Polymer Conformation Research Articles

Cleaning of Wastewater Using Crosslinked Poly(Acrylamide-co-Acrylic Acid) Hydrogels: Analysis of Rotatable Bonds, Binding Energy and Hydrogen Bonding.

The discharge of untreated wastewater, often contaminated by harmful substances, such as industrially used dyes, can provoke environmental and health risks. Among various techniques, the adsorption of dyes, using three-dimensional (3D) networks consisting of hydrophilic polymers (hydrogels), represents a low-cost, clean, and efficient remediation method. Three industrially used dyes, Methylene Blue, Eosin, and Rose Bengal, were selected as models of pollutants. Poly(acrylamide) (poly(AM)) and poly(acrylamide-co-acrylic acid) (poly(AM-co-AA)) networks were chosen as adsorbent materials (hydrogels). These polymers were synthesized by crosslinking the photopolymerization of their respective monomer(s) in an aqueous medium under exposure to UV light. Experimental adsorption measurements revealed substantially higher dye uptakes for poly(AM-co-AA) compared to poly(AM) hydrogels. In this report, a theoretical model based on docking simulations was applied to analyze the conformation of polymers and pollutants in order to investigate some aspects of the adsorption process. In particular, hydrogen and halogen interactions were studied. The presence of strong hydrogen bonding plays a crucial role in the retention of dyes, whereas halogen bonding has a small or negligible effect on adsorption. An evaluation of binding energies allowed us to obtain information about the degree of affinity between polymers and dyes. The number of rotatable bonds in the copolymer exceeds those of poly(AM),meaning that poly(AM-co-AA) is revealed to be more suitable for obtaining a high retention rate for pollutants.

Nonequilibrium free energy during polymer chain growth.

During fast diffusion-influenced polymerization, nonequilibrium behavior of the polymer chains and the surrounding reactive monomers has been reported recently. Based on the laws of thermodynamics, the emerging nonequilibrium structures should be characterizable by some "extra free energy" (excess over the equilibrium Helmholtz free energy). Here, we study the nonequilibrium thermodynamics of chain-growth polymerization of ideal chains in a dispersion of free reactive monomers, using off-lattice, reactive Brownian dynamics computer simulations in conjunction with approximative statistical mechanics and relative entropy (Gibbs-Shannon and Kullback-Leibler) concepts. In the case of fast growing polymers, we indeed report increased nonequilibrium free energies ΔFneq of several kBT compared to equilibrium and near-equilibrium, slowly growing chains. Interestingly, ΔFneq is a non-monotonic function of the degree of polymerization and thus also of time. Our decomposition of the thermodynamic contributions shows that the initial dominant extra free energy is stored in the nonequilibrium inhomogeneous density profiles of the free monomer gas (showing density depletion and wakes) in the vicinity of the active center at the propagating polymer end. At later stages of the polymerization process, we report significant extra contributions stored in the nonequilibrium polymer conformations. Finally, our study implies a nontrivial relaxation kinetics and "restoring" of the extra free energy during the equilibration process after polymerization.

Path integral description of semiflexible active Brownian polymers.

Semiflexible polymers comprised of active Brownian particles (ABPOs) exhibit intriguing activity-driven conformational and dynamical features. Analytically, the generic properties of ABPOs can be obtained in a mean-field description applying the Gaussian semiflexible polymer model. In this article, we derive a path integral representation of the stationary-state distribution function of such ABPOs, based on the stationary-state distribution function of the normal mode amplitudes following from the Langevin equation of motion. The path integral includes characteristic semiflexible polymer contributions from entropy and bending energy, with activity dependent coefficients, and, in addition, activity-induced torsional and higher order correlations along the polymer contour. Focusing on a semiflexible polymer approximation, we determine various properties such as the tangent-vector correlation function, effective persistence length, and the mean-square end-to-end distance. The latter reflects the characteristic features of ABPOs, and good quantitative agreement is obtained with the full solution for larger activities, specifically for flexible polymers. Moreover, the approximation indicates the relevance of torsional and higher order contour correlations for the ABPO conformations. In general, the ABPO path integral illustrates how colored noise (active fluctuations) affects semiflexible polymer conformations in comparison to white noise thermal fluctuations.

