Session Y26: Polymers and Block Copolymers at Interfaces I

Exploration of complex nanostructures in block copolymer

The exploration of three-dimensional (3D) nanostructures in block copolymers involves the manipulation of compositional fluctuations at interfaces, induction of conformational asymmetry, and design of complex architectures. Despite the abundance of such nanostructures identified to date (e.g., triply periodic minimal surfaces and Frank–Kasper phases), the experimental demonstration of thermodynamically stable complex 3D structures with high packing frustration remains limited. In this talk, I would like to show the importance of molecular interactions for stabilizing complex 3D structures and proposes the use of end-group chemistry as a versatile method for realizing thermodynamically stable network structures with high packing frustration in simple linear diblock copolymers. I advocate revising conventional block-copolymer phase diagrams to consider end–end interactions and end-group arrangements. In instances where end-functionalized block copolymers exhibit strong end–end interactions, the lam structures are confined to a narrower phase diagram window. Conversely, a broader phase window enables the stabilization of diverse network structures such as gyroid, diamond, and primitive phases. This approach can reveal unprecedented network structures in various soft materials.   

Surface Mechanical Behavior of Water-Spread PS–PEG Cylindrical Micelles at the Air-Water Interface

Our lab has been studying the air-water interfacial properties of PS-PEG micelles, exploring their potential for treating respiratory distress syndrome (RDS). Initially, our research focused on spherical micelles formed by PS-PEG block copolymers. This presentation presents our latest findings regarding the behavior of cylindrical micelles at the air-water interface. We prepared kinetically-frozen PS-PEG cylindrical micelles via equilibration-nanoprecipitation. Similar to spherical micelles, cylindrical ones demonstrated the ability to generate high surface pressure (>70 mN/m) upon compression, a critical factor for RDS therapy. However, cylindrical micelles exhibited a distinct monolayer-to-bilayer transition, resulting in a secondary plateau in their surface pressure-area isotherm, preceding ultimate collapse during compression—a phenomenon absent in spherical micelles. Additionally, the population of cylindrical micelles influenced increased hysteresis on the surface pressure-area isotherm during compression and expansion cycles. This work demonstrates the potential to optimize therapeutic outcomes by fine-tuning the surface mechanics of PS-PEG micelles through tailored micelle shapes.   

Polymer Evaporative Crystallization on Water Surface

Solvent evaporation is one of the most fundamental processes in soft matter. Structures formed via solvent evaporation are often complex yet tunable via the competition between solute diffusion and solvent evaporation time scales. This work concerns the polymer evaporative crystallization on water surface (ECWS). The dynamic and two-dimensional nature of the water surface offers a unique way to control the crystallization pathway of polymeric materials. Using poly(L-lactic acid) (PLLA) as the model polymer, we demonstrate that both one-dimensional (1D) crystalline filaments and two-dimensional (2D) lamellae are formed via ECWS, in stark contrast to the 2D Langmuir-Blodget monolayer systems as well as polymer solution crystallization. Results show that this biphasic 1D/2D structure is tunable via chemical structures such as molecular weight and polarity of the polymer and processing conditions such as temperature, evaporation speed, and interfacial tension. Only 1D filaments are formed when poly(L-lactic acid)-b-poly(ethylene oxide) (PLLA-b-PEO) block copolymer is employed in ECWS, which is attributed to the strong pinning effect of the block copolymer imparted by the hydrophilic PEO block. Our work demonstrates that ECWS provides a rich platform to tune polymer crystallization pathways.   

pH-Dependence of Gold Nanoparticle Adsorption to a Weak Polyelectrolyte Brush

The ability to tailor nanoparticle adsorption onto surfaces is limited by the understanding of particle-surface interactions and dynamics. Here, we expand upon recent work investigating the pH-mediated size-selectivity of gold nanoparticles (Au NPs) adsorption to poly(2-vinylpyridine) (P2VP) brushes fabricated from diblock copolymer. Now, end-grafted P2VP brushes with molecular weights of 10 and 53 kg/mol are investigated. With increasing brush molecular weight, the brush heights increased and the grafting density decreased. We used AFM to characterize the brush out-of-plane surface structure and in situ ellipsometry to measure the film thicknesses as a function of pH. At pH=4.0 and pH=6.2, we monitored the adsorption of 10- and 20-nm citrate-functionalized Au NPs onto these brushes. We measured the kinetics of Au NP adsorption using QCM-D and imaged NP packing with SEM. Combining these techniques, we found a size-selective pH-mediated response of NP adsorption in these homopolymer P2VP brushes. While grafting density influences adsorption behavior, the ratio of brush height to Au NP diameter controls the adsorption behavior in this system. These findings of selective adsorption allow for enhanced design of separation membrane technologies for nanoscale particles and molecules.   

