对称柔性-半刚性-柔性三嵌段共聚物自组装行为

摘要

基于蠕虫状链模型和高斯链模型的自洽场理论研究了对称柔性-半刚性-柔性三嵌段共聚物在二维空间中的相行为。在设定的计算参数下得到了多种该体系的热力学平衡态相结构，包括各向同性Isotropic相、各向异性Nematic相、层状Smectic-C相、四角柱状相、斜交型冰球状相、断层型冰球状相和稻粒状结构。与刚-柔两嵌段共聚物相比，对称柔性-半刚性-柔性三嵌段共聚物更容易形成Smectic-C相。在柱状相结构中，在柔性嵌段的牵伸作用下半刚性嵌段在液晶相区中相互穿插，从而形成菱形和长方形液晶相区。当取向相互作用较强时，半刚性嵌段沿长方形液晶相区的短边方向出现2次折叠。为进一步探索高分子折叠现象产生的原因以及液晶成核机理奠定了理论基础。%The self-assembly of symmetric coil-semiflexible-coil triblock copolymers in 2 D space was investi-gated using self-consistent field theory(SCFT). The semiflexible block and the coil block of the copolymers were described as wormlike chain model, and Gaussian chain model, respectively. A highly accurate and effi-cient operator splitting pseudospectral algorithm combined with SPHEREPACK software package was employed to solve modified diffusion equations for the propagators of wormlike chains. The anisotropic interactions favoring the parallel alignment between semiflexible segments was described by Maier-Saupe orientational inter-actions μN , and the enthalpic interactions between coil and semiflexible segments were described by isotro-pic Flory-Huggins interactions χN . By increasing χN and volume fraction of the coil blocks, a phase dia-gram including isotropic, nematic, smectic-C, tetragonal cylinders, oblique pucks, broken lamella pucks and rice-shaped phase for symmetric coil-semiflexible-coil triblock copolymers was built. At relatively low coil frac-tions and strong interactions, semctic-C phases were readily formed because the strong chain-stretching ener-gy, which came from the highly ordered orientation of semiflexible blocks, dominates the system free energy. In the meanwhile, the stretch from two coil sides to the bridged semiflexble block results in a higher chain-stretching energy. Therefore, more smectic-C phases formed than that in biblock copolymers. The tetragonal cylinders exhibiting weak segregation were obtained in the region of coil fractions from 0.5 to 0.6 . With the increase of coil fractions and orientational interactions, hexagonal packed pucks were found. In the rhombus and rectangle liquid crystal domain, the coil-semiflexible block junctions were observed at opposite sides, therefore a cross-link conformation of the semiflexible blocks was produced. More interestingly, the semiflexi-ble chain folds twice along the short side of the rectangle liquid crystal domain of the oblique pucks at coil fraction 0.6 and broken lamella pucks at coil fraction 0.7 . Chain folding behavior increases the interfaces be-tween coils and semiflexible blocks allowing for the coils with larger fractions to gain conformational entropy. When the coil fraction is 0.8 , chain folding behavior decreases and the semiflexible blocks orient along the long side of the rectangle liquid crystal domain, forming rice-shaped phase. These simulation results are of paramount importance in rationalizing chain folding phenomena and understanding the nucleation mechanism of liquid crystals.