Random Polyampholytes Research Articles

Hydrophobic fouling-resistant electrospun nanofiber membranes from poly(vinylidene fluoride)/polyampholyte blends.

This study reports the fabrication of non-woven fibrous membranes from electrospinning blended solutions of PVDF with polyampholytes in N,N-dimethylformamide and methanol. Polyampholytes are macromolecules that have both positive and negative charged units in different side groups attached to the backbone. In this study, we used a random polyampholyte amphiphilic copolymer (r-PAC) synthesized by co-polymerizing a hydrophobic monomer in addition to the positive and negative charged monomer units, to reduce the fouling propensity of PVDF electrospun membranes while preserving its inherent hydrophobicity. Blends of PVDF/r-PAC were electrospun across the full range of compositions from 0/100 to 100/0. Scanning electron microscopic analysis showed formation of beaded fibers with average fibril diameters from 0.09-0.18 μm. The variation in the fiber diameters is caused by the change in surface charge density, dynamic viscosity of the solution, and the instability of the Taylor cone. Bead formation was observed in the mats electrospun from less viscous solutions. Wide angle X-ray scattering showed that electrospun fibers of PVDF crystallized into the electro-active β and γ crystal phases, whereas polyampholytes were amorphous. Thermogravimetry showed that the PVDF/r-PAC blends have a multi-step thermal degradation mechanism while PVDF homopolymer showed single-step thermal degradation. Sessile drop contact angle measurements confirmed that fibers possess high hydrophobicity and super-oleophilicity. Adsorptive fouling experiments with a fluorescently labeled protein confirmed that the fiber mats obtained from the PVDF/r-PAC blends resist protein adsorption, exhibiting highly enhanced fouling resistance compared to the fibers obtained from homopolymer PVDF.

Amphiphilic Polyampholytes for Fouling-Resistant and Easily Tunable Membranes.

The versatility of membranes is limited by the narrow range of material chemistries on the market, which cannot address many relevant separations. Expanding their use requires new membrane materials that can be tuned to address separations by providing the desired selectivity and robustness. Self-assembly is a versatile and scalable approach to create tunable membranes with a narrow pore size distribution. This study reports the first examples of a new class of membrane materials that derives state-of-the-art permeability, selectivity, and fouling resistance from the self-assembly of random polyampholyte amphiphilic copolymers. These membranes feature a network of ionic nanodomains that serve as nanochannels for water permeation, framed by hydrophobic nanodomains that preserve their structural integrity. This copolymer design approach enables precise selectivity control. For example, sodium sulfate rejections can be tuned from 5% to 93% with no significant change in the pore size or fouling resistance. Membranes developed here have potential applications in wastewater treatment and chemical separations.

Shape Fluctuations of Random Polyampholyte and Intrinsically Disordered Protein Sequences

Charged polymers with quenched charge sequences adopt pearl-necklace structures due to the interplay between charge disorder and thermal fluctuations. Partially globular pearl necklace conformations from random (+/−) sequences are intrinsically heterogeneous and exhibit various structures characterized by different numbers of pearls. On the basis of the molecular dynamics simulations of random model polyampholytes (PAs), we study dynamics of pearls and associated conformational heterogeneity in pearl structures. Fast nucleation/dissolution of small pearls controls the dynamics of the number of pearls, which is well described by first order kinetic equations. Most of the individual sequences we considered have a rather stable number of large pearls. Only a few sequences allow for fast switchings in the number of large pearls following complex processes (trajectories in parameter space) going through states with different numbers of pearls. Processes are most complex in the tail of the switching time distribution. The specific sequences of two intrinsically disordered proteins (IDPs) are studied along the same lines. Our study shows the resemblance in shape fluctuations between the IDP sequences and the fast-switching PA sequences.

Translocation, Rejection and Trapping of Polyampholytes.

