Chains In Solvent Research Articles

Polymerization/Depolymerization-Induced Self-Assembly under Coupled Equilibria of Polymerization with Self-Assembly.

Depolymerization breaks down polymer chains into monomers like unthreading beads, attracting more attention from a sustainability standpoint. When polymerization reaches equilibrium, polymerization and depolymerization can reversibly proceed by decreasing and increasing the temperature. Here, we demonstrate that such dynamic control of a growing polymer chain in a selective solvent can spontaneously modulate the self-assembly of block copolymer micellar nano-objects. Compared to polymerization-induced self-assembly (PISA), where irreversible growth of a solvophobic polymer block from the end of a solvophilic polymer causes micellization, polymerization/depolymerization-induced self-assembly presented in this study allows us to reversibly regulate the packing parameter of the forming block copolymer and thus induce reversible morphological transitions of the nano-objects by temperature swing. Under the coupled equilibria of polymerization with self-assembly, we found that demixing of the growing polymer block in a more selective solvent entropically facilitates depolymerization at a substantially lower temperature. Taking ring-opening polymerization of δ-valerolactone initiated from the hydroxyl-terminated poly(ethylene oxide) as a model system, we show that polymerization/depolymerization/repolymerization leads to reversible morphological transitions, such as rod-sphere-rod and fiber-rod-fiber, during the heating and cooling cycle and accompanied by changes in macroscopic properties such as viscosity, suggesting their potential as dynamic soft materials.

A molecular dynamics simulation study of glycine/serine octapeptides labeled with 2,3-diazabicyclo[2.2.2]oct-2-ene fluorophore.

The compound 2,3-diazabicyclo[2.2.2]oct-2-ene (DBO) is a versatile fluorophore widely used in Förster resonance energy transfer (FRET) spectroscopy studies due to its remarkable sensitivity, enabling precise donor-acceptor distance measurements, even for short peptides. Integrating time-resolved and FRET spectroscopies with molecular dynamics simulations provides a robust approach to unravel the structure and dynamics of biopolymers in a solution. This study investigates the structural behavior of three octapeptide variants: Trp-(Gly-Ser)3-Dbo, Trp-(GlyGly)3-Dbo, and Trp-(SerSer)3-Dbo, where Dbo represents the DBO-containing modified aspartic acid, using molecular dynamics simulations. Glycine- and serine-rich amino acid fragments, common in flexible protein regions, play essential roles in functional properties. Results show excellent agreement between end-to-end distances, orientational factors from simulations, and the available experimental and theoretical data, validating the reliability of the GROMOS force field model. The end-to-end distribution, modeled using three Gaussian distributions, reveals a complex shape, confirmed by cluster analysis highlighting a limited number of significant conformations dominating the peptide landscape. All peptides predominantly adopt a disordered state in the solvent, yet exhibit a compact shape, aligning with the model of disordered polypeptide chains in poor solvents. Conformations show marginal dependence on chain composition, with Ser-only chains exhibiting slightly more elongation. This study enhances our understanding of peptide behavior, providing valuable insights into their structural dynamics in solution.

Investigating with molecular dynamics, self-assembly of enantiomeric forms of PLA/functionalized-PLA in different solvent conditions

Poly-lactic acid is a versatile biopolymer with excellent mechanical and biological properties, making it suitable for various industrial applications. Enantiomeric forms of PLA can self-assemble into macromolecular nanoparticles with tunable functionalities, further enhancing their potential applications. In this study, the solvated behavior of single and multiple chains of enantiomeric forms of native and functionalized PLA chains in different solvents is investigated via MD simulations to understand the molecular level interactions between them. Self-assembled polymeric structures are analyzed based on the evaluation of their radius of gyration and solvent accessible surface area. Molecular interactions between PLA-PLA and PLA-solvent are explored by examining the number of inter-molecular h-bonds, radial distribution functions and PLA-solvent vdW/coulombic interactions. Outcomes provide insights into the comparative stability of PLA enantiomeric forms in different solvents, the role of solvents in self-assembly, the anticipated impact of functionalization on the self-assembly process and the favorability of stereocomplexation in different solvents.

