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Register a new userBundle formation in parallel aligned polymers with competing interactions
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Aggregation of like-charged polymers is widely observed in biological and soft matter systems. In many systems, bundles are formed when a short-range attraction of diverse physical origin like charge-bridging, hydrogen-bonding or hydrophobic interaction, overcomes the longer- range charge repulsion. In this Letter, we present a general mechanism of bundle formation in these systems as the breaking of the translational invariance in parallel aligned polymers with competing interactions of this type. We derive a criterion for finite-sized bundle formation as well as for macroscopic phase separation (formation of infinite bundles).
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When polyelectrolyte-neutral block copolymers are mixed in solutions to oppositely charged species (e.g. surfactant micelles, macromolecules, proteins etc), there is the formation of stable supermicellar aggregates combining both components. The resulting colloidal complexes exhibit a core-shell structure and the mechanism yielding to their formation is electrostatic self-assembly. In this contribution, we report on the structural properties of supermicellar aggregates made from yttrium-based inorganic nanoparticles (radius 2 nm) and polyelectrolyte-neutral block copolymers in aqueous solutions. The yttrium hydroxyacetate particles were chosen as a model system for inorganic colloids, and also for their use in industrial applications as precursors for ceramic and opto-electronic materials. The copolymers placed under scrutiny are the water soluble and asymmetric poly(sodium acrylate)poly(acrylamide) diblocks. Using static and dynamical light scattering experiments, we demonstrate the analogy between surfactant micelles and nanoparticles in the complexation phenomenon with oppositely charged polymers. We also determine the sizes and the aggregation numbers of the hybrid organic-inorganic complexes. Several additional properties are discussed, such as the remarkable stability of the hybrid aggregates and the dependence of their sizes on the mixing conditions.
We consider fluids where the attractive interaction at distances slightly larger than the particle size is dominated at larger distances by a repulsive contribution. A previous investigation of the effects of the competition between attraction and repulsion on the liquid-vapour transition and on the correlations is extended to the study of the stability of liquid-vapour phase separation with respect to freezing. We find that this long-range repulsive part of the interaction expands the region where the fluid-solid transition preempts the liquid-vapour one, so that the critical point becomes metastable at longer attraction ranges than those required for purely attractive potentials. Moreover, the large density fluctuations that occur near the liquid-vapour critical point are greatly enhanced by the competition between attractive and repulsive forces, and encompass a much wider region than in the attractive case. The decay of correlations for states where the compressibility is large is governed by two characteristic lengths, and the usual Ornstein-Zernike picture breaks down except for the very neighborhood of the critical point, where one length reduces to the commonly adopted correlation length, while the other one saturates at a finite value.
The thermodynamic and elastic properties of a flexible polymer in the presence of dipole interactions are studied via Monte Carlo simulations. The structural coil-globular, solid-globular, and solid-solid transitions are mapped in the hyperphase diagram, parameterized by the dipole concentration, $eta$, and temperature, $T$. Polymer flexibility is usually quantified by the persistent length, $ell_p$, which is defined as the length on which the bond-bond correlation is lost. Non-monotonic flexibility of polymeric complexes as a function of $eta$ has been interpreted as a cooperative effect under the Worm-Like Chain model. Instead of the usual exponential behavior, $langle Cleft(kright)ranglepropto e^{-k/ell_p}$, here we show that the bond-bond correlation follows a power law decay, $langle Cleft(kright)rangleapprox c_0k^{-omega}$. The power law regime holds even at the coil-globular transition, where a Gaussian limit is expected, originated from non-leading terms due to monomer-monomer connectivity. The exponent $omega$ monotonically converges to the mbox{SAW} limit for large $eta$, if the isotherm pathway is constructed at the coil phase. The deviation from ideality in better probed at the chain segment size, and the expected $Theta-$condition at the $(T,eta)$ pathway near the coil-globular transition is not observed.
A simple model was constructed to describe the polar ordering of non-centrosymmetric supramolecular aggregates formed by self assembling triblock rodcoil polymers. The aggregates are modeled as dipoles in a lattice with an Ising-like penalty associated with reversing the orientation of nearest neighbor dipoles. The choice of the potentials is based on experimental results and structural features of the supramolecular objects. For films of finite thickness, we find a periodic structure along an arbitrary direction perpendicular to the substrate normal, where the repeat unit is composed of two equal width domains with dipole up and dipole down configuration. When a short range interaction between the surface and the dipoles is included the balance between the up and down dipole domains is broken. Our results suggest that due to surface effects, films of finite thickness have a none zero macroscopic polarization, and that the polarization per unit volume appears to be a function of film thickness.
Using monomer-resolved Molecular Dynamics simulations and theoretical arguments based on the radial dependence of the osmotic pressure in the interior of a star, we systematically investigate the effective interactions between hard, colloidal particles and star polymers in a good solvent. The relevant parameters are the size ratio q between the stars and the colloids, as well as the number of polymeric arms f (functionality) attached to the common center of the star. By covering a wide range of qs ranging from zero (star against a flat wall) up to about 0.75, we establish analytical forms for the star-colloid interaction which are in excellent agreement with simulation results. A modified expression for the star-star interaction for low functionalities, f < 10 is also introduced.
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