Polyampholytes (PA) are a special class of polymers comprising both positive and negative monomers along their sequence. Most proteins have positive and negative residues and are PAs. Proteins have a well-defined sequence while synthetic PAs have a random charge sequence. We investigated the translocation behavior of random polyampholyte chains through a pore under the action of an electric field by means of Monte Carlo simulations. The simulations incorporated a realistic translocation potential profile along an extended asymmetric pore and translocation was studied for both directions of engagement. The study was conducted from the perspective of statistics for disordered systems. The translocation behavior (translocation vs. rejection) was recorded for all sequences comprised of N = 20 charged monomers. The results were compared with those for random sequences of N = 40 to better demonstrate asymptotic laws. At early times, rejection was mainly controlled by the charge sequence of the head part, but late translocation/rejection was governed by the escape from a trapped state over an antagonistic barrier built up along the sequence. The probability distribution of translocation times from all successful attempts revealed a power-law tail. At finite times, there was a population of trapped sequences that relaxed very slowly (logarithmically) with time. If a subensemble of sequences with prescribed net charge was considered the power-law decay was steeper for a more favorable net charge. Our findings were rationalized by theoretical arguments developed for long chains. We also provided operational criteria for the translocation behavior of a sequence, explaining the selection by the translocation process. From the perspective of protein translocation, our findings can help rationalize the behavior of intrinsically disordered proteins (IDPs), which can be modeled as polyampholytes. Most IDP sequences have a strong net charge favoring translocation. Even for sequences with those large net charges, the translocation times remained very dispersed and the translocation was highly sequence-selective.

Macromolecular complexes of polyampholytes

Abstract The macromolecular complexes of random, regular, graft, block and dendritic polyampholytes with respect to transition metal ions, surfactants, dyes, polyelectrolytes, and proteins are discussed in this review. Application aspects of macromolecular complexes of polyampholytes in biotechnology, medicine, nanotechnology, catalysis are demonstrated.

Synthesis of Diblock Polyampholyte PAMPS-b-PMAPTAC and Its Adsorption on Bentonite.

To study the adsorption of polyampholyte on bentonite (Bent), a block polyampholyte, PAMPS-b-PMAPTAC, comprised of 2-Acrylamido-2-Methylpropane Sulfonic Acid (AMPS) units and Methacrylamido Propyl Trimethyl Ammonium Chloride (MAPTAC) units, was synthesized using reversible addition-fragmentation chain transfer polymerization (RAFT) method. The block polyampholyte samples were characterized by FTIR, 1H NMR and Gel Permeation Chromatography (GPC). The microstructure of block polyampholyte and random polyampholyte in deionized water indicated that uneven distribution of charged groups increased the entanglement of polymer chains. Addition of salt weakened the electrostatic interactions among charged groups, and, therefore, increased the zeta potential of polyampholyte in aqueous solutions. The adsorptive behaviors of PAMPS-b-PMAPTAC on Bent were studied using elemental analysis, and the effects of external factors were considered. The adsorption equilibrium of polymers on Bent was reached after 12 h. Increased temperature and increased salinity exerted a positive and negative effect on the adsorption of polyampholyte, respectively. The molecular weight played as the decisive factor for the adsorption of polyampholyte in the absence of NaCl, while the content of cationic groups acted as the main factor in the presence of NaCl. Block polyampholyte exhibited higher adsorption than random polyampholyte in the absence of salt. XRD results also indicated that block polyampholyte had a better intercalation effect than random polyampholyte.

Charge fluctuation effects on the shape of flexible polyampholytes with applications to intrinsically disordered proteins

Random polyampholytes (PAs) contain positively and negatively charged monomers that are distributed randomly along the polymer chain. The interaction between charges is assumed to be given by the Debye-Huckel potential. We show that the size of the PA is determined by an interplay between electrostatic interactions, giving rise to the polyelectrolyte effect due to net charge per monomer (σ) and an effective attractive PA interaction due to charge fluctuations, δσ. The interplay between these terms gives rise to non-monotonic dependence of the radius of gyration, R g , on the inverse Debye length, κ, when PA effects are important ( ). In the opposite limit, R g decreases monotonically with increasing κ. Simulations of PA chains, using a charged bead-spring model, further corroborate our theoretical predictions. The simulations unambiguously show that conformational heterogeneity manifests itself among sequences that have identical PA parameters. A clear implication is that the phases of PA sequences, and by inference intrinsically disordered proteins (IDPs), cannot be determined using only the bare PA parameters (σ and δσ). The theory is used to calculate the changes in R g on N, the number of residues for a set of IDPs. For a certain class of IDPs, with N between 24 and 441, the size grows as R g ∼ N 0.6, which agrees with data from small angle X-ray scattering experiments.

