Emerging trends in the dynamics of polyelectrolyte complexes

Polyelectrolyte complex-based materials for separations: progress, challenges and opportunities

Polyelectrolyte complex (PEC) based materials could provide a sustainable alternative to conventional materials, especially for separation applications. However, reproducible production remains a challenge due to the many parameters influencing the polyelectrolyte complexation process, eventually affecting the properties and performance of the final material. Here, we provide an overview of how different parameters affect polyelectrolyte complexation and discuss promising PEC-based materials for separation applications, i.e., porous membranes, functional and barrier coatings, adhesives, saloplastics, and extraction media. Additionally, we highlight the challenges and opportunities and discuss what is needed to get to the next level. We envision that collaboration between experimentalists and theoreticians can leverage experimental datasets with accurate descriptions of all the parameters for multiscale modelling, machine learning and artificial intelligence approaches that can be used to design PEC materials and predict their properties.

Development of a novel polyelectrolyte complex nanocomposite of modified chitosan and karaya gum for co-delivery of 5-fluorouracil and curcumin for cancer therapy

Combination chemotherapy is a relatively recent and preferred method for cancer treatment. Sustained delivery of dual drugs can be achieved with a suitable matrix. In the present work, a pH-responsive polyelectrolyte complex (PEC) of trimethylchitosan and carboxymethylkaraya gum containing silver nanoparticles (SNps) has been developed as a matrix material for co-delivery of the drugs, 5-fluorouracil (5-Fu) and curcumin (Cur). The experimental conditions have been optimized for high yield and high swelling of the PEC nanocomposite. 1H-NMR, FT-IR, FE-SEM, P-XRD, HR-TEM, EDS, TGA techniques and zeta potential measurements have been employed in the physico-chemical characterization of the nanocomposite material. The presence of SNps with an average diameter of 16.57 ± 1.25 nm influenced the surface structure and hydrophilicity of the PEC. The swelling study indicated higher swelling at pH 7.4 than at pH 1.2. The two drugs, 5-Fu and Cur, were successfully entrapped and released from the nanocomposite in a sustained manner. Cytotoxicity studies performed with the MCF-10A cell line confirmed the biocompatibility of the nanocomposite and those with the MCF-7 cell line indicated the synergistic effect of the dual drugs in controlling cancer cell growth. The overall study indicates the usefulness of the PEC nanocomposite made from modified polysaccharides, chitosan and karaya gum as a promising material for the development of a dual drug delivery system for cancer treatment.

The Whole Is Greater than the Sum of Its Parts - Challenges and Perspectives in Polyelectrolytes

Polyelectrolytes offer unique properties for biological applications due to their charged nature and high water solubility. Here, the challenges in their synthesis and characterization techniques are reviewed, emphasizing that their strong interactions with the surrounding media and counterions must be considered when working with this interesting class of materials. Their potential in complexation for gene delivery, their unique stealth and anti-fouling properties, and their more specific interactions with amino acid transporters for cancer therapy are highlighted. The underlying mechanisms responsible for their biological efficacy, including the proton sponge effect for endosomal release and their interactions with cellular membranes, are addressed. For polyelectrolytes with a high level of usage, an overview is given of their historical context. This Perspective outlines the potential of polyelectrolytes for innovative applications in the field of biomedicine. Considering the physicochemical characteristics of this class of materials, this work strives to elucidate the distinctive properties and applications of polyelectrolytes.

