Session F42: Polymer Assembly I

Chain exchange kinetics of block copolymer micelles in ionic liquids

The chain exchange kinetics of block copolymer micelles has been studied using time-resolved small-angle neutron scattering (TR-SANS), a key tool in determining the average micelle composition in contrast-matched solvents. In this work, PMMA-\textit{block}-PnBMA was selected as the model block copolymer, which has a LCST behavior in the common ionic liquids, [EMIM][TFSI] and [BMIM][TFSI]. We examined the chain exchange kinetics of three PMMA-\textit{block}-PnBMA copolymers, with identical PMMA block length ($M_{\mathrm{PMMA}} \quad =$ 25000) and different PnBMA block lengths ($M_{\mathrm{PnBMA}} \quad =$ 24000, 35000 and 53000); the Flory-Huggins interaction parameter ($\chi )$ between the core (PnBMA) and the solvent were varied by mixing [EMIM][TFSI] and [BMIM][TFSI] in different ratios. We found that the relaxation of the initial segregation of h- and d- micelles followed the same form with the time as previously developed by our group. Assuming that single chain expulsion is the rate limiting step, the thermal barrier was found to depend linearly on the core block length ($N_{\mathrm{core}})$. Furthermore, the effect of $\chi $ on the chain exchange kinetics will also be discussed.   

Self-assembly of Coordination Macroions \textbf{---} the Effect of Small Polymer Chains

In the presence of small simple counterions, macroions with modest charge density could self-assemble into one-layer hollow spherical structures. The driving force to form this vesicle-like structure is considered as counter-ion mediated attractions. On the other hand, overall hydrophilic transition metal-organic macrocations, consisting of hydrophobic ligands (pyridine-based) and hydrophilic palladium (Pd) metal ions, can be used as nanosized macrocations. One of the positively charged coordination macroions, Pd$_{12}$L$_{24}$ (Pd $=$ Pd (II), L $=$ (1,3-di(pyridin-4-yl)benzene), is famous for its controllable and precise assembly structure and possible functional sites. By functionalizing the organic ligand with polymer ethylene glycol (PEG), the macroions could possess larger size thus lower charge density. Detecting their self-assembled structures will tell the difference of their solution behavior as well as the discrepancy of macroion clusters. In summary, the small non-charged PEG chains have been observed to reduce the surface charge density, and furthermore, significantly change the solution behavior.   

Simulation and Numerical Modeling of the Self-assembly of an Optoelectronic Peptide

We report molecular dynamics simulations of the self-assembly of synthetic $\pi$-conjugated oligopeptides into optoelectronic nanostructures. The electronic properties provide the basis for an array of organic electronic devices, such as light-emitting diodes, field-effect transistors, and solar cells. Control of the structure, stability, and kinetics of self-assembled organic electronics by tuning monomer chemistry and environmental conditions presents a powerful route to the fabrication of biocompatible “designer materials.” We have performed coarse-grained simulations of the self-assembly of several hundred peptides over microsecond time scales to probe the morphology and kinetics of aggregation with molecular-level detail. We have subsequently used this simulation data to parameterize a kinetic aggregation model based on Smoluchowski coagulation theory to enable prediction of aggregation dynamics on millisecond time scales. These numerical models are now being integrated into a multi-physics model of peptide aggregation in a microfluidic flow cell developed by our experimental collaborators to model the self-assembly of diverse peptide architectures under tailored flow-fields for the fabrication of biocompatible assemblies with defined morphology and optoelectronic function.   

Complexation Between Cationic Diblock Copolymers and Plasmid DNA

Deoxyribonucleic acids (DNA), as polyanions, can spontaneously bind with polycations to form polyelectrolyte complexes. When the polycation is a diblock copolymer with one cationic block and one uncharged hydrophilic block, the polyelectrolyte complexes formed with plasmid DNA (pDNA) are often colloidally stable, and show great promise in the field of polymeric gene therapy. While the resulting properties (size, stability, and toxicity to biological systems) of the complexes have been studied for numerous cationic diblocks, the fundamentals of the pDNA-diblock binding process have not been extensively investigated. Herein, we report how the cationic block content of a diblock influences the pDNA-diblock interactions. pDNA with 7164 base pairs and poly(2-deoxy-2-methacrylamido glucopyranose)-block-poly(N-(2-aminoethyl) methacrylamide) (PMAG-b-PAEMA) are used as the model pDNA and cationic diblock, respectively. To vary the cationic block content, two PMAG-b-PAEMA copolymers with similar PMAG block lengths but distinct PAEMA block lengths and a PAEMA homopolymer are utilized. We show that the enthalpy change from pDNA-diblock interactions is dependent on the cationic diblock composition, and is closely associated with both the binding strength and the pDNA tertiary structure.   