Stochastic Resonance of a Confined Flexible Polymer

AbstractThe dynamics of a flexible polymer confined in two connected rectangular cavities with a small structureless hole in between subject to an oscillatory force has been studied via overdamped Brownian dynamics simulations. The system exhibits a stochastic resonance (SR) phenomenon which arises from both geometric and entropic effects. SR only occurs when the amplitude and frequency of the external force are greater and smaller than the critical values respectively. Another important factor is the lateral dimension of the cavities, which affects the chain's timely access to the hole at a given thermal noise via lateral diffusion before the direction reversal of the periodic force. The entropic effect associated with the polymer conformation emerges when the pore width is sufficiently small relative to the chain size. SR may take place at smaller thermal noise for longer chains due to the bead connectivity that assists the chain migration through the hole.

Mechanofluorescent Polymer Brush Surfaces that Spatially Resolve Surface Solvation.

Polymer brushes, consisting of densely end-tethered polymers to a surface, can exhibit rapid and sharp conformational transitions due to specific stimuli, which offer intriguing possibilities for surface-based sensing of the stimuli. The key toward unlocking these possibilities is the development of methods to readily transduce signals from polymer conformational changes. Herein, we report on single-fluorophore integrated ultrathin (<40 nm) polymer brush surfaces that exhibit changing fluorescence properties based on polymer conformation. The basis of our methods is the change in occupied volume as the polymer brush undergoes a collapse transition, which enhances the effective concentration and aggregation of the integrated fluorophores, leading to a self-quenching of the fluorophores' fluorescence and thereby reduced fluorescence lifetimes. By using fluorescence lifetime imaging microscopy, we reveal spatial details on polymer brush conformational transitions across complex interfaces, including at the air-water-solid interface and at the interface of immiscible liquids that solvate the surface. Furthermore, our method identifies the swelling of polymer brushes from outside of a direct droplet (i.e., the polymer phase with vapor above), which is controlled by humidity. These solvation-sensitive surfaces offer a strong potential for surface-based sensing of stimuli-induced phase transitions of polymer brushes with spatially resolved output in high resolution.

The second Vassiliev measure of uniform random walks and polygons in confined space

Biopolymers, like chromatin, are often confined in small volumes. Confinement has a great effect on polymer conformations, including polymer entanglement. Polymer chains and other filamentous structures can be represented by polygonal curves in three-space. In this manuscript, we examine the topological complexity of polygonal chains in three-space and in confinement as a function of their length. We model polygonal chains by equilateral random walks in three-space and by uniform random walks (URWs) in confinement. For the topological characterization, we use the second Vassiliev measure. This is an integer topological invariant for polygons and a continuous functions over the real numbers, as a function of the chain coordinates for open polygonal chains. For URWs in confined space, we prove that the average value of the Vassiliev measure in the space of configurations increases as O(n 2) with the length of the walks or polygons. We verify this result numerically and our numerical results also show that the mean value of the second Vassiliev measure of equilateral random walks in three-space increases as O(n). These results reveal the rate at which knotting of open curves and not simply entanglement are affected by confinement.

Self-induced hydrodynamic coil-stretch transition of active polymers.

We analyze the conformational dynamics and statistical properties of an active polymer model. The polymer is described as a freely jointed bead-rod chain subject to stochastic active force dipoles that act on the suspending solvent where they drive long-ranged fluid flows. Using Langevin simulations of isolated chains in unconfined domains, we show how the coupling of active flows with polymer conformations leads to emergent dynamics. Systems with contractile dipoles behave similarly to passive Brownian chains with enhanced fluctuations due to dipolar flows. In systems with extensile dipoles, however, our simulations uncover an active coil-stretch transition whereby the polymer spontaneously unfolds and stretches out in its own self-induced hydrodynamic flow, and we characterize this transition in terms of a dimensionless activity parameter comparing active dipolar forces to thermal fluctuations. We discuss our findings in the context of the classic coil-stretch transition of passive polymers in extensional flows and complement our simulations with a simple kinetic model for an active trimer.

SANS Study of PPPO in Mixed Solvents and Impact on Polymer Nanoprecipitation.

