Computer Simulation Study On Polyelectrolyte Complexation

When oppositely charged polyelectrolytes are mixed in aqueous solution,due to electrostatic interaction,the system will separate into a polymer-rich condensed phase and a coexisting polymer dilute phase under suitable conditions.When the condensed phase is in a liquid state,this liquid-liquid separation phenomenon is known as polyelectrolyte complex coacervation（PECC）.Polyelectrolyte complex coacervation has important applications in many fields;for example,it is commonly used in food science,cosmetics,paper,textiles to paints,adhesives,and pharmaceutical technology.Many studies have shown that polyelectrolyte complex coacervation is affected by many factors,such as polycation/polyanion mixing ratio,ionic strength（salt concentration）,polymer concentration,p H and temperature.However,due to the numerous factors affecting polyelectrolyte complex coacervation and complex interactions,the current understanding of complex coacervation is still far from enough,and there is still a large space for exploration on the understanding of the behavior of polyelectrolyte complex condensed phase.Among them,the influence of charged sequence along the polyelectrolyte backbone on complex coacervation is still unclear.Therefore,in this thesis,we will use computer simulation method to systematically study the influence of charged sequence on the chain conformation and thermodynamic properties of polyelectrolyte complex coacervation.Specifically,we will investigate the physical properties of polyelectrolyte complex coacervation with different charged sequences both in the absence and in the presence of salt.The key to study polyelectrolyte complex coaservation is how to understand the complex behavior of two polyelectrolyte chains with opposite charges.Therefore,in this thesis,based on a coarse-grained polymer model,we will employ Monte Carlo simulation method to study the complex behavior of polyelectrolytes with opposite charges.In the simulation,the main physical parameters controlling the chain structure and thermodynamic properties of the system are as follows:temperature,two degree of polymerization of polyelectrolyte chain,charge fraction of the chain,order of different types of monomers on the chain and salt ion concentration.In order to clearly reveal the influence of different physical parameters on polyelectrolyte complexing behavior,we fixed other parameters by changing only one parameter and designed a variety of Monte Carlo trial move to obtain the equilibrium conformation of the system under different interaction parameters.Then the complexing behavior of polyelectrolyte chain with opposite charge was analyzed quantitatively by calculating the mean square radius of rotation and monomer density distribution of polymer chain.Specifically,this paper mainly studies the following two cases:（1）Firstly,the complexation behavior of polyelectrolyte chains with different charged sequences in the absence of salt was studied,and five different sequence systems were designed,each of which contained different sequences with certain changing rules.The first system considered is the complex behavior of（C_nQ_l）_m-type polycationic chain and fully charged polyanionic chain,where C and Q respectively represent polycationic and neutral monomers,n and l respectively represent the number of charged and neutral monomers in a block,m is the total number of blocks.In this system,we increase the number of C and Q,Under the condition of constant chain length,m decreases at the same time.The simulation results show that when the number of charged monomers in every polyelectrolyte chain increases,that is,when n and l increases,the two chains tend to be more closely complexed.The second system studies the influence of the position of a single charged monomer.There are eight charged monomers and eight neutral monomers,and the sequence is divided into two groups.The first group can understand the arrangement order of C₁QC₂Q in every cycle,where C₁and C₂are the number of charged monomers,and C₁+C₂=8,the number of neutral monomers Q is four,and each time a charged monomer is moved to a new sequence.The second group is to change the position of neutral monomers,which can be understood as the order of CQ₁CQ₂,Q₁and Q₂are the number of neutral monomers,the number of neutral monomers C is four.Each time we move a neutral monomer to a new sequence.The simulation results show that when we move the distribution position of a monomer（whether charged or uncharged）,the complexation behavior of the two chains is greatly affected.The closer the isolated charged monomer is to the charged chain segment,the more the two chains will be complexed due to the effect of electrostatic synergy.In addition to these two systems above,we also designed other sequence groups,including controlling that the middle part of the polycationic chain is 50 neutral monomers,and changing the number of charged monomers at both ends respectively.We can understand that the polycationic chain is of type C₁QC₂,where Q=50 remains unchanged,and C₁and C₂are both the number of charged monomers and C₁+C₂=50.We take C₁=0,C₁=10,C₁=15 and C₁=20 respectively.We find that the complexation behavior is stronger when the charged monomers are evenly distributed at both ends of the chain than when the charged monomers are distributed at one end of the chain alone.Finally,when the sequence of polyanion chain and polycation chain is the same,the increase of n in the block of（C_nQ_l）_mtype sequence has little effect on the complexation behavior.（2）Subsequently,the complexation behavior of polyelectrolyte chains with different charged sequences in the presence of salt was studied,we will study three sequence groups.First,when the polycationic chain and polyanion chain were fully charged,the simulation results showed that with the increase of salt concentration in the simulation system,the two chains gradually dispersed from the tight complexation state.The second group is the sequence group in which the charged monomers at both ends of the chain gradually increase in the same amount.We find that when the number of charged monomers is small,the increase of salt ions makes the two chains change from the state of dissociation to the state of uncomplexation;when the number of charged monomers is large,the two chains also show the trend of gradually separating.But bringing the two chains to a dissociation state requires more salt ions.The last group is（C_nQ_l）_m-type polycationic chain and fully charged polyanion chain.In this case,with the increase of salt ions,the complex behavior is first promoted,and further increase of salt ions will weaken the complex behavior.Through this study,we have obtained a deeper understanding of the charged sequence control of polyelectrolyte chain,and more comprehensive analysis of the factors affecting the complex behavior of polyelectrolyte chain,which will provide more suggestions for the subsequent application of polyelectrolyte composite coacervation based micellar preparation and many other applications.