Temperature-dependent chlorosomal self-aggregation of bacteriochlorophyll-d analogs with a branched alkyl chain in a single 1-chlorooctane solvent

Abstract Recently, the supramolecular polymerization of chlorophyll pigments mimicking a natural light-harvesting apparatus (chlorosome) was demonstrated in low-polar organic solvents or aqueous solutions. To obtain the most aggregation models, a concentrated solution of the pigments in a polar organic solvent was diluted with a large amount of a nonpolar organic solvent or water. Here, bacteriochlorophyll-d analogs possessing branched alkyl chains of different lengths at the peripheral 17-propionate residue on the core chlorin π-system were prepared and their highly soluble chlorosomal supramolecules were produced in single 1-chlorooctane as a low-polar solvent. Temperature-dependent electronic absorption and circular dichroism spectroscopies were employed to analyze their self-aggregation and disaggregation mechanisms. The synthetic analogs were monomeric at high temperatures and self-aggregated during cooling of the hot solution through a non-sigmoidal pathway. The obtained chlorosomal self-aggregates were reversibly monomerized by heating the homogeneous solution. The disaggregation pathway was fitted to an isodesmic model whose melting points depended on the alkyl-chain lengths.

Local swelling effect of flexible entangled polymer chains in athermal solvents

Solutions of flexible, entangled polymer chains dissolved in athermal solvents have been widely studied; however, the influence of the local swelling effects on the overall dynamics of such systems remains incompletely understood. In this study, we performed coarse-grained simulations to compute the concentration dependence of plateau modulus of such systems. Initially, we examined the concentration dependence of entanglement length, denoted as Ne, through simulations. Our findings revealed a concentration scaling relationship for Ne, demonstrating Ne=AΦ−5/4+B, where A and B are chain length independent constants, and Φ is polymer concentration. To account for the local swelling effects, we employed the concept of blobs to represent the chains confined in athermal solvents. Each blob was characterized by a diameter ξ, a number of connected beads g, and a volume Ωb∼ξ3. Our simulations showed that the blob diameter followed the excluded volume relationship, with ξ∼g3/5. By combining the local swelling effect and the non-zero constant B, we derived a concentration scaling relationship for the plateau modulus G of solutions of flexible polymer chains in athermal solvents: G∼Φ(Ne/g)Ωb∼Φ2.30. This scaling exponent aligns with experimental observations ranging from dilute to highly concentrated systems, as well as our simulations, where values of 2.0–2.3 were observed.

Conformational and static properties of tagged chains in solvents: effect of chain connectivity in solvent molecules.

Polymer chains immersed in different solvent molecules exhibit diverse properties due to multiple spatiotemporal scales and complex interactions. Using molecular dynamics simulations, we study the conformational and static properties of tagged chains in different solvent molecules. Two types of solvent molecules were examined: one type consisted of chain molecules connected by bonds, while the other type consisted of individual bead molecules without any bonds. The only difference between the two solvent molecules lies in the chain connectivity. Our results show a compression of the tagged chains with the addition of bead or chain molecules. Chain molecule confinement induces a stronger compression compared to bead molecule confinement. In chain solvent molecules, the tagged chain's radius of gyration reached a minimum at a monomer volume fraction of ∼0.3. Notably, the probability distributions of chain size remain unchanged at different solvent densities, irrespective of whether the solvent consists of beads or polymers. Furthermore, as solvent density increases, a crossover from a unimodal to a bimodal distribution of bond angles is observed, indicating the presence of both compressed and expanded regions within the chain. The effective monomer-solvent interaction is obtained by calculating the partial radial distribution function and the potential of the mean force. In chain solvents, the correlation hole effect results in a reduced number of nearest neighbors around tagged monomers compared to bead solvents. The calculation of pore size distribution reveals that the solvent nonhomogeneity induced by chain connectivity leads to a broader distribution of pore sizes and larger pore dimensions at low volume fractions. These findings provide a deeper understanding of the conformational behavior of polymer chains in different solvent environments.