Complexation of AB$^{\mathrm{+}}$, AB$^{\mathrm{+}}$C, ACB$^{\mathrm{+}}$, and A(B$^{\mathrm{+}}$-stat-C) block copolymer micelles with poly(styrene sulfonate) as models for tunable gene delivery vectors

We investigate the complexation of poly(styrene sulfonate) with micelles with mixed cationic/hydrophilic coronas as models for tunable gene delivery vectors. The micelles are self-assembled from AB$^{\mathrm{+}}$, AB$^{\mathrm{+}}$C, ACB$^{\mathrm{+}}$, and A(B$^{\mathrm{+}}$-\textit{stat}-C) block polymer architectures, where the hydrophobic A blocks (poly(styrene)) form the micelle cores, and the cationic B blocks (poly(dimethylamino ethyl methacrylate)) and hydrophilic, nonionic C blocks (poly(poly(ethylene glycol) methyl ether methacrylate)) form the coronas. We find that hydrophilic units do not change the colloidal stability of the complexes, and complexes based on all four micelle architectures form broad, multimodal size distributions. While complexes based on the AB$^{\mathrm{+}}$, AB$^{\mathrm{+}}$C, and ACB$^{\mathrm{+}}$polymer architectures are kinetically trapped at low ionic strength, however, those based on the A(B$^{\mathrm{+}}$-stat-C) architecture rapidly rearrange into single-micelle complexes when the linear polyanion is in excess. This suggests that the randomly-placed hydrophilic units break up the ion pairing between the cationic and anionic chains and promote formation of over-charged complexes. Design of the micelle architecture may thus provide a powerful way control the structure and stability of micelle-polyelectrolyte complexes for gene delivery applications.   

Computational Insight into Solvent Effects on Conformation and Assembly of Structured Ionic Polymer

Structured ionomers are in the core of numerous current and potential new applications including clean energy, water purification membranes and sensors. The ability to facilitate ions and solvents transport is a key to their function and is controlled by their structure. One effective path for structural control is tuning their conformation by solvent interactions. Here, the confirmation and association of an ABCBA co-polymer where C is a randomly sulfonated polystyrene with sulfonation fractions $f \quad =$ 0 to 0.55, B is poly (ethylene-r-propylene), and A is poly (t-butyl styrene), in n-propanol are studied by molecular dynamic simulation. In contrast to the collapsed conformation of the ionizable block in hydrophobic solvents, we find that it remains swollen. Similar to hydrophobic solutions the co-polymers aggregate to form an ionizable core surrounded by extended hydrophobic chains. In contrast to the ``locked-in'' ionizable segments observed in cyclohexane/heptane, here the ionic clusters remain dynamic.   

Continuous monitoring of structural dynamics in polymer assemblies.

Due to their flexibility, optical methods are preferred approaches for monitoring the dynamics and mechanical properties of scattering systems such as polymer solutions, colloidal suspensions, and complex media in general. In particular, their potential noninvasiveness is critical for the passive assessment of dynamic processes. Practical implementations however suffer sometimes from limitations due to effects such as multiple scattering or strong attenuation. Here we introduce an optical technique that overcomes some of these limitations and permits accessing the dynamics of complex colloidal systems under realistic conditions and inherent external influences. This interferometric technique operates based on the coherence-gated isolation of single scattering and allows for the spatially-resolved evaluation of the system's dynamics in optically isolated picoliter-sized volumes. This effective isolation permits a fully passive characterization of nonstationary dynamic processes in complex systems including aggregation and self-assembling, sedimentation, structural evolution and phase transitions, interface dynamics, and dynamics in inhomogeneous or stratified solvents.   