Sticky Science: Using Complex Coacervate Adhesives for Biomedical Applications

Tissue adhesives are used for various medical applications, including wound closure, bleeding control, and bone healing. Currently available options often show weak adhesion or cause adverse effects. Recently, there has been an increasing interest in complex coacervates as medical adhesives. Complex coacervates are formed by mixing oppositely charged macromolecules that associate and undergo liquid-liquid phase separation, in which the dense bottom phase is the complex coacervate. Complex coacervates are strong and often biocompatible, and show strong underwater adhesion. The properties of the resulting materials are tunable by intrinsic factors such as polymer chemistry, molecular weight, charge density, and topology of the macromolecules, as well as extrinsic factors such as temperature, pH, and salt concentration. Therefore, complex coacervates are interesting new candidates for medical adhesives. In this review, it is described how complex coacervates form and how different factors influence their behavior. Next, an overview of recent studies on complex coacervates in the context of medical adhesives is presented. The application of complex coacervates as hemostatic or embolic agents, skin or bone repair adhesives, and soft tissue sealants is discussed. Lastly, additional possibilities for utilizing these materials in the future are discussed.

Hydrogels Based on Polyelectrolyte Complexes: Underlying Principles and Biomedical Applications

Ionic complexes of electrostatically charged biomacromolecules are key players in various biological processes like nucleotide transportation, organelle formation, and protein folding. These complexes, abundant in biological systems, contribute to the function, responsiveness, and mechanical properties of organisms. Coherent with these natural phenomena, hydrogels formed through the complexation of oppositely charged polymers exhibit unique attributes, such as rapid self-assembly, hierarchical microstructures, tunable properties, and protective encapsulation. Consequently, polyelectrolyte complex (PEC) hydrogels have garnered considerable interest, emerging as an up-and-coming platform for various biomedical applications. This review outlines the underlying principles governing PEC hydrogels. The classification of polyelectrolytes and the self-assembly of PEC hydrogels are discussed, including the factors influencing their self-assembly process. Recent developments of PEC hydrogels for biomedical applications, including drug delivery, tissue engineering, wound healing and management, and wearable sensors, are summarized. This review concludes with the prospective directions for the next generation of PEC hydrogel research.

Polyelectrolyte complexes based on a novel and sustainable hemicellulose-rich lignosulphonate for drug delivery applications

Polyelectrolyte complexes (PECs) are polymeric structures formed by the self-assembly of oppositely charged polymers. Novel biomaterials based on PECs are currently under investigation as drug delivery systems, among other applications. This strategy leverages the ability of PECs to entrap drugs under mild conditions and control their release. In this study, we combined a novel and sustainably produced hemicellulose-rich lignosulphonate polymer (EH, negatively charged) with polyethyleneimine (PEI) or chitosan (CH, positively charged) and agar for the development of drug-releasing PECs. A preliminary screening demonstrated the effect of several parameters (polyelectrolyte ratio, temperature, and type of polycation) on PECs formation. From this, selected formulations were further characterized in terms of thermal properties, surface morphology at the microscale, stability, and ability to load and release methylene blue (MB) as a model drug. EH/PEI complexes had a more pronounced gel-like behaviour compared to the EH/CH complexes. Differential scanning calorimetry (DSC) results supported the establishment of polymeric interactions during complexation. Overall, PECs' stability was positively affected by low pH, ratios close to 1:1, and the addition of agar. PECs with higher EH content showed a higher MB loading, likely promoted by stronger electrostatic interactions. The EH/CH formulation enriched with agar showed the best sustained release profile of MB during the first 30 h in a pH-dependent environment simulating the gastrointestinal tract. Overall, we defined the conditions to formulate novel PECs based on a sustainable hemicellulose-rich lignosulphonate for potential applications in drug delivery, which promotes the valuable synergy between sustainability and the biomedical field.

Counter Anion Type Influences the Glass Transition Temperature of Polyelectrolyte Complexes

