Inter-domain Spacing Control <i>via</i> an Interdigitating Structure to Bilayers in Lamellae-Forming Star-Block Copolymers

Abstract

Herein, we aim to examine the topological effects of block copolymer (BCP) architecture on the self-assembly of lamellae-forming star-BCPs composed of polystyrene (PS) and poly(dimethylsiloxane) (PDMS) blocks with equivalent arm length and therefore almost identical volume fraction. An interesting wet-brush-like lamellar phase with an interdigitating structure for chain packing was found in the solution-cast (PS-b-PDMS)n (n = 1, 3, or 4) samples regardless of the value of n which corresponds to the number of PS-b-PDMS arms attached. While the temperature is gradually increasing, an order–order transition from the interdigitating structure to bilayers in the self-assembled lamellar phase can be observed in the bulk state, exhibiting approximately 50% increase on d-spacing. These results implicitly indicate that it is possible to acquire the smaller spacing of microphase-separated lamellae from casting. Also, as examined by in situ temperature-resolved small-angle X-ray scattering, transformation occurs once the temperature is over the glass transition of PS and the formation of stable lamellae with bilayers is able to be expedited by increasing the arm number because of the low degree of formation of wet brushes benefited by the topological effects. Moreover, an interesting transition was found in which the forming interdigitating chain packing can be restructured after cooling down from the stable lamellae, while the thermal treatment is not able to completely disentangle the polymer chains. Such an observation is an additional evidence for the suggested mechanism and corresponding kinetics for the formation of lamellar phases with such a large variation on d-spacing. This discovery provides an insight for the transformation mechanisms of the self-assembly of BCPs; it indicates the strong dependence of the self-assembling process on the topological effects from star-block architecture, making these materials valuable for the engineering of nanostructured BCPs with temperature-responsive d-spacing variation.