Morphology Control Of Poly(styrene)-block-poly(L-lactide)(PS-b-PLLA)Copolymer Thin Films

Block copolymer is a kind of polymer that made up of two or more polymer blocks with different chemical structure and their blocks are connected by chemical bonds.Because of the block copolymer can form different nanostructures by micro-phase separation,block copolymer has wide application prospect in the fields of energy,photoelectric,biological and catalytic.Therefore,understanding the mechanisms of structure formation and regulating the structure and morphology in block copolymer films are significantly important to fabrication of new nanostructured functional materials.It is also an important research direction in polymer materials science and engineering.In this thesis,we choose poly(styrene)-b-poly(L-lactide)block copolymer(PS-b-PLLA)as a model material to investigate the solvent,concentration,and addition of homopolymers on the crystallization,micro-phase separation and dewetting of PS-b-PLLA thin films with atomic force microscope(AFM),two-dimensional grazing incident wide angle X-ray scattering(2D GIWAXS),X-ray photoelectron spectrometry(XPS).Our results showed that the crystallization and dewetting of PS-b-PLLA thin films are strongly influenced by solvent properties and mixed solvent composition ratio.When using selective solvent for PS block(such as toluene,chlorobenzene)or selective solvents for PLLA block(such as acetone),the morphology formation of PS-b-PLLA thin films were dominated by the PLLA crystallization process,and the formed crystals have fibrillar crystal morphology which can be adjusted by solvent composition ratio of toluene/THF mixed solvent.However,when using good solvent for both PS and PLLA(such as trichloromethane and tetrahydrofuran),the morphology formation of PS-b-PLLA thin films were dominated by dewetting and micro-phase separation process and can obtain dewetting induced mesoporous structures.The pore size and depth were regulated by the solvent composition ratio of toluene/THF mixed solvent.In addition,the crystalline and dewetting morphology of PS-b-PLLA can also be further regulated by the addition of the PS or PLLA homopolymers.The research works in thesis can provide a new insight into understand the morphology formation mechanisms of crystalline block copolymer thin films for fabrication nanostructured block copolymer thin films.