Session F05: Polymers and Block Copolymers at Interfaces II

Tuning pore size in block copolymer thin film for ultrafiltration applications

Porous membranes are commonly utilized in ultrafiltration (UF) applications such as water and biotechnological purification due to their capacity to filter nanometer size molecules. Block copolymers (BCPs) have been recognized as a particularly promising class of materials for membrane applications based on their ability to form films with dense arrays of uniform nanoscale pores by self-assembly. Tuning pore size and chemistry makes it possible to tailor self-assembled membrane functionality for specific applications. In this study, the self-assembled domain size in polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) thin films was controlled by blending P4VP homopolymers that selectively segregate to minority domains; the effect of homopolymer molecular weight and mass fraction on self-assembled domain orientation and pore size after homopolymer removal by selective solvents were investigated. Pore sizes were further reduced by incorporating metal oxides into pore walls via a vapor phase infiltration process achieving sub 10 nm pore diameters.

    

Antibacterial properties of nanostructured surfaces via the selfassembly of block copolymers: (I) Effects of surface interactions

Here we report synergistic nanostructured surfaces combining bactericidal and bacteria releasing properties against Escherichia coli (E. coli, a Gram-negative bacterium) and Listeria monocytogenes (L. monocytogenes, a Gram-positive bacterium). In order for the surface to emerge the dual functionalities, it is important to optimize the surface morphology and interaction. We use polystyrene-block-poly(methyl methacrylate) (PS-block-PMMA) diblock copolymers and fabricate vertically oriented cylindrical PS structures on silicon substrates (“PS-nanopillars”). To improve the biocompatibility of the PS nanopillars, a very thin layer (~ 3 nm-thick) of titanium oxide (TiO₂) was deposited by using atomic layer deposition (“TiO₂ nanopillars”). The results demonstrate that the TiO₂ nanopillars exhibit further improved bactericidal and bacteria releasing properties against E. coli, and the dual properties also emerge against L. monocytogenes. To understand the mechanisms associated with the surface interaction of the nanopillars, coarse-grained molecular dynamics simulations of a lipid bilayer (as a simplified model for E. coli) in contact with a substrate containing cylindrical pillars with different bacterium/substrate interactions are also performed. The detailed mechanism underlying the multifaceted property of the nanosurfaces will be discussed.

    

Antibacterial properties of nanostructured surfaces via the self-assembly of block copolymers: (II) Effects of surface morphology

The adhesion of unwanted pathogenic bacteria creates a great challenge and health risks when designing implantable medical devices. In response, significant efforts have been made to design effective antibacterial surface coatings. Antibacterial surfaces are typically based on liquid- repellent or bactericidal properties. However, either single property alone has a disadvantage in practical applications. Therefore, the development of dual-functional surface coatings is needed but is still challenging due to the different mechanisms underlying the respective functions. Here we develop synergistic surfaces combining bacteria-releasing and bactericidal properties against Escherichia coli (a Gram-negative bacterium) and Listeria monocytogenes (a Gram-positive bacterium). Hexagonally packed vertically oriented cylindrical polystyrene structures are fabricated via the self-assembly of polystyrene-block-poly(methyl methacrylate) diblock copolymers on silicon substrates. How the surface morphology (lamella and cylinder) and the domain size affect the bacteria-releasing and bactericidal properties are investigated. The results demonstrate that the cylindrical nanostructures show higher efficacy, and there is a critical domain size at which the dual properties emerge.   

Comparing polyelectrolyte complexes formed from linear polycations and linear, star, and bottlebrush polyanions

Polyelectrolyte complexes form by the interactions between linear polyanions and polycations systems have been the subject of much research. Some limitations of linear commercial homo-polyelectrolyte are related to charge density and ion pairing density. Using synthetic bottlebrush (BB-) and star (s-) homo-polyelectrolytes, the ion charge density will be significantly increased and the architecture of the polymer will strongly influence the spatial distribution of charges locally and globally. In addition, the anchoring of multiple charged chains to a single backbone or a single junction point may also affect the accessibility of charges along the “bristles” or the arms due to the proximity of other charged chains.   We use ring opening metathesis polymerization (ROMP) to synthesize star and bottlebrush polyanionic polyacrylic acid, s-PAA, and BB-PAA, respectively. Titration of the s-PAA and BB-PAA allowed us to assess the pKa of these polymers, providing insight into the accessibility of the charges.   By mixing L-PAA, s-PAA, or BB-PAA into the aqueous solution of polyethylene glycol (PEG) and dextran (DEX), and linear polyallylamine hydrochloride (L-PAH) into an aqueous solution of PEG and DEX, where the PEG and DEX solution concentrations were set to binodal concentrations at the termini of the tie line of the phase diagram.  This allowed the L-PAA, s-PAA, or BB-PAA and the L-PAH to diffuse across the interface of this aqueous two-phase system absent phase separation of the solutions comprising the solutes. Using DLS/SLS, optical and fluorescence microscopy, and turbidity the characteristics of the L-PAH/L-PAA, L-PAH/s-PAA, and L-PAH/BB-PAA complexes were compared.

