Polyampholyte Chains Research Articles

Long-lived metastable knots in polyampholyte chains.

Knots in proteins and DNA are known to have significant effect on their equilibrium and dynamic properties as well as on their function. While knot dynamics and thermodynamics in electrically neutral and uniformly charged polymer chains are relatively well understood, proteins are generally polyampholytes, with varied charge distributions along their backbones. Here we use simulations of knotted polymer chains to show that variation in the charge distribution on a polyampholyte chain with zero net charge leads to significant variation in the resulting knot dynamics, with some charge distributions resulting in long-lived metastable knots that escape the (open-ended) chain on a timescale that is much longer than that for knots in electrically neutral chains. The knot dynamics in such systems can be described, quantitatively, using a simple one-dimensional model where the knot undergoes biased Brownian motion along a "reaction coordinate", equal to the knot size, in the presence of a potential of mean force. In this picture, long-lived knots result from charge sequences that create large electrostatic barriers to knot escape. This model allows us to predict knot lifetimes even when those times are not directly accessible by simulations.

Polysaccharide-based polyampholyte complex formation: Investigating the role of intra-chain interactions

The difference in inter-chain and intra-chain electrostatic attraction was investigated in polyelectrolyte and polyampholyte electrostatic complex formation. Three polymers with similar backbone molecular structures including chitosan (Ch) polycation, carboxymethyl cellulose (CMCe) polyanion, and carboxymethyl chitosan (CMCh) polyampholyte were used for this purpose. The turbidimetric, water content, and rheological measurements for polyampholyte self-complex showed more dependence on the ionic strength rather than the polyelectrolyte one. The degree of dissociation (α), dissociation constant (pKa), and intrinsic persistence length were calculated by applying the Katchalsky-Lifson model to potentiometric data. We studied the gyration radii as a function of Debye length and observed the polyampholyte chain contractions due to the intra-chain electrostatic attractions, which minimize the entropic gain of the inter-chain complex formation. This is in accordance with the decrease in pKa by αc for CMCh which is the opposite of that for the Ch and CMCe samples. We also found that the polyampholyte has less intrinsic and electrostatic persistence length compared with both polyanion and polycation with similar chain structures indicating the impact of the inter-chain electrostatic interaction on the complex properties. This study deepens our insight about the behavior of CMCh and the nature of difference between CMCh and Ch/CMCe electrostatic complexes.

Influence of Charge Block Length on Conformation and Solution Behavior of Polyampholytes.

We investigate the effect of charge block length on polyampholyte chain conformation and phase behavior using small-angle X-ray scattering (SAXS) and implicit-solvent molecular simulations. To this end, we use solid phase peptide synthesis to precision-tailor a series of polyampholytes consisting of l-glutamic acid (E) and l-lysine (K) monomers arranged in alternating blocks from 2 to 16 monomers. We observe that the polyampholytes tend to phase separate as block size increases. With addition of NaCl, phase separated polyampholytes exhibit a salting-in effect dependent on charge block length. Fourier-transform infrared (FTIR) spectroscopy reveals the presence of intramolecular hydrogen bonds that are disrupted upon the addition of NaCl, implicating both electrostatic interactions and hydrogen bonding in the phase behavior. SAXS spectra at no-added salt conditions show minimal dependence of charge block length on the radius of gyration (Rg) for soluble polyampholytes, but local chain stiffening is found to be dependent on charge block length. With increasing NaCl, consistent with electrostatic screening, all polyampholytes expand and behave as neutral or swollen chains in good solvent conditions. Molecular simulations are qualitatively consistent with experiments. Implications for understanding intracellular condensates and material design are noted.

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.

Dynamics of a single polyampholyte chain.

Polymers that feature both positive and negative charges along chains, known as polyampholytes, represent a class of materials that hold promise for a new generation of energy storage devices, the designof which will require knowledge of the underlying structure and dynamics. Here, we develop a theory based on the Rouse model for the dynamic structure factor of a single polyampholyte chain in the weak coupling regime (negligible intramolecular electrostatics) or subjected to weak external electric fields (governed by linear response). Neglecting effects of small ions, we find deviations in scaling from the classic Rouse theory and make predictions for scattering experiments performed on polyampholytes. We find that, under weak coupling with arbitrarily strong fields, the dynamics are highly dependent on the charge distribution and consequently look at two representative examples-random charge densities and periodic charge densities-with different scaling properties. Under weak fields, the dynamics are largely independent of charge distribution. Finally, we investigate the influence of hydrodynamic effects and the implications of including inertial effects in the model.