ARGET-ATRP synthesis and swelling response of compositionally varied poly(methacrylic acid-co-N,N-diethylaminoethyl methacrylate) polyampholyte brushes.

Modifying the composition of polyampholytes, which are comprised of charge-positive and charge-negative repeat units, directly contributes to trade-offs between charge and structure, which are externally regulated by solution pH and added salt. Here, the relative ratio of anionic and cationic comonomers is varied to tailor the stimuli-responsiveness of poly(methacrylic acid-co-N,N-diethylaminoethyl methacrylate) (P(MAA-co-DEAEMA)) brushes to changes in solution pH and an added zwitterion. These systems display a strong dependence on excess repeating units of either type and the random incorporation appears to facilitate self-neutralization of charges. Pseudo-living growth with smooth comonomer incorporation is achieved using activators regenerated by electron transfer atom transfer radical polymerization (ARGET ATRP), creating well-defined brushes. In situ ellipsometry measurements of solvated brush thickness indicate that at low and high pH, the brushes display polyelectrolyte behavior with a strong compositional dependence, but at intermediate pH values, the brushes show the characteristic collapse attributed to self-neutralization of polyampholytes. The polyampholyte brushes maintain these patterns of behavior across all compositions and in the presence of an added zwitterion, which contributes additional hydrophobic character as evidenced by decreases in the swollen layer thicknesses. The response of the P(MAA-co-DEAEMA) brushes to the organic osmolyte betaine is consistent with its tendency to stabilize proteins and peptides in a kosmotropic fashion. These studies add perspective to efforts to manipulate sequence in polyampholytic polymers, support broader efforts to tailor interfacial soft films for applications in biotechnology and sensing, and understand aggregation and stability of biological polymers.

Critical adsorption of periodic and random polyampholytes onto charged surfaces.

How different are the properties of critical adsorption of polyampholytes and polyelectrolytes onto charged surfaces? How important are the details of polyampholyte charge distribution on the onset of critical adsorption transition? What are the scaling relations governing the dependence of critical surface charge density on salt concentration in the surrounding solution? Here, we employ Metropolis Monte Carlo simulations and uncover the scaling relations for critical adsorption for quenched periodic and random charge distributions along the polyampholyte chains. We also evaluate and discuss the dependence of the adsorbed layer width on solution salinity and details of the charge distribution. We contrast our findings to the known results for polyelectrolyte adsorption onto oppositely charged surfaces, in particular, their dependence on electrolyte concentration.

Design of a C-b-(A-co-B)-b-C telechelic polyampholyte pH-responsive gelator

We report the synthesis and the pH dependent structural and rheological properties of a telechelic polyampholyte associative polymer, composed of a random polyampholyte central block, end-capped by shorter hydrophobic blocks [C-b-(A-co-B)-b-C block/random terpolymer type].

Cryoprotective properties of completely synthetic polyampholytes via reversible addition-fragmentation chain transfer (RAFT) polymerization and the effects of hydrophobicity

A completely synthetic polyampholyte cryoprotectant was developed with cationic and anionic monomers by reversible addition-fragmentation chain transfer polymerization. The neutralized random polyampholyte, which had an equal composition ratio of monomers, showed high cryoprotective properties in mammalian cells. Introduction of a small amount of hydrophobic monomer enhanced cell viability after cryopreservation, indicating the importance of hydrophobicity. Leakage experiments confirmed that these polyampholytes protected the cell membrane during cryopreservation. Due to low cytotoxicity, this polyampholyte has the potential to replace the convention cryoprotective agent dimethyl sulfoxide. The present study is the first to show that we can design a polymeric cryoprotectant that will protect the cell membrane during freezing using appropriate polymerization techniques.

