Session X9: Self Assembly of Polymers: Solutions, Confinement and External Fields

Role of corona block in molecular exchange in block copolymer micelles

Self-assembled block polymer micelles are used in a variety of applications including drug delivery and viscosity modification as additives to motor lubricants. Previous work with dilute solutions of poly(styrene-b-ethylene-alt-propylene) (PS-PEP) diblock copolymer micelle has resulted in a theoretical model that accounts for the dramatic influence of the PS core block length and dispersity on chain exchange kinetics in squalane, a selective neutral solvent for PEP. This model, which accounts for the significant effect of core block length and its polydispersity on chain exchange kinetics, does not address the role of the corona block length. This presentation will describe the consequences of varying the size of the PEP corona block at constant core molecular weight on the rate of chain exchange based on time-resolved small-angle neutron scattering (TR-SANS) measurements. These results have informed an improved model that explicitly includes a corona term.   

Understanding Unimer Exchange Processes in Block Copolymer Micelles using NMR Diffusometry, Time-Resolved NMR, and SANS

Our team seeks to understand dynamic behaviors of block copolymer micelles and their interplay with encapsulated cargo molecules. Quantifying unimer and cargo exchange rates micelles can provide critical information for determining mechanisms of unimer exchange as well as designing systems for specific cargo release dynamics. We are exploring the utility of NMR spectroscopy and diffusometry techniques as complements to existing SANS and fluorescence methods. One promising new method involves time-resolved NMR spin relaxation measurements, wherein mixing of fully protonated and 2H-labeled PEO-b-PCL micelles solutions shows an increase in spin-lattice relaxation time (T1) with time after mixing. This is due to a weakening in magnetic environment surrounding 1H spins as 2H-bearing unimers join fully protonated micelles. We are measuring time constants for unimer exchange of minutes to hours, and we expect to resolve times of \textless 1 min. This method can work on any solution NMR spectrometer and with minimal perturbation to chemical structure (as in dye-labelled fluorescence methods). Multimodal NMR can complement existing characterization tools, expanding and accelerating dynamics measurements for polymer micelle, nanogel, and nanoparticle developers.   

Linear Diblock Copolymer Micellization Kinetics Probed by Integrated Microfluidic Device and Small-angle X-ray Scattering

Polymeric nanoparticles (NPs) for delivery of active pharmaceutical compounds are under rapid development as the need for advanced drug-delivery systems increase. Micelles, self-assembled from amphiphilic block copolymers, attracted attention because of their biocompatibility, better stability, and potential functionalities of targeting delivery. Nanoprecipitation, driven by solvent replacement, is one of the major processes for generating polymeric NPs. In order to optimize the structure of the NPs, the kinetics of micellization and drug nucleation and growth need to be well understood. However, the structural evolution is challenging to follow experimentally. With the development of synchrotron X-ray sources, time-resolved SAXS (TR-SAXS) integrated with a stopped-flow apparatus has become possible with a temporal resolution in the 100 millisecond time range. In this study, we have integrated a microfluidic mixer with TR-SAXS to catch the millisecond kinetics. Regimes of nuclei formation, merging, and polymer insertion were observed.   

Simulation of Dynamical Processes in Block Copolymer Micelles

Micellar solutions exhibit a "fast" dynamical process associated with single-molecular insertion and expulsion and a "slow" process associated with birth and death of entire micelles. We report an analysis of simulations of highly asymmetric "hairy" micelles that is designed to quantify rates of both types of process, and to elucidate mechanisms. By modeling diffusion of the micelle aggregation number on a well characterized free energy surface, we are able to calculate and compare relaxation for different possible mechanisms for the slow process, which would otherwise be computationally inaccessible. This method allows us to compare the rate for step-wise association and dissociation to the rate of the competing mechanism of micelle fission and fusion.   

General Mechanism of Morphology Transition and Spreading Area-dependent Phase Diagram of Block Copolymer Self-assembly at the Air/Water Interface

Block copolymers (BCPs) can be self-assembled forming periodic nanostructures, which have been employed in many applications. While general agreements exist for the phase diagrams of BCP self-assembly in bulk or thin films, a fundamental understanding of BCP structures at the air/water interface still remain elusive. The current study explains morphology transition of BCPs with relative fraction of each block at the air/water interface: block fraction is the only parameter to control the morphology. In this study, we show morphology transitions from spherical to cylindrical and planar structures with neat polystyrene-b-poly(2-vinylpyridine) (PS-b-P2VP) via reducing the spreading area of BCP solution at the air/water interface. For example, PS-b-P2VP in a fixed block fraction known to form only spheres can experience sphere to cylinder or lamellar transitions depending on the spreading area at the air/water interface. Suggesting a new parameter to control the interfacial assembly of BCPs, a complete phase diagram is drawn with two paramters: relative block fraction and spreading area. We also explain the morphology transition with the combinational description of dewetting mechanism and spring effect of hydrophilic block.   