Dynamics of an aqueous suspension of short hyaluronic acid chains near a DPPC bilayer.

The synergy between hyaluronic acid (HA) and lipid molecules plays a crucial role in synovial fluids, cell coatings, etc. Diseased cells in cancer and arthritis show changes in HA concentration and chain size, impacting the viscoelastic and mechanical properties of the cells. Although the solution behavior of HA is known in experiments, a molecular-level understanding of the role of HA in the dynamics at the interface of HA-water and the cellular boundary is lacking. Here, we perform atomistic molecular dynamics simulation of short HA chains in an explicit water solvent in the presence of a DPPC bilayer, relevant in pathological cases. We identify a stable interface between HA-water and the bilayer where the water molecules are in contact with the bilayer and the HA chains are located away without any direct contact. Both translation and rotation of the interfacial waters in contact with the lipid bilayer and translation of the HA chains exhibit subdiffusive behavior. The diffusive behavior sets in slightly away from the bilayer, where the diffusion coefficients of water and HA decrease monotonically with increase in HA concentration. On the contrary, the dependence on HA chain size is only marginal due to enhanced chain flexibility as their size increases.

Polymer translocation: effects of periodically driven confinement.

We study the influence of confinement on the dynamics of translocation of a linear polymer chain in a good solvent through a cone-shaped pore. Using the Langevin dynamics simulations, we calculate both the first attempt time and translocation time as a function of the position of the back wall and apex angle α. As the in vivo confining environment is inherently dynamic, we extended the present study to explore the consequences of a periodically driven back wall and apex angles on the translocation dynamics. Our findings reveal that the translocation time initially decreases as the driving frequency increases, but increases after a certain frequency. The frequency at which the translocation time is found to be minimum is referred to as the resonance activation. Analyzing the distribution of translocation times around this frequency renders interesting information about the translocation process. We further explore the translocation dynamics by calculating the residence time of individual monomers, shedding light on the microscopic aspects of the process.

Self-assembly of amphiphilic asymmetric comb-like copolymers with responsive rigid side chains.

Amphiphilic asymmetric comb-like copolymers (AACCs) exhibit distinct self-assembly behaviours due to their unique architecture. However, the synthetic difficulties of well-defined AACCs have prohibited a systematic understanding of the architecture-morphology relationship. In this work, we conducted dissipative particle dynamics simulations to investigate the self-assembly behaviours of AACCs with responsive rigid side chains in selective solvents. The effects of side chain length, number of branches, and spacers on the morphology of aggregates were investigated by mapping out morphology diagrams. Besides, the numbers and surface areas of aggregates clearly depicted the morphological transitions during the self-assembly process. Moreover, the rod-to-coil conformation transitions were simulated to explore the stimuli-responsive behaviour of the AACCs with responsive rigid side chains by adjusting the bond angle parameter of the rigid chains. The results indicated that without the support of the rigid chains, the assembly structure collapsed, leading to the tube-to-channelized micelles and one-compartment-to-multicompartment vesicle morphology transformations. The simulation results are consistent with earlier experimental results, which can provide theoretical guidance for assembly toward desired nanostructures.

Self-Assembly of Symmetric Copolymers in Slits with Inert and Attractive Walls.

Although the behavior of the confined semi-dilute solutions of self-assembling copolymers represents an important topic of basic and applied research, it has eluded the interest of scientists. Extensive series of dissipative particle dynamics simulations have been performed on semi-dilute solutions of A5B5 chains in a selective solvent for A in slits using a DL-MESO simulation package. Simulations of corresponding bulk systems were performed for comparison. This study shows that the associates in the semi-dilute bulk solutions are partly structurally organized. Mild steric constraints in slits with non-attractive walls hardly affect the size of the associates, but they promote their structural arrangement in layers parallel to the slit walls. Attractive walls noticeably affect the association process. In slits with mildly attractive walls, the adsorption competes with the association process. At elevated concentrations, the associates start to form in wide slits when the walls are sparsely covered by separated associates, and the association process prevents the full coverage of the surface. In slits with strongly attractive walls, adsorption is the dominant behavior. The associates form in wide slits at elevated concentrations only after the walls are completely and continuously covered by the adsorbed chains.

