Confinement and Processing Can Alter the Morphology and Periodicity of Bottlebrush Block Copolymers in Thin Films
Daniel Sunday, Moshe Dolejsi, Alice B. Chang, Lee J. Richter, Ruipeng Li, Paul Nealey, Robert H. Grubbs
Bottlebrush block copolymers (BBCPs) are intriguing architectural variations on linear BCPs with highly tunable structure. Conﬁnement can have a signiﬁcant impact on polymer assembly, giving rise to changes in morphology, assembly kinetics, and properties like the glass transition. Given that conﬁnement leads to signiﬁcant changes in the persistence length of bottlebrush homopolymers, it is reasonable to expect that BBCPs will see signiﬁcant changes in their structure and periodicity relative to the bulk morphology. Understanding how conﬁnement inﬂuences assembly will be important for designing BBCPs for thin ﬁlm applications including membranes, integrated photonic structures, and potentially BCP lithography. In order to study the eﬀects of conﬁnement on BBCP conformation and morphology, a blade coating was used to prepare ﬁlms with continuous variation in ﬁlm thickness. Unlike thin ﬁlms of linear BCPs, islands/holes were not observed, and instead mixtures of parallel and perpendicular morphologies emerge after annealing. The lamellar periodicity (L0) of the morphologies is found to be thickness dependent, increasing L0 with decreasing ﬁlm thickness for blade coated ﬁlms. Films coated out of tetrahydrofuran (THF) resulted in a single well-deﬁned lamellar periodicity, veriﬁed through atomic force microscopy (AFM) and grazing incidence small-angle X-ray scattering (GISAXS), which increases dramatically from the bulk value (30.6 nm) and continues to increase as the ﬁlm thickness decreases. The largest observed L0 was 65.5 nm, and this closely approaches the estimated upper limit of 67 nm corresponding to a fully extended backbone in a bilayer arrangement. Films coated out of propylene glycol methyl ether acetate (PGMEA) resulted in a mixture of perpendicular lamellae and a smaller, likely cylindrical morphology. The lamellar portion of the ﬁlm shows the same thickness dependence as the lamellae observed in the THF coated ﬁlms. The scaling of the lamellar L0 with respect to ﬁlm thickness follows predictions for conﬁned semiﬂexible polymers with weak excluded volume interactions and can be related to models for conﬁnement of DNA. Spin coated ﬁlms shows the same reduction in periodicity, although at very diﬀerent ﬁlm thicknesses. This result suggests that the material has shallow free-energy barriers to transitioning between diﬀerent L0 and morphologies, a property that could be taken advantage of for patterning diverse structures with a single material.
, Dolejsi, M.
, Chang, A.
, Richter, L.
, Li, R.
, Nealey, P.
and Grubbs, R.
Confinement and Processing Can Alter the Morphology and Periodicity of Bottlebrush Block Copolymers in Thin Films, ACS Nano, [online], https://doi.org/10.1021/acsnano.0c07777
(Accessed September 22, 2023)