Synthesis Of Polymer Single-chain Nanoparticle With High Compactness In Co-solvent Condition:A Computer Simulation Study

In recent years,a new type of nanoparticle--single chain nanoparticle(SCNP),has attracted a lot of interest.Single-chain nanoparticles(SCNPs)are unimolecular soft nano-objects,consisting of individual polymer chains collapsed to a certain degree by intramolecular bonding(i.e.,covalent,noncovalent,or reversible covalent bonds)at high dilution.Because of its unique properties,SCNPs have been extensively studied and have found potential applications in many fields,such as bio-imaging,nanomedicine,catalysis,rheology etc.More importantly,the folding/collapse process of SCNP is similar to the folding process of proteins.Therefore,the study of the folding/collapse process and mechanism of SCNP is of both great theoretical and practical significance for understanding protein folding,DNA packaging and intra-chain complexation.However,the folding degree/process of SCNPs is difficult to control in the synthesis process,which can be attributed to the fact that the inherent self-avoiding character of the precursor chain under good solvent condition will result in a higher reaction probability for the functional reactive groups separated by shorter contour distances.In such a condition,the bonding probability of the two active crosslinking units at a short contour distance along the chain backbone are much higher than those which are far away from each other.Such reaction condition often results in the local spheroidization and,therefore,the formations of loosely packed structures.How to inhibit the local spheroidization and improve the compact-ness of SCNPs is therefore a major challenge for the syntheses of SCNPs.To solve the above problems,the main purpose of this thesis is to study the influence of co-solvent effect on the preparation of single chain nanoparticles byusing molecular dynamics simulation technology.Recently,it has been found that in the mixed solution of two good solvents,due to the tendentious adsorption of one of the solvent molecules on the surface of the polymer linkage,the polymer chain can undergo a “swell-collapse-swell” transition under different proportion of mixed solvents.We demonstrate that the co-non-solvency effect can be utilized to fabricate SCNPs with high compactness.Simulations are also performed in a single-component good solvent condition for comparison.In particular,we analyze the topological polydispersity,size,shape parameters(asphericity and prolateness)of formed SCNPs.We demonstrate that we can effectively improve the compactness of SCNPs by utilizing the collapsed precursor chain conformation caused by co-non-solvency effect under co-solvent conditions.The results show that the inherent self-avoiding character of the precursor chain in the good single component solvent that can largely improve the probability of the cross-linking reactions at short contour distances,so that they can be local spheroidization along the main chain.The cosolvent effect has great influence on the synthesis of SCNPs,that a pre-collapse of the polymer chain conformation in a cosolvent condition can largely inhibit the cross-linking probability between near-distance cross-linking reactions along the chain backbone and therefore the non-idea “necklace”-like structure formations.To some extent,it prevents the local spheroidization of SCNPs,and better realizes the preparation of "spherical" SCNP,so as to obtain more compact SCNP without increasing the active crosslinker.We hope our simulation work can be helpful for the experimentally fabrication of SCNPs with high compactness.