Computational Design of Patchy Particles with Complex Surface Patterning

Synthesizing reconfigurable nanoscale synthons with predictive control over shape, size, and interparticle interactions is a holy grail for designing stimuli-responsive self-assembled materials. However, grand challenges in their rational design lie in the large space of potential experimental parameters and complex synthetic protocols. Here, we define a strategy for designing complex, reconfigurable building blocks that addresses the above limitations. Specifically, we engineer triblock, star-like polymers and leverage their microphase separation to control surface patterning on nanoparticles. Our theory characterizes the structural organization of grafted polymers as a function of parameters such as grafting density, chain length, block fractions, and core shape/sizes. Stripe-like patterning and corner/edge patch formation on arbitrarily shaped cores are readily accessible using our framework, all of which can be a priori predicted. We then employ assembly simulations to show that the resulting particles provide tighter control over structural and orientational orderings during self-assembly by overriding face-face alignments tendencies intrinsic to each core geometry. Our theories provide unique insights into nanoscale synthesis, allowing for a priori design of complex building blocks that can target novel assemblies for novel materials fabrication.   

Impact of Nanoparticle Curvature on Adsorbed Polymer Chain Structure and Local Glass Transition Properties

Polymer properties are commonly enhanced via the addition of nanofiller, resulting in polymer nanocomposites with broad applications. Previous work suggests that a key feature governing the tuning of nanocomposite properties is the interfacial region between the nanoparticle and the polymer matrix, particularly where polymer chains experience frustrated packing during physical adsorption on the nanoparticle surface. Accordingly, nanoparticle morphology is expected to impact adsorbed chain packing and subsequent nanocomposite properties. Precise characterization and tuning of this interface has remained unrealized, however, as conventional characterization techniques are limited in spatial resolution of local polymer properties. In this study, fluorescence spectroscopy and transmission electron microscopy are combined to enable direct imaging and measurement of local structure and properties of polymer chains adsorbed on nanoparticles. Varying nanoparticle size allows us to elucidate how curvature impacts adsorbed chain structures and their corresponding local glass transition properties. This enhanced understanding paves the way for more intentional and effective approaches in designing nanocomposites.   

Size-Dependent Electrostatic Adsorption of Polymer-Grafted Gold Nanoparticles on Polyelectrolyte Brushes

In this study, we demonstrate that by tuning the salt concentration a polyelectrolyte brush selectively adsorbs larger nanoparticles. Specifically, we successfully created a positively charged polyelectrolyte brush by quaternizing pyridine groups of poly(2-vinylpyridine) brushes using methyl iodide. Using X-ray photoelectron spectroscopy and X-ray reflectivity we characterize the charge fraction and out-of-plane structure of the polyelectrolyte brush. A quartz crystal microbalance with dissipation monitored the adsorption kinetics and thermodynamics of polyethylene glycol-grafted, negatively-charged gold nanoparticles (diameters of 12 and 20 nm) as a function of salt concentration. In a salt-free solution, the polyelectrolyte brush adsorbs gold nanoparticles at both nanoparticle sizes. As the salinity increases, the number of adsorbed nanoparticles monotonically decreases and eventually becomes negligible at high salinity. Interestingly, there is a range of salt concentrations where the decrease in nanoparticle adsorption is more pronounced for smaller particles, leading to size-selective adsorption of the 20-nm nanoparticles. As further proof of selectivity, when the polyelectrolyte brush is immersed in a mixture of large and small nanoparticles, the brush selectively captures the larger particles. In addition, we demonstrate that the size distribution of synthesized gold nanoparticles can be reduced by selectively removing larger particles.   

Molecular Dynamics Study of the Interactions between Polyethylene Nanoplastic Particles and Lipid Membranes

Plastic wastes can break down into micro and nano-sized particles, which can enter biological tissues and potentially impact human health. The interactions of micro and nano-plastics with biological systems, such as cell membranes, however, are not well understood. In this work, we investigate the interactions between polyethylene (PE) nanoplastic particles and model POPC lipid bilayers in water using all-atom (CHARMM36 force field) and coarse-grained (MARTINI) molecular dynamics (MD) simulations. We show that atomistic resolution is critical for quantifying the polymer-membrane interactions. The all-atom PE nanoparticle is semicrystalline with crystalline surfaces exposed to water. The MARTINI polymer nanoparticle, however, is a spherical liquid droplet. The distinct morphologies give rise to different nanoplastic-membrane interactions. We compute the potentials of the mean force of both model systems and show their differences throughout the intermolecular distance between the PE nanoplastic particle and the POPC lipid bilayer. While the interaction between a PE particle and the lipid bilayer is purely repulsive in the all-atom simulations, the coarse-grained PE particle can penetrate the lipid membrane and dissolve in the hydrophobic core of POPC. We speculate that the crystallinity of nanoplastics plays an important role in the polymer-membrane interactions.   