    

Excluded volume tunes water diffusivity near non-polar surfaces

Water behavior near soft interfaces plays a central role in a wide range of applications from drug delivery and catalysis to sensing and water purification. While well established that water thermodynamics facilitate surface-solute interactions critical to soft material performance, controlling water behavior in systems in which functionalities can be flexibly incorporated remains a longstanding challenge. This work uses versatile sequence-defined polypeptoids to demonstrate that water behavior can be tuned near polymeric surfaces. This system simultaneously offers both a route to control functional group position and a unique opportunity to map water diffusivity moving away from a surface with Overhauser dynamic nuclear polarization. Our results demonstrate that excluded volume causes water dynamics to slow in proximity to non-hydrogen bonding surfaces – an important consideration in the design of functional soft materials.   

Unraveling metastable nanostructures of isotactic polypropylene on solids

 Owing to strong demands for newly emerging polymer-based nanotechnologies, many sophisticated devices are moving towards nanometer scales, while maintaining exceptional performance capabilities. The key to breakthroughs in such advanced polymer-based technologies is a better understanding of a solid-polymer melt (SPM) interface which determines their stability/reliability and properties/functionalities. In this talk, we focus on substrate-induced heterogeneous nanostructures of semicrystalline polymers that exist on solid surfaces even above the bulk melting temperatures. Isotactic polypropylene (iPP) and two different planar substrates, i.e., silicon and carbon-layer (~ 20 nm thick) coated silicon substrates were used as rational models. The SPM interfaces were exposed to the air by solvent rinsing of supported iPP thin films (about 100 nm thick) thoroughly. In-situ grazing incidence small-angle X-ray scattering and X-ray diffraction measurements were performed to study the temperature dependence of the crystalline and lamellar structures at the SPM interface. The details will be discussed in the presentation.

    

pH-Mediated Size-Selective Adsorption of Gold Nanoparticles to Polymer Brushes

The design of smart functional surfaces that interact with nanoparticles is essential to precisely control adsorption, desorption, and diffusion dynamics during the nanoparticle transport in a variety of nanoscale applications. Here, we demonstrate that a weak polyelectrolyte brush can be used as a functional surface to selectively adsorb nanoparticles according to their size by adjusting the pH of the buffer solution. We first developed an effective brush preparation method using a symmetric polystyrene-b-poly(2-vinylpyridine) (PS-b-P2VP) block copolymer. We characterized the uniform P2VP brush layer using X-ray reflectivity and atomic force microscopy. The buffer pH effectively adjusts the interactions between the citrate-coated AuNPs surfaces and P2VP brushes. Low pH (~4.0) yields highly stretched brush chain conformations with sufficiently attractive interaction pairs, while high pH (~6.5) produces slightly stretched brushes with limited attractive interaction pairs. Quartz crystal microbalance with dissipation monitored the adsorption thermodynamics as a function of AuNP diameter (11 and 21 nm) and buffer pH. We find that high pH provides limited penetration depth for nanoparticles and promotes size selectivity for 11-nm AuNP adsorption. This selectivity for the smaller AuNPs was also observed in mixed AuNP suspensions, thus demonstrating the potential application of P2VP brushes for size separation of nanoparticles.   

Durable superhydrophilic coatings via surface-initiated photopolymerization

The covalent attachment of polymers has emerged as a powerful strategy for the preparation of multi-functional surfaces. For this work, we describe new chemistries to engineer durable superhydrophilic polymer brush films that provide anti-fogging properties. We use both tribology to study the mechanical properties of the resulting coatings and use vibrational sum frequency generation (SFG) spectroscopy to study how the conformation of hydrophilic polymer brushes changes in response to environmental conditions, i.e., changes in humidity (in air) and upon exposure to liquid water. Three hydrophilic brushes were prepared on silicon oxide surfaces by surface-initiated reversible deactivation radical polymerization of containing cationic (quaternary ammonium), anionic (sulfonate), and zwitterionic (containing both) monomers. The average tilt angle of methyl groups was analyzed and used to deduce the chain conformations of the polymer brushes. In air, the brush films absorb water and swell with increasing humidity. This is accompanied by the rotation of interfacial polymer chains. The degree of water uptake and chain conformation vary with the nature of the charged hydrophilic moieties. The hydrophilic polymer brush surfaces appear to remain relatively dry except in near-condensation conditions. In water, the quaternary ammonium groups of cationic and zwitterionic brushes are aligned nearly parallel to the surface. The anionic brush chains appear to assume nearly random conformations in water.   

