Confinement and Processing Can Alter the Morphology and Periodicity of Bottlebrush Block Copolymers in Thin Films

Abstract

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 (L 0 ) of the morphologies is found to be thickness dependent, increasing L 0 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 L 0 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 L 0 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 L 0 and morphologies, a property that could be taken advantage of for patterning diverse structures with a single material.

Rapid, interface-driven domain orientation in bottlebrush diblock copolymer films during thermal annealing

“…4c). The narrow standard deviation is expected, as these film thicknesses are large enough that confinement effects 56 are not expected to lead to meaningful chain compression or stretching. The observed azimuthal variation in scattering intensity (Fig.…”

Resonant soft X‐ray scattering in polymer science

“…A recent study has used this capability to investigate how the length of periodicity in bottlebrush BCP thin films increases via chain extension due to confinement in everthinner films. 128 Another unique capability of RSoXS was highlighted in another early study where chemical organization within an ABC triblock copolymer nanostructure was solved without the need for staining. 55 Instead, the chemical organization was determined by setting the X-ray photon energy to values where the refractive index for pairs of the blocks was matched.…”

“…Scattering signal fidelity and structural analyses compared well between these thin films and the >1 mm thick films measured in standard hard X‐ray scattering. A recent study has used this capability to investigate how the length of periodicity in bottlebrush BCP thin films increases via chain extension due to confinement in ever‐thinner films 128 . Another unique capability of RSoXS was highlighted in another early study where chemical organization within an ABC triblock copolymer nanostructure was solved without the need for staining 55 .…”

Solvent Vapor Annealing, Defect Analysis, and Optimization of Self-Assembly of Block Copolymers Using Machine Learning Approaches

“…To achieve the desired nanopattern via self-assembly, not only are the annealing parameters critical but the initial thickness of the applied BCP monolayer must be precise, typically within one or two nanometers. ,− The ideal thickness for a given BCP is unique and depends upon the composition of the BCP and factors such as surface functionalization and energy, the use of a topcoat, and other factors. ,,,,, Both experimental and computational results strongly link initial film thickness with the resulting self-assembled structure and persistent defects. ,, Small fluctuations of thickness within a monolayer film can lead to different self-assembled structures as unfavorably thin domain thicknesses frustrate packing due to the buildup of strain. , The periodicity of structures formed from self-assembled BCP nanopatterns may be dependent upon film thickness and other processing parameters, as has been recently shown with bottlebrush BCPs. , …”