Abstract
The crystallization of helical nanostructure resulting from the self-assembly of a chiral diblock copolymer, poly(styrene)-b-poly(L-lactide) (PS-PLLA), is studied. Various crystalline PS-PLLA nanostructures are obtained by controlling the crystallization temperature of PLLA (T <sub>c,PLLA</sub> ), at which crystalline helices and crystalline cylinders occur while T <sub>c,PLLA</sub> < T <sub>g,PS</sub> (the glass transition temperature of PS) and T <sub>c,PLLA</sub> ≥ T <sub>g,PS</sub> , respectively. As evidenced by selected-area electron diffraction and two-dimensional X-ray diffraction results, the PLLA crystallites under confinement reveal a unique anisotropic character regardless of the crystallization temperature. On the basis ofobserved uniaxial scattering results the PLLA crystallites grown within the microdomains are identified as crystals with preferential growth directions either along the [100] or along the [110]-axes of the PLLA crystalline unit cell, at which the molecular chains and the growth direction are normal and parallel to the central axes of helices, respectively. The formation of this exclusive crystalline growth is attributed to the spatial confinement effect for crystallization. While T <sub>c,PLLA</sub> < T <sub>g,PS</sub> , owing to the directed crystallization by helical confinement, the preferential crystalline growth leads to the crystallization following a helical track with growth direction parallel to the central axes of helices through a twisting mechanism. Consequently, winding crystals with specific crystallographic orientation within the helical microdomains can be found. By contrast, while T <sub>c,PLLA</sub> ≥ T <sub>g,PS</sub> , the preferential growth may modulate the curvature of microdomains by shifting the molecular chains to access the fest path for crystalline growth due to the increase in chain mobility. As a result, a spring-like behavior ofthe helical nanostructurecan bedriven by crystallization so as to dictate the transformation of helices, resulting in crystalline cylinders that might be applicable to the design of switchable large-strain actuators. © 2009 WILEY-VCH Verlag GmbH & Co. KGaA.