Self-Healable Porous Polyampholyte Hydrogels with Higher Water Content as Cell Culture Scaffolds for Tissue Engineering Applications.

In this paper, we first demonstrate the control of the film pore size using neutral hydrophilic 2-hydroxyethyl methacrylate (HEMA) content. To improve the mechanical properties of a polyampholyte (PA), both HEMA and the cross-linker N,N'-methylenebisacrylamide (Bis-Am) were introduced into the PA chain. The predesigned copolymers showed great mechanical properties and optical behavior. The introduction of HEMA significantly increased the water content of the polymer, leading to the formation of porous structures in xerogels. The dynamic interaction between the positive and negative termini of the PA endowed the hydrogels with self-healing ability. The synthesized chemically cross-linked PA gels showed high stability in saline solution. The biocompatibility of the PA gels was confirmed using a cytotoxicity test of cells attached to the synthesized PA-X-2 and PA/HEMA-90/10-X-0.5. The results of this investigation indicate that the synthesized PA gels are applicable as a polymeric scaffold for cell culture.

Charge reversal in the collapse of polyelectrolyte star brushes under an electric field

We have performed Langevin dynamics simulations of the collapse of charged star brushes under an external electric field. It was found that the emergence of charge reversal of collapsed charged monomers on the grafting electrode is governed by the total net charge on a single grafted ordinary polyelectrolyte or polyampholyte chain. The mean value of the total net charge on a grafted star chain required to induce charge reversal on the grafting electrode decreases with increasing Bjerrum length. However, at very high Bjerrum length, the grafted star chains with ionizable functional groups are effectively neutral. Thus, the phenomenon of charge reversal becomes irrelevant at very high Bjerrum length. Simulation results revealed that the thickness of the adsorption layer is nearly independent of the average charge fraction, arm length and number of arms of grafted star chains as well as Bjerrum length. In the regime of charge reversal, simulation results showed that the degree of charge overcompensation increases with increasing charge fraction, but is nearly irrespective of the arm length and number of arms of grafted star chains. The degree of charge overcompensation was found to be a non-monotonic function of Bjerrum length and it reaches a maximum at an intermediate value of Bjerrum length. The effects of the characteristic parameters of grafted star chains and the applied electric field on the internal stratification of polyelectrolyte star brushes were also examined.

(Invited) Flexible and Self-Healing Aqueous Supercapacitors By Polyampholyte Gel Electrolytes with Biochar Electrodes and Their Unique Low Temperature Properties

