Lamella reorientation in thin films of a symmetric poly(l-lactic acid)-block-polystyrene upon crystallization at different temperatures |
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Authors: | Jun Fu Longjian Xue Caiyuan Pan Yanchun Han |
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Affiliation: | a State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, PR China b Department of Polymer Science and Engineering, University of Science and Technology of China, Hefei 230026, PR China |
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Abstract: | The thin films of a symmetric crystalline-coil diblock copolymer of poly(l-lactic acid) and polystyrene (PLLA-b-PS) formed lamellae parallel to the substrate surface in melt. When annealed at temperatures well above the glass transition temperature of PLLA block (TgPLLA), the PLLA chains started to crystallize, leading to reorientation of lamellae. Such reorientation behavior exhibited dependence on the correlation between the crystallization temperature (Tc), the glass transition temperature of PS (TgPS), the peak melting point of PLLA crystals (TmPLLA), and the end melting point of PLLA crystals (Tm,endPLLA). When annealed at (Tc=) 80 °C (Tc < TgPS < TODT, order-disorder transition temperature), 123 °C (TgPS < Tc < TmPLLA < TODT), 165 °C (TgPS < TmPLLA < Tc < Tm,endPLLA < TODT), the parallel lamellae became perpendicular to the substrate surface, exclusively starting at the edge of surface relief patterns. Meanwhile, the corresponding lamellar spacing was significantly enhanced. The PLLA crystallization between PS layers was hypothesized to account for the lamella reorientation during annealing. The crystallization, chain conformation, and possible chain folding mechanisms were discussed, based on detailed analysis of the lamellar structure before and after crystallization. |
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Keywords: | Block copolymer Crystallization Thin films |
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