We investigate the conformation of poly(2,6-diphenyl-p-phenylene oxide) (PPPO) in good and mixed solvents by small-angle neutron scattering (SANS) across its ternary phase diagram. Dichloromethane was selected as a “good” solvent and heptane as a “poor” solvent whose addition eventually induces demixing and polymer precipitation. Below the overlap concentration c*, the polymer conformation is found to be well described by the polymer-excluded volume model and above by the Ornstein–Zernike expression with a correlation length ξ which depends on the concentration and solvent/nonsolvent ratio. We quantify the decrease in polymer radius of gyration Rg, increase in ξ, and effective χ parameter approaching the phase boundary. Upon flash nanoprecipitation, the characteristic particle radius (estimated by scanning electron microscopy, SEM) is found to scale with polymer concentration as well as with nonsolvent content. Significantly, the solution volume per precipitated particle remains nearly constant at all polymer concentrations. Overall, our findings correlate ternary solution structure with the fabrication of polymer nanoparticles by nonsolvent-induced phase separation and precipitation.

Back Cover: PDMS with Tunable Side Group Mobility and Its Highly Permeable Membrane for Removal of Aromatic Compounds (Angew. Chem. Int. Ed. 6/2022)

Aromatics-permeable membranes made from polydimethylsiloxane with tunable side group mobilities are reported by Gongping Liu and co-workers in their Research Article (e202111810). Phenyl groups were introduced as rigid molecular spacers to endow the methyl groups with high mobility and reconstruct the polymer conformation, creating a highway for the fast transport of aromatic molecules, and thus realizing efficient removal of aromatic compounds from aqueous solution.

Superparamagnetic Iron Oxide Nanoparticles Decorated Mesoporous Silica Nanosystem for Combined Antibiofilm Therapy.

A crucial challenge to face in the treatment of biofilm-associated infection is the ability of bacteria to develop resistance to traditional antimicrobial therapies based on the administration of antibiotics alone. This study aims to apply magnetic hyperthermia together with controlled antibiotic delivery from a unique magnetic-responsive nanocarrier for a combination therapy against biofilm. The design of the nanosystem is based on antibiotic-loaded mesoporous silica nanoparticles (MSNs) externally functionalized with a thermo-responsive polymer capping layer, and decorated in the outermost surface with superparamagnetic iron oxide nanoparticles (SPIONs). The SPIONs are able to generate heat upon application of an alternating magnetic field (AMF), reaching the temperature needed to induce a change in the polymer conformation from linear to globular, therefore triggering pore uncapping and the antibiotic cargo release. The microbiological assays indicated that exposure of E. coli biofilms to 200 µg/mL of the nanosystem and the application of an AMF (202 kHz, 30 mT) decreased the number of viable bacteria by 4 log10 units compared with the control. The results of the present study show that combined hyperthermia and antibiotic treatment is a promising approach for the effective management of biofilm-associated infections.

Effect of pendant stereostructure on backbone conformation and enantioseparation ability of helical polyacetylene-based chiral stationary phases.

Six proline-derived acetylene monomers bearing either two stereocenters (S-mR, S-mS, R-mS, Rac-mS and S-mRac) or one stereocenter (S-mBn) were obtained from commercially available N-(tert-butoxycarbonyl)-prolinal. Under the catalysis of Rh-diene complex, they were converted to the corresponding optically active helical polymers, S-pR, S-pS, R-pS, Rac-pS, S-pRac, and S-pBn. The correlations between configuration and position of stereocenters in pendants with the polymer conformation as well as chiral resolution performance were systematically explored by a combination of nuclear magnetic resonance (NMR), Raman, UV-Vis absorption, electronic/vibration circular dichroism spectroscopies, high-performance liquid chromatography (HPLC), and computational simulation. The configuration of the stereocenter adjacent to polymer mainchain determined the sense of helical conformation and the elution order of analytes, while that of the remote one affected the arrangement of pendants and the scope of analytes that could be discriminated. Among 18 aromatic analytes selected, S-pR could discriminate 10, while S-pS recognized 12. The racemization of adjacent or remote stereocenters greatly reduced the scope of analytes that could be resolved. Based on computer simulations, S-pS had larger recognition space than S-pR, favoring the steric fit with the racemates containing axial chirality. The strength and number of intermolecular hydrogen bondings between enantiomers and CSPs predominantly determined the chiral discrimination.

