Association behavior of binary polymer mixtures under elongational flow

The influence of elongational flow on the association behavior of binary mixtures of functionalized polymers capable of forming single reversible orientationally dependent bonds, such as hydrogen bonds, is studied analytically. Applying a mean-field approach with an external potential representing the effect of the elongational flow, the orientation distribution functions for the dumbbell model and the freely jointed model of a polymer chain were obtained. Two opposite factors determine the association of “linear” diblock copolymerlike chains: the unfavorable extra stretching under flow of associated polymer chains and the favorable orientation of the chains (segments) along the flow direction. The former dominates and the fraction of associated “linear” chains decreases with increasing flow rate. For mixtures of polymers which are capable of forming associated T-chains, the association also decreases, however, more slowly, and this time due to unfavorable orientational effects. If the formation of associated linear and T-polymers as well as complex linear/T-polymers is possible, a strong preference for the formation of associated T-chains is found. At high flow rates any type of association becomes unfavorable

The Influence of Elongational Flow on Hydrogen Bond Formation and Stability of the Homogeneous Phase of Binary Hydrogen-Bonded Polymer Blends

Macrophase separation tendency induced by flow in binary blends of polymers capable of single hydrogen bonding between one of the chain ends is studied analytically. To describe the conformational and orientational properties of a polymer chain a simple dumbbell model is applied. It is demonstrated that with an increase of flow rate the association rate decreases because of the extra stretching of the associated chain compared with the two initial homopolymer chains. On the other hand flow promotes association by improving the chain orientation for hydrogen bonding. As a result, at relatively weak flow the homogeneous state becomes less stable due to the decrease in the fraction of hydrogen bonded diblock copolymer-like chains. At larger flow rates the fraction of hydrogen bonded chains slightly increases enhancing to some extend the stability of the homogeneous phase.

Molecular Structure and Co-solvent Distribution in PPO–PEO and Pluronic Micelles

The structure and properties of micelles formed by diblock and triblock copolymers containing polypropylene oxide (PPO) and polyethylene oxide (PEO) in aqueous solutions are affected by chain architecture and have important implications for applications, e.g., in the biomedical area. Using atomistic molecular dynamics simulations, we investigate and compare the molecular structure of diblock copolymer PPO29PEO26, Pluronic L64, and reverse Pluronic 17R4 micelles formed by block copolymers of the same length and composition but different distributions of PPO and PEO blocks in pure aqueous solution or with 5% (by volume) added co-solvents. We show that while the diblock copolymer forms a tightly packed mostly spherical micelle, Pluronic L64 micelles are non-spherical and contain 10–18% (by volume) water in the loosely packed PPO core partially interpenetrated by the PEO block. Reverse Pluronic 17R4 micelles are rather small but relatively well-packed. Addition of 5% (by volume) alcohol to aqueous micelle solutions results in a minimal change in the case of ethanol, while addition of butanol or hexanol leads to an increase of water content in the core and alcohol accumulation at the core–corona interface for the PPO29PEO26 micelle. For L64 micelles, alcohol makes micelles more spherical but enhances defects, e.g., concentrates water in the core center or enhances PEO penetration, depending on the aggregation number. For 17R4 micelles, butanol and especially hexanol penetrate into the micelle core, swelling it. Even stronger core swelling occurs upon addition of 5% (by volume) isobutyric acid to aqueous solution of PPO29PEO26 micelles. We show that the extent of co-solvent penetration and its distribution within the micelles strongly depend on co-solvent hydrophobicity, the capability of hydrogen bond formation with the polymer and micelle architecture, factors that can affect micelle properties and performance in practical applications

Tadpole and Mixed Linear/Tadpole Micelles of Diblock Copolymers: Thermodynamics and Chain Exchange Kinetics

Chain architecture is known to control macromolecular self-assembly and furthermore affect in a more complex way nanostructure stability. Equilibrium properties and chain exchange kinetics between micelles formed by tadpole-shaped diblock copolymers containing a loop-shaped hydrophobic block and a linear hydrophilic block are investigated using dissipative particle dynamics simulations. We found that tadpoles form micelles of smaller size and aggregation number than the corresponding linear diblock copolymers and have faster chain exchange kinetics, demonstrating that chain architecture can alter its effective hydrophobicity. Similar observations are made for linear diblock copolymer with a less hydrophobic core block indicating that the more compact conformation of tadpole core-forming block makes it “less hydrophobic”. We show that tadpole and linear block copolymers form mixed micelles with tadpoles (or less hydrophobic chains) located on the periphery of the micelle core. The chain exchange kinetics between mixed micelles is found to be quicker than in linear diblock copolymer micelles and slower than in tadpole micelles. Tadpole escape or less hydrophobic chain exchange between mixed micelles occurs slower (in part due to the shielding role that these chains play) than in the corresponding pure micelles, while linear more hydrophobic chain exchange only slightly changes, suggesting that the exchange kinetics of the individual components can be affected differently by mixing

Lipid Nanodisc-Templated Self-Assembly of Gold Nanoparticles into Strings and Rings

Gold nanoparticles (AuNPs) exhibit strong fluorescent and electromagnetic properties, which can be enhanced upon clustering and used in therapeutic, imaging, and sensing applications. A combination of gold nanoparticles with lipid nanodiscs can be attractive for AuNP self-assembly and useful in biomedical applications. Using molecular dynamics simulations we show that lipid nanodiscs can serve as templates for AuNP clustering into rings and string-like structures. We demonstrate that equilibrium encapsulation of 1 nm hydrophobically modified AuNPs into lipid nanodiscs composed of a mixture of dipalmitoyl­phosphatidyl­choline (DPPC) and dihexanoyl­phosphatidyl­choline (DHPC) lipids occurs at the rim and results in formation of a ring of gold. The interior of the nanodisc is inaccessible to AuNPs due to the DPPC liquid crystalline order. With temperature increase the lipid order diminishes, initiating the nanodisc transformation into a vesicle, upon which encapsulated AuNPs cluster into a close-packed string or nanoring, thereby stalling the vesiculation process at a “round vase” or cup-like stage depending on the AuNP concentration. In contrast, encapsulation of AuNPs by an equilibrium lipid vesicle results in its deformation with randomly clustered AuNPs, in agreement with experimental observations. We characterize the AuNP cluster size and surface-to-surface pair distribution, both of which impact the AuNP luminescent properties. We investigate the effect of alkane tether length on the nanodisc stability and AuNP clustering inside the nanodiscs and vesicles. Our results show that lipid nanodiscs can enhance gold cluster formation, which can be further exploited in imaging applications