Quenched polyampholytes provide a novel class of tough hydrogel that has self-healing ability, strong adhesion, and mechanical flexibility. Understanding the structure of polymer chains in the hydrogel and the phase behavior of water therein has broad impact on various applications, such as lubrication, adhesion, and electrical conductivity, as well as the hydrogel’s low temperature properties. In this paper, the structure of polymer chains of hydrogels made of a model charge-balanced polyampholyte, a random copolymer of poly(4-vinylbenzenesulfonate-co-[3-(methacryloylamino) propyl] trimethylammonium chloride), was investigated by small- and wide-angle x-ray scattering (SAXS and WAXS). The SAXS results suggested a networked globule structure in the charge-balanced polyampholyte hydrogels prevented freezing of water in the hydrogel, while the evidence of non-frozen water at low temperatures, such as –45 °C was monitored by solid-state 2H NMR. Correspondingly, we observed high ionic conductivity at low temperatures using electrochemical impedance spectroscopy (EIS). Interestingly, multiple freezing-thawing cycles did not impact the phase behavior of water in the hydrogel. We also found evidence that the crosslinked network structure of the polyampholyte chains disrupts the crystalline growth of ice, resulting in ‘slush-like’ ice formation. Utilizing the scientific investigations, a flexible and self-healing supercapacitor with high energy density in low temperature operation was fabricated using a polyampholyte hydrogel electrolyte. The electrode material was a biochar (produced from the low-temperature pyrolysis of biological wastes) bound by self-assembled reduced graphene oxide. Our novel sample preparation technique to enable the self-reinforcing wrapping of reduced graphene oxide sheets is turned out to have universal efficacy. The procedures of this technique will be introduced in the presentation. At the room temperature, the fabricated supercapacitor showed high energy density of 30 Wh/kg with 90% capacitance retention after 5000 charge-discharge cycles at room temperature at a power density of 50 W/kg. At –30 °C, the supercapacitor exhibited an energy density of 10.5 Wh/kg at a power density of 500 W/kg, which is threefold increase compared to control sample that do not incorporate polyampholyte hydrogel electrolyte. We also harnessed the tunable optical property of the polyampholyte hydrogel to fabricate a smart window. Specifically, we modulated the overall hydrophilicity/phobicity of polyampholyte chains when synthesizing the random copolymer and adjusted the upper critical solution temperature (UCST) at high precision, thus achieved a fine-tuning of UCST between 15 and 65 °C. Finally, we developed a stretchable, high-contrast, optically tunable stretchable window which consists of the PA hydrogel and a printed stretchable electric heater by our own ink recipe. In summary, we performed fundamental studies on the phase behavior of polymer chains and water molecules in quenched polyampholyte hydrogels by using synchrotron SAXS/WAXS, solid-state NMR, EIS, and DSC. We utilized the understanding in energy storage and smart window applications, both of which are unconventional for the application of tough hydrogels. In addition, we utilized renewable resource of biochar to fabricate low-cost, high-performance electrode material.

Complexation Behavior of Polyelectrolytes and Polyampholytes.

We perform grand canonical Monte Carlo simulations to study the pH titrations of isolated polyampholytes and polyelectrolyte-polyampholyte complexes in dilute solutions. Our simulations indicate that the electrostatic interactions promote the coexistence of opposite charges along the polyampholyte chain during titration. The repulsion between excess charges typically dominates the electrostatic interaction and leads to polymer stretching. Salt ions can screen the repulsion between excess charges as well as the fluctuation-induced attraction between opposite charges, and therefore make the variation between titration curves of polyampholytes and the ideal (no electrostatic interactions) curves less significant. We observe that this screening of charge repulsion decreases the chain size. The presence of pearl-necklace configuration of polyampholytes is diminished by the addition of salt. Similar simulations for the polyelectrolyte-polyampholyte system show that the resulting complexes are generally stable in the low pH region. In comparison to ideal case, electrostatic interactions strongly influence the acid-base properties of polyampholyte chains in the adsorbed state by reducing the presence of the coexistence domain of both positive and negative charges in the titration curves. We attribute the complex formation between polyelectrolyte and polyampholyte chains in the high pH region to, e.g., the high salt content. The pH variation leads to abrupt transition between adsorbed and desorbed states. Independent of charge sequence, a polyampholyte chain in a complex is usually located at one of the ends of the polyelectrolyte chain.

Electrohydrodynamics of spherical polyampholyte-grafted nanoparticles: Multiscale simulations by coupling of molecular dynamics and lattice-boltzmann method

We perform multiscale simulations based on the coupling of molecular dynamics and lattice-Boltzmann (LB) method to study the electrohydrodynamics of a polyampholyte-grafted spherical nanoparticle. The long-range hydrodynamic interactions are modeled by coupling the movement of particles to a LB fluid. Our results indicate that the net-neutral soft particle moves with a nonzero mobility under applied electric fields. We systematically explore the effects of different parameters, including the chain length, grafting density, electric field, and charge sequence, on the structures of the polymer layer and the electrophoretic mobility of the soft particle. It shows that the mobility of nanoparticles has remarkable dependence on these parameters. We find that the deformation of the polyampholyte chains and the ion distribution play dominant roles in modulating the electrokinetic behavior of the polyampholyte-grafted particle. The enhancement or reduction in the accumulation of counterions around monomers can be attributed to the polymer layer structure and the conformational transition of the chains in the electric field. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017

Flexible and Self-Healing Aqueous Supercapacitors for Low Temperature Applications: Polyampholyte Gel Electrolytes with Biochar Electrodes

