Solvents Of Varying Quality Research Articles

Spherical Polymer Brushes in Solvents of Variable Quality: An Experimental Insight by TEM Imaging

The spherical micelle and vesicle composed of [PW12O40](3-) and poly(styrene-b-4-vinylpyridinium methyl iodide) are regarded as a model system to study spherical polymer brushes (SPBs) in solvents of various quality. The pure repulsions occur for the brush chains in the chloroform solution and chloroform/methanol mixture with a methanol volume ratio of 9.1%, where the grafted polystyrene chains have a relatively extended conformation. Further increase in the methanol concentrations leads to the presence of the intra/inter-brush van der Waals attractions. Transmission electron microscopy studies show that there is a coexistence of isolated and oligomeric SPBs and multi-SPB aggregates (MSAs) with the methanol content from 17% to 23%. Only MSAs are detected with the increasing methanol content. Both the corona and core shrink significantly in the isolated and oligomeric SPBs and MSAs. The full interpenetration of the grafted chains is observed between the cores in the oligomeric SPBs and MSAs.

Impact of solvation on equilibrium conformation of polymer brushes in solvent mixtures

We have investigated the response of dextran brushes in aqueous dimethyl sulfoxide (DMSO) mixtures under normal compression using the extended surface forces apparatus. The hydrophilic, tethered polymer chains extend significantly in the studied aqueous mixtures and two brush-covered surfaces exhibit repulsive forces upon compression against each other; these attributes are relevant for both anti-biofouling and lubrication properties. An estimation of the height of the dextran brushes with varying solvent quality is made from measurements of the onset of the repulsion: the brush swells significantly in pure water, a decrease in swelling being observed with an increase in DMSO concentration (up to ∼50 vol%), followed by a more pronounced swelling at higher DMSO concentrations. This behavior is in qualitative agreement with the solubility of bulk dextran polymer in aqueous DMSO solutions. Besides the conformational changes, an increase in the compressibility of the dextran brushes upon increasing the DMSO concentration was observed and explained by the change in the solvation structure of the polymer brush. We describe the equilibrium configuration of the polymer brush under marginal/good solvent conditions with the help of a mean-field approach, considering both solvent quality and a solvation-structure-dependent bond length. This work shows that an appropriate selection of polymer brush and solvent mixture can be used to tune the conformational properties of the polymer brushes at the interface, as well as the consequent repulsive and frictional forces.

Interaction Between Polymer Brush-Coated Spherical Nanoparticles: Effect of Solvent Quality

The interaction between two spherical polymer brushes in solvents of variable quality is studied by molecular dynamics simulation and by self-consistent field theory, varying both the radius of the spherical particles and their distance, as well as the grafting density and the chain length of the end-grafted flexible polymer chains. Both the potential of mean force between the particles as a function of their distance is computed, for various choices of the parameters mentioned above, and the structural characteristics are discussed (density profiles, average end-to-end distance of grafted chains, etc.) It is found that for rather short chain lengths and not too large grafting densities, isolated spherical brushes in the poor solvent regime exhibit incomplete coverage of the nanoparticle, rather than a complete coverage, i.e., a core–shell structure where the nanoparticle is covered homogeneously by a dense polymeric film. This latter case occurs for long chains and/or high grafting densities. When two su...

Phase diagrams of knotted and unknotted ring polymers

The phase diagram for a lattice ring polymer under applied force, with variable solvent quality, for different topological knot states, is determined for the first time. In addition to eliminating pseudophases where the polymer is flattened into a single layer, it is found that nontrivial knots result in additional pseudophases under tensile force conditions.

Effect of the solvent quality on the structural rearrangement of spherical brushes: coarse-grained models

A coarse-grained model for flexible polymers end-grafted to repulsive spherical nanoparticles is studied for various polymer lengths, grafting densities, and nanoparticle sizes by molecular dynamics simulations, considering variable solvent quality in the framework of an implicit solvent treatment. Below the theta point, the tuning of the temperature strongly influences the coverage of the nanoparticle surface by collapsed single chains or clusters of several chains. The shape and size of the aggregates depend on the number of monomers and surface density of the polymers. Specifically we analyzed the effect of the solvent quality on the density profiles and radius of gyration of the single molecule. The stretching of the chains parallel to the surface, required for the cluster formation, leads to a slightly non-monotonic temperature variation of the polymer linear dimensions at intermediate chain lengths. For theta conditions, however, the chains are still stretched in the radial direction. In particular for long chains a crossover to the star polymer limit occurs that is studied in detail with self-consistent field theory. At a theoretical level we extended our study exploring the case of a spherical brush embedded in a polymer melt. For different choices of the Flory–Huggins parameter between free and grafted chains, the amount of monomers of the free chains adsorbed on the nanoparticle surface is predicted via self-consistent field calculations.