Salt acts as a plasticizer in polyelectrolyte complexes (PECs), which impacts the physical, thermal, and mechanical properties, thus having implications in applications, such as drug delivery, energy storage, and smart coatings. Added salt disrupts polycation-polyanion intrinsic ion pairs, lowering a hydrated PEC's glass transition temperature (Tg). However, the relative influence of counterion type on the PEC's Tg is not well understood. Here, the effect of anion type (NaCl, NaBr, NaNO3, and NaI) on the Tg of solid-like, hydrated PECs composed of poly(diallydimethylammonium) (PDADMA)-poly(styrenesulfonate) (PSS) is investigated. With increasing the chaotropic nature of the salt anion, the Tg decreases. The relative differences are attributed to the doping level, the amount of bound water, the mobility of water molecules within the PECs, and the strength of interactions between the PEs. For all studied salt concentrations and salt types, the Tg followed the scaling of -1/Tg ≈ ln([IP]/[H2O]), in which [IP]/[H2O] is the ratio of intrinsic pairs to water. The scaling estimates that about 7 to 17% of the intrinsic ion pairs should be weakened for the PEC to partake in a glass transition. Put together, this study highlights that the Tg in PECs is impacted by the salt anion, but the mechanism of the glass transition remains unchanged.

Tuning the underwater adhesiveness of antibacterial polysaccharides complex coacervates

HYPOTHESIS

Adjusting the water content and mechanical properties of polyelectrolyte coacervates for optimal underwater adhesion requires simultaneous control of the macromolecular design and the type and concentration of the salt used. Using synthetic or bio-inspired polymers to make coacervates often involves complicated chemistries and large variations in salt concentration. The underwater adhesiveness of simple, bio-sourced coacervates can be tuned with relatively small variations in salt concentration. Bio-sourced polymers can also impart beneficial biological activities to the final material.

EXPERIMENTS

We made complex coacervates from charged chitosan (CHI) and hyaluronic acid (HA) with NaCl as the salt. Their water content and viscoelastic properties were investigated to identify the formulation with optimal underwater adhesion in physiological conditions. The coacervates were also studied in antibacterial and cytotoxicity experiments.

FINDINGS

As predicted by linear rheology, the CHI-HA coacervates at 0.1 and 0.2 M NaCl had the highest pull-off adhesion strengths of 44.4 and 40.3 kPa in their respective supernatants. In-situ physical hardening of the 0.2 M coacervate upon a salt switch in 0.1 M NaCl resulted in a pull-off adhesion strength of 62.9 kPa. This material maintained its adhesive properties in physiological conditions. Finally, the optimal adhesive was found to be non-cytotoxic and inherently antimicrobial through a chitosan release-killing mechanism.

Evaporation-Induced Polyelectrolyte Complexation: The Role of Base Volatility and Cosolvents

Film formation is a vital step for coating applications where a homogeneous, defect-free solid phase should be obtained, starting from a liquid casting formulation. Recently, an alternative waterborne-coating approach was proposed, based on the formation of a polyelectrolyte complex film. In this approach, an evaporating base induces a pH change during drying that initiates the complexation of oppositely charged polyelectrolytes, followed by further densification. In previous studies, ammonia was used as the evaporative base, leading to relatively fast evaporation and resulting in films showing significant brittleness, which tended to crack at low relative humidity or larger thicknesses. We hypothesize that slower complexation and/or evaporation can reduce the problematic stress build-up in the prepared polyelectrolyte complex coatings. For this reason, we studied the changes in the film formation process when there are different bases and cosolvents. We found that reducing the evaporation rate by changing ammonia to the slower evaporating dimethylamine or by adding DMSO as a cosolvent, led to less internal stress build-up during film formation, which could be beneficial for film application. Indeed, films prepared with ammonia showed cracking after 1 h, while films prepared with dimethylamine only showed cracking after one month. The fast evaporation of ammonia was also found to cause a temporary turbid phase, indicating phase separation, while for the slower evaporating bases, this did not occur. All prepared films remained sensitive to humidity, which poses the next challenge for these promising coatings.

