Thin Film Phase Behavior of Bottlebrush/Linear Polymer Blends

Abstract

Bottlebrush polymers contain polymeric side-chains attached to a linear polymer backbone, and they are currently of interest for a variety of potential applications that include drug delivery, tailored surface wettability, and self-assembled photonics. These polymers are challenging to synthesize in large quantities, so for practical applications, it is of interest to study their properties as additives for low-cost linear polymers. In this work, we examine the phase behavior of bottlebrush polystyrene (PS) and linear deuterated polystyrene (dPS) in thin films. These nearly athermal systems exhibit wetting and dewetting transitions that drive bottlebrush dispersion or aggregation, respectively, and these effects depend on the relative degrees of polymerization of matrix chains N m to those of bottlebrush side-chains N sc . When N m /N sc is low (≤1.6), the bottlebrushes are dispersed throughout the film thickness with a slight excess at the free surface and substrate interfaces. When N m /N sc is high (≥8), the bottlebrushes are depleted from the interior of the film and strongly segregated at the interfaces. The interfacial excess is driven by an entropic depletion attraction effect: larger branched molecules are adsorbed (attracted) to the interfaces, and the linear chains are displaced to the film's interior where they gain conformational entropy. The bottlebrushes prefer to accumulate at the silicon substrate over the air interface, and this may be driven by the more restrictive condition of a hard boundary or weak van der Waals interactions with the underlying silicon. These studies demonstrate that low concentrations of certain bottlebrush polymer architectures can generate brushlike surfaces and interfaces in any thermoplastic material through a spontaneous, entropy-driven segregation process. ■ INTRODUCTIONBottlebrush polymers contain polymeric side-chains attached to a linear polymer backbone, and controlled polymerization techniques enable the preparation of fully grafted bottlebrush polymers with tunable backbone length, side-chain length, and side-chain composition. 1−5 This synthetic tunability has motivated a number of recent studies into the potential applications of bottlebrush polymers, including drug delivery, 6−8 polymer photonics, 9−11 lubricants and rheology modifiers, 2,12−14 and surface coatings. 15,16 These applications take advantage of unique aspects of bottlebrush polymers, including an extended backbone, densely grafted and flexible side-chains, and a very high entanglement molecular weight. 14 However, bottlebrush polymers are specialty materials that are difficult to synthesize in large quantities. Therefore, it is of interest to study their properties as additives in blends with low-cost, linear polymers.A number of recent studies have focused on a related set of materials, blends of polymer coated nanoparticles with linear polymers, as detailed in recent reviews. 17,18 Polymer-coated nanoparticles have a layer of linear polymers end-grafted to the nanoparticle surface which control...

Structure, function, self-assembly, and applications of bottlebrush copolymers

“…Polymer brush films 142,143 , polymer-coated nanoparticles 144,145 , and branched polymers 146 are effective for reducing surface fouling, enhancing film properties, or as additives for compatibilizers. Bottlebrush polymers have been shown to exhibit similar properties and spontaneously segregate to interfaces 91 , but their potential for modifying films and interfaces, including liquid-liquid interfaces, is unclear. With proper design and processing, bottlebrush additives may be able to provide antifouling, self-healing, or stimuli-responsive interfacial properties similar to other brush-like polymers and polymeric nanoparticles.…”

“…Stein and Verduzco et al demonstrated that this could be used as an effective method to drive bottlebrushes to interfaces (see Fig. 5B) 91 , which might be desirable for applications such as antifouling. Xia et al studied the dynamics of the side-chains and backbone using electron paramagnetic resonance (EPR).…”

Surface segregation driven by molecular architecture asymmetry in polymer blends

“…In contrast, the Cahn-Hilliard model incorporating substantial empirical information on the pressure-volume-temperature behavior of the two components is more successful in rationalizing the observed dependences of surface energy on composition. Stein and collaborators [24] have very recently studied the interfacial segregation of bottlebrush copolymers having norborene backbones and polystyrene arms in blends with linear chains.The hypothesis regarding the roles of the chain ends and branch points mentioned above in connection with the linear response theory can be tested using neutron reflectometry …”

Thermal Approaches to Perpendicular Block Copolymer Microdomains in Thin Films: A Review and Appraisal

“…The effect of the size of organic NPs seemed secondary to the networking density, as seen in the effective substrate neutralization by intramolecularly cross‐linked single‐chain NPs. The segregation effects resulting from architectural asymmetry between an athermal (i.e., chemically identical) mixture of polymer matrix and additive has been confirmed in both theory and experiment …”