Dependence of the relaxation properties of polypropylene solutions on the concentration of organic solvents: molecular dynamics simulation

The microscopic relaxation of polymer chains in organic solvents has been studied by the molecular dynamics method. The simulation was carried out for the mixtures of polypropylene with three different organic solvents (acetone, cyclopentane, acetonitrile). The dependence of the relaxation time and the glass transition temperature of the polymer on the mass fraction of the solvent is obtained.

Light scattering study of intrachain conformational relaxation of hyperbranched polystyrene chains in a marginal solvent

So far, much less attention has been paid to the dynamic behavior of branched polymers under marginal solvent condition. This work reports the first study of the intrachain conformational relaxation of hyperbranched polystyrene (PS) chains under the marginal solvent condition in the dilute regime. Specifically, the solvents of ethyl acetate (EA) and toluene (Tol) are selected for comparison. By a combination of static and dynamic light scattering measurements, a few findings have been revealed: 1) the weight average of molar masses (Mw) of hyperbranched PS chains are determined to be 7.33 × 107 and 8.02 × 107 g/mol in Tol and EA, respectively, implying the single chain state under both solvent conditions; 2）the value of reduced first cumulant (Γ*) at very high q-regime, i.e., Γ*(∞), is determined to be around 0.026 ± 0.002 in EA, which is not only smaller than the smallest Γ*(∞) ≈ 0.052 ± 0.002 reported for linear chains in literature under theta and good solvent conditions, but also much smaller than the reported Γ*(∞) ≈ 0.033 ± 0.003 for hyperbranched chains in literature under good solvent condition, and the result implies the reduced hydrodynamic interactions. Although the excluded volume effect is relatively weak in the marginal solvent, the present study supports that the existence of weak binary interactions between segments still influence the conformational behavior significantly.

Mechanical response of a thick poroelastic gel in contactless colloidal-probe rheology

When a rigid object approaches a soft material in a viscous fluid, hydrodynamic stresses arise in the lubricated contact region and deform the soft material. The elastic deformation modifies in turn the flow, hence generating a soft-lubrication coupling. Moreover, soft elastomers and gels are often porous. These materials may be filled with solvent or uncrosslinked polymer chains, and might be permeable to the surrounding fluid, which further complexifies the description. Here, we derive the point-force response of a semi-infinite and permeable poroelastic substrate. Then, we use this fundamental solution in order to address the specific poroelastic lubrication coupling associated with contactless colloidal-probe methods. In particular, we derive the conservative and dissipative components of the force associated with the oscillating vertical motion of a sphere close to the poroelastic substrate. Our results may be relevant for dynamic surface force apparatus and contactless colloidal-probe atomic force microscopy experiments on soft, living and/or fragile materials, such as swollen hydrogels and biological membranes.

Conformations and Solvation of Synthetic Polymers in Water by Generalized Born Implicit-Solvent Molecular Dynamics Simulations: Stereoisomers of Poly(acrylic acid) and Poly(methacrylic acid)

