Session B02: Polymer Structure Formation and Dynamics in Solution I: Structure and Assembly

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Block copolymer micelles are versatile materials with potential in many applications such as drug delivery. However, the dynamics of how block copolymer chains actually self-assemble into nanostructures has not been well documented. In this work, 1,2-polybutadiene-b-poly(ethylene oxide) (PB-PEO) was first dissolved in a mixture of the PEO-selective ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide and a neutral solvent dichloromethane. Dynamic light scattering and small angle X-ray scattering were then employed to monitor the development and size changes in micelles as a function of solvent selectivity as dichloromethane slowly evaporated. The effect of different dichloromethane evaporation rates was studied. The sigmoidal character of the obtained data suggested multiple equilibration processes are involved, including chain exchange and possibly micelle fusion.   

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Poloxamers are widely used in cosmetics and drug delivery due to their tunable structure and properties via molecular weight, block length, and block ratio. The sphere-to-rod transition in aqueous poloxamers is well-characterized; however, the molecular parameters and solution conditions controlling further growth and elongation into wormlike micelles (WLMs) are unknown. Here, we systematically evaluate rod formation and growth in a series of aqueous poloxamers using linear rheology and SANS to identify parameters critical to WLM formation. Important molecular and microstructural features include amphiphile size, micelle cross-section curvature, and solvent penetration. Despite vastly different amphiphile molecular weights, all poloxamers forming rods have nearly identical cross-sectional areas. However, a high relative curvature reduces the likelihood of further growth and WLM formation. Salt type and concentration also play a key role, where increasing salinity decreases the temperature for rod formation, but also narrows the temperature range over which WLMs are formed. This research sheds light on the fundamental role of poloxamer subunit and solution conditions on assembly and growth processes, providing a comprehensive dataset for validating thermodynamic models.   

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Bottlebrush polymers are a class of branched macromolecules that have side-chains densely-grafted on a linear backbone. Bottlebrush block copolymers can rapidly undergo self-assembly into microphase separated structures in solution. However, it remains challenging to efficiently model the side-chain degrees of freedom in multi-chain bottlebrush systems. We have recently shown how to map an explicit side-chain bottlebrush to an implicit side-chain representation; this discrete worm-like cylinder (dWLCy) model simplifies the branched molecular structure to only a few coarse-grained parameters that can be modeled much more efficiently. We refine the dWLCy model by incorporating a set of coarse-grained pair potentials, informed by a scaling theory that captures inter-bottlebrush interactions. Using this new model, we perform non-dilute Molecular Dynamics simulations of bottlebrush block copolymer solution assembly. As the solution concentration increases, we observe the transition from a disordered phase to a lamellar phase, immediately preceded by strong compositional fluctuations. We compare directly with experimental results, showing agreement between these simulation results and X-ray scattering characterization of bottlebrush block copolymer solutions.   

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We synthesized two types of amphiphilic diblock copolypeptoids, i.e., poly(N-methyl glycine)-b-poly(N-octyl glycine) (PNMG-b-PNOG) and poly(N-methyl glycine)-b-poly(N-2-ethyl-1-hexyl glycine) (PNMG-b-PNEHG), to investigate the influence of N-substituent (side chain) structure on the hierarchical self-assembly of diblock copolypeptoids in solution. With linear aliphatic N-substituent, the board-like PNOG blocks crystallize into a highly ordered lamellar structures, resulting in the hierarchical self-assembly of PNMG-b-PNOG into 3D microflowers comprised of stacked nanoribbons. By contrast, the PNEHG blocks bearing bulky branched aliphatic N-substituents are rod-like and prefer to stack into a columnar hexagonal liquid-crystalline mesophase, which drives PNMG-b-PNEHG to self-assemble into 2D hexagonal nanosheets in solution. These results show that the N-substituent architecture (i.e., linear versus branched) has a tremendous impact on the solution self-assembly of diblock copolypeptoids, which can serve as an effective strategy to tune the geometry and hierarchical structure of polypeptoid-based nanomaterials.   

