Solvents Of Varying Quality Research Articles

Reply to the 'Comment on "Effects of topological constraints on linked ring polymers in solvents of varying quality"' by M. Tsige and H. Guo, Soft Matter, 2024, 20, DOI: 10.1039/D3SM01614E.

In the preceding Comment, Drs Tsige and Guo compare their findings about the θ-temperatures of linear chains, ring polymers and poly[n]catenanes with those of previous work by us [Z. A. Dehaghani, I. Chubak, C. N. Likos and M. R. Ejtehadi, Soft Matter, 2020, 16, 3029-3038] and point out that the ordering they obtain for these three quantities is, for large degrees of polymerisation, the reverse of the one we had found in our own investigations. We thank the authors of the Comment for their remarks and we appreciate their detailed investigations, which emphasise the importance of understanding the properties of topological polymers and their behaviour under varying solvent quality. We point out, however, that the discrepancy found by Tsige and Guo is only apparent because it pertains to the Θ-temperature of rings and poly[n]catenanes with the same overall molecular weight, whereas in our work we compared the Θ-temperature of a constituent ring of the poly[n]catenane with that of the entire mechanically linked macromulecule.

Probing conformational properties of conjugated polymers in dilute solutions under variable solvent quality via coarse‐grained modeling

AbstractConjugated polymers (CPs), characterized by rigid conjugation backbones and flexible peripheral side chains, hold significant promise in various organic optoelectronic applications. In this study, we employ coarse‐grained molecular dynamics (CG‐MD) simulations to investigate the intricate interplay of solvent quality, temperature, and chain architecture (e.g., side‐chain length and molecular mass) on the conformational behaviors of CPs in dilute solutions. Our research uncovers distinctive conformational behaviors under varying solvent conditions, highlighting the versatile nature of polymer chains, which can adopt extended configurations in good solvents and transition to aggregated states in poor solvents. Additionally, the mass scaling exponent , a robust structural descriptor, consistently described CPs behavior across diverse architectures and solvent conditions. Furthermore, our study shows that a CP with longer side‐chain exhibits improved solubility, which is further confirmed by experimental observations. Moreover, our analysis of the shape descriptor provided valuable insights into the symmetry and dimensionality of CPs under varying solvent conditions. These findings offer a comprehensive understanding of conformational behaviors of CPs in dilute solution, which are helpful in guiding the conformational design of polymer for specific applications.

Effects of solvent conditions on the self-assembly of heterotrimeric collagen-like peptide (CLP) triple helices: a coarse-grained simulation study.

We perform coarse-grained (CG) molecular dynamics (MD) simulations to investigate the self-assembly of collagen-like peptide (CLP) triple helices into fibrillar structures and percolated networks as a function of solvent quality. The focus of this study is on CLP triple helices whose strands are different lengths (i.e., heterotrimers), leading to dangling 'sticky ends'. These 'sticky ends' are segments of the CLP strands that have unbonded hydrogen-bonding donor/acceptor sites that drive heterotrimeric CLP triple helices to physically associate with one another, leading to assembly into higher-order structures. We use a validated CG model for CLP in implicit solvent and capture varying solvent quality through changing strength of attraction between CG beads representing the amino acids in the CLP strands. Our CG MD simulations show that, at lower CLP concentrations, CLP heterotrimers assemble into fibrils and, at higher CLP concentrations, into percolated networks. At higher concentrations, decreasing solvent quality causes (i) the formation of heterogeneous network structures with a lower degree of branching at network junctions and (ii) increases in the diameter of network strands and pore sizes. We also observe a nonmonotonic effect of solvent quality on distances between network junctions due to the balance between heterotrimer end-end associations driven by hydrogen bonding and side-side associations driven by worsening solvent quality. Below the percolation threshold, we observe that decreasing solvent quality leads to the formation of fibrils composed of multiple aligned CLP triple helices, while the number of 'sticky ends' governs the spatial extent (radius of gyration) of the assembled fibrils.