We present the GB-OBC model as an approach for implicit-solvent MD simulations of a synthetic macromolecule in water. The model is tested and found to be successful in reproducing the chain dimensions and predicting the free energy of solvation of carboxylic acid vinyl polymers. The influence of stereochemistry and the hydrophobic nature of the polymer was investigated as a function of chain length (20 < N < 600) for poly(acrylic acid) (PAA) and poly(methacrylic acid) (PMA). The dimensionless parameters of the GB-OBC model were parameterized to be applicable to PAA and PMA. Scaling relations for chain dimensions obtained using implicit-solvent MD simulations in this study are in good agreement with those from experiments, theory of solvated chains in good solvents, and all-atom MD simulations in explicit water. Results show that ⟨Rg2⟩/NL2 is greater for the atactic chain as compared to the isotactic chain, for PAA as well as PMA. ⟨Rg2⟩/NL2 values of chains attain constancy in water for N = 200, with the values being greater for PMA. The PMA chain is conformationally more perturbed than the PAA chain, for both isotactic and atactic stereochemistry. The solvation free energy ΔGhyd of PAA and PMA in water is negative for all chain lengths (N = 20-600) and becomes more favorable with an increase in molecular weight. The ΔGhyd values for isotactic and atactic chains are identical at lower values of N but differ slightly for N > 300. Irrespective of the hydrophobic nature of the polymer, the atactic chain is thermodynamically more soluble in water as compared to the isotactic chain. The isotactic chain is less hydrophilic as compared to the atactic chain due to the closer proximity of the COOH groups along the backbone. This implicit solvent method is an effective way to accurately simulate the configurational properties and solvation of synthetic polymers in water.

Coil-globule transition in two-dimensional polymer chains in an explicit solvent.

The structure of two-dimensional polymer chains in a solvent at different temperatures is still far from being fully understood. Computer simulations of high-density macromolecular systems require the use of appropriate algorithms, and therefore the simulations were carried out using the Cooperative Motion Algorithm. The polymer model studied was exactly two-dimensional, coarse-grained and based on a triangular lattice. The theta temperature and temperature of coil-to-globule transition, and critical exponents were determined. The differences between the structure of such a disk and that of a chain in a dense polymer liquid were shown.

Excluded-Volume Interactions in Field-Theoretic Simulations: Multiconvolutions and Model Equivalence.

To deal with divergences of functional integrals in field-theoretic simulations (FTS) of complex fluids, the microscopic density is often smeared by being replaced by a convoluted one, typically using a Gaussian masking function. The smearing changes radically the nature of nonbonded interactions of the original microscopic density and results in a regularized model that is free of ultraviolet (UV) divergences. In this work, we first resolve a few fundamental issues related with the use of masking functions for δ-interactions in FTS and then we detail a new methodology that builds on the concept of multiconvoluted inverse potentials and a principle of model equivalence for statistical weights to accommodate more physically relevant interactions in FTS. The capabilities of the new approach are highlighted by examining the Gaussian-regularized Edwards model (GREM) and the Yukawa potential. A successful test calculation of the excess chemical potential of a polymer chain in a good solvent with the GREM illustrates the power of the new theoretical framework.

Solvent Quality Dependent Osmotic Pressure of Polymer Solutions in Two Dimensions.

Confined in two-dimensional planes, polymer chains comprising dense monolayer solutions are segregated from each other because of topological interaction. Although the segregation is inherent in two dimensions (2D), the solution may display different properties depending on the solvent quality. Among others, it is well-known in both theory and experiment that the osmotic pressure (Π) in the semidilute regime displays solvent quality dependent increases with the area fraction (ϕ) (or monomer concentration, ρ), that is, Π ∼ ϕ3 for good solvents and Π ∼ ϕ8 for Θ solvents. The osmotic pressure can be associated with the Flory exponent (or the correlation length exponent) for the chain size and the pair distribution function of monomers; however, they do not necessarily offer a detailed microscopic picture leading to the difference. To gain microscopic understanding into the different surface pressure isotherms of polymer solutions under the two distinct solvent conditions, we study the chain configurations of the polymer solution based on our numerical simulations that semiquantitatively reproduce the expected scaling behaviors. Notably, at the same value of ϕ, polymer chains in a Θ solvent occupy the surface in a more inhomogeneous manner than the chains in good solvent, yielding on average a greater and more heterogeneous interstitial void size, which is related to the fact that the polymer in the Θ solvent has a greater correlation length. The polymer configurations and interstitial voids visualized and quantitatively analyzed in this study offer microscopic understanding to the origin of the solvent quality dependent osmotic pressure of 2D polymer solutions.

