We present a combined experimental and computational study of surface segregation in thin films of nearly athermal blends of linear and bottlebrush polymers. The lengths of bottlebrush backbone (N b), bottlebrush side chain (N sc), and linear polystyrene host (N m) are systematically varied to examine the effects of polymer architecture on phase behavior. From the experiments, combinations of architectural parameters are identified that produce enrichment and depletion of bottlebrush at the polymer–air interface. These surface segregation behaviors are consistent with entropy-dominated thermodynamics. In addition, the experiments reveal conditions where bottlebrush and linear polymers are equally preferred at the surface. Simulations based on the self-consistent field theory (SCFT) qualitatively capture the three types of surface segregation behaviors and highlight the delicate interplay of entropic and enthalpic effects. Our investigations demonstrate that controlling both entropic and enthalpic driving forces is critical for the design of surface-active bottlebrush polymer additives.

The composition of polymer blends near interfaces can differ from the average blend composition because the attraction of each polymer toward surfaces is controlled by its chemistry, size, and architecture. In this work, we studied thin film blends of bottlebrush copolymers and linear homopolymers to understand the enthalpic and entropic effects that drive preferential segregation of one constituent to film interfaces. Bottlebrush copolymers containing polystyrene (PS) and poly­(methyl methacrylate) (PMMA) side chains were blended with either linear PS or linear PMMA, and time-of-flight secondary ion mass spectroscopy was used to quantify the distribution of bottlebrushes through the film thickness as a function of homopolymer type, homopolymer molecular weight, and processing conditions. We found that the bottlebrush copolymers segregated to air and substrate interfaces above a critical molecular weight of the linear homopolymer, consistent with an entropic preference for chain ends and shorter chains toward the interfaces. This segregation was used to tailor the surface wettability of blend films using bottlebrush additives as a minority component. Modeling using self-consistent field theory highlighted effects of conformational entropy and enthalpic interactions in driving almost complete segregation from the interior of the films toward interfaces. Furthermore, enthalpic interactions were predicted to cause lateral phase segregation in cases where the homopolymer is preferred over the bottlebrush copolymer at the substrate, an effect that was also observed in experiments. This study demonstrates that bottlebrush copolymer additives can be designed to spontaneously segregate to surfaces in thermal blends, providing a possible route to decouple surface properties from bulk properties.

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With a view to understanding how a dilute sprinkling of strong hydrogen bonds affect the bulk properties and the colligative properties of polymers, we examine three polymer adsorption problems. They are (1) a copolymer between two adsorbing plates, (2) a comb polymer between two adsorbing surfaces, and (3) a star polymer between two absorbing surfaces. Matrix methods are used to solve each of these problems. Numerical results for a block copolymer with a periodic repeat of a single strong bond and r weak bonds show that the adsorption is not well represented by a model which replaces the segments by those of an average energy. This means that the specifics of molecular architecture cannot be ignored as has been done in so much of the previously published work on adsorption. Because the matrices do not commute, block polymers and random copolymers of the same composition are expected to show different adsorption profiles. IntroductionWe are interested generally in how a relatively few but very strong bonds affect the properties of polymer systems.l One example is a two-component polymer blend that has a few hydrogen bonds per molecule between the unlike polymers. The hydrogen bonds have the effect of compatibilizing the polymers.2 Another example is a bulk copolymer which contains a few strong bonds per molecule that act as cross-links between the chains, thus creating a rubbery material rather than just a viscous material.3 Each of these systems is important industrially.In order to gain further insight into the sparse but strong bonds problem (SBSB) in bulk systems, we thought it best to proceed methodically by treating the two-molecule problem, then the three-molecule problem, and so on until the n-molecule problem is solved. However, even the two-molecule problem is difficult as one sees readily since the two-molecule problem is closely related to the double-strained DNA problem (imperfect matching model). The imperfect matching DNA model has been solved when the bonds are all the same ~t r e n g t h ,~ and the perfect matching model has been solved for copolymer^,^ but for the case of a sparse sprinkling of strong bonds (the copolymer problem) the imperfect matching model is not solved. So we thought it best to gain insight by looking for any exactly solvable problem that contains SBSB. It is to be stressed that we are looking for exact solutions; mean-field treatments will miss the characteristic features of the SBSB problem.The adsorption of a polymer lying between two parallel plates is an exactly solvable lattice model problem6 (self-excluded volume is not treated). A trivial generalization of this model allows us to treat the general linear copolymer problem exactly, and by choosing a copolymer with only a few A monomers that are strongly adsorbed and many B monomers that are weakly adsorbed, we immediately have an entry into an exactly solvable SBSB problem.We have also extended the lattice model of a chain between two parallel plates to architectures other than pure linear since for these materials al...

Bottlebrush polymers can be used to introduce novel surface properties including hydrophilicity, stimuli-responsiveness, and reduced friction forces. However, simple, general, and efficient approaches to cross-linking bottlebrush polymer films and coatings are limited. Here, we report that bottlebrush polymers with an unsaturated polynorbornene backbone and thiolterminated side chains can be cross-linked on demand by UV irradiation to produce uniform and insoluble bottlebrush polymer coatings. To quantify the kinetics and efficiency of cross-linking by UV exposure (254 nm), we measured the normalized residual thickness (NRT) of bottlebrush and linear polymer films after UV exposure and solvent washing. For bottlebrush polymers with thiol-terminated polystyrene (PS) side chains, the NRT exceeded 60% for a UV dose of 1.0 J/cm 2 , while unfunctionalized linear PS required a dose of 7.9 J/cm 2 to achieve similar NRT values. Rapid UVinduced cross-linking of the bottlebrush PS was attributed to the thiol−ene coupling of the thiol-terminated side chains with the unsaturated polynorbornene backbones, as demonstrated through FTIR measurements and control studies involving bottlebrush polymers with saturated backbones. To establish the broader applicability of this approach, UV-induced cross-linking was demonstrated for thin films of bottlebrush polymers with thiol-terminated poly(methyl acrylate) (BB-PMMA-SH) side chains and those with poly(ethylene glycol) (BB-PEG) and poly(lactic acid) (BB-PLA) side chains which do not contain thiol end groups. UVinduced cross-linking of BB-PEG and BB-PLA films required the use of a multifunctional thiol additive. Finally, we demonstrated that bottlebrush polymer multilayers can be fabricated through sequential deposition and UV-induced cross-linking of different bottlebrush polymer chemistries. The cross-linking process outlined in this work is simple, general, and efficient and produces solvent-resistant coatings that preserve the unique properties and functions of bottlebrush polymers.