Chiral Regulation Of Self-Assembly Of Block Copolymers To Form Helical Structures

Single helical structures with chirality can be seen everywhere in daily life.Most natural organisms represented by DNA double helix structure exhibit unique single chiral characteristics after a long evolution process,while the homogeneous helix structure has been preserved through natural selection in the historical evolution,becoming a very stable structure,which contains profound chiral selection and regulation mechanism.As a typical soft material,the self-assembly of block copolymers to form nanoscale helical structures under cylindrical constraint can be used as an ideal model for chiral selection research,which can help people understand the law of chiral selection to a certain extent.However,as block copolymers are typical soft matter,their "soft" nature make the self-assembly process easy to be affected by the external environment,resulting in a large number of defects in the final phase structure,so it is difficult to obtain large-scale homogeneous single helix.The large-scale orderly homogeneous helix is obtained by applying regulation and control,which corresponds to the role and selection of external natural environment to some extent.Therefore,the study of phase transition mechanism in the regulatory process can be used to understand chiral selection to a certain extent.In addition,in the field of new material preparation,the self-assembly of block copolymers to obtain nanoscale target phase structure is also a promising preparation method.Therefore,this dissertation studies the chiral control of selfassembly of block copolymer to form helical structure,which has certain theoretical significance and potential practical application significance.The achiral cylinder-forming diblock copolymers are confined in nanopore.Even in the phase region where the single helix is stable,the single helix keeping order over a long range is not obtained.To solve this problem,two chiral pattern induction fields are proposed in this dissertation.The influencing factors are analyzed using time-dependent Ginzburg Landau theory and self-consistent field theory,and the method of directed self-assembly of the whole helix phase region is given.1.A suggestion is proposed to introduce the chiral helical patch pattern into the inner surface of the nanopore,and the effect of the patch pattern induction field on the single helix structure was studied when the diameter of the cylinder changed.When the patch pattern arrangement is sparse and the cylindrical diameter is small,D ≤6.90Rg,even if the weak patch field strength k=1.2,100%homogeneous single helix structure can be formed.When the pore size exceeds D=6.90Rg,the probability of defects increases rapidly,because the increase of pore size leads to the weak induction effect of patch pattern field on block copolymers,and the failure to effectively regulate the thermodynamic path of block copolymers,so that they enter some defect states with high free energy.With continued enlargement of the diameter,even in the phase region where the single helix is relatively stable,the long-range ordered single helix structure can hardly be obtained,indicating that the increase of the restricted diameter has a great influence on the induction effect of the patch pattern.In addition,by increasing the field intensity of the patch(k=1.3～1.8),the probability of single helix structure is still very low,indicating that increasing the field intensity of the patch induced pattern cannot offset the influence caused by the increase in the diameter of the cylinder,and also indicating that the induction scheme of sparse pattern induced patch has an effective action distance.By increasing the number of patches,the perfect structure can be obtained in the larger diameter D≤7.4Rg to a certain extent,and each patch is in the "blank space" of the helix structure,indicating that the patch pattern does not affect the thermodynamic path of self-assembly.However,with the increase of patches,when Np≥ 20,the proportion of perfect spiral structure decreases,indicating that certain relaxation and adjustment space is needed in the self-assembly process of helix structure.In general,patch pattern induction scheme is only suitable for small diameter.2.An improved helical belt induction scheme was proposed,that is,a chiral helical belt was added on the inner surface of the cylinder to make the preparation and regulation more flexible.By changing the number of spiral pitch,it is found that although the complete spiral belt is applied to the inner surface of the cylinder,it has a certain probability to form a single helix structure with equal handedness under different induced field strengths,but the overall proportion is very low.When the number of pitch of the spiral belt is n=1 and the induced field intensity increases to k=2.0,the proportion of single spiral structure reaches 100%.This is because the spiral belt does not cover the entire confined surface of the cylinder,and there is sufficient relaxation space in the process of self-assembly to form the spiral structure,which is conducive to the evolution of the system to the steady-state structure with low free energy.However,this scheme cannot maintain long range order.When V0=0.15 is adjusted on the cylinder surface,the induced field decreases as a whole,and the formation ratio of perfect single helix structure increases to more than 50%.At this time,the defects are mainly concentrated on the bottom surface of the upper and lower columns.Considering the extrusion of large components,the perfect single helix structure can be obtained at fA=0.21 when k=1.2～1.8,and the free energy of the system is the lowest when k=1.8,but the probability of obtaining this stable structure is only 80%.It shows that it is difficult to obtain steady-state structure in large-scale structures.Increasing the helical band bandwidth is equivalent to increasing the acting area of the induction field.Although the free energy of the system can be reduced,the formation probability of single helical structure does not improve.Too wide bandwidth will affect the position of B block during phase separation.The stable phase structure of B block can be adsorbed with the help of wide bandwidth when the diameter is large.3.Combined with the influencing factors,the induction scheme in the whole helix phase region is given.Under the restriction of a cylinder with a small diameter,in the phase region of 6.40 Rg ≤D<6.90Rg,a patch pattern induced field with helical distribution can be selected to realize the long-range single chiral single helix structure.In phase 6.90Rg≤D<7.8Rg,spiral belt field k=1.4,bandwidth h=3,can realize perfect induction effect.When the diameter is in the 7.8Rg ≤D<8.6Rg phase,the induction effect of the helical band with a bandwidth of h=3 becomes limited.By increasing the induction field intensity,k=1.5,a single helical structure can be obtained.When the diameter is increased to D=8.0Rg,the bandwidth needs to be increased to h=4 and the induction field k=1.6 to form a single helix structure.When the phase boundary D=8.4Rg,the bandwidth increases by h=5,and the induced field increases to k=2.5.The single chiral single helix structure can be realized by the induction of the helical band.However,the induced field changes the dynamical path of the system,so hat the internal helical structure is located on the surface of the helical band,which is a metastable state with relatively high free energy.