Session M04: Polymers and Block Copolymers at Interfaces III

Engineering fluid-fluid interfaces through processing and multicomponent adsorption

Systems involving deformable interfaces between immiscible fluids offer a significant challenge for materials design and processing.   Static interfacial/surface tension is often the only parameter considered in the design of systems with fluid-fluid interfaces.   In foams, emulsions, blends, sprays, droplet-based microfluidic devices and many other applications, the dynamic nature of surface active species and deformation of interfaces requires a more detailed characterization of the interfacial transport, dynamic interfacial properties and interfacial structure.  Macroscopic properties and the ability to tune and control phenomena requires an improved understanding of the time-dependent properties of the interfacial tension and interfacial mechanics.  We have developed tools and approaches to quantify the impact of surface active species on interfacial behavior.  Surfactant-nanoparticle complexes, polymer-surfactant aggregates and proteins show the potential of interfacial processing in controlling interfacial properties.  The use of sequential adsorption, differences in transport timescales and variability in reversibility of different species allows interfaces to be engineered.  This talk will provide the motivation to use microscale interfaces for efficient analysis of complex interfacial phenomena and how that relates to the material properties of interface-dominated materials.

    

Effect on interfacial tension of multiblock polymers at immiscible homopolymer interfaces

Block copolymers have been investigated as compatibilizers in immiscible homopolymer blends for over 40 years. A critical aspect of their performance is their ability to emulsify thermodynamically dissimilar polymers, as manifested by a reduction in the interfacial tension between the two homopolymers. While significant theoretical and experimental effort has been devoted to understanding how diblock copolymers reduce interfacial tension, considerably less effort has been devoted to studying the reduction in interfacial tension induced by multiblock copolymers when they adsorb at interfaces. To address this question, we will present interfacial tension data obtained via molecular dynamics simulations of a coarse-grained polymer model, focusing on the extent to which this effect is controlled by the architecture of the multiblock polymers.

    

Self-Assembly of Amphiphilic Janus Particles at Oil/Water Interfaces

Recent experimental developments have led to a variety of easily scalable techniques for the synthesis of morphologically and chemically anisotropic polymer colloids. One such technique, seeded emulsion polymerization (SEP), has been used to produce a variety of amphiphilic polymer Janus particles for use as pH responsive solid surfactants. These studies have led to an understanding of the emulsification behavior of these colloids and factors controlling emulsion type and size. However, there has been less work focused on understanding the mechanism and factors governing the interfacial assembly of Janus particles. We investigated the interfacial behavior of these chemically and morphologically heterogeneous polymer colloids using pendant drop tensiometry. Systematic studies were performed as a function of colloid concentration, system pH, and colloid morphology. The interfacial behavior of these systems was found to be not only sensitive to their environmental conditions, but also to the type and degree of anisotropy. Our studies elucidate the way these anisotropic colloids adsorb to and assemble at the interface and re-arrange to maximize the reduction of the interfacial tension.   

Aggregation and pressure effects of asphaltene and resin molecules at oil–water interfaces: a coarse-grained molecular dynamics and free energy study

Coarse-grained molecular dynamics simulations were used to investigate the aggregation of asphaltene and resin molecules in oils and their deposition to oil–water interfaces. Resin, "interfacially-active" asphaltenes, and "bulk-like" asphaltenes are considered as solutes in organic phases consisting of aromatics or saturates. Resins and asphaltenes formed aggregates with a spacing of 0.46 nm between stacked polycyclic sheets. Whether in the aromatic or saturated solvent, resin molecules did not interact with the interface, but its aggregates remained in the bulk. The degree of surface activity of asphaltenes was found to increase with the polarity of their chemical groups, and decrease with the aromatics content of the solvent. Axial stress profiles were measured to calculate the interfacial tension of each system. The tension of interfaces of crude oil with water was found to depend on aromatics content. The free energy of deposition of asphaltenes and resin molecules to the interface was measured using well-tempered metadynamics, in which it was found that "interfacially-active" asphaltenes possess greater stability at the oil-water interface than "bulk-like" asphaltenes, and the organic solvent influences the favorability of deposition.   