Self-assembly and structural relaxation in 'patch-clasping' nanoparticles

Polymer-grafted ‘patchy’ nanoparticles (NPs) can self-assemble into open network structures, making them great building blocks for tunable metamaterials. However, a fundamental understanding of the underlying driving forces remains elusive. Here, we combine simulation and experiments to study the assembly dynamics of a model system of triangular gold nanoprisms functionalized with polystyrene-polyacrylic acid block copolymers (PS-b-PAA). We show that inter-NP bonds formed between interacting polymeric patches are longitudinally robust and rotationally flexible. Scaling theory reveals that inter-NP bond formation is driven by chain reorganization between interacting patches. This enables the development of a coarse-grained model that reproduces experimental dynamics of network chains formed from multiple NPs. Simulations reveal that the network’s rotational relaxation, and therefore reconfiguration, is influenced by chain architecture and relative bond orientations between NPs. Our results suggest that tuning the relative orientations between patchy nanoparticles during bond formation can provide a powerful handle for controlling the dynamical responses of reconfigurable nanomaterials.   

Unraveling the role of phenyl groups on the packing process of polystyrene chains bound to a solid surface

In response to the growing demand for precisely controllable nanotechnologies, emerging polymer-based materials are transitioning to nanometer scales while maintaining outstanding performance. Enabling further advancements in polymer-based technologies may hinge on a deeper understanding of a buried solid-polymer melt interface, which governs both their structural stability and properties. In this talk, we focus on revealing substrate-induced, short-range nanostructures of polymer melts strongly bound ("flattened chains") to solid surfaces. Atactic polystyrene (aPS) flattened chains prepared on silicon substrates were used as a rational model. Following an established solvent rinsing method, the final thicknesses of the aPS flattened layers measured about 2 nm. In-situ grazing incidence small-angle X-ray scattering measurements were conducted to study how amorphous aPS chains are arranged and packed in the lateral direction. In addition, atomistic molecular dynamics simulations were performed to complement the experimental results and further characterize the experimentally unobservable local chain conformations of aPS backbone and phenyl groups. The integrated results delineate the role of pi-pi interactions in the flattening process of bound polymer chains.   

Probing the buried structure at the silica/rubber interface

Silica (SiO₂) fillers have been recently paid great attention due to their potential for improving the processability of rubbers and are being used as partial or even complete replacements for carbon black (CB) fillers in automobile tires. Since the chemical interaction between SiO₂ and PB is much weaker than that between CB and PB, silane coupling agents are typically compounded together into an elastomer matrix, resulting in the formation of a monolayer on the filler surface. However, the detailed structure of the filler-rubber interface including the formation of bound rubber (BR) remains unsolved. To provide insight into it, we mimic the SiO₂/rubber interface using a planar silicon (Si) substrate with a native oxide layer. Monodisperse polybutadiene (PB) and a bis[3-(triethoxysilyl)propyl]tetrasulfide (Si69) were used as a rational model. PB thin films (~200 nm in thickness) onto the Si substrates were prepared by spin coating of a PB/Si69 (with 16 parts per hundred rubber) and toluene mixture solution and were subsequently annealed at 140°C/1hr to facilitate silanization. The films were then leached by toluene to extract the BR layer on the substrate. An X-ray reflectivity technique allows detailed characterization of the BR and silane coupling layer. The details will be discussed in the presentation.   

Controlled Disordering in Metal-Infiltrated Block Copolymer Nanopatterns

Disordered structures in the nanoscale can be of particular interest in photonics owing to their unique capability to localize wave-like quasiparticles – a renowned phenomenon known as Anderson localization. Aims to apply such an intriguing phenomenon to devices such as random lasers and plasmonic waveguides have motivated intensive study on structures accommodating intentionally introduced defects. However, reproducible fabrication of nanostructures with a controlled degree of disorder still remains challenging.

In this study, we present nanostructures with systematic variation of disorder employing the metal-infiltrated block copolymer system. In particular, we deliberately introduced defects into a single crystalline hexagonal array built on a sphere-forming block copolymer thin film. Here, disordering was induced by the adjustment of annealing temperature, and various processing parameters were proposed to broaden the spectrum of disorder. Such tunable parameters include the relative ratio of different metal precursors incorporated into the core blocks and the blending ratio with homopolymers. The results of this study potentially offer a platform for designing and engineering disordered systems that could mediate quasiparticle localization.