Computer simulation of bottle-brush polymers with flexible backbone: Good solvent versus theta solvent conditions

By molecular dynamics simulation of a coarse-grained bead-spring-type model for a cylindrical molecular brush with a backbone chain of N(b) effective monomers to which with grafting density σ side chains with N effective monomers are tethered, several characteristic length scales are studied for variable solvent quality. Side chain lengths are in the range 5 ≤ N ≤ 40, backbone chain lengths are in the range 50 ≤ N(b) ≤ 200, and we perform a comparison to results for the bond fluctuation model on the simple cubic lattice (for which much longer chains are accessible, N(b) ≤ 1027, and which corresponds to an athermal, very good, solvent). We obtain linear dimensions of the side chains and the backbone chain and discuss their N-dependence in terms of power laws and the associated effective exponents. We show that even at the theta point the side chains are considerably stretched, their linear dimension depending on the solvent quality only weakly. Effective persistence lengths are extracted both from the orientational correlations and from the backbone end-to-end distance; it is shown that different measures of the persistence length (which would all agree for Gaussian chains) are not mutually consistent with each other and depend distinctly both on N(b) and the solvent quality. A brief discussion of pertinent experiments is given.

Phase diagram of the uniaxial and biaxial soft–core Gay–Berne model

Classical molecular dynamics simulations have been used to explore the phase diagrams for a family of attractive-repulsive soft-core Gay-Berne models [R. Berardi, C. Zannoni, J. S. Lintuvuori, and M. R. Wilson, J. Chem. Phys. 131, 174107 (2009)] and determine the effect of particle softness, i.e., of a moderately repulsive short-range interaction, on the order parameters and phase behaviour of model systems of uniaxial and biaxial ellipsoidal particles. We have found that isotropic, uniaxial, and biaxial nematic and smectic phases are obtained for the model. Extensive calculations of the nematic region of the phase diagram show that endowing mesogenic particles with such soft repulsive interactions affect the stability range of the nematic phases, and in the case of phase biaxiality it also shifts it to lower temperatures. For colloidal particles, stabilised by surface functionalisation, (e.g., with polymer chains), we suggest that it should be possible to tune liquid crystal behaviour to increase the range of stability of uniaxial and biaxial phases (by varying solvent quality). We calculate second virial coefficients and show that they are a useful means of characterising the change in effective softness for such systems. For thermotropic liquid crystals, the introduction of softness in the interactions between mesogens with overall biaxial shape (e.g., through appropriate conformational flexibility) could provide a pathway for the actual chemical synthesis of stable room-temperature biaxial nematics.

Nanopores with Solvent-Sensitive Polymer Brushes: A Dissipative Particle Dynamics Simulation

Dissipative particle dynamics (DPD) is used to investigate solvent flow controlled by a cylindrical nanofluidic channel grafted with polymer brushes. Results for monomer and solvent beads density for different solvent quality were obtained, as well as the height and inclination angle of the polymer brushes in Poiseuille flow. The velocity profile and solvent permeability in Poiseuille flow for varying solvent quality were also examined. The polymer brushes expand and inhibit flow under good solvent conditions. However, under poor solvent conditions the chains collapse to open the channel. Additionally, the dependence of the permeability on the grafting density is given. The results indicate that the system is capable of changing the permeability and flow rate drastically upon changing the solvent quality.

Universal Behavior of Dendrimer Solutions

The effect of molecular topology on the structural and dynamic properties of dendrimers with different generation numbers and molecular weights is studied in solvents of varying solvent quality and...

Stable responsive diblock copolymer micelles for rheology control

Stable block copolymer micelles with crosslinked rubbery cores serve as model soft colloids with tunable rheology. We demonstrate this by studying systematically two such micelles: one with a small polyisoprene core and long polystyrene hairs that dominate the response of the system, and another with a larger polybutadiene core and shorter polystyrene hairs, exhibiting colloid-like behavior. In addition to mass concentration, we use temperature as a means to vary the volume fraction and show that achieving the same volume fraction by these two different routes yields different macroscopic properties. The number density of the particles and osmotic pressure effects are the origin of the difference. The response of these micelles to varying solvent quality conditions has unexpected rheological consequences: remarkably, for concentrated colloid-like micelles, the viscosity exhibits a non-monotonic temperature dependence. This is attributed to particle swelling and changing particle interactions with temperature. It is thus evident that such systems provide ways for tailoring the flow of soft colloids.

