The molecular weights and chain-rigidities of block copolymers (BCPs) can strongly influence their self-assembly behavior, particularly when the BCPs are under confinement. We investigate the self-assembly of bottlebrush block copolymers (BBCPs) confined in evaporative emulsions with varying molecular weights. A series of symmetric BBCPs, where polystyrene and polylactide side-chains are grafted onto the polynorbornene (PNB) backbone, are synthesized with different degrees of polymerization of the PNB (NPNB). Morphological transitions from onion-like concentric particles into striped ellipsoids occur as the NPNB of the BBCP increases, which is also predicted from coarse-grained simulations of BBCP droplets by an implicit solvent model. This transition is understood by the combined effects of i) an elevated entropic penalty associated with bending lamella domains of larger molecular weight BBCP particles and ii) the favorable parallel alignment of the backbone-chains at the free surface. Furthermore, the morphological evolutions of onion-like and ellipsoidal particles are compared. Unlike the onion-like BBCP particles, ellipsoidal BBCP particles are formed by the unique axial development of ring-like lamellae domains on the particle surface, followed by the radial propagation into the particle center. Finally, the shape-anisotropies of the ellipsoidal BBCP particles are analyzed as functions of NPNB and particle volume. These BBCP particles demonstrate promising potential for various applications that require unique rheological, optical, and responsive properties.

Three-dimensional confined assembly of bottlebrush block copolymers: the effect of molecular weight on the particle morphology

ACS Spring Meeting 2021

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