Inclusion Kinetics of Polyrotaxanes

Inclusion complex (IC) formation of $\alpha$-cyclodextrin ($\alpha$-CD) and poly(ethylene glycol) (PEG) brush in water was investigated by Surface Plasmon Resonance Spectroscopy(SPR), neutron reflectometry(NR) and grazing incident wide angle X-ray scattering(GISANS). Spontaneous IC formation of $\alpha$-CD with PEG (polyrotaxanes) is believed to be due to hydrophobic interaction between the hydrophobic interior of $\alpha$-CD and PEG; however, the detail of the IC formation kinetics has not been observed because IC formation results in aggregation and precipitation of the complex. SPR revealed that IC formation occurs after induction period, which often appears in crystallization. When concentration of $\alpha$-CD solution is 10\%, IC consisting randomly oriented $\alpha$-CD polycrystal appeared. In contrast, when the concentration of $\alpha$-CD solution is 5\%, a uniform 10-nm-thick IC layer with $\alpha$-CD stacked perpendicular to the substrate appeared. 10-nm-thick IC was also found in the diluted PEG brush in contact with a 10\% $\alpha$-CD solution. The characteristic 10-nm-thick layer is related to the folded crystalline structure of $\alpha$-CD on PEG brush. Such crystallization was proved to be the main driving force for IC formation.   

Crystallization in Micellar Cores: confinement effects and dynamics

It is well known that liquids confined to small nanoscopic pores and droplets exhibit thermal behavior very different from bulk samples. Here we demonstrate that n-alkanes forming 2-3 nm small micellar cores are considerably affected by confinement in analogue with hard confined systems. We study micelles form by self-assembly of a series of well-defined n-Alkyl-PEO polymers in aqueous solutions [1]. By using small-angle X-ray scattering (SAXS), densiometry and differential scanning calorimetry (DSC), we show that n-alkane exhibit a first-order phase transition i.e. melting. Correlating the structural and thermodynamic data, we find that a melting depression can be accurately described by the Gibbs-Thomson equation [2]. $\backslash $f1 The effect of core crystallinity on the molecular exchange kinetics is investigated using time-resolved small-angle neutron scattering (TR-SANS) [3-6]. We show that there are considerable entropic and enthalpic contributions from the chain packing that affect the kinetic stability of micelles. [7]$\backslash $pard[1] T. Zinn \textit{et al}., \textit{Soft Matter}, 2014, \textbf{10}, 5212.$\backslash $pard[2] T. Zinn, L. Willner and R. Lund, \textit{Phys. Rev. Lett.} \textbf{113} (2014) 238305.3] R. Lund, L. Willner, D. Richter, \textit{Adv. Polym. Sci.} \textbf{204} (2013) 51.5] R. Lund\textit{ et al}. Phys. Rev. Lett., 2006, 96, 068302.6] S.-H. Choi \textit{et al}. \textit{Phys. Rev. Lett.} 104 (2010) 1.7] T. Zinn, L. Willner, V. Pipich, D. Richter and R. Lund, \textit{ACS Macro Lett.}, 2015, \textbf{4}, 651--655.   

Assembly, Conformation, and Thermodynamics of Star-Branched Poly(N-isopropylacrylamide) (PNIPAM) in Solution

The synthetic route and molecular architecture of poly(N-isopropylacrylamide) (PNIPAM) affects its assembly and thermoresponsive behavior. Here, the structure and assembly of star-branched PNIPAM is studied in water using a new model based on the random phase approximation (RPA), and small-angle neutron scattering (SANS) for $f$ = 3, 4, and 6 arm star polymers, synthesized using either click chemistry, RAFT, or ATRP. Unlike linear PNIPAM, we find that for star-branched polymers, the radius of gyration scales with the degree of polymerization $N$ as $R_{g} \sim N^{0.65}$— indicating physical excluded volume effects in the polymer chain conformation. $R_{g}$ decreases monotonically as the system temperature approaches the lower critical solution temperature (LCST). Interesting, PNIPAM chains begin to associate well below the LCST, depending on whether the polymers were synthesized via ATRP or RAFT, and a strong structure factor peak can be observed as T increases. Finally, the Flory-Huggins interaction parameters between PNIPAM and water are extracted from the scattering data, and indicate that molecular architecture does not substantially influence the interaction between water and PNIPAM. These observations are compared to recent studies in the literature.   

Monodisperse Block Copolymer Particles with Controllable Size, Shape, and Nanostructure.