Electrochemical Switching of Conformation of Random Polyampholyte Brushes Grafted onto Polypyrrole

We demonstrate the possibilities for subtle control over redox-driven surface switching that could be obtained by altering the balance between hydrophobic, ionic, and dipolar components of polymer brushes that are grafted onto electrochemically active conducting polymers (ECP). We extend our previous work on the conformation switching of polyzwitterionic brushes grafted onto ECP to the case of ABC random polyampholyte brushes: here, a statistical near-neutral copolymer of methyl methacrylate (MMA), methacryloxyethyltrimethylammonium chloride (METAC), and 3-sulfopropyl methacrylate potassium salt (SPMA). The statistical polyampholyte differs from the polyzwitterion in that it is not strictly neutral and the charges do not have a defined spatial relationship to one another. The consequence is a significant change in the switching behavior that is also dependent on the salt concentration in the electrolyte. The results can be understood by reference to the theory of polyampholyte conformation in solution given by Higgs and Joanny modified to apply to a surface-bound brush. Three states of the polyampholyte brush are deduced from electrochemical impedance spectroscopy (EIS): collapsed, partially collapsed, and expanded. At low salt concentration, the behavior was the opposite of that of the polyzwitterion: the brush switched between partially collapsed with the ECP reduced and expanded with the ECP oxidized. With increase of salt concentration, the switch changed, to collapsed with the ECP oxidized and partially collapsed with ECP reduced. At still higher salt concentration, the switch changed back again, to partially collapsed with the ECP reduced and expanded with the ECP oxidized. Measurements of surface wetting under electrochemical control supported the interpretation. The behavior can be contrasted with that of zwitterionic brushes, which show a switch between collapsed with ECP oxidized and expanded with ECP reduced, independent of salt concentration over the same range (10(-3)-2 M NaCl) as that studied here, and that of zwitterionic-hydrophobic block copolymers where the switch is suppressed at low salt concentration. The results illustrate the significant range of behavior that can be engineered into these electrochemically switchable systems.

Model Random Polyampholytes from Nonpolar Methacrylic Esters

Copolymerizations of nonpolar methacrylic esters followed by functional group conversions were employed to produce random polyampholytes. Random terpolymers of Solketal methacrylate (SMA):tert-butyl methacrylate (tBMA):N,N-dimethylaminoethyl methacrylate (DMAEMA) were prepared from 80:10:10 and 54:23:23 mol % mixtures of monomers by RAFT polymerization in 1,4-dioxane. The terpolymers were treated with iodomethane to convert the DMAEMA units to quaternary ammonium ions and then with concentrated aqueous HCl/1,4-dioxane to convert the SMA units to glyceryl methacrylate units and the tBMA units to methacrylic acid units. 1H NMR analyses of the unreacted monomers in reaction mixtures after partial polymerization and compositions of the terpolymers calculated from reactivity ratios support random structures for the terpolymers. 1H NMR analyses of the polymers support complete conversions of the nonpolar monomer units in the terpolymers to ionic and hydrophilic monomer units in the polyampholytes. SEC analyses ...

Stimuli-Responsive Interfaces Using Random Polyampholyte Brushes

We study the pH-responsive behavior of random polyampholyte brushes synthesized by surface-initiated Atom Transfer Radical Polymerization. We define the “annealed” polyampholyte as the random copolymer that contains weak acid (methacrylic acid) and weak base (2-(dimethylamino)ethyl methacrylate) units. The “semi-annealed” polyampholyte is defined as the random copolymer that contains the methacrylic acid and a strong base unit (the quaternized amine). The structure of the polyampholyte brushes in aqueous solutions at different pH is determined using neutron reflectivity. The monomer density profile and therefore the brush thickness are obtained. Polymer brushes with various grafting densities are compared. We demonstrate that the structure of the polyampholyte brush depends strongly on the net charge of the chains. With an excess of charge, the brush is as stretched as a polyelectrolyte brush, and the swelling behavior is in good agreement with the scaling laws. At the isoelectric point corresponding to a...

