Effect Of Additives On Self-assembly Of Amphiphilic Block Copolymers In Aqueous Solution

The self-assembly of amphiphilic block copolymers(BCPs)in selective solvents is increasingly attracting the interest of researchers in the past decades due to their potential applications in biopharmaceutical delivery,microreactor chemistry and nanostructure templates.There are many factors affecting the self-assembly of BCP in solution,such as block length,micelle concentration,temperature,solvent composition,additives and so on.In this thesis,we selected additives as influencing factors to explore the effect of amphiphilic block copolymers on self-assembly in aqueous solution.Compared with other influencing factors,the advantage of selecting additives is that the experimental conditions are simple and thus we could avoid the time-consuming synthesis of block copolymer with different components.The specific work contents are as follows:1.In the first part,two block copolymers polystyrene-b-poly(4-vinylpyridine)(PS-b-P4VP)with different lengths of hydrophilic segments were synthesized by reversible addition-fragmentation chain transfer polymerization(RAFT).Subsequent quaternization with ethyl bromide obtains the ion-pair copolymer polystyrene-b-poly(quatemized 4-vinylpyridine)/ethyl bromide[PS-b-q(P4VP)-Br].They are originally self-assembled into so-called "large compound micelles" in aqueous solution.After ion exchange of I-,SCN" and PF6-for Br-,morphological transitions are controlled by a competition of solvation and size effects of the newly introduced counterions.The solvation effect is dominated in a relatively short corona-forming block.Thus,sphere-to-wormlike micelles,sphere-to-vesicle,and sphere-to-precipitate transitions are observed.In contrast,due to a long corona-forming block,size effects become more profound after adding SCN-,thereby no morphological transition occurs.A morphological transition is characterized after partial exchange of I-for Br-for both diblocks.A sphere-to-wormlike particles transition is observed only after most Br-is replaced by I-.2.In the second part,the triblock copolymer poly(ethylene oxide)-b-poly(?-caprolacto ne)-b-Poly(2-(dimethylamino)ethyl methacrylate)(PEO-b-PCL-b-PDM)was synthesized by ring-opening polymerization and RAFT polymerization,followed by quaternization with ethyl bromide to obtain the ion-pair copolymer[PEO-b-PCL-b-q(PDM)-Br][q(BCP)-Br].It initially self-assembled into spherical micelles in aqueous solution.However,after compl ete exchange Br-for more hydrophobic counterions such as I-,SCN-,PF6-,OTf-,morph ological transations from the sphere to worm-like and sphere to platelet were obderved.Moreover,we also found that the aspect ratio of the obtained platelets also increased as the hydrophobicity of the counterion increased.3.In the third part,the triblock copolymer PEO-b-PCL-b-P4VP was synthesized by ring-opening polymerization and RAFT polymerization.It is originally self-assembled into crystalline lamellar micelles in aqueous solution.Subsequently,the effect of varying pH and organic diacids on morphological transition is investigated in detail.Lamellae-to-cylinder-to-sphere transitions are observed after decreasing the pH or with the addition of organic diacids with different chain spacers.The decreasing pH causes increasing hydrophilicity of the P4VP block,while adding organic diacids results in an increasing corona swelling of the P4VP segment,both of which lead to a decreasing crystallinity of the poly(?-caprolactone)core.Consequently,morphological variety changing from lamellar to worm-like to spherical micelles can be achieved.