Shape-anisotropic particles are important class of novel colloidal building block for their functionality is more strongly governed by their shape, size and nanostructure compared to conventional spherical particles. Recently, facile strategy for producing non-spherical polymeric particles by interfacial engineering received significant attention. However, achieving uniform size distribution of particles together with controlled shape and nanostructure has not been achieved. Here, we introduce versatile system for producing monodisperse BCP particles with controlled size, shape and morphology. Polystyrene-b-polybutadiene (PS-b-PB) self-assembled to either onion-like or striped ellipsoid particle, where final structure is governed by amount of adsorbed sodium dodecyl sulfate (SDS) surfactant at the particle/surrounding interface. Further control of molecular weight and particle size enabled fine-tuning of aspect ratio of ellipsoid particle. Underlying physics of free energy for morphology formation and entropic penalty associated with bending BCP chains strongly affects particle structure and specification.   

Shape-designed single-polymer micelles: a proof-of-concept simulation

Much effort has been directed towards self-assembling nanostructures. Strong, local interactions between specific building blocks often determine these structures (e.g., globular proteins). We seek to produce designed structures that are instead determined by collective effects of weak interactions (e.g., surfactant self-assembly). Such structures may reversibly change conformation or disassemble in response to changing solvent conditions, and, being soft, have potential to adapt to fluctuating or unknown application-imposed shape requirements. Concretely, we aim to realize such a structure in the form of a single polymer micelle---an amphiphilic polymer exhibiting a condensed, phase-segregated conformation when immersed in solvent. Connecting all amphiphiles into a single chain provides geometric constraints controlling the surface curvature profile, thus dictating a non-trivial shape. We present 2D Monte Carlo simulation results demonstrating the feasibility of such soft, shape-designed micelles. Preliminary results demonstrate a stable concave ``dimple" in a micelle composed of a single A-B multiblock linear copolymer. We discuss both current limitations on shape robustness and effects of block asymmetry, block molecular weights and overall chain length on micelle shape.   

Mechanism of polymer nanoparticle formation by nanoprecipitation

Nanoprecipitation method is a simple and convenient way to produce nanoparticles from polymers in solution. The control of nanoparticle size and size distribution plays a pivotal role in the use of nanoprecipitation for drug delivery. We investigated various factors and initial conditions that affect the particle size, such as the initial solute concentration, solvent/non-solvent ratio and the molecular weight of the polymer samples. The results shed light on the mechanisms of particle formation and phase separation which occurs in nanoprecipitation. Spinodal decomposition, which takes place as a result of a quench to a sufficiently high supersaturation, is believed to be the main mechanism that governs the phase separation. In addition, the glass transition of the polymers will also be a key factor that contributes to the thermodynamics and kinetics of the phase separation and the resulting particle size and morphology.   

Assembly and Structural Evolution of Micelleplexes

Cationic micelles complex with DNA to form micelleplexes, which are attractive vehicles for gene delivery. We investigate the formation and structural evolution of micelleplexes in buffered solutions. The micelles are composed of poly((2-dimethylamino)ethyl methacrylate)-block-poly(n-butyl methacrylate). The formation of the micelleplexes is monitored via turbidimetric titration. With DNA oligomers, solutions of the complexes are homogeneous until near the charge neutral point, at which point the complexes precipitate. With plasmid DNA, more than a stoichiometric amount of DNA is needed to reach the inhomogeneous region, which suggests that binding is partially inhibited. This inhibition is not fully relieved when the plasmid DNA is linearized, suggesting that the stiffness of the DNA is the main source of the inhibition. With micelles in excess, the micelleplexes formed at low ionic strength exhibit bimodal size distributions and remain stable in solution. With DNA in excess, soluble micelleplexes aggregate over time and precipitate. We explain the structural evolution of the micelleplexes as an interplay between kinetic trapping and thermodynamic equilibrium, and compare the results for DNA with those for a flexible polyanion.   

Soft Patchy Particles of Block Copolymers from Interface-Engineered Emulsions

We report a simple and practical method for creating colloidal patchy particles with a variety of three-dimensional shapes~\textit{via}~the evaporation-induced assembly of polystyrene-$b$-poly(4-vinylpyridine) (PS-$b$-P4VP) block copolymer (BCP) in an oil-in-water emulsion. Depending on the particle volume, a series of patchy particles in the shapes of snowmen, dumbbells, triangles, tetrahedra, and raspberry can be prepared, which are then precisely tuned by modulating the interfacial interaction at the particle/water interface using a mixture of two different surfactants. In this talk, theoretic calculations of free energy of the system based on the strong segregation theory(SST) will be mainly discussed to support the experimental observation of various soft patchy particles and identified the underlying principles of their formation with tunable 3D structures.