Conformation Transition Of A Single Homopolymer Chain In Binary Mixed Solvents By Monte Carlo Simulation

Binary mixed solvents are widely used in engineering applications,such as the design of stimulus-responsive polymers,the optimization of hydrogel properties,and polymer electrospinnig.For example,in the design of hydrogels,silk fibroin hydrogels with high mechanical properties were prepared by using water and hexafluoroisopropanol to induce conformational transformation.Underlying these important applications,the conformational change of polymer chain is a key factor to determine the properties of the system.A large number of studies have shown that polymer chains exhibit a variety of conformational changes in binary mixed solvents,including cosolvency,cononsolvency,and the conformational transition near the critical point of the mixed solvents.At present,cosolvency and cononsolvency have got a more comprehensive understanding.However,systematic inverstigation on the conformational changes of polymers when approaching the critical point of the binary-solvent mixture and the underlying mechanism is still lack.Therefore,in this thesis,we develop a novel Monte Carlo simulation algorithm to investigate the conformational transition of a single homopolymer chain close to the critical point of a mixture of binary solvents.In view of the complexity of this system,we adopt a simple lattice model with a minimal parameter set,which makes it possible to systematically study the conformational change in the whole parameter space.We consider a homopolymer chain of N segments,solvent A and solvent B in a simple cubic lattice of volume L³.The self-avoiding walk is used to model the homopolymer single chain.We further introduce a non-bonded interaction for two different components located at the nearest neighbor sites.Thus,we have three interaction parameters:the interaction between solvent A and segment,?_{p A},the interaction between solvent B and segment,?_{p B},and the interaction between solvent A and solvent B,?_AB.In this thesis,we develop a novel Monte Carlo algorithm,which allows us to study the conformational variation of a single homopolymer single chain in the?_ABparameter space.Based on this model,we study the following three cases:(1)First,we study the ideal case in which the interaction between solvent A and polymer segment is the same as that between solvent B and polymer segment.In other words,there is no difference between solvent A and solvent B with regards to the single homopolymer chain.We calculate the dependence of the mean-square radius of gyration on the nearest-neighbor interaction between solvent A and solvent B.Our simulation results show that,with increasing the nearest-neighbor interaction between solvent A and solvent B,the single homopolymer chain firstly expands,and then slightly shrinks when approaching the critical point of binary solvents.(2)Next,we consider the case where one solvent has a preference to the polymer segments.In this case,we assume that the interaction between solvent B and the segment is stronger than that between solvent A and the segment.Our simulation results show that the conformational changes of a single homopolymer chain is closely related to the strength of mutual attraction between solvent B and segment.When the mutual attraction between solvent B and polymer segment is weak,the homopolymer chain expands firstly,and the expansion is more obvious near the critical point.With increasing the mutual attraction between solvent B and chain segment,the expansion trend of a single chain weakens,and the single chain shrinks and re-expands slightly after slight expansion.When the mutual attraction between solvent B and chain segment is strong,the single homopolymer chain firstly collapses and then expands.(3)Third,we examine the cases where both solvents are poor for the polymer segment,but the repulsion strength of the latter is greater than that of the former.Our simulation results show that the single chain expands in a binary solvent with stronger repulsion,and the expansion is more obvious when it is very close to the critical point of phase separation.To quantitatively explain the conformation changes of a single homopolymer chain in a mixture of binary solvents,in this thesis,we also develop a theory to examine the three cases specified above,and find that the theoretical calculation results are qualitatively consistent with the Monte Carlo simulation results.In summary,in this thesis we systematically examine the conformational changes of a single homopolymer chain near the critical point of a binary mixed solvents,and construct the physical picture of the conformational changes of the single chain near the critical point of a binary mixed solvents.Our simulation results expound novel mechanism on the conformational transition phenomenon of polymers near the critical point of the binary mixed solvent in the experiment,and provide some instructive suggestions for rational design of stimulation-responsive polymer materials and other fields.