Microphase separation of carbohydrate-based star-block copolymers with sub-10 nm periodicity†
Block copolymers (BCPs) with a high Flory–Huggins interaction parameter (χ) and a low degree of polymerization (N) are being investigated to create microphase-separated structures with sub-10 nm periodicity (d). However, the lowered N value also leads to undesirable properties. In this paper, we demonstrate that the use of a star-block architecture is an effective way to achieve microphase separation with sub-10 nm d without reducing the molecular weight. Thus, three-, four-, and six-armed star-block copolymers consisting of poly(ε-caprolactone) (PCL) and maltotriose (MT), i.e., (PCL-b-MT)x (x = 3, 4, and 6, respectively), with comparable arm lengths and MT volume fractions were synthesized in three steps involving the ring-opening polymerization of ε-caprolactone, chain end modification, and click reaction. Small angle X-ray scattering experiments revealed that the (PCL-b-MT)x microphase separated into hexagonally close-packed cylindrical structures with a d of 6–8 nm. The d value and morphology of (PCL-b-MT)x are essentially the same as those of the corresponding arm unit, i.e., linear diblock copolymers (PCL-b-MTs). In other words, the d value can be fixed at less than 10 nm even though the total molecular weight of the BCP is increased by increasing the arm number. In addition, (PCL-b-MT)x had the advantages of an increased order–disorder transition temperature as well as better quality of the nanostructure formed in the thin film state compared to PCL-b-MT. Overall, well-ordered microphase-separated structures with a d value less than 10 nm were obtained, while the total molecular weight of the BCPs is higher than 10 000 g mol−1.