Computer Simulation Study On The Self-Assembly Of Alternating Copolymers

Polymer self-assembly is a simple and effective method for fabricating polymer nanomaterials,which has been widely used in various fields such as material science,nanomedicine,bioscience,chemical sensing and so on.Chain architecture plays a key role in determining the self-assembly behaviors of polymers,and essentially endows polymers with unique characteristics in the self-assembly.As an important kind of linear polymers,alternating copolymers have strict alternating structures,due to which alternating copolymers show very special properties and applications.However,limited by the synthesize difficulties,there are few types of amphiphilic alternating copolymers,and the study of alternating copolymer self-assembly is seriously restricted.With the development of advanced polymerization methods,the types of amphiphilic alternating copolymers are greatly enriched,which promotes the study of alternating copolymer self-assembly.Alternating copolymers have shown very special features in self-assembly.Meanwhile,more and more puzzling questions have emerged and are hindering the deep development of alternating copolymer self-assembly.For instance,it is still unclear about the thermodynamics and kinetics of alternating copolymer self-assembly,polymer chain behaviors in the self-assembly and how to regulate the self-assembly process of alternating copolymers.More efforts should be made to solve these microscopic problems,which are difficult for current experimental characterization techniques.With the fast development of computer software and hardware,computer simulation has been the most effective method for studying self-assembly behaviors in microscopic view.It has been widely used for explaining the self-assembly mechanism,studying the thermodynamics and kinetics of self-assembly,predicting self-assembly morphologies and even computer-aided designing of new systems.In this study,we apply computer simulation method to the self-assembly of alternating copolymers,to explore the thermodynamic rules and kinetics of alternating copolymer self-assembly.We build several morphological phase diagrams and explore the unique characteristics of alternating copolymer self-assembly.Our study can provide powerful theoretical support for the development of alternating copolymer self-assembly.The main contents are shown as follows.1.A study on the self-assembly behaviors of amphiphilic alternating copolymers in solution.The self-assembly behaviors of coil-alt-coil alternating copolymers have been studied by employing dissipative particle dynamics(DPD)simulation.The general model was built and then several morphological diagrams were constructed as the functions of polymer concentrations,different hydrophilicity,different hydrophobicity and different incompatibility of polymers.Several kinds of assemblies have been obtained such as channelized micelles,bicontinuous micelles,vesicles,nanotubes,cylindrical micelles,spherical micelles,among others.The evolution of different assemblies was integrated into one roadmap,along which the formation mechanism and structures of assemblies were studied.Specially,we put forward the folded-chain model for self-assembly of alternating copolymers.In addition,we summarized the unique characteristics of alternating copolymer self-assembly,including ultra-fine and uniform feature sizes,various unimolecular assemblies and special molecular weight and molecular weight distribution-independence.The comparison with experimental data has demonstrated the reliability of our simulation results.2.A study on the self-assembly behaviors of rod-alt-coil alternating copolymers in solution.The self-assembly behaviors of rod-alt-coil alternating copolymers have been studied by employing DPD simulation.Based on the general model,a morphological phase diagram was built by regulating the lengths of coil and rod segments.Several assemblies have been obtained such as large compound micelles,vesicles,lamellae,perforated lamellae,bicontinuous micelles,cylindrical micelles,among others.Taking order parameters as criteria,we explored the rod alignment in the assemblies,where the folded-chain model was adopted.In addition,the effects of coil and rod lengths were studied,which showed that longer rod segments promoted the ordered alignment while longer coil segments do the opposite.?-? interaction also showed great effects on the self-assembly behaviors and could seriously promote the ordered alignment of rods.Finally,the unique characteristics of alternating copolymer self-assembly were explored.In particular,we found that the alternating structures were helpful for the ordered alignment of rod segments,which is valuable for the design of ordered supramolecular materials.3.A study on the block alternating copolymers for fabricating multigeometry nanoparticles.Multigeometry nanoparticles(MGNs)have a high level of complexity and have shown great potential in material science.However,up to now,the self-assembly strategy and the polymer category to fabricate MGNs are quite limited,and it is still a big challenge to get MGNs in a controlled way.By employing DPD simulations,we designed the novel block alternating copolymers,which could self-assemble into MGNs.In the block alternating copolymers,each block was an individual alternating copolymer.By regulating the composition of each block,a morphological diagram was built,in which 16 kinds of MGNs were obtained.In the self-assembly of block alternating copolymers,the two blocks can self-assemble independently into single geometries,which are forced to connect together to form MGNs due to the limitation of the covalent bonding.We named this process a“covalent-bonding-forced orthogonal self-assembly” strategy.Two basic requirements should be satisfied to ensure the success of this strategy: high enough degree of polymerization of each block and high enough incompatibility between the two blocks.In addition,the fine structures of MGNs can be regulated through controlling the polymer concentration and the volume ratio of the two blocks.Finally,we extended this strategy to the triblock alternating copolymer system,in which more complicated ternary MGNs were obtained.4.A study on the “high ?-high N” alternating copolymer system for accessing sub-5nm nanopatterns.“High ?-low N” block copolymer has been widely used for fabricating sub-10 nm nanopatterns.However,too low molecular weight results in the drop of polymer properties,which limits the applications of these materials.By employing molecular dynamics(MD)and DPD simulations,we designed the novel “high ?-high N” alternating copolymer system for fabricating nanopatterns with sub-5 nm feature sizes.We first built and validated the coarse-grained(CG)models of high-? alternating copolymers by parameterizing with atomistic MD simulation.Then,the CG models were used for studying the microphase separation of high-? alternating copolymer melts.Several mesophases were obtained including lamellae,perforated lamellae,hexagonally packed cylinders,disordered spherical micelles,among others.The feature sizes of these mesophases were all less than 5 nm.Polymer chain packing and conformation in the mesophases were also explored,which showed great differences with those of block copolymers.In addition,we evaluated the effects of molecular weight(MW)and molecular weight distribution(MWD).It was found that both the morphologies and feature sizes of mesophases showed special MW-and MWD-independence once exceeding a certain threshold value.“High ?-high N” alternating copolymers can provide sub-5 nm freature size together with the high enough molecular weight,which is valuable for nanolithography.