Theoretical Study On The Complex Structures Self-assembled By Block Copolymers Under Confinements

Block copolymer self-assembly plays a significant role in the preparation of functional materials with ordered nanostructure,which are increasingly favored by researchers because of their wide application prospect in various fields,such as drug delivery,photonic crystals,sensing devices and so on.However,the self-assembly process of most block copolymers is not completely carried out in the bulk,and is often affected by external boundary conditions.The most typical one is confined by the geometric container wall.Under the confinement,the conformation and interface energy of the block copolymers would be changed,which can form nanostructures different from those in the bulk.Therefore,an efficient way to enrich the self-assembled structures of block copolymer is to impose some geometric confinement on them.In this thesis,the geometrical confinements are imposed on block copolymer melts in order to obtain perforated lamellar and helical structures,and the formation mechanism of which is studied using self-consistent field theory.One of the target structures of this thesis is the perforated lamella(PL).According to the structural characteristics of this structure,we introduce parallel substrate(or film)confinement in the AB-type diblock copolymer system and explore the key parameters that favor the stable existence of PL by changing the topology of copolymer chains.Firstly,the AmB multi-arm star copolymer was designed to study the influence of the number of arms,m,acting on the stability of PL structure,and the phase diagram with respect to the volume fraction of A-block(fA)and film thickness(D)for m 1,2 and 4 respectively are constructed by comparing the free energy of these structures.It is found that the increase of m is beneficial to broaden the stability region of PL structure on fA,but the region increased insignificantly when m>2.Therefore,fixing m=2,we design the ABAT-type block copolymers by changing the parameter ? to tuneing the tethering positions of one A block on the B block,and study the effect of ? on the stability of the PL structure.By constructing a set of phase diagrams about fA and D for 0<??1,the optimal ? that is favorable to the stability of the PL structure is obtained.And by analyzing the different free energy contributions to the PL structure,the distribution of chain segments,and the period variation of structures at different ?,it is concluded that the stability of the PL structure is directly affected by its spatial distribution in the phase domain.Another target structure of this thesis is the helical structure,according to which we introduce a cylindrically confined model.In order to obtain complex helical structures with a larger number of helices,we first designed the "frustrated" ABC triblock copolymers,i.e.,polymer cylindrically confined systems with the interaction parameter of ?_ACN<<?_ABN??_BCN.We confirmed that the frustrated interaction also has a significant effect on the stability of the helical structure,which cannot be obtained when ?_ACN is close to ?_ABN(and ?BCA).By constructing the phase diagram with respect to the volume fraction(fC)and the confined size(D)of the C-block,the helical structure with the number of helixes from 1?5 is obtained,and the regulation that the number of helices increases with the decrease of fc is derived,which is opposite to the change trend in the bulk.By analyzing the free energy contributions and the cross-sectional profile of different helical structures in the bulk phase and under cylindrical confinement,it is found that the shape of the helical columns with different helical structures has an important effect on their stability.In this system,with the decrease of fc,the C component cannot continue to maintain the columnar structure,thus inhibiting the formation of helical structures with a higher number of helices.Therefore,we added C homopolymer on the basis of this system,that is,designing a cylindrically confined A1B1C1/C2 blending system,mainly exploring the effects of the chain length ?,content ?H of the added homopolymer as well as the confined size D on the stability of the helical structure by constructing D-?H and ?-?H phase diagrams,and a series of helical structures with different helix numbers were obtained,including the six-helix structure that has never been reported.The analysis revealed that the self-assembled shape of the homopolymer at the center of the cylinder directly affects the stability of the different helical structures formed by the B component.The result show that the phase behavior of the block copolymer under confined conditions is determined by a combination of the composition of the copolymer,the topology of the molecular chain and the confined environment.Therefore,we can obtain the desired target structures by regulating the relevant parameters in the system,which provides a theoretical reference for the experimental design of nanostructures.