Study On The Morphology And Orientation Of Self-assembled Polystyrene-b-polylactide (PS-b-PLA) Thin Films

In block copolymer (BCP) thin films, the blocks can usually assembleinto certain nanostructures assuming different morphologies andorientations through microphase separation, which is controlled by themolecule parameters、the thickness of the film and other external fields. AsPLA can be easily removed through chemical or physical procedureswithout destroying the intrinsic structure of the matrix, PLA containingblock polymers are widely studied.In this dissertation, the Polystyrene-b-Polylactide (PS-b-PLA) thinfilms were prepared by spin-coating and annealed at different temperaturesunder vacuum condition. It was found that when the films were annealed at150°C, the PLA block assembled into the short rod-like cylinders withinthe PS matrix. After being annealed at170°C for3h, the elongatedcylinders aligned with their long axes parallel to the substrate could beobtained. However, the cylinders with their long axes perpendicular to thesubstrate surface could be acquired when the annealing time was30h at170°C. It was demonstrated that the morphologies of short rod-like andelongated cylinders could be reversed by switching the annealingtemperature between150°C and170°C. After the degradation of PLA with diluted sodium hydroxide in the mixture of water and methanol, the porousPS thin films were obtained further.Additionally, the PS-b-PLA thin films were also prepared bysolution-casting. Through controlling the evaporation rate of the solventthe PLA cylinders with different orientations within PS matrix weregenerated. When the chloroform solution of PS-b-PLA was used,perpendicular oriented PLA cylinders were observed due might to the fastevaporation rate (~110nL/s) of chloroform. When the evaporation rate wasdown to~25nL/s the as-formed PLA cylinders oriented into a mixedmanner, perpendicular and parallel to the substrate. When the evaporationrate was slow (~9nL/s), the cylinders were fully parallel to the substrate. Ifthe chlorobenzene solution of PS-b-PLA was used, the formedmorphologies and orientations under different evaporation rates werenearly the same.