Chain conformations and phase separation in polymer solutions with varying solvent quality

AbstractMolecular dynamics simulations are used to investigate the conformations of a single polymer chain, represented by the Kremer‐Grest bead‐spring model, in a solution with a Lennard‐Jones liquid as the solvent when the interaction strength between the polymer and solvent is varied. Results show that when the polymer‐solvent interaction is unfavorable, the chain collapses as one would expect in a poor solvent. For more attractive polymer‐solvent interactions, the solvent quality improves and the chain is increasingly solvated and exhibits ideal and then swollen conformations. However, as the polymer‐solvent interaction strength is increased further to be more than about twice the strength of the polymer‐polymer and solvent‐solvent interactions, the chain exhibits an unexpected collapsing behavior. Correspondingly, for strong polymer‐solvent attractions, phase separation is observed in the solutions of multiple chains. These results indicate that the solvent becomes effectively poor again at very attractive polymer‐solvent interactions. Nonetheless, the mechanism of chain collapsing and phase separation in this limit differs from the case with a poor solvent rendered by unfavorable polymer‐solvent interactions. In the latter, the solvent is excluded from the domain of the collapsed chains while in the former, the solvent is still present in the pervaded volume of a collapsed chain or in the polymer‐rich domain that phase separates from the pure solvent. In the limit of strong polymer‐solvent attractions, the solvent behaves as a glue to stick monomers together, causing a single chain to collapse and multiple chains to aggregate and phase separate.

Phase Behavior of Lyotropic Liquid-Crystalline Polymers under Varying Solvent Conditions: Effect of External Fields on the Phase Diagram.

In this work, we report a Density Functional Theory based study of phase behavior of lyotropic liquid-crystalline polymers under both good and varying solvent conditions in the presence of external electric or magnetic field. Our microscopic model for the good solvent case is based on the tangent hard-sphere chain with bond-bending potential to account for the chain stiffness; the variable solvent quality is modeled by adding attractive monomer-monomer interactions. The phase diagrams are constructed in three intensive variables (temperature, pressure, and field strength), and are characterized by the presence of critical and triple lines, which originate from the critical and triple points of the corresponding zero-field case. The merging of critical and triple lines results in the appearance of the "double critical" and "critical triple" points, already known from the earlier studies of the phase behavior of spin fluids in magnetic fields. The important difference of the present model from the spin fluids is due to the finite stiffness of the polymer chains (characterized by their persistence length), which adds an additional parameter controlling the morphology of the phase diagrams.

Modeling short-chain branched polyethylenes in dilute solution under variable solvent quality conditions: Basic configurational properties

We perform molecular dynamics (MD) simulations of a coarse-grained model of linear low-density polyethylenes (LLDPEs) having short-chain branching in an implicit solvent and calculate basic configurational properties (radius of gyration Rg, hydrodynamic radius Rh, and intrinsic viscosity [η]) using the path-integration program ZENO. Solvent quality effects in our model are accounted for through the introduction of an attractive polymer-polymer interaction potential with solvent quality “strength” λ that is tuned from a “good” solvent limit, where the inter-polymer interactions are purely repulsive, to a “very poor” solvent regime, where the polymers collapse due to their self-attractive intermolecular interactions. We also identify a compensation condition or “θ point” in our model between these limits, where the polymers are “ideal” in the sense in which attractive interactions effectively nullify inter-polymer repulsions so that the chains behave akin to Gaussian polymer chains in their average conformational properties. In order to compare with experiments performed under variable solvent conditions, we define a dimensionless measure of excluded volume interaction parameter δ in terms of the mass scaling exponent ν for Rg or νh for Rh, which can be determined from both experiment and simulation. We illustrate our variable solvent approach to estimate polymer solution properties in the case of polyethylenes (PEs) and evaluate the δ value that corresponds to the experimental solvent conditions. Our combined use of MD and ZENO allows for the numerical estimation of polymer solution properties for polymers having general monomer structures and solvent qualities in a computationally tractable fashion and should be useful as a general computational tool for polymer structural characterization.

Phase behavior of flexible and semiflexible polymers in solvents of varying quality.

The interplay of nematic order and phase separation in solutions of semiflexible polymers in solvents of variable quality is investigated by density functional theory (DFT) and molecular dynamics (MD) simulations. We studied coarse-grained models, with a bond-angle potential to control chain stiffness, for chain lengths comparable to the persistence length of the chains. We varied both the density of the monomeric units and the effective temperature that controls the quality of the implicit solvent. For very stiff chains, only a single transition from an isotropic fluid to a nematic is found, with a phase diagram of "swan-neck" topology. For less stiff chains, however, also unmixing between isotropic fluids of different concentration, ending in a critical point, occurs for temperatures above a triple point. The associated critical behavior is examined in the MD simulations and found compatible with Ising universality. Apart from this critical behavior, DFT calculations agree qualitatively with the MD simulations.

Phase transitions in solvent-dependent polymer adsorption in three dimensions.