Synthesis of random polyampholyte brushes by atom transfer radical polymerization

AbstractWe report the synthesis of random polyampholyte brushes containing 2‐(dimethylamino)ethyl methacrylate (DMAEMA) and methacrylic acid (MAA). The preparation of polyampholyte brushes is performed by the “grafting from” strategy using surface‐initiated atom transfer radical polymerization (ATRP). The first step consists in the formation of the self‐assembled monolayer of the ATRP initiator. Secondly, the chains are grown from the surface by controlled/“living” radical polymerization. The random copolymer brushes and the corresponding homopolymers brushes containing 2‐(dimethylamino)ethyl methacrylate and tert‐butyl methacrylate (tBuMA) are prepared. The last step is the deprotection of the tBuMA form to the MAA segment by in situ hydrolysis reaction. The annealed DMAEMA group can also be converted to the quenched form by in situ quaternization reaction. This results in the formation of “annealed” and “semiannealed” polyampholyte brushes. The “annealed” polyampholyte corresponds to the random copolymer that contains only annealed units, weak acid and weak base. The “semiannealed” polyampholyte consists of the mixture of annealed (weak acid) and quenched (quaternized segment) units. Polyampholyte brushes with various grafting densities are synthesized and carefully characterized using surface techniques such as ellipsometry and FTIR‐ATR. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 4305–4319, 2008

A Monte Carlo Study of Weak Polyampholytes: Stiffness and Primary Structure Influences on Titration Curves and Chain Conformations

The conformation and titration curves of weak polyampholytes are examined using Monte Carlo simulations with screened Coulomb potentials in the Grand Canonical ensemble. Two different types of monomers are considered. Depending on the solution pH, monomers A are weak acidic sites that can either be negatively charged or uncharged (as carboxylic groups), whereas monomers B are weak basic sites that can either be positively charged or uncharged (as amino groups). The influence of the chain stiffness, primary structure, and ionic concentration on the acid/base properties of the polyampholyte chains are systematically investigated. By adjusting the pH values, titration curves and then the fractions of positively and negatively ionized charged monomers are calculated. Stiffness influence is estimated by comparing two models of chain: a fully flexible and a rod-like polyampholyte. Different primary structures such as statistical (diblock, octablock, and alternating) and random polyampholytes are also considered. We demonstrate that the primary structure plays important roles in the acid/base properties as well as the charge distribution along the polymer backbone of a statistical rod-like polyampholyte. When flexible polyampholytes are considered, polyampholyte conformations promote the attractive electrostatic interactions between positively and negatively charged monomers, hence leading to more or less compact conformations and acid/base properties relatively different in comparison to the rod-like polyampholytes. Various conformations such as extended, globular, and pearl-necklace conformations are found in good agreement with the literature by adjusting the interaction parameter between monomers and monomer stoichiometry.

Molecular Dynamics Simulations of Polyelectrolyte−Polyampholyte Complexes. Effect of Solvent Quality and Salt Concentration

Complexation between polyelectrolyte and polyampholyte chains in poor solvent conditions for the polyelectrolyte backbone has been studied by molecular dynamics simulations. In a poor solvent a polyelectrolyte forms a necklace-like structure consisting of polymeric globules (beads) connected by strings of monomers. The simulation results can be explained by assuming the existence of two different mechanisms leading to the necklace formation. In the case of weak electrostatic interactions, the necklace formation is driven by optimization of short-range monomer-monomer attraction and electrostatic repulsion between charged monomers on the polymer backbone. In the case of strong electrostatic interactions, the necklace structure appears as a result of counterion condensation. While the short-range attractions between monomers are still important, the correlation-induced attraction between condensed counterions and charged monomers and electrostatic repulsion between uncompensated charges provide significant contribution to optimization of the necklace structure. Upon forming a complex with both random and diblock polyampholytes, a polyelectrolyte chain changes its necklace conformation by forming one huge bead. The collapse of the polyelectrolyte chain occurs due to the neutralization of the polyelectrolyte charge by polyampholytes. In the case of the random polyampholyte, the more positively charged sections of the chain mix with negatively charged polyelectrolyte forming the globular bead while more negatively charged chain sections form loops surrounding the collapsed core of the aggregate. In the case of diblock polyampholyte, the positively charged block, a part of the negatively charged block, and a polyelectrolyte chain form a core of the aggregate with a substantial section of the negatively charged block sticking out from the collapsed core of the aggregate. In both cases the core of the aggregate has a layered structure that is characterized by the variations in the excess of concentration of monomers belonging to polyampholyte and polyelectrolyte chains throughout the core radius. These structures appear as a result of optimization of the net electrostatic energy of the complex and short-range attractive interactions between monomers of the polyelectrolyte chain.

