publications

2023

1. Surface enrichment dictates block copolymer orientation

   Suwon Bae, Marcus M. Noack, and Kevin G. Yager

   Nanoscale, Feb 2023

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   Orientation of block copolymer (BCP) morphology in thin films is critical to applications as nanostructured coatings. Despite being well-studied, the ability to control BCP orientation across all possible block constituents remains challenging. Here, we deploy coarse-grained molecular dynamics simulations to study diblock copolymer ordering in thin films, focusing on chain makeup, substrate surface energy, and surface tension disparity between the two constituent blocks. We explore the multi-dimensional parameter space of ordering using a machine-learning approach, where an autonomous loop using a Gaussian process (GP) control algorithm iteratively selects high-value simulations to compute. The GP kernel was engineered to capture known symmetries. The trained GP model serves as both a complete map of system response, and a robust means of extracting material knowledge. We demonstrate that the vertical orientation of BCP phases depends on several counter-balancing energetic contributions, including entropic and enthalpic material enrichment at interfaces, distortion of morphological objects through the film depth, and of course interfacial energies. BCP lamellae are found more resistant to these effects, and thus more robustly form vertical orientations across a broad range of conditions; while BCP cylinders are found to be highly sensitive to surface tension disparity.

   @article{bae_surface_2023,
     author = {Bae, Suwon and Noack, Marcus M. and Yager, Kevin G.},
     doi = {10.1039/D3NR00095H},
     issn = {2040-3364, 2040-3372},
     journal = {Nanoscale},
     language = {en},
     month = feb,
     number = {15},
     pages = {6901--6912},
     title = {Surface enrichment dictates block copolymer orientation},
     volume = {15},
     year = {2023},
   }

2. Autonomous discovery of emergent morphologies in directed self-assembly of block copolymer blends

   Gregory S. Doerk, Aaron Stein, Suwon Bae, and 3 more authors

   Science Advances, Jan 2023

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   The directed self-assembly (DSA) of block copolymers (BCPs) is a powerful approach to fabricate complex nanostructure arrays, but finding morphologies that emerge with changes in polymer architecture, composition, or assembly constraints remains daunting because of the increased dimensionality of the DSA design space. Here, we demonstrate machine-guided discovery of emergent morphologies from a cylinder/lamellae BCP blend directed by a chemical grating template, conducted without direct human intervention on a synchrotron x-ray scattering beamline. This approach maps the morphology-template phase space in a fraction of the time required by manual characterization and highlights regions deserving more detailed investigation. These studies reveal localized, template-directed partitioning of coexisting lamella- and cylinder-like subdomains at the template period length scale, manifesting as previously unknown morphologies such as aligned alternating subdomains, bilayers, or a “ladder” morphology. This work underscores the pivotal role that autonomous characterization can play in advancing the paradigm of DSA.

   @article{doerk_autonomous_2023,
     author = {Doerk, Gregory S. and Stein, Aaron and Bae, Suwon and Noack, Marcus M. and Fukuto, Masafumi and Yager, Kevin G.},
     doi = {10.1126/sciadv.add3687},
     issn = {2375-2548},
     journal = {Science Advances},
     language = {en},
     month = jan,
     number = {2},
     pages = {eadd3687},
     title = {Autonomous discovery of emergent morphologies in directed self-assembly of block copolymer blends},
     volume = {9},
     year = {2023},
   }

2022

1. Chain Redistribution Stabilizes Coexistence Phases in Block Copolymer Blends

   Suwon Bae, and Kevin G. Yager

   ACS Nano, Sep 2022

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   The nanoscale morphologies of block copolymer (BCP) thin films are determined by chain architecture. Experimental studies of thin film blends of different BCP chain types have demonstrated that blending can stabilize new motifs, such as coexistence phases. Here, we deploy coarsegrained molecular dynamics (MD) simulations in order to better understand the self-assembly behavior of BCP blend thin films. We consider blends of lamella- and cylinder-forming BCP chains, studying their morphological makeup, the chain distribution within the morphology, and the underlying polymer chain conformations. Our simulations show that there are local concentration deviations at the scale of the morphological objects that dictate the local structure, and that BCP chains redistribute within the morphology so as to stabilize the structure. Underlying these effects are measurable distortions in the BCP chain conformations. The conformational freedom afforded by BCP blending stabilizes defects and allows coexistence phases to appear, while also leading to kinetic trapping effects. These results highlight the power of blending in designing the morphology that forms.

