Regulation And Mechanism Of Weak Intermolecular Interaction On The Morphology Of PEO-PPO-PEO Block Copolymer Aggregates

Amphiphilic PEO-PPO-PEO block copolymer has wide applications in many fields due to its designable,biocompatible and sensitively responsive properties.Understanding the morphology of block copolymer aggregates in selective solvent and its formation mechanism is crucial to its application.Although the aggregation behaviors of PEO-PPO-PEO block copolymer have been extensively studied,knowledge on mechanism of regulating the aggregates morphology is still lacking.The weak intermolecular interaction is known to be vital to the directional fabrication of nano-micro structure.Therefore,regulating the aggregation behaviors of PEO-PPO-PEO block copolymer by weak intermolecular interaction not only is beneficial to deepen our understanding of the formation mechanism of aggregates,but would provide a simple and effective approach for controlling the aggregates morphology.In this dissertation,the influence of various small molecules on the aggregation behaviors of PEO-PPO-PEO block copolymer in aqueous solution was comprehensively investigated by combining dynamical light scattering,cryo-transmission electron microscopy,nuclear magnetic resonance and molecular dynamics simulations.The effect of various weak interactions between small molecules and PEO-PPO-PEO block copolymers on the morphology was unveiled and the underlying mechanism was provided.The research contents and results were mainly composed of four parts that were presented as follows:(1)Using gallates with different alkyl chain lengths as model small molecules,the regulation and mechanism of the intermolecular hydrophobic interaction on the morphology of PEO-PPO-PEO aggregates were studied.The results showed that as the alkyl chain of gallate grows,its hydrophobic interaction with the PPO block was significantly enhanced,so that they were located at different positions of the micelle.Among them,methyl gallate molecules with the shortest alkyl chain were distributed in the core and shell of micelles,as well as in solvent,which could only cause an increase in micellar size but cannot induce the change in morphology.The aromatic ring,ester group and adjacent methylene group of propyl gallate with a longer alkyl chain were located at the core-shell interface of micelle,and the remaining alkyl chain was situated at the core of the micelle.This positioning caused part of the PPO core to be exposed to water,which destroyed the stability of micelles and induced the fusion and aggregated of micelles to form large micelles or even micelle clusters.The strong hydrophobic interaction between the octyl gallate with the longest alkyl chain and the PPO block made it mainly located in the PPO core of the micelle and its aromatic ring situated at the core-shell interface,which brought about the sphere-rod morphological transition of the micelle.(2)Adopting propyl benzoate with different numbers of phenolic hydroxyl groups as model small molecules,the effect of intermolecular hydrogen bonding on the morphology of PEO-PPO-PEO aggregates was studied.The results showed that propyl benzoate without phenolic hydroxyl had no hydrogen bonding interaction with PEO blocks,and only had hydrophobic interaction with PPO blocks,made it completely located in the core of the micelle and had no effect on micellar morphology.For propyl paraben with a phenolic hydroxyl group,the hydrogen bonding interaction between phenolic hydroxyl and PEO blocks caused its aromatic ring to be located at the core-shell interface of micelle and the alkyl chain lied in the core of micelles,thus lead to a morphological transition of sphere-long worm-unilamellar vesicle.Hydrogen bonding interaction between propyl gallate with three phenolic hydroxyl and PEO blocks further enhanced.So compared with propyl paraben,the position of propyl gallate was closer to the micellar shell,which could induce the formation of large micelles and micelle clusters.Molecular dynamics simulations results showed that although the hydrophobic interactions between small molecules and PPO blocks were significantly stronger than their hydrogen bonding interaction with PEO blocks,the morphologies of aggregates could also be controlled by changing the strength of hydrogen bonding interaction.(3)Taking n-octanol,n-octylamine and n-octanoic acid as the representatives,the regulation of PEO-PPO-PEO aggregate morphology by small molecules with different hydrogen bond donors was further investigated.All three kinds of small molecules could increase the size of P123 micelles,but n-octanol could not cause change in micelle morphologies,while n-octylamine and n-octanoic acid induced morphological transition of spherical-short wormlike and spherical-long wormlike-unilamellar vesicle,respectively.Although the interaction sites between small molecules and P123 block copolymer were same,the difference in the interaction intensity made them have different positions within micelle.Thereinto,the interaction between n-octanoic acid and PEO and PPO blocks was the strongest,followed by n-octylamine,and the interaction of n-octanol with PEO and PPO blocks was the weakest.(4)The unique structure of ionic liquid enabled them to bear multiple weak interactions.This dissertation finally investigated the effect of cationic surface active ionic liquids(SAILs)on the aggregation behavior of PEO-PPO-PEO in aqueous solutions.C8mimBr,CsPyBr and CsMPB had no effect on the CMT of F127 block copolymer,but could affect the composition of micelles.Addition of SAILs increased the cloud point of F127 solution,and the ability of SAILs to increase the cloud point enhanced with the growth of its alkyl chain.The effect of SAILs on F127 micelles was closely dependent on their concentration.When the concentration of SAILs was low(<CMC),different types of SAILs entered F127 micelles through various interactions to form smaller and charged mixed micelles.When the concentration of SAILs was higher(>CMC),the enhanced electrostatic repulsion on the core-shell interface decomposed mixed micelles,SAILs aggregated to form micelles and F127 monomolecular chain had contact with SAILs micelles through different interaction sites.