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Assembly of peptide-based amphiphilic polymer chains in solution provides a route for the design of materials for a variety of applications (e.g., drug delivery vesicles). The engineering of these materials depends on the precise characterization of the assembled structures, often obtained through small angle scattering techniques. The interpretation of these scattering profiles typically relies on analytical models for conventional shapes that may not capture the system geometry at hand. This calls for a method that can tie scattering profile features directly to molecular details in complex nanostructures without needing off-the-shelf scattering models. To address this need, we apply recent extensions of Computational Reverse-Engineering Analysis for Scattering Experiments (CREASE) to a system of polymer-peptide conjugates that self-assemble into bilayer and vesicle structures. Taking in scattering intensity profiles and polymer chemistries as inputs, CREASE combines genetic algorithm and molecular reconstruction simulations to determine the peptide amphiphile bilayer composition, vesicle dimensions (e.g. core diameter, layer thicknesses) and molecular level packing within the nanostructure.   

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Block copolymers provide a remarkably versatile platform for achieving desired nanostructures by self-assembly, with lengthscales ranging from a few nanometers up to several hundred nanometers. While block copolymers generally adopt the morphologies familiar in small molecule surfactants and lipids (i.e., spherical micelles, worm-like micelles, and vesicles), one key difference is that polymeric micelles are typically not at equilibrium. The primary reason is the large number of repeat units in the insoluble block, Ncore, which makes the thermodynamic penalty for extracting a single chain (“unimer exchange”) substantial. As a consequence, the critical micelle concentration is rarely accessed experimentally; however, in the proximity of a critical micelle temperature, equilibration is possible. We use time-resolved small angle neutron scattering to obtain a detailed picture of the mechanisms and time scales for chain exchange, for systems near equilibrium. The dependence of the rate of exchange on the key variables – concentration, temperature, N_core, N_corona – will be discussed. Interestingly, almost none of the observed features are captured by available theory. Then, when micelles are significantly larger or smaller than the equilibrium size, fragmentation and fusion mechanisms, respectively, can become operative. We will describe measurements using dynamic light scattering, small-angle X-ray scattering, and liquid-phase TEM to follow the fragmentation process in detail.   

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Self-growing polymers can be used as building blocks for materials having a wide range of desirable properties, e.g. self-healing, controllable gradient in material properties, programmable deformation etc. To this end, we present a minimalistic theoretical model of two types of self-growing polymer gels, one comprising of an interpenetrating network (IPN) of two parent-chains, and the other one consisting of a random copolymer network (RCN), with the copolymers being formed by an inter-chain exchange between the IPN units. When placed in a monomer solution, these gels exhibit sustained growth by assimilating monomeric units from the solution into their network. We show how this swelling could be controlled by tuning the cross-link density as well as the chemical properties of the monomer and solvent units in the ambient solution.   

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Methylcellulose (MC) is a commercially important water-soluble polysaccharide, which thermoreversibly gels upon heating. Although MC has been studied and exploited in applications for many decades, it has only recently been discovered that the gelation occurs via self-assembly of the polymer chains into ca. 15 nm diameter fibrils, which percolate into a network. The network structure dictates the properties and mechanical behavior of the resulting hydrogel. The addition of salt to MC gels has been an area of academic and commercial interest. MC solutions containing salts exhibit an increase or decrease in the gelation temperature, generally following the Hofmeister series. We build upon those investigations and explore the effect of salt on MC fibril structure. We demonstrate the effect of salt on the gelation and dissolution temperatures using rheology and cloud point measurements. From SAXS and cryo-TEM, we show that salty MC gels are also comprised of fibrils. Fitting the SAXS curves to a semiflexible cylinder model, we demonstrate that the fibril diameter decreases monotonically with increasing salt molarity, largely independent of the salt anion or cation type. We propose two different mechanisms of fibril diameter reduction with the addition of salt.   

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Block copolymers (BCPs) have sustained academic and industrial interest due to unparalleled tunability in properties for widespread applications; however, inability to control long-range ordering limits their utility. We recently discovered anomalous field-induced phase transitions in industrially-relevant BCP solutions (20-30 wt%) using weak magnetic fields (B > 0.5 T), which can be used to control their self-assembly and long-range ordering. These solutions exhibit an anomalous transition from a low viscosity fluid (10^-2 Pa*s) to an ordered soft solid (10⁵ Pa*s) under weak magnetic fields. To determine the mechanism behind these phase transitions, we systematically studied the role of solvent quality and hydrogen bonding on the magnetorheological response and resulting induced structure. The dynamics of each system during magnetization and subsequent relaxation suggest that hydrogen bonding and induced chain conformation play significant roles in the presence of, and induction time required for, anomalous field-induced behavior. Understanding the mechanism behind this unexpected phase behavior in polymer solutions provides a new strategy for designing and processing advanced BCP materials.   