A flexible and self-healing supercapacitor with high energy density in low temperature operation was fabricated using a combination of biochar-based composite electrodes and a polyampholyte hydrogel electrolyte. Polyampholytes, a novel class of tough hydrogel, provide self-healing ability and mechanical flexibility, as well as low temperature operation for the aqueous electrolyte. Biochar is a carbon material produced from the low-temperature pyrolysis of biological wastes; the incorporation of reduced graphene oxide conferred mechanical integrity and electrical conductivity and hence the electrodes are called biochar-reduced-graphene-oxide (BC-RGO) electrodes. The fabricated supercapacitor showed high energy density of 30 Wh/kg with ~90% capacitance retention after 5000 charge–discharge cycles at room temperature at a power density of 50 W/kg. At −30 °C, the supercapacitor exhibited an energy density of 10.5 Wh/kg at a power density of 500 W/kg. The mechanism of the low-temperature performance excellence is likely to be associated with the concept of non-freezable water near the hydrophilic polymer chains, which can motivate future researches on the phase behaviour of water near polyampholyte chains. We conclude that the combination of the BC-RGO electrode and the polyampholyte hydrogel electrolyte is promising for supercapacitors for flexible electronics and for low temperature environments.

Morphologies of spherical polyampholyte brushes: Effects of counterion valence and charged monomer sequence

We study the conformational behavior of spherical brushes composed of polyampholyte chains end-grafted onto spherical particles in the presence of monovalent and tetravalent counterions using molecular dynamics simulations. The overall net charge for each chain consisting of both positively and negatively charged segments is zero. The dependences of the structural properties of the brushes on the chain stiffness, grafting density and charge sequence along the chains are examined systemically. For the diblock brushes, our results indicate that increasing the chain rigidity leads to a significant effect of counterion valence and a reduced collapsing degree of the brushes. At high chain stiffness, the number of high-valent counterions trapped in the brushes is diminished compared to the cases with monovalent counterions, corresponding to lower osmotic pressure. For the flexible brushes, the short-range and electrostatic interactions depending on the charge sequence, determine the conformational transition of the brushes, whereas the effect of counterion valence becomes weaker. In the presence of high-valent counterions, the brushes which consist of polyampholyte chains with blocks of medium length, adopt a slightly stretched conformation owing to relatively strong electrostatic correlation between high-valent counterions and oppositely charged monomers.

Multi-tunable self-assembled morphologies of stimuli-responsive diblock polyampholyte films on solid substrates.

Intelligent polymers, due to their sensitive stimuli response to changes in the environment, have gained increasing amounts of attention over recent years and have become a popular topic in polymer materials science. In this study, the aggregation behaviors and stimuli responses of the stimuli-responsive diblock polyampholyte poly(2-(dimethylamino)ethyl methacrylate)-b-poly(acrylic acid) (PDMAEMA-b-PAA) are conveniently tunable by introducing pH changes, temperature changes, salt addition and surfactant neutralization. Under different pH values, either globular or fractal self-assemblies can be observed in which the latter have crystal properties. Salts and alkalis can promote the fractal aggregation growth because their deposited crystals can act as nucleation sites for the polyampholyte chains. A combination of fluorescence spectroscopy, atom force microscopy and transmission electron microscopy revealed that the presence of anionic surfactants in the PDMAEMA-b-PAA solutions promoted the solubility of the polyampholyte, consequently leading to a more homogeneous distribution of the polymer chains on the solid substrate upon drying the mixtures. The fractal formation was suppressed for the studied Gemini surfactants, and a higher surfactant hydrophobicity results in an earlier start of the formation of the fractal pattern.

Conformational transitions of a weak polyampholyte.