Moderated ionic bonding for water-free recyclable polyelectrolyte complex materials

While nature extensively uses electrostatic bonding between oppositely charged polymers to assemble and stabilize materials, harnessing these interactions in synthetic systems has been challenging. Synthetic materials cross-linked with a high density of ionic bonds, such as polyelectrolyte complexes, only function properly when their charge interactions are attenuated in the presence of ample amounts of water; dehydrating these materials creates such strong Coulombic bonding that they become brittle, non-thermoplastic, and virtually impossible to process. We present a strategy to intrinsically moderate the electrostatic bond strengths in apolar polymeric solids by the covalent grafting of attenuator spacers to the charge carrying moieties. This produces a class of polyelectrolyte materials that have a charge density of 100%, are processable and malleable without requiring water, are highly solvent- and water-resistant, and are fully recyclable. These materials, which we coin "compleximers," marry the properties of thermoplastics and thermosets using tailored ionic bonding alone.

Evaluation of the impact of tragacanth/xanthan gum interpolymer complexation with chitosan on pharmaceutical performance of gels with secnidazole as potential periodontal treatment

Periodontitis consists a group of dental disorders that affect about 70 % of the world population. The therapy mainly relies on mechanical removing bacterial biofilm, nevertheless, local or systemic antibacterial agents play a key role in treating the acute conditions. Secnidazole is a newer derivative of commonly used metronidazole with high safety profile and broad spectrum of antimicrobial activity. The aim of the study was to evaluate the applicability of polyelectrolyte complex-based hydrogels composed of anionic tragacanth with addition of xanthan gum and cationic chitosan as carriers for buccal/intra pocket delivery of secnidazole. Prepared hydrogels with 5 % and 10 % (w/w) drug content were evaluated pharmaceutically towards inter alia physicomechanical, rheological and thermal properties, drug release kinetics, swelling behavior or antimicrobial activity. Cytotoxicity against human primary umbilical vein endothelial cells was also assessed with two independent method. Stable compositions with secnidazole were obtained, however, various miscibility of the drug with the polymers was noted. By adding chitosan, antibacterial activity and swelling performance of the gels were improved, nevertheless, drop of the mucoadhesiveness was also recorded. Hydrogels with 5 % secnidazole were selected as effective antimicrobial compositions with the highest cytocompatibility. They might be considered as promising for oromucosal application with special attention given to SEC as an alternative locally administered antimicrobial agent.

A Perspective on the Glass Transition and the Dynamics of Polyelectrolyte Multilayers and Complexes

Polyelectrolyte multilayers (PEMs) or polyelectrolyte complexes (PECs), formed by layer-by-layer assembly or the mixing of oppositely charged polyelectrolytes (PEs) in aqueous solution, respectively, have potential applications in health, energy, and the environment. PEMs and PECs are very tunable because their structure and properties are influenced by factors such as pH, ionic strength, salt type, humidity, and temperature. Therefore, it is increasingly important to understand how these factors affect PECs and PEMs on a molecular level. In this Feature Article, we summarize our contributions to the field in the development of approaches to quantify the swelling, thermal properties, and dynamic mechanical properties of PEMs and PECs. First, the role of water as a plasticizer and in the glass-transition temperature (Tg) in both strong poly(diallyldimethylammonium)/poly(sodium 4-styrenesulfonate) (PDADMA/PSS) and weak poly(allylamine hydrochloride)/poly(acrylic acid) (PAH/PAA) systems is presented. Then, factors influencing the dynamics of PECs and PEMs are discussed. We also reflect on the swelling of PEMs in response to different salts and solvent additives. Last, the nature of water's microenvironment in PEMs/PECs is discussed. A special emphasis is placed on experimental techniques, along with molecular simulations. Taken together, this review presents an outlook and offers recommendations for future research directions, such as studying the additional effects of hydrogen-bonding hydrophobic interactions.