Nucleation of a double diamond twin

Many twin boundary (TB) defects are found in the tubular network double diamond (DD) phase, suggesting these are readily formed, low energy structures. The DD phase consists of two inter-catenated networks comprised of basic loops of 6 tetrahedral nodes. The two networks are identical and achiral, just translationally shifted. The TB is a (222) plane with one network offset from the boundary plane while the other network has nodes on the mirror. The structure of the offset network is precisely the same as a (111) twin in carbon diamond, while for the other network of the DD, the TB causes formation of 2 new types of mirror symmetric nodes (alternating pentahedral and trihedral nodes) which link into a hexagonal mesh to form the TB.¹ Terminated TBs allow investigation of the formation of the twin. 3D reconstruction at the edge of the TB provides direct visualization of the nucleation pathway from DD crystal into DD twin. Observations show that one set of tetrahedral nodes on one network splits into two trihedral nodes, one of which then combines with another tetrahedral node to form a pentahedral node leaving a trihedral node in-between each pentahedral node. On the other network, each tetrahedral node again splits into two trihedral nodes, but then each of these trihedral nodes combines with another adjacent trihedral node arising from a different tetrahedral node to form a new tetrahedral node that connects across the boundary to a mirror symmetric partner.

    

Interface modification with (co)polyethers: surface-initiated polymerization and grafting

Polyethers are important polymers that are applied in a wide variety of technological contexts: from polymer electrolytes for lithium-ion batteries to stabilizers in mRNA vaccines and from surface coatings that prevent fouling to pre-cursors for commercial polyurethane foams. Applying polyethers at interfaces with precise properties is vital to realizing these and other technologies. Polyethers are often synthesized from the polymerization of functional epoxides. However, most epoxide polymerization methods lack some control (e.g., compositional, architectural, end group, etc.), which makes polyether design difficult. Furthermore, these polymerization methods are oftentimes not compatible with facile surface modification. In this talk, I will discuss recent efforts my lab has made in the synthesis of polyethers with tunable properties and how we have placed these polyethers at interfaces to enhance properties. Specifically, I will discuss recent efforts in preparing amphoteric membranes for redox flow batteries through copolyether compositional control and grafting, new ways to polymerize epoxides directly from the surface of silica nanoparticles, and how to tune polymer end group functionality to facilitate polyether end-grafting to surfaces. This work is enabled by a robust and sustainable aluminum-based polymerization platform that is easy to use. I hope this talk will inspire others to utilize the techniques discussed here to apply polyethers in their own research.

    

Machine Learning Driven Calculation of Hydrogen Bonding Inside Densely Grafted Polyelectrolyte Brush Layers

Recent all-atom molecular dynamic (MD) simulations have revealed behavior and properties of the counterions and the water molecules supported by the polyelectrolyte (PE) brushes. Such efforts, however, are limited by the fact that very often a generic definition is used to describe the properties of water and ions inside the brush layer. For example, while defining the water-water hydrogen bonds (HBs) inside the brush layer, it is often disregarded that these HBs will behave differently between locations outside and inside the brush layer since water connectivity is severely disrupted inside the brush layer. Here we address this gap and present the use of an unsupervised machine learning (ML) approach, which is based on clustering algorithm and uses the all-atom MD simulation generated equilibrium coordinates of the water molecules as input, to predict the water-water HBs inside cationic and anionic PE brush layers. Our calculations enable us to (1) compare the clusters formed inside and outside the brush layer and identify the corresponding disruption of the hydrogen bonding inside the brush layer and (2) to quantify the possible PE-brush-induced confinement-driven changes to the average "hydrogen – acceptor-oxygen – donor-oxygen" angle that defines the HBs.   

Multiscale modeling of grain-boundary motion in cylinder-forming block copolymers

Structure formation in block copolymer systems typically results in a multigrain state and the late-stage morphology can be conceived as an assembly of grain boundaries that separate regions of the equilibrium phase that differ in their orientation and registration. The motion of grain boundaries dictates the late-stage coarsening kinetics.

Using the combination of (i) a soft, coarse-grained, particle-based model, (ii) a free-energy functional, and (iii) a lattice model of local, metastable states, we study the structure and motion of a grain boundary between two orthogonal grains of cylindrical domains in asymmetric block copolymers. The particle-based simulation provides direct insights into the elementary class of thermally activated transitions of the self-assembled morphology in the course of grain-boundary translation. These processes are correlated in space and time. We identify a minimal set of transitions, whose free-energy changes and barriers are obtained by describing the system by a free-energy functional and calculating the minimum free-energy path. The spatiotemporal correlation arises from the dependence of the free-energy characteristics on the local environment. We use this information to devise a lattice model of the correlated processes involved in grain-boundary motion. This allows us to investigate the grain-boundary motion by kinetic Monte-Carlo (kMC) simulation and determine its free-energy landscape. Grain-boundary motion proceeds by nucleating a two-dimensional, anisotropic cluster inside the plane of the grain boundary.