Using grand canonical Monte Carlo simulations of a flexible polyelectrolyte where the charges are in contact with a reservoir of constant chemical potential given by the solution pH, we study the behavior of weak polyelectrolytes in poor and good solvent conditions for polymer backbone. We address the titration behavior and conformational properties of a flexible diblock polyampholyte chain formed of two oppositely charged weak polyelectrolyte blocks, each containing equal number of identical monomers. The change of solution pH induces charge asymmetry in a diblock polyampholyte. For diblock polyampholyte chains in poor solvents, we demonstrate that a discontinuous transition between extended (tadpole) and collapsed (globular) conformational states is attainable by varying the solution pH. The double-minima structure in the probability distribution of the free energy provides direct evidence for the first-order like nature of this transition. At the isoelectric point electrostatically driven coil-globule transition of diblock polyampholytes in good solvents is found to consist of different regimes identified with increasing electrostatic interaction strength. At pH values above or below the isoelectric point diblock chains are found to have polyelectrolyte-like behavior due to repulsion between uncompensated charges along the chain.

Thermo-/pH-responsive behaviours of base-rich diblock polyampholytes in aqueous solution: experiment and simulation

We report on the thermo-responsive and pH-responsive behaviours exhibited by two poly(acrylic acid)-poly(2-(dimethylamino)ethyl methacrylate) diblock copolymers with unbalanced acid–base compositions, PAA40-PDMAEMA120 and PAA40-PDMAEMA285, in water, a selective solvent. Using a combination of ultraviolet–visible (UV–vis) spectroscopy and dynamic light scattering, the dual-stimuli responsive behaviours can be clearly detected. The ζ-potential decreases sharply for a symmetrical counterpart PAA60-PDMAEMA70 solution, while it changes evenly for PAA40-PDMAEMA120 and PAA40-PDMAEMA285 solutions because of the asymmetrical compositions. The lower critical solution temperature of the PAA40-PDMAEMA285 solution is always less than that of the PAA40-PDMAEMA120 solution at the same solution concentration and pH values. The molecular dynamics simulation is used to explore the molecular mechanism of the responsive behaviour. The simulation also evidences the presence of dual-stimuli responsive behaviour of the copolymer with respect to changes in temperature and pH in aqueous solutions. The radius of gyration Rg and the end-to-end distance Re of the asymmetric polyampholyte A20-B60 are smaller than those of the symmetric one A40-B40 at all simulated temperatures, which indicates increased aggregation behaviour and the compact aggregates of asymmetric polyampholyte chains. The effect of polymer molecular mass MW on the aggregation behaviour seems to be more profound than that of polymer composition, which may be attributed to the fact that increasing the molecular weight of the copolymer increases the variability of the micelle sizes. Additionally, hydrophobic interaction plays a key role in the aggregation process.

Responsive Behaviors of Diblock Polyampholyte Brushes within Self-Consistent Field Theory

Self-consistent field theory (SCFT) calculation has been performed to study the structure and stimuli-responsive behaviors of diblock polyampholyte (PA) brushes. Two kinds of brushes are considered: one formed by PA chains consisting of two strong polyelectrolyte blocks (system i) and the other formed by PA chains of a grafted strong acid block and an ungrafted weak base block (system ii). Density profiles and brush thickness are obtained. The chain trajectory (average position of each polymer segment) is also calculated to characterize the conformation of the grafted chains. For system i, the ungrafted blocks loop backward at low salt concentration and extend out at high salt concentration. For system ii, the charge fraction of the annealing block is independent of pH and becomes dependent on it at high salt concentration. As a result, pH has no effect on the brush structure at low salt concentration and takes effect at high salt concentration. That the salt concentration can switch on and off the responses of the PA brush to the pH stimuli may find application in building functional surfaces.

Formation of Complexes between Nanoparticles and Weak Polyampholyte Chains. Monte Carlo Simulations

The electrostatic driven complex formation between a weak polyampholyte chain and one positively charged nanoparticle is investigated using Monte Carlo simulations. The influence of parameters such as the polyampholyte contour length, number and size of blocks, nanoparticle surface charge density, and solution properties, such as the pH and ionic concentration, on the PA titration curves is investigated. It is shown that the presence of one positively charged nanoparticle significantly modifies the acid/base properties of the weak polyampholyte by, on the one hand, promoting the formation of negatively charged monomers and, on the other hand, limiting the number of positively charged monomers. The electrostatic interactions of this system can be modified by pH, ionic concentration, and nanoparticle surface charge. The competition between attractive and repulsive, intramolecular and intermolecular electrostatic interactions leads to a wide range of possible PA conformations at the nanoparticle surface, which have a direct impact on the nanoparticle stabilized or destabilized solutions. Extended conformations, electrostatic rosettes, and dense multiplayer structures are observed. Nonetheless, the intramolecular interactions between the positively and negatively charged PA monomers, in particular at the isoelectric point, are found to play important and subtle roles for both the isolated and adsorbed chain conformations. It is also found that nanoparticle charge inversion is an important ingredient for the formation of multilayer structures at the nanoparticle surface.

