25 research outputs found
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 âlinearâ 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 âlinearâ 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
Role of Competitive PEOâWater and WaterâWater Hydrogen Bonding in Aqueous Solution PEO Behavior
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 PPOâPEO 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 10â18% (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 coreâcorona
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
The OrderâDisorder Transition and the Disordered Micelle Regime in Sphere-Forming Block Copolymer Melts
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 âless hydrophobicâ.
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 dipalmitoylÂphosphatidylÂcholine
(DPPC) and dihexanoylÂphosphatidylÂcholine (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 âround
vaseâ 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