We consider the phase diagram of self-avoiding walks (SAWs) on the simple cubic lattice subject to surface and bulk interactions, modeling an adsorbing surface and variable solvent quality for a polymer in dilute solution, respectively. We simulate SAWs at specific interaction strengths to focus on locating certain transitions and their critical behavior. By collating these new results with previous results we sketch the complete phase diagram and show how the adsorption transition is affected by changing the bulk interaction strength. This expands on recent work considering how adsorption is affected by solvent quality. We demonstrate that changes in the adsorption crossover exponent coincide with phase boundaries.

Critical Conditions of Adhesion and Separation of Functionalized Nanoparticles on Polymer Grafted Substrates

Emerging technologies of production and processing of functionalized nanoparticles (NP) require advanced methods of NP characterization and separation. While various methods are available for NP separation by size, there are no efficient methods for NP separation by surface chemistry. Using extensive dissipative particle dynamics simulations, this work investigates the mechanisms of NP adhesion and flow in polymer brush (PB)-grafted pore channels searching for the conditions for size-independent separation of NPs that are similar to the critical conditions of liquid chromatography of polymers. We consider interactions of NPs functionalized by hydrophilic and hydrophobic ligands with PBs, in which conformation and adhesion properties are controlled by the solvent quality varied with the composition of thermodynamically good and poor solvent components. The NP-PB adhesion is characterized by the free energy landscape calculated by the ghost tweezers simulation method that mimics the experimental technique of optical tweezers. The NP Henry constant and the respective partition coefficient are calculated depending on the NP size and ligand composition at varying solvent quality. Our findings demonstrate that, with the decrease of solvent quality, the NP elution undergoes a transition from the size-exclusion mode with larger NPs having shorter retention time to the adsorption mode with the reverse order of elution. This transition, which occurs in a narrow range of solvent composition, signifies the so-called “critical” point of adsorption that strongly depends on the NP functionalization. The dynamics of NP axial dispersion in the isocratic and gradient elution modes is characterized employing a convective-diffusion model. We show that the NPs can be effectively separated by surface chemistry at the critical points of adsorption, using the gradient mode of interaction NP chromatography with controlled variation of the solvent composition.

Fabrication of Flexible Hydrogel Sheets Featuring Periodically Spaced Circular Holes with Continuously Adjustable Size in Real Time.

We report on the formation of stimuli-responsive structured hydrogel thin films whose pattern geometry can be adjusted on demand and tuned reversibly by varying solvent quality or by changing temperature. The hydrogel films, ∼100 nm in thickness, were prepared by depositing layers of random copolymers comprising N-isopropylacrylamide and ultraviolet (UV)-active methacryloyloxybenzophenone units onto solid substrates. A two-beam interference pattern technique was used to cross-link the selected areas of the film; any unreacted material was extracted using ethanol after UV light-assisted cross-linking. In this way, we produced nanoholes, perfectly ordered structures with a narrow size distribution, negligible tortuosity, adjustable periodicity, and a high density. The diameter of the circular holes ranged from a few micrometers down to several tens of nanometers; the hole periodicity could be adjusted readily by changing the optical period of the UV interference pattern. The holes were reversibly closed and opened by swelling/deswelling the polymer networks in the presence of ethanol and water, respectively, at various temperatures. The reversible regulation of the hole diameter can be repeated many times within a few seconds. The hydrogel sheet with circular holes periodically arranged may also be transferred onto different substrates and be employed as tunable templates for the deposition of desired substances.

Phase diagram of twist storing lattice polymers in variable solvent quality

When double stranded DNA is turned in experiments it undergoes a transition. We use an interacting self-avoiding walk on a three-dimensional fcc lattice weighted by writhe to relate to these experiments and treat this problem via simulations. We provide evidence for the existence of a thermodynamic phase transition induced by writhe and examine related phase diagrams taking solvent quality and stretching into account.

Stepwise self-assembly of a block copolymer-platinum(ii) complex hybrid in solvents of variable quality: from worm-like micelles to free-standing sheets to vesicle-like nanostructures.

The self-assembly process of formation of worm-like micelles of a block copolymer-platinum(ii) complex hybrid is investigated with respect to the influence of solvent quality. When the solvent quality is moderately weakened, unilamellar free-standing sheets are achieved, in which the worm-like micelles snap off to form star micelles together with a few short worms. Extremely worsened solvent quality leads to unilamellar vesicle-like nanostructures, onto which only star micelles emerged. With the intermediate solvent quality, the sheets coexist with the vesicle-like nanostructures. This is well correlated with mechanistic insights regarding the morphological transition from sheet- to vesicle-like nanoassemblies. In these aggregates, short worms and star micelles still hold their core-shell structures. Furthermore, these unconventional superstructures are well interrelated with their luminescence properties. This result challenges the conventional paradigm of the amphiphilic self-assembly of surfactants and block copolymers in selective solvents, where they form bilayered nanostructures and are required universally to be rearranged during the morphological transition from micelles to vesicles.