Dual-stimuli responsive behaviors of diblock polyampholyte PDMAEMA-b-PAA in aqueous solution

Two poly(2-(dimethylamino)ethyl methacrylate)-b-poly(acrylic acid) diblock copolymers, PDMAEMA 84-b-PAA 18 and PDMAEMA 50-b-PAA 18, were synthesized by the atom transfer radical polymerization (ATRP) and their dual-stimuli responsive behaviors to the changes in temperature and pH in aqueous solutions were investigated by UV–vis spectroscopy, dynamic light scattering (DLS), 1H NMR spectroscopy and surface tension measurement. Different from PDMAEMA 84-b-PAA 18 solutions where no aggregation is observed between pH 7.0 and 9.5, the PDMAEMA 50-b-PAA 18 aggregates can exist in this broad pH range due to the hydrophobic interactions among the charge-balanced polyampholyte chains. At high pH, e.g., 11.0, the DMAEMA segments collapse to form the core of micelles due to the hydrophobic property of the de-protonized DMAEMA stabilized with the highly ionized AA segments on the surface of the micelles upon heating. At pH around the IEP, e.g., 9.5, large micelles can be formed in PDMAEMA 84-b-PAA 18 solution upon heating, just like that at pH 11.0, while PDMAEMA 50-b-PAA 18 first formed the micelles due to the electrostatic attraction between ionized AA segments and protonated DMAEMA segments, but the aggregation of the micelles was hardly happened upon heating due to the smaller DMAEMA segment. Moreover, LCST can be exactly estimated by surface tension experiment.

A novel multi-responsive polyampholyte composite hydrogel with excellent mechanical strength and rapid shrinking rate

Series of hydrophilic core–shell microgels with cross-linked poly(N-isopropylacrylamide) (PNIPAAm) as core and poly(vinyl amine) (PVAm) as shell are synthesized via surfactant-free emulsion polymerization. Then, the microgels are treated with a small amount of potassium persulfate (KPS) to generate free radicals on the amine nitrogens of PVAm, which subsequently initiate the graft copolymerization of acrylic acid (AA), acryloyloxyethyl trimethyl ammonium chloride (DAC), and acrylamide (AAm) onto microgels to prepare multi-responsive composite hydrogels. The composite hydrogels consist of cross-linked ungrafted polyampholyte chains as the first network and microgels with grafted polyampholyte chains as graft point and second network and show surprising mechanical strength and rapid response rate. The investigation shows the compress strength of composite hydrogels is up to 17–30 MPa, which is 60–100 times higher than that of the hydrogel matrix. The composite hydrogel shows reversible switch of transmittance when traveling the lowest critical temperature (LCST) of microgels. When the composite hydrogel swollen in pH 2.86 solution at ambient condition is immersed into the pH 7.00 solution at 45 °C, a rapid dynamic shrinking can be observed. And the character time ( τ) of shrinking dynamic of composite hydrogel is 251.9 min, which is less than that of hydrogel matrix ( τ = 2273.7 min).

Molecular dynamics simulation of semiflexible polyampholyte brushes--The effect of charged monomers sequence

Planar brushes formed by end-grafted semiflexible polyampholyte chains, each chain containing an equal number of positively and negatively charged monomers, are studied using molecular dynamics simulations. Keeping the length of the chains fixed, the dependences of the average brush thickness and equilibrium statistics of the brush conformations on the grafting density and the salt concentration are obtained with various sequences of charged monomers. When similarly charged monomers of the chains are arranged in longer blocks, the average brush thickness is smaller and the dependence of brush properties on the grafting density and the salt concentration is stronger. With such long blocks of similarly charged monomers, the anchored chains bond to each other in the vicinity of the grafting surface at low grafting densities and buckle toward the grafting surface at high grafting densities.