Molecular dynamics study of polymer conformation as a function of concentration and solvent quality

A coarse-grained model for molecular dynamics simulations of polymer solutions with variable solvent quality is proposed. This model allows solvent quality to be varied over the whole range from very poor to very good solvent conditions by varying a single parameter. The model has several advantages. All interactions are short ranged and repulsive, making the model very computationally efficient compared with other explicit solvent models that include the long-ranged attractive part of the interactions; the solvent is included explicitly, ensuring that the theta condition corresponds to a genuine cancellation of the solvent-mediated polymer-polymer interactions; and hydrodynamic interactions and entanglement can occur for all solvent conditions. The theta point is determined and the conformational properties of a 50-bead chain system are investigated over the whole concentration range from the dilute limit to the melt as a function of solvent quality.

Bethe approximation for a DNA-like self-avoiding walk model with variable solvent quality

The phase diagram and critical behavior of a simple toy model for DNA zipping/unzipping are examined in the framework of the Bethe approximation. The effects of solvent quality are included and found to lead to a variety of different thermodynamic behaviors.

Interactions between polymer brushes in solvents of variable quality: A density functional theory study

We present a density functional theory study of interactions between sterically stabilized colloidal particles in solvents of variable quality. Both flat and spherical polymer brushes are considered, as well as both monatomic and polymeric solvents. It is shown that the interaction between sterically stabilized particles can be tuned from repulsive to attractive by varying the solvent quality, the relative length of free and grafted chains, and by employing a mixed brush consisting of both well and poorly solvated chains.

One‐ and Two‐Component Bottle‐Brush Polymers: Simulations Compared to Theoretical Predictions

AbstractScaling predictions for bottle‐brush polymers with a rigid backbone and flexible side chains under good solvent conditions are discussed and their validity is assessed by a comparison with Monte Carlo simulations of a simple lattice model. It is shown that typically only a rather weak stretching of the side chains is realized, and then the scaling predictions are not applicable. Also two‐component bottle brush polymers are considered, where two types (A,B) of side chains are grafted, assuming that monomers of different kind repel each other. In this case, variable solvent quality is allowed. Theories predict “Janus cylinder”‐type phase separation along the backbone in this case. The Monte Carlo simulations, using the pruned‐enriched Rosenbluth method (PERM) give evidence that the phase separation between an A‐rich part of the cylindrical molecule and a B‐rich part can only occur locally. The correlation length of this microphase separation can be controlled by the solvent quality. This lack of a phase transition is interpreted by an analogy with models for ferromagnets in one space dimension.magnified image

Influences of local polymer–solvent π–π-interaction on dynamics of phenyl ring rotation and its role on photophysics of conjugated polymer

In this report, the role of local polymer–solvent π–π-interaction on rotational dynamics of phenyl rings of poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylenevinylene] (MEH-PPV) is examined by utilizing nuclear magnetic resonance (NMR) spectroscopy. We have found that an aromatic solvent can couple strongly with the phenyl rings via favorable local π–π-interaction, causing a restriction of the ring rotation. The dynamic process takes place at much faster rate in a non-aromatic solvent. NMR line shape analysis indicates the existence of two side chain configurations with relatively long life-time in toluene and pyridine while a single time-averaged configuration is detected in chloroform and tetrahydrofuran. Addition of chloroform or cyclohexane to a solution of MEH-PPV in toluene is accompanied by an increase of the rotational dynamics. This indicates that the expansion or collapse of main chain upon varying solvent quality play a minor role on the rotational dynamics. The relationship between the dynamics of ring rotation and photophysics of MEH-PPV in solution is discussed.

Simulation of Phase Transitions of Single Polymer Chains: Recent Advances

AbstractSummary:The behaviour of a flexible polymer chain in solvents of variable quality in dilute solution is discussed both in the bulk and in the presence of an adsorbing wall. Monte Carlo simulations of coarse‐grained bead‐spring models and of the bond fluctuation model are presented and interpreted in terms of phenomenological theories and scaling concepts. Particular attention is paid to the behaviour of the polymer chain when the temperature of the polymer solution gets lower than the Theta temperature. It is argued that the adsorption transition line at the Theta temperature splits into lines of wetting and drying transitions of polymer globules attached to the wall. In addition, it is shown that the coil‐globule transition is followed by a second transition, which in the bond fluctuation model is a crystallization into a regular lattice structure. Performing a finite size scaling analysis on the two transitions it is shown that (for the chosen model) both transitions coincide in the thermodynamic limit, corresponding to a direct collapse of the random coil into the crystal without intermediate coil‐globule transition. The implications of this result for the standard mean field or tricritical theory of the coil‐globule transition based on a truncated virial expansion are discussed.