Rigid Polymers at Interface: Competition of Interfacial Forces and Molecular Architecture

Confinement of conjugated polymers to spontaneously formed aggregates and nanoparticles is strongly affected by their rigid architecture and determines their photophysics through conformational constraints. Their rigid architecture dominates spontaneous assemblies, whereas in nanoparticles the polymer chains are forced into far-from-equilibrium conformations. For many of their applications, conjugated polymers reside at solid interfaces where interactions arising from the polymer architecture compete with interfacial effects. AFM and X-ray of highly rigid dialkyl poly (para-phenylene ethynylene)s (PPEs) at an interface as the films are solvent annealed in presence of toluene vapor are compared with thermal annealing. With increasing annealing time, larger aggregates are formed for both, dictated by the inherent polymer architecture. Solvent annealing affects selectively the backbone and the substituting size chains impact the internal packing at early exposure times. Solvents unlock the interfacial interactions, forming architecture dominated assemblies.   

Polymer Architecture Effects in Confined Geometry: Molecular Dynamics Simulation Study

Luminescent rigid polymers confined into nanoparticles, or polydots, are emerging as a promising tool for nano medicine. The constrained architecture of a rigid backbone trapped in nano-dimensions results in photophysics that differs from that of spontaneously assembled rigid polymers. Incorporating ionizable functionalities in the polymers, often required for therapeutics, impacts the polymer conformation in solution. Here we report fully atomistic molecular dynamics simulations on the structure of dialkyl $p$-phenylene ethynylene confined into polydots. We find that the structure and thermal stability of polydots are sensitive to both the molecular weight $n$ and the carboxylation fraction $f$. At room temperature$,$ polydots remain confined regardless of $n$ and f$.$ However, as temperature is increased, polydots with lower $n$ or $f$ rearrange whereas polydots with higher $n$ or $f $remain confined, though no direct clustering of the ionic groups was observed.   

Study on the mesophase development of pressure-responsive ABC triblock copolymers

Here we focus on the revelation of new nanoscale morphologies for a molten compressible polymeric surfactant through a compressible self-consistent field approach. A linear ABC block copolymer is set to allow a disparity in the propensities for curved interfaces and in pressure responses of ij-pairs. Under these conditions, the copolymer evolves into noble morphologies at selected segregation levels such as networks with tetrapod connections, rectangularly packed cylinders in a 2-dimensional array, and also body-centered cubic phases. Those new structures are considered to turn up by interplay between disparity in the densities of block domains and packing frustration. Comparison with the classical mesophase structures is also given.   

Shifting the Phase Boundary with Electric Fields to Jump In and Out of the Phase Diagram at Constant Temperature

Understanding the phase behavior of polymer blends and block copolymers under the presence of electric fields is important for advanced applications containing electrodes such as organic photovoltaics and batteries, as well as for field-directed assembly and alignment of domains. We have recently demonstrated that electric fields enhance the miscibility of polystyrene (PS) / poly(vinyl methyl ether blends) (PVME) blends [J. Chem. Phys. 2014, 141, 134908], shifting the phase separation temperature Ts(E) up by 13.5 ± 1.4 K for electric field strengths of E = 1.7 MV/m. Experimentally this effect is much larger than the traditional predictions from adding the standard electrostatic energy term for mixtures to the free energy of mixing. However, accounting for the energy penalty of dielectric interfaces between domains created during phase separation, the primary factor that drives alignment of domains, may also be responsible for the change in miscibility. Here we investigate the dynamics of repeatedly jumping the system from the one-phase to the two-phase region and demonstrate that this can be done at a constant temperature simply by turning the electric field on and off, illustrating electric-field-induced remixing in the two-phase region.   

Electric Field Effects in a Polarizable Model of Diblock Copolymer Melts

We present a new molecularly-informed statistical field theory model of inhomogeneous polarizable soft matter. Using a Drude oscillator model, we construct polarizable, and optionally charged, molecular units. Within this field theory, models containing an arbitrary number of small-molecule or polymeric components can be constructed. This framework is amenable to a number of theoretical techniques, including analysis within the Gaussian fluctuation approximation and fully-fluctuating field-theoretic simulation. We construct a diblock copolymer melt within the proposed framework, distinguishing `A' and `B' monomers only by their respective polarizabilities. Using the former analytical methodology, we demonstrate that electrostatic fluctuations can induce phase separation for monomers of contrasting polarizability. We subsequently apply the latter numerical technique to investigate the order-disorder transition of a block copolymer melt experiencing an externally-applied electric field, and we consider the role of coupled electrostatic and compositional fluctuations therein.   

Understanding of the charge carriers dynamics in poly(p-phenylene) under external electric and magnetic fields.

Conducting polymers have emerged as highly attractive materials with very diverse applications among which, the charge and spin transport are of central importance. Due to the large electron-phonon coupling in these one-dimensional systems, charge is thought to be transported mainly in the form of polarons, in which trapped charge localizes itself with an associated structural distortion. The dynamics of these charge carriers which carry spin \textonehalf (polarons) are complex and an improved insight into the underlying processes is vital for improving device performance. We address the effect of electric and magnetic fields on the electronic excitations in doped poly(p-phenylene), PPP, using the Su-Schrieffer-Heeger (SSH) tight-binding model. The electric field is included in the Hamiltonian through the time-dependent vector potential via Peierls substitution of the phase factor and magnetic field via Zeeman term. The Zeeman splitting caused by the magnetic field breaks the spin degeneracy of the energy levels. Our calculations reveal three regimes of electric field, based on the charge-phonon coupling, the coupling of charge to acoustic and optical phonons as well as dissociation of the polarons.