在气液界面上压缩诱导聚(L-乳酸)膜的结构变化（英文）

Structural Transition of Poly(L-lactic acid) Film Induced by Compression at Air/Water Interface

聚乳酸(PLA)是一种环境友好及生物可降解的聚合物,其界面性质受到了广泛关注.本文以LangmuirBlodgett(LB)膜天平、原子力显微镜(AFM)研究了聚(L-乳酸)(PLLA)在气液界面上的性质.表面压-面积(π-A)等温曲线的结果表明,在膜压缩的初始阶段,表面压逐渐增大;当膜压为9.0 m N·m-1时,曲线出现了一个平台,其重复单元的面积大约在0.11-0.17 nm2之间.原子力显微镜的结果发现,在压缩过程中,膜结构发生了明显的变化:平台刚出现时,膜内出现了大量的纤维结构;在平台区内,界面上出现了多层膜结构.特别地,当表面压为20.0 m N·m-1时,PLLA在界面上可形成约6.0 nm厚的薄膜.由此可见,PLLA等温线中的平台与其膜结构的变化紧密相关.这有别于普通双亲分子的性质,即这类双亲分子π-A等温线中的平台通常表示它们在二维空间上发生了单分子膜的相转变.

Poly（lactic acid） （PLA） has attracted considerable interest as an environmentally friendly and biodegradable polymer. The properties of poly（L-lactic acid） （PLLA） at an air/water interface were studied based on the Langmuir-Blodgett （LB） film balance and atomic force microscopy （AFM）. The surface pressure-area （E-A） isotherm indicated that the surface pressure of PLLA initially increased as the interfacial film was compressed; at Tr=9.0 mN· m^-1, a plateau was observed in the TT--A isotherm, in which the area of the repeat unit was in the approximate range 0.11-0.17 nm^2, The AFM results showed that there is a clear structural transition in the PLLA film during the compression： （i） at the beginning of the plateau, a number of fibrils are present at the air/water interface and （ii） multilayer structures （at least bilayer, i.e., the underlying layer and top layer consisting of fibrils） is formed in the plateau region. In particular, when Tr=20.0 mN·m^-1, a thin film of PLLA of thickness about 6.0 nm was fabricated. Our findings suggest that the plateau in the PLLA Tr-A isotherm is closely related to a change in the film structure from monolayer to multilayer at the air/water interface. This is significantly different from the behavior of conventional amphiphiles, because the plateau in amphiphiles TT--A isotherm is equivalent to a phase transition of monolayers derived from amphiphiles in a two-dimensional plane.