Abstract
The thin-film samples of polystyrene-b-poly(L-lactide) diblock copolymer (PS-PLLA) with fPLLAv=0.35 were prepared by solution casting in dichloromethane (CH2Cl2; 0.05wt%). As evidenced by scanning probe microscopy (SPM) and transmission electron microscopy (TEM), a rod-like structure can be obtained from quenched PS-PLLA melt at 175°C. On the basis of staining effect and energetic consideration, the structures quenched from melt were further identified the formation of core-shell cylinder structure. In contrast to the formation of nanohelical phase from the self-assembly of the PS-PLLA, the occurrence of the tertiary hierarchical superstructure, namely core-shell cylinder texture, is obviously different to the quaternary helical structure phase. Notably, the PLLA component is intrinsically a crystallizable polymer as a result of the regularity of the chiral configuration. The crystallization event is thus carried out to examine the effect of crystallization on hierarchical structures. Surprisingly, a helix-like texture for the PS-PLLA thin-film samples was observed by SPM and TEM after crystallization at which periodic height profile can be clearly identified by SPM for surface analysis and TEM for shadowed images; suggesting that the formation of helical superstructure. The periodic contrast can also be clearly identified in the inner-core microdomains of the core-shell texture by TEM for stained images and is comparable to the pitch length of helical texture. Similar results can also be obtained for samples crystallized at different temperatures; further confirming that the formation of helical superstructure is driven by crystallization. In addition, the core-shell cylinder structures were also formed at higher annealing temperatures. The results indicate the differences between crystallization and annealing effect on hierarchical self-assembly. Moreover, the crystalline helical superstructure gradually transforms into core-shell cylinder structure during melting. The morphological transformation from core-shell to helical superstructure is thus identified as a reversible mechanism. The crystallization process was also performed for PLLA homopolymers, and lamellar single crystals were observed by SPM and TEM; suggesting that the mutual repulsion between PS and PLLA blocks play an important role for the formation of helix-like curvature. To summarize the experimental results and similar to the concepts of chiral self-assembly, we suggest that the formation of helical textures is driven by an intrinsic bending force in addition to twisting due to molecular chirality. Moreover, in comparison with the results of amorphous texture, the helical assembly is attributed to the crystallization effect from the PLLA blocks. As a result, we suggest that the crystallization-enhanced chiral strength and microphase separation due to mutual repulsion are the major origins which provide the twisting force and bending moment to cause the spontaneous torsion of the edges of helical ribbon so as to form the helical shape. A probable mechanism with respect to molecular dispositions is thus proposed for the formation of hierarchical helical superstructure.