PDAAM250,upon addition of 0.3 M NaCl,the spherical micelles transformed into short rod-like micelles,which further transformed into reticulated morphology when the concentration of NaCl is further increased to 0.8 M.For PSSS50-PDAAM600 with longer hydrophobic block,,dense reticulated morphology with relatively uniform holes was formed at a rather low NaCl concentration of about 0.1 M.Further increase the concentration of NaCl to 0.8 M,octopus-like morphology were observed.In contrast to the above process in which NaCl was added in the micellar solution,bead-like mophorlogy were observed in the case where NaCl was added before the PISA reaction.It was concluded that adding salt after reaction successfully induced the self-assembly morphological transition of PSSS50-PDAAMx diblock copolymers.