Per- and Polyfluoroalkyl Substances (PFAS) Foams: Effect of Interfacial Properties

Per- and polyfluoroalkyl substances (PFAS) are a group of partially or fully fluorinated organic compounds. They have been widely used in many applications, such as non-stick cookware, aqueous film-forming foams (AFFFs), coating materials, and surfactants, due to their excellent thermal and chemical stability. However, PFAS are toxic and accumulative in organisms and the environment. Water resources have been contaminated by PFAS owing to their surfactancy and tendency of being transported through the air-water interfaces. While foam fractionation is one of the promising remediations to remove surfactants and colloids from water resources, it is currently not viable for PFAS removal. Therefore, fundamental studies on the correlations between the properties of PFAS aqueous foams and their interfacial properties, such as dilatational rheological properties, are required and necessary for improving the effectiveness of the foam fractionation process. In the present work, the foaming capacity of long-chain and short-chain PFAS in the presence of electrolytes is studied. In addition, the PFAS foam properties in terms of liquid drainage and bubble coarsening are analyzed. Then, the air-water interfacial properties of PFAS are studied by performing dilatational rheology and measuring dynamic surface tension to further evaluate their effects on the PFAS aqueous foam properties.

    

The Effect of Core Tg on the Air–Water Surface Mechanical Properties of Water–Spread Block Copolymer Micelles

In the pursuit of the development of a novel polymer lung surfactant therapeutic, proper design and understanding of the air–water surface mechanical properties of the polymer surfactant particles must be achieved. To this end, the effect of the core T_g of amphiphilic block copolymer micelles on their corresponding air-water surface mechanical behavior of various hydrophobic core chemistries, all having poly(ethylene glycol) as the hydrophilic block, was studied. At temperatures below T_g, the solid core domains are kinetically ‘frozen’ and prevented from rearranging at the interface. X-ray reflectivity data suggests the core domain remains largely submerged in the water subphase. The rise in surface pressure upon compression is a result of the osmotic pressure build up in the PEG corona. At temperatures above Tg, a transition in the surface-mechanical behavior occurs which is dependent upon the interfacial properties of the core domain. For core chemistries which have a high interfacial tension for both air and water, like poly(styrene), the micelle structure transitions to form surface micelles. For core chemistries which have lower interfacial tension with air than water such as poly(tert butyl methacrylate), the core domain is able to spread on the water surface to form a continuous film. In both cases, transitions are shown to be irreversible, and the temperature of the transition in the surface-mechanical behavior agrees well with core T_g measured using a previously disclosed T₂ NMR method.       

Sequence-Defined Polymer Brushes as Patternable Surface Modification Monolayers for Semiconductor/Bio Interfaces

Control over semiconductor/bio interfaces is a key enabler for biological nanofabrication pathways and new applications at the intersection of semiconductor technology and synthetic biology. Conventional surface functionalization methods such as silane chemistries and self-assembled monolayers (SAMs) may offer only a limited level of customization, while polymer brushes offer a wider range of chemistries and maintain compatibility with lithographic techniques. Here we developed a class of bioinspired, sequence-defined polymers–polypeptoids, as designer polymer brushes for surface modification of lithographic substrates. The polypeptoid brushes are demonstrated to be compatible with both lithographic patterning workflows and processes involving biomolecules. We designed polypeptoids with a hydroxyl group that enables efficient melt grafting onto silicon substrates to form 1–2 nm monolayers under lithographically relevant conditions. Chemical contrast patterns consisting of polypeptoid and poly(methyl methacrylate) brushes were generated with length scales defined by electron-beam lithography, which display selective adsorption of biomolecular building blocks such as DNA origami, with the polypeptoid brush grafted regions exhibiting much higher affinity to DNA origami compared to commonly used SiO₂ surfaces. We further show that the polypeptoid brush affinity to DNA origami can be tuned by manipulating monomer chemistry and sequence within the polypeptoid chains.   

Symmetry-Breaking Patch Formation on Triangular Gold Nanoparticles

Synthesizing patchy particles with predictive control over patch size, shape, placement and number is a holy grail of nanoparticle assembly research. Here, using gold triangular nanoprisms and polystyrene-b-polyacrylic acid as a model system, we show that polymers can be driven to selectively adsorb onto different surface locations on nanoparticles, resulting in patchy nanoparticles with broken symmetry. We rationalize the underlying assembly mechanism using scaling theories that accurately predict our experimental observations at all levels – from particle-level patch pattern, nanoscopic size and shape of patches, to the self-limited assemblies. Both experimental strategies and theoretical predictions readily extend to particles of other shapes. Our work provides a novel approach to leverage polymer interaction with nanoscale surfaces to drive asymmetric grafting in functional nanomaterials.