Swelling of asymmetric pom-pom polymers in dilute solutions

In this paper we continue our recent analysis [K. Haydukivska et al., J. Mol. Liq., 2021, 328, 115456] of complex molecules with two branching points at both ends of the linear backbone with f1 and f2 side arms starting from them, known as the pom-pom polymers. Here, we analyze the asymmetric case, f1 ≠ f2, by applying both the analytical approach, based on the direct polymer renormalization, and computer simulations using both dissipative particle dynamics and Monte Carlo methods. We study the role played by the molecular asymmetry of average polymer conformations, considering the infinite dilution regime and good solvent conditions.The quantitative estimates are reported for the set of universal size and shape characteristics of such molecules and for their individual branches, all the functions of f1 and f2. In particular, we evaluate the size ratio of the gyration radii of symmetric and asymmetric pom-pom topologies with the same molecular weight and quantitatively reveal an increase of the effective size of a molecule caused by its asymmetry. We also introduce and analyse the asymmetry factor and estimate the shift of the center of mass caused by the presence of side stars, which can serve as another characteristic of the asymmetry of pom-pom structure.

Controlling mechanical properties of ultrahigh molecular weight ion gels by chemical structure of ionic liquids and monomers.

A new class of ion gels, termed ultrahigh molecular weight (UHMW) gels, formed by physical entanglement of ultrahigh molecular weight polymers in ionic liquids, are synthesised using facile one step radical polymerisation with significantly low initiator conditions, and exhibit superior mechanical characteristics such as stretchability, recyclability, and room temperature self-healing ability. In this study, UHMW gels are synthesised using various combinations of monomer and IL structures, and the effect of their chemical structures on the physicochemical properties of UHMW gels are thoroughly investigated. UHMW polymers are prepared in situ for all combinations of ILs and monomers used in this study, indicating the wide applicability of this fabrication strategy. The structure-property relationships between chemical structures and mechanical properties of UHMW gels are investigated in detail. Furthermore, the differences in self-healing efficiency of UHMW gels depending on the chemical structure is discussed in terms of individual polymer conformation and polymer-polymer interaction based on molecular dynamics simulations.

Emergent conformational properties of end-tailored transversely propelling polymers.

We study the dynamics and conformations of a single active semiflexible polymer whose monomers experience a propulsion force perpendicular to the local tangent, with the end beads being different from the inner beads ("end-tailored"). Using Langevin simulations, we demonstrate that, apart from sideways motion, the relative propulsion strength between the end beads and the polymer backbone significantly changes the conformational properties of the polymers as a function of bending stiffness, end-tailoring and propulsion force. Expectedly, for slower ends the polymer curves away from the moving direction, while faster ends lead to opposite curving, in both cases slightly reducing the center of mass velocity compared to a straight fiber. Interestingly, for faster end beads there is a rich and dynamic morphology diagram: the polymer ends may get folded together to 2D loops or hairpin-like conformations that rotate due to their asymmetry in shape and periodic flapping motion around a rather straight state during full propulsion is also possible. We rationalize the simulations using scaling and kinematic arguments and present the state diagram of the conformations. Sideways propelled fibers comprise a rather unexplored and versatile class of self-propellers, and their study will open novel ways for designing, e.g. motile actuators or mixers in soft robotics.

Interface engineering of calligraphic ink mediated conformal polymer fibers for advanced flexible supercapacitors

A novel and universal interface engineering strategy was developed for constructing conformal organic–inorganic–organic composite fiber electrodes from renewable marine polysaccharides to assemble high-performance symmetrical FESCs.

Ising density functional theory for weak polyelectrolytes with strong coupling of ionization and intrachain correlations.

We report a theoretical framework for weak polyelectrolytes by combining the polymer density functional theory with the Ising model for charge regulation. The so-called Ising density functional theory provides an accurate description of the effects of polymer conformation on the ionization of individual segments and is able to account for both the intra- and interchain correlations due to the excluded-volume effects, chain connectivity, and electrostatic interactions. Theoretical predictions of the titration behavior and microscopic structure of ionizable polymers are found to be in excellent agreement with the experiment.