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As a water-soluble cellulose ether, methylcellulose (MC) is widely used as a binder and viscosity modifier in a variety of applications from pharmaceuticals to construction materials. Many applications exploit the ability of MC to gel thermoreversibly above its lower critical solution temperature (LCST) (~50 °C), correlated to the formation of nanofibrils of uniform diameter. Our recent work has shown that the fibril structure can be altered by grafting poly(ethylene glycol) (PEG) chains onto MC, and that fibrils are suppressed at high grafting densities. To expand our understanding of fibril formation, we have chemically modified MC with poly(N-isopropylacrylamide) (PNIPAm) at various grafting densities. PNIPAm also displays an LCST in water, but at a lower temperature (~32 °C). In this study, we present the effect of PNIPAm on the chain conformation and fiber formation of MC. Using dynamic and static light scattering, the temperature dependent changes in polymer size and solvent quality are investigated. Cryogenic electron microscopy and small-angle X-ray scattering are used to observe and quantify changes in fibril structure. Similar to PEG-grafted MC, at 80 °C the fibril length decreases with increasing PNIPAm grafting density, until fibril formation is entirely suppressed.   

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The process of single chain exchange is the dominant mechanism for micelle equilibration near equilibrium, and understanding it is important for applications such as drug delivery and oil-based lubrication technology. Previous methods of extracting an activation barrier to chain exchange in block copolymer micelles involve fitting a calculated intensity relaxation function to time-resolved small angle neutron scattering measurements; however, this method does not provide information about the shape of the barrier. We utilize umbrella sampling to probe the full free energy profile of expulsion of a single chain from a diblock copolymer micelle in solvent. Our simulations are carried out using dissipative particle dynamics, and the weighted histogram analysis method (WHAM) is utilized to extract the free energy profile as a chain is held at various distances from the micelle center-of-mass via a biasing potential. The free energy profile is characterized as a function of interaction energy between the core block and solvent. Further, the dependence of the free energy barrier on aggregation number is explored. Contrary to past approaches, this study allows for determination of the full free energy landscape of chain expulsion.   

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The nanostructure and dynamics of supramolecular polymers of computationally designed peptidic bundlemers is presented using a combination of Small-Angle Neutron Scattering (SANS), Neutron Spin Echo (NSE) and high-resolution Transmission Electron Microscopy (TEM). The bundlemers are oligomers of four identical alpha-helical peptides that are packed in an antiparallel fashion to form a robust cylindrical building block. Via a hybrid physical-covalent pathway and by employing linkers of different length and geometry, the bundlemers are linked into ultra-rigid rod-like polybundlemers or semi-rigid worm-like chains of polybundlemers. The two polymer systems show distinct scattering signatures in SANS and the differences in rigidity are also readily viewable in TEM micrographs. NSE indicates that the inter-bundlemer dynamics is impacted by the linker type. A dynamical decay rate, Γ ~ Q ² (Q is the scattering vector) is recorded for the rigid rod-like polymers for the entire probed Q-regime. In contrast, semi-rigid chains show a Q-dependent dynamic signature, with a Γ ~ Q ² dependence at high-Q which deviates to a Γ ~ Q ^8/3 dependence in intermediate-Q regime. This indicates Zilman-Granek bending modes are at play at inter-bundlemer length scales in the semi-rigid polybundlemer system.   

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Cosolvents are commonly used at various stages in preparing micelles from amphiphilic diblock copolymers (DBC). Yet the cosolvent effects on the micellization has been rarely investigated. In this study, the Field-Accelerated Monte Carlo simulation and the self-consistent field calculations are employed in the grand canonical ensemble to examine the influence of adding a mutual solvent (a good solvent for both polymer blocks) on the DBC micelles formed in a selective solvent. It is found that the aggregation number (AG) of the micelles can decrease or increase within a wide range relative to the cosolvent-free state. The decrease in AG happens when the cosolvent acts as a surfactant that helps to reduce the interfacial energy by residing at the core-corona interface. The increase in AG is observed, however, when the corona block favors being in more contact with the cosolvent than with the solvent. This cononsolvency behavior of the corona block becomes more pronounced at higher molecular weight, leading to greater AG. Our study reveals the intriguing interplay of solvent and cosolvent in controlling the micellization of DBC, and highlights the importance of understanding polymer behaviors in solvent mixtures toward modulating polymer assemblies in the solution phase.