   @article{bae_chain_2022,
     author = {Bae, Suwon and Yager, Kevin G.},
     doi = {10.1021/acsnano.2c07448},
     issn = {1936-0851, 1936-086X},
     journal = {ACS Nano},
     language = {en},
     month = sep,
     number = {10},
     pages = {17107--17115},
     title = {Chain {Redistribution} {Stabilizes} {Coexistence} {Phases} in {Block} {Copolymer} {Blends}},
     volume = {16},
     year = {2022},
   }

2. Priming self-assembly pathways by stacking block copolymers

   Sebastian T. Russell, Suwon Bae, Ashwanth Subramanian, and 5 more authors

   Nature Communications, Nov 2022

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   Block copolymers spontaneously self-assemble into well-defined nanoscale morphologies. Yet equilibrium assembly gives rise to a limited set of structures. Non-equilibrium strategies can, in principle, expand diversity by exploiting self-assembly’s responsive nature. In this vein, we developed a pathway priming strategy combining control of thin film initial configurations and ordering history. We sequentially coat distinct materials to form prescribed initial states, and use thermal annealing to evolve these manifestly non-equilibrium states through the assembly landscape, traversing normally inaccessible transient structures. We explore the enormous associated hyperspace, spanning processing (annealing temperature and time), material (composition and molecular weight), and layering (thickness and order) dimensions. We demonstrate a library of exotic non-native morphologies, including vertically-oriented perforated lamellae, aqueduct structures (vertical lamellar walls with substrate-pinned perforations), parapets (crenellated lamellae), and networks of crisscrossing lamellae. This enhanced structural control can be used to modify functional properties, including accessing regimes that surpass their equilibrium analogs.

   @article{russell_priming_2022,
     author = {Russell, Sebastian T. and Bae, Suwon and Subramanian, Ashwanth and Tiwale, Nikhil and Doerk, Gregory and Nam, Chang-Yong and Fukuto, Masafumi and Yager, Kevin G.},
     doi = {10.1038/s41467-022-34729-0},
     issn = {2041-1723},
     journal = {Nature Communications},
     language = {en},
     month = nov,
     number = {1},
     pages = {6947},
     title = {Priming self-assembly pathways by stacking block copolymers},
     volume = {13},
     year = {2022},
   }

2021

1. Film Thickness and Composition Effects in Symmetric Ternary Block Copolymer/Homopolymer Blend Films: Domain Spacing and Orientation

   Kristof Toth, Suwon Bae, Chinedum O. Osuji, and 2 more authors

   Macromolecules, Sep 2021

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   We report thickness and homopolymer molar mass dependencies in thin film blends of a compositionally symmetric 75 kg/mol lamellar polystyrene-block-poly(methyl methacrylate) diblock copolymer (PS-b-PMMA) with near-equal volume fractions of PS and PMMA homopolymers on “neutral” surfaces that promote a vertical domain orientation. PS and PMMA homopolymers with equal molar masses of 1, 3, and 22 kg/mol are blended in a 1:1 mass ratio. To efficiently explore this parameter space, we prepared combinatorial samples with gradient homopolymer concentrations using electrospray deposition (ESD) and gradient film thicknesses using flow coating; these samples were subsequently characterized by grazing-incidence small-angle X-ray scattering and scanning electron microscopy, respectively. Our results demonstrate that the added homopolymers increase or reduce domain spacings with respect to the neat (unblended) block copolymer depending on their molar masses, in line with previous reports; a simple heuristic to estimate the domain scaling in ternary blends based on the homopolymer molar mass agrees well with data for the thinnest blend films studied here. More surprisingly, we observe that the measured domain spacing in blends also depends on film thickness, with thicker films exhibiting larger domain spacings than thinner films at the same homopolymer concentration. Furthermore, thickness gradients at fixed homopolymer concentrations reveal a change in lamellae orientation from vertical to horizontal as film thickness is increased beyond the nominal lamellar domain spacing. Coarse-grained molecular dynamics simulations indicate that this change in orientation is induced by homopolymer segregation to film surfaces. Tying these effects together, we hypothesize that the observed dependence of domain spacing on film thickness is a consequence of changes in the vertical homopolymer composition profile in thicker films during lamellae reorientation and the conformational asymmetry between PMMA and PS segments.