Electric field directed formation of aligned conjugated polymer fibers

Alternating current (AC) electric fields effectively align poly(3-alkylthiophene) (P3AT) fibers during a one-dimensional crystallization process. Structural, mechanical and electrical properties have been probed using microscopy, small angle neutron scattering (SANS), atomic force microscopy (AFM), X-ray diffraction (XRD), rheology, UV-Vis spectroscopy, and dielectric spectroscopy. Optimum frequency and amplitudes were identified for specific P3AT concentrations and for variable solvent quality. Optical microscopy along with SANS and XRD demonstrate alignment persists over both micrometer and nanometer length scales. Small amplitude oscillatory shear rheology also showed that the shear modulus increased for aligned fibers. The structural changes were correlated to improvements in electric conductivity as probed by dielectric spectroscopy. XRD experiments performed with variable sample orientations also showed that the P3HT π-π stacking direction was aligned parallel to the direction of the fiber axis. Electric field alignment was also possible for other alkyl-thiophene polymers and for polymers with simple backbone repeat units.

Hydrodynamic Interactions and Entanglements of Polymer Solutions in Many-Body Dissipative Particle Dynamics.

Using many-body dissipative particle dynamics (MDPD), polymer solutions with concentrations spanning dilute and semidilute regimes are modeled. The parameterization of MDPD interactions for systems with liquid–vapor coexistence is established by mapping to the mean-field Flory–Huggins theory. The characterization of static and dynamic properties of polymer chains is focused on the effects of hydrodynamic interactions and entanglements. The coil–globule transition of polymer chains in dilute solutions is probed by varying solvent quality and measuring the radius of gyration and end-to-end distance. Both static and dynamic scaling relations for polymer chains in poor, theta, and good solvents are in good agreement with the Zimm theory with hydrodynamic interactions considered. Semidilute solutions with polymer volume fractions up to 0.7 exhibit the screening of excluded volume interactions and subsequent shrinking of polymer coils. Furthermore, entanglements become dominant in the semidilute solutions, which inhibit diffusion and relaxation of chains. Quantitative analysis of topology violation confirms that entanglements are correctly captured in the MDPD simulations.

Going beyond the classical amphiphilicity paradigm: the self-assembly of completely hydrophobic polymers into free-standing sheets and hollow nanostructures in solvents of variable quality

Self-assembly is well-known to occur in amphiphiles, and the totally hydrophobic ones are never reported to self-assemble. In this work we report for the first time that the latter can self-assemble into free-standing sheets and hollow spheres in toluene/methanol mixed solvents by modulating the solvent quality. The homopolymers studied in this work are polystyrene (PS), polyphenylacetylene (PPA), and poly(3-hexyl thiophene) (P3HT), representing polymers with different rigidity. All the three form a homogenous solution in toluene, but self-assembly occurs in the toluene/methanol mixed solvents. Micrometer sized free-standing sheets were formed for PS, PPA, and P3HT at methanol volume fractions being 43%, 50%, and 67%, respectively, and hollow spheres were observed for PPA at higher methanol fractions of 75 and 90%. Under the latter solvent conditions, PS forms solid spheres, yet ill-defined aggregates and free-standing sheets coexist in the case of P3HT. This non-solvent induced self-assembly was explained by a delicate balance of two "opposing forces": van der Waals attractive and entropic repulsive forces generated between the segments of these homopolymers within a single chain, between two chains, and among more chains in the solvents of worsened quality.

Self-assembly of star micelle into vesicle in solvents of variable quality: the star micelle retains its core-shell nanostructure in the vesicle.

Intra- and intermolecular interactions of star polymers in dilute solutions are of fundamental importance for both theoretical interest and hierarchical self-assembly into functional nanostructures. Here, star micelles with a polystyrene corona and a small ionic core bearing platinum(II) complexes have been regarded as a model of star polymers to mimic their intra- and interstar interactions and self-assembled behaviors in solvents of weakening quality. In the chloroform/methanol mixture solvents, the star micelles can self-assemble to form vesicles, in which the star micelles shrink significantly and are homogeneously distributed on the vesicle surface. Unlike the morphological evolution of conventional amphiphiles from micellar to vesicular, during which the amphiphilic molecules are commonly reorganized, the star micelles still retain their core-shell nanostructures in the vesicles and the coronal chains of the star micelle between the ionic cores are fully interpenetrated.

