Computer Simulation Studies On The Self-assembly Of Hyperbranched Polymers

Hyperbranched polymers are an important branch of polymer chemistry.The self-assembly of hyperbranched polymers is becoming the focus of attention due to the obvious superiority they has than linear polymers.Molecular self-assembly refers to the process form an aggregation with a certain structure and function spontaneously by non-covalent weak interactions between molecules.The aggregates formed from self-assembly are possessed of features and functionality which the single molecule or low-level aggregates not have.To study the process of self-assembly will be helpful to understand and simulate some processes of life.The molecular self-assembly has been the frontiers of studying the weak interactions between molecules.With the rapid development of computers,multi-scale and cross-scale computer simulations become a very important field of theoretical chemistry.The systems and methods empolyed involves atomic and molecular structure by quantum mechanics(QM),enzyme-catalyzed reaction by the quantum/molecular mechanics(QM/MM)and protein folding by molecular dynamics.The time scales from the femtosecond to millisecond,and the spatial scales from nanometers to microns.However,the above methods in the time scale can not meet the needs when the huge increase in the research system,such as the self-assembly and phase transformation of biofilms and soft materials systems,etc.Therefore,the coarse-grained(CG)mesoscopic method was born,and has been developed rapidly and applied widely in recent years.A challenging problem in the CG approach is how to establish an accurate correspondence from the microscopic scale to the mesoscopic models,which is also the emphasis and difficulty for the current computer simulation.In this thesis,the micro-scale simulations method is proposed based on ABEEMa??/MM(atom-bond electronegativity equalization method fused into molecular mechanics)fluctuating charge force field,which was established and developed by Yang et al.In this model,the charge sites are defined explicitly,and the hydrogen bonding interaction are treated specially.Moreover,the charges of all regions can fluctuate with the surrounding environment,so the model can obtain accurate properties of many-body condensed phase materials system.In meso-scale simulation,we employed dissipative particle dynamics(DPD)as the simulation tool.Meanwhile,a viable simulation method was established to connect microscopic and mesoscopic system,and it was applied to simulate the self-assembly of hyperbranched polymers.In addition,we established ABEEM/MM force field for hydrogen peroxide by fitting the high-level ab initio and experimental data,and further applied it to simulate hydrogen peroxide and the relevant systems.The main contents as following:(1)Simulation study of the self-assembly of hyperbranched polymerWe adopted a systematic coarse-graining strategy that several segments are coarse-grained into a single bead.The Flory-Huggins ? parameters of HBPO,maleic anhydride(MA),succinic anhydride(SA),PEO and PPO are obtained by ABEEM??/MM fluctuating charge force field,and then the interaction parameters between DPD beads are estimated from it.Furthermore,the self-assembly process of hyperbranched polymers HBPO-star-MA/SA,HBPO-star-PEO,HBPO-linear-PEO and HBPO-star-PPO in aqueous solution are investigated by DPD method.So the meso-scale and micro-scale simulation was docking achieved.For HBPO-star-MA/SA system,our results are in agreement with those of experimental structure.HBPO-star-MA assembles to be a spherical micellar structure,while HBPO-star-SA assembles to be a hollow vesicle structure.Moreover,the simulation results show that the vesicle has double-membrane and the formation process is similar to the self-assembled process of the vesicle of the block copolymer reported in the literature,but the assembled structure is not same.From the energy point of view,the whole processes of the micelle and vesicle formation are all the energy minimization process.For HBPO-PEO system,it is found that along with the lowering degree of branching,star-shaped structure changes to the linear structure,and the final self-assembled structure changes from the vesicle to micelle.The HBPO-star-PEO assembles to vesicle,and HBPO-linear-PEO assembles to micelle.The above findings are in agreement with the experimental results.For the linear molecules,we tried to change the chain length and hydrophilic ratio.The results show that the chain length has some influence,but the hydrophilic ratio determined the final assembled structure basically.The main chain rigidity,i.e.the size of the bond angle potential,has little influence on the self-assembled structure in low concentrations(dilute solution),while has a great influence on the self-assembled structure in high concentrations.For HBPO-star-PPO system,we obtained spherical micellar structure at low concentration,and cylindrical micelles at high concentration which have been not reported in experiment.The micro-assembly mechanism was given explicitly.Our studies have shown that the structure of the spherical micelles from simulation is consistent with the proposed experimental structure.The assembly process of spherical micelle is the same as those of HBPO-star-MA and HBPO-linear-PEO.However,the cylindrical micelles are obtained at high concentrations and the assembled process is not the same as that at lower concentrations.Meanwhile,the assembled speed of the cylindrical micelle is much faster than the spherical micelle due to the highly initial concentration.(2)Molecular dynamics simulation of hydrogen peroxide systemWe construct a new ABEEM/MM fluctuating charge potential model for hydrogen peroxide(HP),and applied it to investigate the properties of(H2O2)n(n=1-6)clusters and liquid HP.The ABEEM/MM potential model gives reasonable properties of HP clusters,including geometries,interaction energies,and dipole moments,when comparing with MP2 results.Meanwhile,the average dipole moment,static dielectric constant,heats of vaporization,radial distribution function,and diffusion constant for the dynamic properties of liquid HP at 273 K and 1 atm are fairly consistent with the available experimental data.To the best of our knowledge,this is the first theoretical investigation of condensed HP.Then,the interactions between HP and water are studied by the combination of the ABEEM/MM HP potential with the ABEEM-7P water model.Results show that the optimized geometries,interaction energies and dipole moments of hydrated HP clusters HP(H2O)n(n=1-6)calculated by ABEEM/MM model are fairly well consistent with the MP2/aug-cc-pVTZ//MP2/aug-cc-pVDZ results.The ABEEM/MM results indicate that n=4 is the transition state structure from two-dimentional planar structure to three-dimentional network structure.The variations of the average hydrogen bond length with the increasing number of water molecules given by ABEEM/MM model are agree well with those of ab initio studies.Moreover,it can be confirmed that HP is a good proton donor and poor proton acceptor in aqueous solution by analysis of the radial distribution functions of HP aqueous solution and the charge distributions of HP-water dimer.The optimized geometries,interaction energies,dipole moments,and vibrational frequencies of GHP,CHP,AHP,THP,GC-(HP)n(n=1-2),and AT-(HP)n(n=1-2)complexes are investigated systematically by ABEEM??/MM fluctuating charge potential model and ab initio methods.The ABEEM??/MM model gives reasonable geometries and interaction energies,when compared with the present ab initio methods.For G-C-HP and A-T-HP complexes,the linear coefficient of the interaction energies all reaches 0.998,and the average absolute deviation are 0.95 and 1.42 kcal/mol,respectively,when compared with MP2/6-311++G(2d,2p)//B3LYP/6-311++G(d,p)results.Moreover,the variations of hydrogen bond length of GC and AT base pairs affected by HP molecules computed by ABEEM??/MM model are all obtained reasonable accordance with B3LYP results.All in all,it is a new attempt to apply ABEEM??/MM fluctuating charge force field to obtain the DPD interaction parameters.We not only gained the micro-mechanism about formation process of micelle and vesicle from the effective combination of MD with DPD,but also the reliability of ABEEMa7c/MM model has been validated.With the development of hardware and software of the computer,and the implementation of the parallel program,the ABEEM??/MM polarization force field will play an important role in the in-depth study of macromolecular and biological molecular aqueous solution,self-assembly and transport phenomena in aqueous solution.