The effect of monomer polarizability on the stability and salt partitioning in model coacervates

Coacervation of charged polymer chains has been a topic of major interest in both polymer and biological sciences, as it is a subset of a phenomenon called liquid-liquid phase separation (LLPS). In this process the polymer-rich phase separates from the polymer-lean supernatant while still maintaining its liquid-like properties. LLPS has been shown to play a crucial role in cellular homeostasis by driving the formation of membraneless organelles. It also has the potential to be harnessed to aid in novel therapeutical applications. Recent studies have demonstrated that there is no one simple mechanism which drives LLPS, which is instead a result of the combined effect of electrostatic, dipolar, hydrophobic, and other weak interactions. Using coarse-grained polymer simulations we investigate the relatively unexplored effects of monomer polarizability and spatially varying dielectric constant on LLPS propensity, and these factors affect the properties of the resulting condensates. In order to produce spatial variations in the dielectric constant, all our simulations include explicit solvent and counterions. We demonstrate that polarizability has only a minor effect on the bulk behaviour of the condensates but plays a major role when ion partitioning and microstructure are considered. We observe that the major contribution comes from the nature of the neutral blocks as endowing them with an induced dipole changes their character from hydrophobic to hydrophilic. We hypothesize that the results of this work can aid in guiding future studies concerned with LLPS by providing a general framework and by highlighting important factors which influence LLPS.

Glycan-Presenting Coacervates Derived from Charged Poly(active esters): Preparation, Phase Behavior, and Lectin Capture

This study presents the preparation and phase behavior of glycan-functionalized polyelectrolytes for capturing carbohydrate-binding proteins and bacteria in liquid condensate droplets. The droplets are formed by complex coacervation of poly(active ester)-derived polyanions and polycations. This approach allows for a straightforward modular introduction of charged motifs and specifically interacting units; mannose and galactose oligomers are used here as first examples. The introduction of carbohydrates has a notable effect on the phase separation and the critical salt concentration, potentially by reducing the charge density. Two mannose binding species, concanavalin A (ConA) and Escherichia coli, are shown to not only specifically bind to mannose-functionalized coacervates but also to some degree to unfunctionalized, carbohydrate-free coacervates. This suggests non-carbohydrate-specific charge-charge interactions between the protein/bacteria and the droplets. However, when mannose interactions are inhibited or when non-binding galactose-functionalized polymers are used, interactions are significantly weakened. This confirms specific mannose-mediated binding functionalization and suggests that introducing carbohydrates reduces non-specific charge-charge interactions by a so far unidentified mechanism. Overall, the presented route toward glycan-presenting polyelectrolytes enables new functional liquid condensate droplets with specific biomolecular interactions.

Polymer Complex Fiber: Property, Functionality, and Applications

Polymer complex fibers (PCFs) are a novel kind of fiber material processed from polymer complexes that are assembled through noncovalent interactions. These can realize the synergy of functional components and miscibility on the molecular level. The dynamic character of noncovalent interactions endows PCFs with remarkable properties, such as reversibility, stimuli responsiveness, self-healing, and recyclability, enabling them to be applied in multidisciplinary fields. The objective of this article is to provide a review of recent progress in the field of PCFs. The classification based on chain interactions will be first introduced followed by highlights of the fabrication technologies and properties of PCFs. The effects of composition and preparation method on fiber properties are also discussed, with some emphasis on utilizing these for rational design. Finally, we carefully summarize recent advanced applications of PCFs in the fields of energy storage and sensors, water treatment, biomedical materials, artificial actuators, and biomimetic platforms. This review is expected to deepen the comprehension of PCF materials and open new avenues for developing PCFs with tailor-made properties for advanced application.

Liquid-liquid and liquid-solid separation in self-assembled chitosan-alginate and chitosan-pectin complexes

The complexation between two oppositely charged polyelectrolytes (PE) can lead liquid-liquid (complex coacervates, CC) or liquid-solid (solid precipitates, SP) phase separations. Herein, the effect of pH (2-11) and ionic strength (I, 0.05-1.0 M KCl) on the associative interactions between chitosan (QL)-alginate (SA) and QL-Pectin (Pec), polysaccharides widely used in biotechnology field, is described. pH and I, exhibited significant effect on the structure and phase transitions by modifying the ionization degree (α), pk_a, and associative interactions between PE. Onset of binding was established at pH_c 9, while continued acidification (pH_τ 5.8) led to simultaneous CC and SP exhibiting a maximum turbidity in both systems. At pH_δ 4.0, QL-Pec showed preferably CC structures whereas QL-SA maintained the CC and SP structures. At pH_ω 2, the associative interactions were suppressed due to the low ionization of Pec and SA. I (1.0 M) significantly diminished the interactions in QL-Pec due to charge screening. Molecular weight, second virial coefficient, hydrodynamic size, ionizable groups, and persistence length of polyion, influenced on the phase behavior of QL-Pec and QL-SA systems. Therefore, CC and SP are found simultaneously in both systems, their transitions can be modulated by intrinsic and environmental conditions, expanding the functional properties of complexed polysaccharides.