Interchain ordering structure and main chain conformation analysis of thermal stability in vinyl-addition polynorbornene

The aggregation structure and formation mechanism during film casting using toluene of vinyl-addition poly(norbornene- co -hexylnorbornene)s (P(NB/HNB)s) synthesized using Ni and Pd catalyst were systematically investigated by wide- and small-angle X-ray scattering and gel permeation chromatography–right-angle laser light scattering–viscometry techniques. The correlation of these data with the glass-transition temperature ( T g ) was discussed. The single-chain conformation of P(NB/HNB)s was a flexible, stretched structure with respect to the Gaussian chain in a good solvent, as characterized by an exponent of the Mark–Houwink–Sakurada equation, and P(NB/HNB)s formed thin-rod aggregates with a length of 30 nm in semi-concentrated toluene solution via interchain stacking of the rod-like chains. P(NB/HNB) films cast from toluene solution exhibit interchain ordering structures with distances between 0.9 and 1.7 nm depending on the NB/HNB ratio. These findings show that the interchain ordering is driven by the stacking of the rod-like chains, which resulted in the highly ordered interchain structure in the film. The T g of the Pd-catalyzed polymer films were 20 °C higher than that of the Ni-catalyzed polymer films depending on the interchain structure. There is a strong correlation between interchain ordering structure and T g , which shows that the T g of P(NB/HNB)s is primarily influenced by the van der Waals interaction between main chains. • Aggregation structure and mechanism of vinyl-addition polynorbornenes was investigated. • The polymers formed thin-rod aggregates in semi-concentrated solution and highly ordered interchain structure in the film. • The interchain ordering was driven by the stacking of the rod-like chains during drying process. • There is a strong correlation between T g of the film and the interchain ordering structure.

Effects of Molecular Encapsulation on the Photophysical and Charge Transport Properties of a Naphthalene Diimide Bithiophene Copolymer.

Engineering the molecular structure of conjugated polymers is key to advancing the field of organic electronics. In this work, we synthesized a molecularly encapsulated version of the naphthalene diimide bithiophene copolymer PNDIT2, which is among the most popular high charge mobility organic semiconductors in n-type field-effect transistors and non-fullerene acceptors in organic photovoltaic blends. The encapsulating macrocycles shield the bithiophene units while leaving the naphthalene diimide units available for intermolecular interactions. With respect to PNDIT2, the encapsulated counterpart displays an increased backbone planarity. Molecular encapsulation prevents preaggregation of the polymer chains in common organic solvents, while it permits π-stacking in the solid state and promotes thin film crystallinity through an intermolecular-lock mechanism. Consequently, n-type semiconducting behavior is retained in field-effect transistors, although charge mobility is lower than in PNDIT2 due to the absence of the fibrillar microstructure that originates from preaggregation in solution. Hence, molecularly encapsulating conjugated polymers represent a promising chemical strategy to tune the molecular interaction in solution and the backbone conformation and to consequently control the nanomorphology of casted films without altering the electronic structure of the core polymer.

Strongly enhanced dynamics of a charged Rouse dimer by an external magnetic field.

While the dynamics of dimers and polymer chains in a viscous solvent is well understood within the celebrated Rouse model, the effect of an external magnetic field on the dynamics of a charged chain is much less understood. Here, we generalize the Rouse model for a charged dimer to include the effect of an external magnetic field. Our analytically solvable model allows a fundamental insight into the magneto-generated dynamics of the dimer in the overdamped limit as induced by the Lorentz force. Surprisingly, for a dimer of oppositely charged particles, we find an enormous enhancement of the dynamics of the dimer center, which exhibits even a transient superballistic behavior. This is highly unusual in an overdamped system for there is neither inertia nor any internal or external driving. We attribute this to a significant translation and rotation coupling due to the Lorentz force. We also find that magnetic field reduces the mobility of a dimer along its orientation and its effective rotational diffusion coefficient. In principle, our predictions can be tested by experiments with colloidal particles and complex plasmas.