External-induced self-assembly of semi-flexible polymers on spherical shell

Self-assembly behavior of semiflexible polymers on rigid spherical shell is investigated using the molecular dynamics (MD) simulation method. We find that the conformation of semiflexible polymers depends on the length of polymers and bending energy of polymer chains. For moderate adhesive strength and bending energy, the polymers aggregate into multiple clusters, each of which contains several parallel chains. Simultaneously, the self-assembly conformations, volley-ball, helical and tennis-ball state, form on spherical shell depend on the number of clusters. We quantify the self-assembly conformation and analysis its physical mechanics of formation. This work may provide a theoretical foundation for the future theories of conformation prediction and material fabrication.

Correction to “Significant Improvement of Unipolar n-Type Transistor Performances by Manipulating the Coplanar Backbone Conformation of Electron-Deficient Polymers via Hydrogen Bonding”

Correction to “Significant Improvement of Unipolar n-Type Transistor Performances by Manipulating the Coplanar Backbone Conformation of Electron-Deficient Polymers via Hydrogen Bonding”

Crowding-induced polymer trapping in a channel.

In this work, we report an intriguing phenomenon: crowding-induced polymer trapping in a channel. Using Langevin dynamics simulations and analytical calculations, we find that for a polymer confined in a channel, crowding particles can push a polymer into the channel corner through inducing an effective polymer-corner attraction due to the depletion effect. This phenomenon is referred to as polymer trapping. The occurrence of polymer trapping requires a minimum volume fraction of crowders, ϕ^{*}, which scales as ϕ^{*}∼(a_{c}/L_{p})^{1/3} for a_{c}≫a_{m} and ϕ^{*}∼(a_{c}/L_{p})^{1/3}(a_{c}/a_{m})^{1/2} for a_{c}≪a_{m}, where a_{c} is the crowder diameter, a_{m} is the monomer diameter, and L_{p} is the polymer persistence length. For DNA, ϕ^{*} is estimated to be around 0.25 for crowders with a_{c}=2nm. We find that ϕ^{*} also strongly depends on the shape of the channel cross section, and ϕ^{*} is much smaller for a triangle channel than a square channel. The polymer trapping leads to a nearly fully stretched polymer conformation along a channel corner, which may have practical applications, such as full stretching of DNA for the nanochannel-based genome mapping technology.

Impact of Hydroxypropyl Methylcellulose Acetate Succinate Critical Aggregation Concentration on Celecoxib Supersaturation.

Polymers play an important role in amorphous solid dispersions (ASDs), enhancing stability in the solid state and maintaining supersaturation in aqueous solutions of intrinsically low-water-soluble drug candidates. Hydroxypropyl methylcellulose acetate succinate (HPMCAS) is widely used in ASDs due to its hydrophobic/hydrophilic balance and ionizability of the substituent functionalities. While colloid formation of HPMCAS in solution due to this hydrophobic/hydrophilic balance has been studied, the impact of the polymer conformation (random coil vs aggregated) on drug supersaturation of ASDs is not well understood. To our knowledge, this is the first report where the critical aggregation concentration for three grades of HPMCAS (HF/MF/LF) has been determined via fluorescence spectroscopy using the environment-sensitive probe pyrene. The specific impact of polymer conformation (random coil vs aggregate) on the model drug celecoxib (CLX) has been elucidated with fluorescence quenching and nuclear magnetic resonance (NMR) spectroscopy. A negative deviation of the Stern-Volmer plot indicated that aggregated HPMCAS effectively blocked the quencher's access to CLX. This is further supported by NMR observations, where NMR spectra indicate a larger change of chemical shift of the -NH group of CLX when HPMCAS is above its aggregated concentration, suggesting strong H-bonding interactions between aggregated HPMCAS and CLX. Finally, the supersaturation-precipitation study shows that all three grades of HPMCAS in the aggregated state significantly enhanced CLX supersaturation compared to the nonaggregated state, indicating that polymer aggregation plays a critical role in maintaining drug supersaturation.