A mini-review on bio-inspired polymer self-assembly: single-component and interactive polymer systems

Biology demonstrates meticulous ways to control biomaterials self-assemble into ordered and disordered structures to carry out necessary bioprocesses. Empowering the synthetic polymers to self-assemble like biomaterials is a hallmark of polymer physics studies. Unlike protein engineering, polymer science demystifies self-assembly by purposely embedding particular functional groups into the backbone of the polymer while isolating others. The polymer field has now entered an era of advancing materials design by mimicking nature to a very large extend. For example, we can make sequence-specific polymers to study highly ordered mesostructures similar to studying proteins, and use charged polymers to study liquid-liquid phase separation as in membraneless organelles. This mini-review summarizes recent advances in studying self-assembly using bio-inspired strategies on single-component and multi-component systems. Sequence-defined techniques are used to make on-demand hybrid materials to isolate the effects of chirality and chemistry in synthetic block copolymer self-assembly. In the meantime, sequence patterning leads to more hierarchical assemblies comprised of only hydrophobic and hydrophilic comonomers. The second half of the review discusses complex coacervates formed as a result of the associative charge interactions of oppositely charged polyelectrolytes. The tunable phase behavior and viscoelasticity are unique in studying liquid macrophase separation because the slow polymer relaxation comes primarily from charge interactions. Studies of bio-inspired polymer self-assembly significantly impact how we optimize user-defined materials on a molecular level.

Quantification of Water-Ion Pair Interactions in Polyelectrolyte Multilayers Using a Quartz Crystal Microbalance Method

Water existing within thin polyelectrolyte multilayer (PEM) films has significant influence on their physical, chemical, and thermal properties, having implications for applications including energy storage, smart coatings, and biomedical systems. Ionic strength, salt type, and terminating layer are known to influence PEM swelling. However, knowledge of water’s microenvironment within a PEM, whether that water is affiliated with intrinsic or extrinsic ion pairs, remains lacking. Here, we examine the influence of both assembly and post-assembly conditions on the water–ion pair interactions of poly(styrene sulfonate)/poly(diallyldimethylammonium) (PSS/PDADMA) PEMs in NaCl and KBr. This is accomplished by developing a methodology in which quartz crystal microbalance with dissipation monitoring is applied to estimate the number of water molecules affiliated with an ion pair (i), as well as the hydration coefficient, π_salt^H₂O. PSS/PDADMA PEMs are assembled in varying ionic strengths of either NaCl and KBr and then exposed post-assembly to increasing ionic strengths of matching salt type. A linear relationship between the total amount of water per intrinsic ion pair and the post-assembly salt concentration was obtained at post-assembly salt concentrations >0.5 M, yielding estimates for both i and π_salt^H₂O. We observe higher values of i and π_salt^H₂O in KBr-assembled PEMs due to KBr being more effective in doping the assembly because of KBr’s more chaotropic nature as compared to NaCl. Lastly, when PSS is the terminating layer, i decreases in value due to PSS’s hydrophobic nature. Classical and ab initio molecular dynamics provide a microstructural view as to how NaCl and KBr interact with individual polyelectrolytes and the involved water shells. Put together, this study provides further insight into the understanding of existing water microenvironments in PEMs and the effects of both assembly and post-assembly conditions.