Self-Assembly of Polyoxometalate-Based Starlike Polymers in Solvents of Variable Quality: From Free-Standing Sheet to Vesicle

Polyoxometalate-based supramolecular star polymers, where an anionic polyoxometalate is closely encapsulated by a hydrophobic shell of cation terminated polystyrenes, are conceptually regarded as a model of star polymers to study their intra- and interstar interactions and self-assembled behaviors in solvents of variable quality. These model stars can self-assemble to form unilamellar free-standing sheets in the toluene/methanol mixture solvent with a methanol volume ratio of 50%. An increase in the methanol content to 75% or 80% results in a coexistence of free-standing sheets with vesicles, where several intermediates are captured for the morphological evolution from sheet to vesicle. With further increasing methanol content to 90%, pure vesicle phases form with unilamellar or oligolamellar features. All these aggregates are packed by the totally hydrophobic starlike polymers, where the core–shell structures are retained. This is distinctly different from the bilayered nanostructures formed by amphiphil...

On the potential of mean force of a sterically stabilized dispersion

The potential of mean force (PMF) is the total free energy of a many-body colloidal system, and consequently it includes all the interactions the colloids experience due to collisions with themselves and with the solvent. Here, the PMF of a colloidal dispersion under various circumstances of current interest, such as varying solvent quality, polymer coating thickness, and addition of electrostatic interaction, is obtained from radial distribution functions available from the literature. They are based on implicit-solvent, computer simulation studies of a model TiO2 dispersion that takes into account three major components to the interaction between colloidal particles, namely van der Waals attraction, repulsion between polymer coating layers, and a hard-core particle repulsion. In addition, a screened form of the electrostatic interaction was included. It is argued that optimal conditions for dispersion stability can be derived from a comparative analysis of the PMF under the different situations studied. This thermodynamics-based analysis is believed to be more accessible to specialists working on the development of improved colloidal formulations than that based on the more abstract, radial distribution functions.

Tunable Interactions of Polyoxometalate-Based Brushlike Hybrids in Solvents of Variable Quality: From Self-Recognition to Supramolecular Recognition

The controllable interactions of a spherical polymer brush modeled by a poly(styrene-b-4-vinylpyridinium methyl iodide)-polyoxometalate composite micelle, SVP-6, with a polyoxometalate-based supramolecular star polymer, PSP-4, in solvents of variable quality allow us to tune their self-assembly behaviors from self-recognition to supramolecular recognition. In the former case, isolated, contractive spheres together with a few vesicles formed by PSP-4 coexist with multimicelle aggregates formed by SVP-6, whereas SVP-6 is hosted inside the vesicle of PSP-4 in the latter case. This work represents an important step toward the development and understanding of programmable self-assembly of brushlike polymers into complex materials.

Contribution of chromophores with different numbers of repeat units to overall emission of MEH–PPV: An experimental and simulation study

In the present study, poly[1-methoxy-4-(2′-ethylhexyloxy)-p-phenylenevinylene] (MEH–PPV) was synthesized, modified and characterized by FTIR, NMR, GPC, UV/vis absorption, fluorescence, and mass spectroscopy. The effective conjugation length was controlled by introducing physical and chemical defects into the chain backbone. It was observed that absorption and emission spectra of polymer chains were blue-shifted by importing structural deficiencies. Chemical defects were introduced by controlled oxidation of vinylene double bonds using m-chloroperbenzoic acid and physical defects were induced by tuning polymer–solvent interactions (varying solvent quality). It was shown for the first time that the recorded fluorescence spectrum of MEH–PPV can be reconstructed using the emission data of oligomeric constituents and the contribution of chromophores with different numbers of repeat units to overall emission can be quantified. The effect of excitation energy on the emission pattern was also investigated. It was shown that the emission contribution of shorter chromophores to overall emission is detectable using high excitation energies. Finally, atomistic molecular dynamic (MD) simulation was used to investigate the polymer chain conformation in methanol and chloroform. The results obtained from torsional angles studies confirmed the presence of a great number of induced physical defects in the backbone and, hence, a collapsed conformation of polymer chains in methanol. Evaluations of physical defects along the chain proposed the average number of coplanar phenyl rings of ∼5 and 14 for polymer chains in methanol and chloroform respectively.