Phase behavior of polyampholytes from charged hard-sphere chain model

A molecular thermodynamic theory is developed for polyampholytes from the coarse-grained charged hard-sphere chain model. The phase behavior of polyampholytes with variations in sequence and chain length is satisfactorily predicted by the theory, consistent with simulation results and experimental observations. At a fixed chain length, the phase envelope expands as the sequence of charge distribution becomes less random. With increasing chain length, the phase envelope expands for diblock and random polyampholytes, but shrinks for zwitterionic polyampholytes. The predicted critical temperature, density, and pressure exhibit scaling relations with chain length for all the three (diblock, random, and zwitterionic) polyampholytes.

Phase behaviour of polyampholyte chains from grand canonical Monte Carlo simulations

The phase behaviour of polyampholyte chains has been investigated by grand canonical Monte Carlo simulations. First, the effects of chain length and charge sequence on the critical parameters and phase diagrams of fully-charged but overall neutral polyampholytes up to a chain length of 16 beads were determined. As the chain length increases, the critical temperature increases while the critical volume fraction decreases. The critical temperatures of the diblock polyampholytes are much higher and increase more quickly with increasing chain length than the critical temperatures of random polyampholytes, consistent with current experimental observations and theoretical predictions. This difference is attributed to the localization of like charges in a diblock chain which leads to stronger attraction and aggregation between chains. Cluster size distributions also reveal a stronger tendency of diblock polyampholytes to form larger aggregates than random chains for the same conditions. The phase behaviour of end-charged polyampholytes up to a chain length of 18 beads was also investigated, and both the critical temperature and volume fraction were observed to decrease with increasing chain length. The formation of micelles was not observed up to the longest chain length considered. Instead, the chains form clusters which eventually aggregate into a percolating gel-like structure through chain bridges spanning across different clusters. The sol–gel density–temperature locus has also been determined for the longest chain length considered, and the vapour–liquid transition is found to be between two gel phases.

Molecular Dynamics Simulations of Polyampholyte−Polyelectrolyte Complexes in Solutions

We have studied how the charge distribution along a polyampholyte backbone influences aggregation of polyampholyte and polyelectrolyte chains in dilute and semidilute solutions. Using molecular dynamics (MD) simulations, we have shown that the complexation between polyampholyte and polyelectrolyte chains is due to polarization-induced attractive interactions between molecules. A polyampholyte chain binds to a polyelectrolyte in such a way to maximize the electrostatic attraction between oppositely charged ionic groups and minimize the electrostatic repulsion between similarly charged ones. The charge sequence along the polyampholyte backbone has a profound effect on the complex structure. In dilute solutions, a diblock polyampholyte could form a three-arm starlike complex in which the longest branches of the star are formed either by two sections of the polyelectrolyte chain or by a negatively charged block of the polyampholyte and by a section of the polyelectrolyte chain. There are no such complexes in solutions of random polyampholytes and polyampholytes with short blocky charge sequences. In dilute solutions of moderate polymer concentration polyampholytes with long blocky charge sequences form mixed micellar aggregates containing both polyampholyte and polyelectrolyte chains. In semidilute solutions diblock polyampholytes form a network of micelles spanning the entire system. On the contrary, the structure of multichain aggregates formed by random polyampholytes and polyelectrolytes resembles that of branched polymers with polyampholyte chains cross-linking polyelectrolytes together. The osmotic coefficients of polyampholyte polyelectrolyte mixtures show no dependence on the charge sequence along the polymer backbone, confirming the leading contribution of small ions to osmotic pressure of ionic systems.