Valence-induced jumps in coacervate properties

Spontaneous phase separation, or coacervation, of oppositely charged macromolecules is a powerful and ubiquitous mechanism for the assembly of natural and synthetic materials. Two critical triggering phenomena in coacervation science and technology are highlighted here. The first is the transition from one (mixed) to two (separated) phases of polyelectrolytes coacervated with small molecules upon the addition of one or two charges per molecule. The second is a large jump in coacervate modulus and viscosity mediated by the addition of just one additional charge to a three-charged system. This previously unknown viscoelastic transition is relevant to those aspects of disease states that are characterized by abnormal mechanical properties and irreversible assembly.

Effect of Ethanol and Urea as Solvent Additives on PSS–PDADMA Polyelectrolyte Complexation

The effect of urea and ethanol additives on aqueous solutions of poly(styrenesulfonate) (PSS), poly(diallyldimethylammonium) (PDADMA), and their complexation interactions are examined here via molecular dynamics simulations, interconnected laser Doppler velocimetry, and quartz crystal microbalance with dissipation. It is found that urea and ethanol have significant, yet opposite influences on PSS and PDADMA solvation and interactions. Notably, ethanol is systematically depleted from solvating the charge groups but condenses at the hydrophobic backbone of PSS. As a consequence of the poorer solvation environment for the ionic groups, ethanol significantly increases the extent of counterion condensation. On the other hand, urea readily solvates both polyelectrolytes and replaces water in solvation. For PSS, urea causes disruption of the hydrogen bonding of the PSS headgroup with water. In PSS–PDADMA complexation, these differences influence changes in the binding configurations relative to the case of pure water. Specifically, added ethanol leads to loosening of the complex caused by the enhancement of counterion condensation; added urea pushes polyelectrolyte chains further apart because of the formation of a persistent solvation shell. In total, we find that the effects of urea and ethanol rise from changes in the microscopic-level solvation environment and conformation resulting from solvating water being replaced by the additive. The differences cannot be explained purely via considering relative permittivity and continuum level electrostatic screening. Taken together, the findings could bear significance in tuning polyelectrolyte materials’ mechanical and swelling characteristics via solution additives.

Hybrid polarizable simulations of a conventional hydrophobic polyelectrolyte. Toward a theoretical tool for green science innovation

Hybrid modeling approaches based on all-atom force fields to handle a solute and coarse-grained models to account for the solvent are promising numerical tools that can be used to understand the properties of large and multi-components solutions and thus to speed up the development of new industrial products that obey the standard of green and sustainable chemistry. Here, we discuss the ability of a full polarizable hybrid approach coupled to a standard molecular dynamics scheme to model the behavior in the aqueous phase and at infinite dilution conditions of a standard hydrophobic polyelectrolyte polymer whose charge is neutralized by explicit counterions. Beyond the standard picture of a polyelectrolyte behavior governed by an interplay between opposite intra-polyelectrolyte and inter-polyelectrolyte/counterion Coulombic effects, our simulations show the key role played by both intra-solute polarization effects and long range solute/solvent electrostatics to stabilize compact globular conformations of that polyelectrolyte. Our full polarizable hybrid modeling approach is thus a new theoretical tool well suited to be used in digital strategies for accelerating innovation for green science, for instance.

Characterization of Monte Carlo Dynamic/Kinetic Properties of Local Structure in Bond Fluctuation Model of Polymer System

We report the results of the characterization of local Monte Carlo (MC) dynamics of an equilibrium bond fluctuation model polymer matrix (BFM), in time interval typical for MC simulations of non-linear optical phenomena in host-guest systems. The study contributes to the physical picture of the dynamical aspects of quasi-binary mosaic states characterized previously in the static regime. The polymer dynamics was studied at three temperatures (below, above and close to the glass transition), using time-dependent generalization of the static parameters which characterize local free volume and local mobility of the matrix. Those parameters play the central role in the kinetic MC model of host-guest systems. The analysis was done in terms of the probability distributions of instantaneous and time-averaged local parameters. The main result is the characterization of time scales characteristic of various local structural processes. Slowing down effects close to the glass transition are clearly marked. The approach yields an elegant geometric criterion for the glass transition temperature. A simplified quantitative physical picture of the dynamics of guest molecules dispersed in BFM matrix at low temperatures offers a starting point for stochastic modeling of host-guest systems.