Monte Carlo simulation of dendrimers in variable solvent quality.

We study via lattice Monte Carlo simulation and Flory theory the properties of g=1-6 dendrimers in variable solvent quality. For all the generations studied, we find that the radius of gyration R(g) collapses significantly (factor of 2) going from athermal to extreme poor solvent conditions, indicating that varying solvent quality is an effective means of controlling dendrimer size. We also find that in athermal, theta, and extreme poor solvent conditions, the radius of gyration of dendrimers scales with the total number of monomers roughly as R(g) approximately N(1/3). However, a more careful analysis shows that in athermal and theta solvents, there is, in fact, a small but systematic deviation of R(g) from R(g) approximately N(1/3) scaling and the simulation data is described better by the Flory theory prediction of R(g) approximately N(1/5)[(g+1)m](2/5) in athermal solvents and R(g) approximately N(1/4)[(g+1)m](1/4) in theta solvents. We also find for our simulation data that stronger deviations from constant density scaling are possible, with scaling behavior as shallow as R(g) approximately N(0.26) possible for solvent conditions in between theta and the completely collapsed state. It is evident therefore that dendrimers do not obey (or even approximately obey) R(g) approximately N(1/3) scaling under all solvent conditions. Under all solvent conditions, we find that the intramolecular density is dense corelike (i.e., the density maximum is in the interior of the dendrimer) and terminal groups are delocalized throughout the dendrimer.

On the kinetics of nematic ordering in solutions of semiflexible macromolecules: a Monte Carlo simulation

Abstract The occurrence of nematic liquid-crystalline ordering in semidilute and concentrated solutions of semiflexible macromolecules has been studied by means of grand canonical Monte Carlo computer simulations using the bond fluctuation model and the configurational bias scheme. Chain length was equal to 20 monomer units, while the persistence length was about 5 monomer units. We used an intramolecular stiffness potential depending on the angle between successive bonds along the chain and on the bond length, and an attractive interaction between monomer units to model variable solvent quality. We have monitored the processes of appearance and destruction of monodomain and multidomain nematic configurations. Our findings are that the first stages of both the ordering and disordering processes occur upon sufficient oversaturation through the spinodal ordering scenario. Possible screening of nucleation processes and the applicability of our model to real kinetics are discussed. Results of our simulations are visualized in six movies.

Phase diagram of solutions of stiff-chain macromolecules: A Monte Carlo simulation

The phase diagram of solutions of semiflexible macromolecules consisting of N=20 effective monomer units has been studied by means of grand canonical Monte Carlo computer simulation using the bond fluctuation model and the configurational bias scheme. Two different types of interactions were taken into account: an intramolecular potential controlling the chain stiffness and an attractive interaction between effective monomer units to model variable solvent quality. A very broad region of density from dilute solution to melt has been covered in the simulation. The phase diagram shows a strong increase of the density difference between isotropic dilute and nematic dense phases upon increasing attraction between monomer units (i.e., decreasing the temperature). We discuss methods of locating the isotropic–nematic transition point and the effects of the vicinity of the polymer–solvent critical point on the properties of the liquid crystalline transition.

Influence of solvent quality on effective pair potentials between polymers in solution.

Solutions of interacting linear polymers are mapped onto a system of "soft" spherical particles interacting via an effective pair potential. This coarse-graining reduces the individual monomer-level description to a problem involving only the center of mass (c.m.) of the polymer coils. The effective pair potentials are derived by inverting the c.m. pair distribution function, generated in Monte Carlo simulations, using the hypernetted chain closure. The method, previously devised for the self-avoiding walk model of polymers in good solvent, is extended to the case of polymers in solvents of variable quality by adding a finite nearest-neighbor monomer-monomer attraction to the previous model and varying the temperature. The resulting effective pair potential is found to depend strongly on temperature and polymer concentration. At low concentration the effective interaction becomes increasingly attractive as the temperature decreases, eventually violating thermodynamic stability criteria. However, as polymer concentration is increased at fixed temperature, the effective interaction reverts to mostly repulsive behavior. These issues help to illustrate some fundamental difficulties encountered when coarse-graining complex systems via effective pair potentials.