   @article{toth_film_2021,
     author = {Toth, Kristof and Bae, Suwon and Osuji, Chinedum O. and Yager, Kevin G. and Doerk, Gregory S.},
     doi = {10.1021/acs.macromol.1c01032},
     issn = {0024-9297, 1520-5835},
     journal = {Macromolecules},
     language = {en},
     month = sep,
     number = {17},
     pages = {7970--7986},
     title = {Film {Thickness} and {Composition} {Effects} in {Symmetric} {Ternary} {Block} {Copolymer}/{Homopolymer} {Blend} {Films}: {Domain} {Spacing} and {Orientation}},
     volume = {54},
     year = {2021},
   }

2019

1. Highly Stretchable Polymers: Mechanical Properties Improvement by Balancing Intra‐ and Intermolecular Interactions

   Or Galant, Suwon Bae, Meredith N. Silberstein, and 1 more author

   Advanced Functional Materials, Apr 2019

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   The mechanical properties of polymers are highly dependent on the mobility of the underlying chains. Changes in polymer architecture can affect inter- and intramolecular interactions, resulting in different chain dynamics. Herein, an enhancement in the mechanical properties of poly(butylmethacrylate) is induced by folding the polymer chains through covalent intramolecular crosslinking (CL). Intramolecular CL causes an increase in intramolecular interactions and inhibition of intermolecular interactions. In both the glassy and rubbery states, this molecular rearrangement increases material stiffness. In the glassy state, this molecular rearrangement also leads to reduced failure strain, but surprisingly, in the rubbery state, the large strain elasticity is actually increased. An intermediate intramolecular CL degree, where there is a balance between intra- and intermolecular interactions, shows optimal mechanical properties. Molecular dynamics simulations are used to confirm and provide molecular mechanisms to explain the experimental results.

   @article{galant_highly_2019,
     author = {Galant, Or and Bae, Suwon and Silberstein, Meredith N. and Diesendruck, Charles E.},
     doi = {10.1002/adfm.201901806},
     issn = {1616-301X, 1616-3028},
     journal = {Advanced Functional Materials},
     language = {en},
     month = apr,
     pages = {1901806},
     shorttitle = {Highly {Stretchable} {Polymers}},
     title = {Highly {Stretchable} {Polymers}: {Mechanical} {Properties} {Improvement} by {Balancing} {Intra}‐ and {Intermolecular} {Interactions}},
     year = {2019},
   }

2018

1. The Effect of Intrachain Cross-Linking on the Thermomechanical Behavior of Bulk Polymers Assembled Solely from Single Chain Polymer Nanoparticles

   Suwon Bae, Or Galant, Charles E. Diesendruck, and 1 more author

   Macromolecules, Sep 2018

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   Chemical cross-linking of polymer chains is a powerful means for tailoring the thermomechanical properties of bulk plastics. Nonetheless, upon cross-linking, processability is reduced as the plastic becomes thermoset. Here, molecular dynamics simulations are used to study the effects of intramolecular chemical cross-linking on chain topology and thermomechanical properties of a bulk, thermoplastic polymer. Polyethylene (PE)-like plastics are assembled purely from chains which have undergone a set level of intrachain cross-linking (to form single chain polymer nanoparticles, SCPNs). We have analyzed the chain topology at an equilibrated state in terms of chain unfolding and entanglement by radius of gyration and primitive path, respectively. The extents of both chain unfolding and chain entanglement were found to decrease with increasing intrachain cross-linking ratio. By applying simulated cooling, uniaxial tension, and uniaxial compression, we characterized the thermomechanical properties at the glassy state. The simulated mechanical testing shows that the bulk polymer becomes stiffer, stronger, and more brittle as the intrachain cross-linking ratio is increased. We observe that the failure of the SCPN-based bulk polymers is a consequence of separation between SCPNs. This study successfully elucidates the effect of intramolecular crosslinking on the thermomechanical properties at bulk, as a clear correlation is shown between the amount of covalent intrachain collapse and interchain interactions.

