(604c) Segregation of Bottlebrush Copolymer Additives to Thin Film Interfaces | AIChE

(604c) Segregation of Bottlebrush Copolymer Additives to Thin Film Interfaces

Authors 

Mei, H. - Presenter, Rice University
Law, T., The University of Tennessee Knoxville
Stein, G., The University of Tennessee Knoxville
Li, J., Rice University
Verduzco, R., Rice University
In polymer blends, a combination of enthalpic and entropic effects can lead to enrichment of the interfaces by one constituent. Bottlebrush polymers provide a convenient system for studying these effects because their composition, side-chain length, and number of side chains are synthetically tunable. Here, we study surface segregation in non-athermal blends of bottlebrush copolymer and linear homopolymer. The bottlebrush copolymer is synthesized with a mixture of polystyrene (PS) and poly(methyl methacrylate) (PMMA) side-chains grafted to a polynorbornene backbone, and the linear homopolymer is either PS or PMMA. Time-of-flight secondary ion mass spectroscopy (ToF-SIMS) is used to quantify the spatial distribution of bottlebrush copolymers throughout a thin film and the surface excesses at both interfaces as a function of the homopolymer type and molecular weight, as well as annealing time. Above a critical molecular weight of the linear homopolymer, the bottlebrush copolymer segregates at both the air surface and silicon substrate, which reflects an entropic preference for chain ends near the interfaces. Indicated by interface excess data, the extent of segregation at each boundary is much larger than observed in prior studies athermal blends, especially for the air surface, and the bulk film is nearly depleted of bottlebrush copolymers after annealing. Segregation to the air interface is strongest for as-cast films, indicating that processing effects enhance segregation to the air surface. A slight difference of the bottlebrush copolymer enrichment at substrate for linear PS and PMMA matrix is observed, which is attributed to the enthalpic interaction with the substrate. The films with linear PS remain flat through prolonged annealing above the glass transition temperature, and the bottlebrush-enriched boundaries do not show signs of phase separation. However, when the matrix is comprised of PMMA, the surface layer might show microscale phase-separation. This study demonstrates that bottlebrush copolymer additives can provide a route to decouple the surface functions of a film from the bulk chemistry, even when the additive is enthalpically unfavorable.

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