Aqueous Formulation of Concentrated Semiconductive Fluid Using Polyelectrolyte Coacervation

Conjugated polyelectrolytes (CPEs), which combine π-conjugated backbones with ionic side chains, are intrinsically soluble in polar solvents and have demonstrated tunability with respect to solution processability and optoelectronic performance. However, this class of polymers often suffers from limited solubility in water. Here, we demonstrate how polyelectrolyte coacervation can be utilized for aqueous processing of conjugated polymers at extremely high polymer loading. Sampling various mixing conditions, we identify compositions that enable the formation of complex coacervates of an alkoxysulfonate-substituted PEDOT (PEDOT-S) with poly(3-methyl-1-propylimidazolylacrylamide) (PA-MPI). The resulting coacervate is a viscous fluid containing 50% w/v polymer and can be readily blade-coated into films of 4 ± 0.5 μm thick. Subsequent acid doping of the film increased the electrical conductivity of the coacervate to twice that of a doped film of neat PEDOT-S. This higher conductivity of the doped coacervate film suggests an enhancement in charge carrier transport along PEDOT-S backbone, in agreement with spectroscopic data, which shows an enhancement in the conjugation length of PEDOT-S upon coacervation. This study illustrates the utilization of electrostatic interactions in aqueous processing of conjugated polymers, which will be useful in large-scale industrial processing of semiconductive materials using limited solvent and with added enhancements to optoelectronic properties.

Systematic Modification of the Glass Transition Temperature of Ion-Pair Comonomer Based Polyelectrolytes and Ionomers by Copolymerization with a Chemically Similar Cationic Monomer

Ion-pair comonomers (IPCs) where both the anion and cation contain polymerizable functional groups offer a route to prepare polyampholyte, ion-containing polymers. Polymerizing vinyl functional groups by free-radical polymerization produces bridging ion-pairs that act as non-covalent crosslinks between backbone segments. In particular the homopolymerization of the IPC vinyl benzyl tri-n-octylphosphonium styrene sulfonate produces a stiff, glassy polymer with a glass transition temperature (Tg) of 191 °C, while copolymerization with a non-ionic acrylate produces microphase separates ionomers with ion-rich and ion-poor domains. This work investigates the tuning of the Tg of the polyelectrolyte or ion-rich domains of the ionomers by copolymerizing with vinyl benzyl tri-n-octylphosphonium p-toluene sulfonic acid. This chemically similar repeat unit with pendant rather than bridging ion-pairs lowers the Tg compared to the polyelectrolyte or ionomer containing only the IPC segments. Rheological measurements were used to characterize the thermomechanical behavior and Tg of different copolymers. The Tg variation in the polyelectrolyte vs. weight fraction IPC could be fit with either the Gordon-Taylor or Couchman-Karasz equation. Copolymerization of IPC with a chemically similar cationic monomer offers a viable route to systematically vary the Tg of the resulting polymers useful for tailoring the material properties in applications such as elastomers or shape memory polymers.

Electrostatically Driven Topological Freezing of Polymer Diffusion at Intermediate Confinements

Breaking the paradigm that polymers in crowded aqueous media obey Einstein's law of diffusion, we report a localized nondiffusive hierarchical metastable state at intermediate confinements. Combining electrostatic and topological effects, we can tune the propensity of this new universality class in a quasicoacervate gel system consisting of guest polyamino acid chains inside an oppositely charged host hydrogel. Our observations offer strategies for controlled release and retention of macromolecules in aqueous crowded media, while opening a new direction for understanding topologically frustrated dynamics in polymers and other soft matter systems.