   @article{bae_effect_2018,
     author = {Bae, Suwon and Galant, Or and Diesendruck, Charles E. and Silberstein, Meredith N.},
     doi = {10.1021/acs.macromol.8b01027},
     issn = {0024-9297, 1520-5835},
     journal = {Macromolecules},
     language = {en},
     month = sep,
     number = {18},
     pages = {7160--7168},
     title = {The {Effect} of {Intrachain} {Cross}-{Linking} on the {Thermomechanical} {Behavior} of {Bulk} {Polymers} {Assembled} {Solely} from {Single} {Chain} {Polymer} {Nanoparticles}},
     volume = {51},
     year = {2018},
   }

2. Modulating metallopolymer mechanical properties by controlling metal ligand crosslinking

   Yuval Vidavsky, Suwon Bae, and Meredith N. Silberstein

   Journal of Polymer Science Part A: Polymer Chemistry, Jun 2018

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   Adjustable materials based on controlling the metal ligand interactions in metal containing polymers are prepared and characterized. Copper(II) carboxylate is used to crosslink a stretchable acrylic polymer, resulting in increased stiffness and strength. Moreover, modulation of the material properties is accomplished by attaching different ligands to the copper cations.

   @article{vidavsky_modulating_2018,
     author = {Vidavsky, Yuval and Bae, Suwon and Silberstein, Meredith N.},
     doi = {10.1002/pola.28994},
     issn = {0887624X},
     journal = {Journal of Polymer Science Part A: Polymer Chemistry},
     language = {en},
     month = jun,
     number = {11},
     pages = {1117--1122},
     title = {Modulating metallopolymer mechanical properties by controlling metal ligand crosslinking},
     volume = {56},
     year = {2018},
   }

2017

1. Mechanical and Thermomechanical Characterization of Glassy Thermoplastics with Intrachain Cross-Links

   Or Galant, Suwon Bae, Feng Wang, and 3 more authors

   Macromolecules, Sep 2017

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   The mechanical properties of an amorphous polymer are a consequence of the inter- and intramolecular interactions which are typically similar. Yet, polymers can be made with covalent intramolecular cross-links, leading to stronger intramolecular interactions. Here, a study on the effect of this intramolecular cross-linking on the mechanical properties of a glassy polymer is shown. A linear poly(methyl methacrylate) with defined size was cross-linked at different ratios and assembled into solid samples by solvent casting. Mechanical testing indicates that intramolecular cross-linking does not affect the mechanical properties at the elastic region but does influence these properties at the plastic region. Interestingly, intramolecular cross-linking leads to different effects than “regular” interchain cross-linking. The results are consistent with a reduction in chain entanglement, reducing toughness and to a lesser extent strength.

   @article{galant_mechanical_2017,
     author = {Galant, Or and Bae, Suwon and Wang, Feng and Levy, Avishai and Silberstein, Meredith N. and Diesendruck, Charles E.},
     doi = {10.1021/acs.macromol.7b01472},
     issn = {0024-9297, 1520-5835},
     journal = {Macromolecules},
     language = {en},
     month = sep,
     number = {17},
     pages = {6415--6420},
     title = {Mechanical and {Thermomechanical} {Characterization} of {Glassy} {Thermoplastics} with {Intrachain} {Cross}-{Links}},
     volume = {50},
     year = {2017},
   }

2. Tailoring single chain polymer nanoparticle thermo-mechanical behavior by cross-link density

   Suwon Bae, Or Galant, Charles E. Diesendruck, and 1 more author

   Soft Matter, Sep 2017

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   Single chain polymer nanoparticles (SCPNs) are formed from intrachain cross-linking of a single polymer chain, making SCPN distinct from other polymer nanoparticles for which the shape is predefined before polymerization. The degree of cross-linking in large part determines the internal architecture of the SCPNs and therefore their mechanical and thermomechanical properties. Here, we use molecular dynamics (MD) simulations to study thermomechanical behavior of individual SCPNs with different underlying structures by varying the ratio of cross-linking and the degree of polymerization. We characterize the particles in terms of shape, structure, glass transition temperature, mobility, and stress response to compressive loading. The results indicate that the constituent monomers of SCPNs become less mobile as the degree of cross-linking is increased corresponding to lower diffusivity and higher stress at a given temperature.

   @article{bae_tailoring_2017,
     author = {Bae, Suwon and Galant, Or and Diesendruck, Charles E. and Silberstein, Meredith N.},
     doi = {10.1039/C7SM00360A},
     issn = {1744-683X, 1744-6848},
     journal = {Soft Matter},
     language = {en},
     number = {15},
     pages = {2808--2816},
     title = {Tailoring single chain polymer nanoparticle thermo-mechanical behavior by cross-link density},
     volume = {13},
     year = {2017},
   }