    

Structural and dynamic properties of poly(styrene-co-maleic anhydride) at the interface of conjugated polymers

Conjugated polymers are employed in a variety of application areas due to their bright fluorescence and strong biocompatability.  For these applications, readily fabricated conjugated polymer nanoparticles (CPNs) are formed using capping agents to provide stable and soluble conjugated polymer formulations. Post-assembly functional modification and chemical doping can be used to further tune CPN properties for target applications.

The nanoscale structure of conjugated polymer materials emerges from interactions between different molecules, and holds significant control over their much-prized optical properties.  Reliably controlling the optical properties of conjugated polymers upon aggregation and assembly remains a challenge. The structural properties of the interface between the capping agents and the conjugated polymer play a key role in the ability to then functionalise the surface of the conjugated polymer formulation with any specific molecules required for tuning the delivery of a CPN to a desired target.  In this presentation, I will present the results of a series of all-atom molecular dynamics simulations that we have conducted in order to provide a molecular scale understanding of the structural and dynamic properties of poly(styrene-co-maleic anhydride) (PSMA), which is a common capping agents for CPNs, on the interface of the conjugated polymer, poly(9,9-di-n-octylfluorene-alt-benzothiadiazole) (F8BT).  I will compare the properties of the PSMA capped interface with those which are found when using another commonly used capping agent, Brij O10.  Finally I will discuss the effect of the difference in the interfaces formed on the functionalisation of these polymer interfaces.

    

Monomer distributions in Dyck Path models of grafted polymers

Polymer coatings on surfaces or on suspended or colloid particles have applications in the steric stabilisation of colloid dispersions, or in drug delivery systems (such as nanoparticle-polymer systems, or drug eluding stents). In these applications a polymer is grafted onto a surface or a hard wall and it coils away from the surface to form a polymer layer. In a drug delivery system the drug absorbs in this layer until it is released at a target site. If the polymer is hydrophylic and in an aqueous solution then it forms a thicker and less dense coating, and if it is hydrophobic it forms a thinner and denser layer. These physical properties affect the adsorption of molecules into the layer. In this talk this is modelled using a directed Dyck path model of a grafted linear polymer. The density profile of monomers along the path is determined in the scaling limit for the path in good or in poor quality solvents. In either case the density profile is determined by calculating the exact probability distribution of monomers in the scaling limit as a function of their position along the Dyck path. This shows that, in a good solvent, the entropic repulsion between the Dyck path and a hard wall creates a low density regime next to the hard wall wherein particles can adsorbed and be stabilised until they are released. This will be, for example, the case in nanoparticle-polymer system for targeted drug delivery.   

Zwitterionic polymer grafted from silica surface for the study of mucosal transport

Mucus is a complex viscoelastic and adhesive substance which serves as a dynamic barrier that protects tissue against infection and invasion by viruses and bacteria. Recently, transmucosal drug delivery has shown promising advantages over other existing delivery routes, including improved bioavailability and therapeutic effects. However, particles with low diffusivities in mucus cannot reach deep mucus layers and are eliminated by mucus clearance mechanism, causing an impediment for drug delivery. It is also known that zwitterionic polymers (ZW) including poly(carboxybetaine) (PCB) or poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC) moiety have good mucopenetration properties. Despite that, fundamental studies and understanding of specific and non-specific interactions between mucus and ZW polymers are still lacking. On the other hand, boronic acids have been used as mucoadhesive components in delivery systems due to their ability to form high-affinity complexes with 1,2-diols such as sialic acids present in mucus. Herein, we studied the interactions between mucus and zwitterionic polymers with PMPC, and the combined effects of boronic acid chemistry, surface presentation, and pH on particle-mucus interactions. We optimized the grafting synthesis of ZW polymers from glass slides and silica particles as well as the further functionalization with different boronic acids. Surface characterization is done by XPS, contact angle, ellipsometry, DLS, SEM and TGA.