Structure of salted and discharged globules of hydrophobic polyelectrolytes adsorbed from aqueous solutions

In this article, we compare structures of protonated poly(2-vinylpyridine) globules (2D compact coils on the surface) adsorbed on the mica surface from aqueous solution when the shrinking is brought about either by discharging the molecules at an elevated pH or by adding monovalent and polyvalent salts. We study the structure of the PE coils using in situ atomic force microscopy experiments in aqueous solutions in a liquid cell. The abrupt coil-to-globule transition caused by pH changes and the discharge of polymer chains resulted in compact globules. If the pH corresponding to extended coil conformation remains unchanged, the coil shrinks due to the added salt. The size of the globule in the latter case corresponds to the unperturbed dimension of the polymer coil. There is no essential difference in the dimensions of the globules as obtained in the presence of monovalent and multivalent counterions for the studied ionic strength. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1623–1627, 2010     

Investigation On the Behavior of Poly(acrylic acid) in Water Solution and Its Effect on the Rheology of α- Al2O3 Suspension

The influence of pH, relative molecular weight and concentration of poly(acrylic acid) (PAA) on the viscosity of PAA solution was investigated. It was found that the rheology of PAA solution was closely related to the ionization and conformation of PAA chains. The increasing rigidity and the stretching of polymer chain resulted in a maximum viscosity at pH=8. The viscosity change of PAAsolution with addition of alumina powder was studied. It was found that the suspension viscosity reached its minimum when the amount of ceramic powder and PAA was at a certain ratio. At the same time, the viscosity minimum was also influenced by the particle size of powder.     

Light Scattering Study of the Antibody–Poly(methacrylic acid) and Antibody–Poly(acrylic acid) Conjugates in Aqueous Solutions

The effect of the conformational state of the polymer coil on the properties of protein–polymer conjugates has been studied for the conjugates of antibody (monoclonal antibody from 6C5 clone against inactivated rabbit muscle glyceraldehyde‐3‐phosphate dehydrogenase; Ab) with poly(methacrylic acid) (PMAA) or poly‐(acrylic acid) (PAA). The pH‐dependencies of molecular properties and structural parameters of aqueous solutions (radius of gyration, intensity of scattered light, hydrodynamic diameter, and polydispersity index) of Ab, PMAA, and PAA and their conjugates, i. e., Ab‐PMAA and Ab‐PAA, have been studied using static and dynamic light scattering techniques. While free Ab aggregates in solution and precipitates at its isoelectric point, the covalent attachment of a charged polymer to Ab prevents its association and shifts the precipitation point towards more acidic values (from pH 5.95 for Ab to pH ˜ 4.8 for Ab‐PMAA). The predominant role of the conformational status of the polymer in the process of conjugate precipitation has been considered. Contrary to the precipitation of Ab‐PMAA, the formation of stable colloidal particles was suggested for Ab‐PAA at pH < 4.8. In the conjugates, polymer chains surround the protein globule in an extremely compact manner while Ab significantly affects the polymer conformation. The essentially larger hydrodynamic radii of conjugates, when compared with their radii of gyration, confirm the strong interaction of conjugates with solvent molecules.     

Fluorescence correlation spectroscopy studies of diffusion of a weak polyelectrolyte in aqueous solutions

We apply fluorescent correlation spectroscopy (FCS) to investigate solution dynamics of a synthetic polyelectrolyte, i.e., a weak polycarboxylic acid in aqueous solutions. The technique brings single molecule sensitivity and molecular specificity to dynamic measurements of polyelectrolyte solutions. Translational diffusion of Alexa-labeled poly(methacrylic acid), PMAA*, chains was studied in very dilute, 10(-4) mg/ml, solutions as a function of solution pH and ionic strength. The observed changes in diffusion coefficients were consistent with about twofold expansion of PMAA* coils when pH was changed from 5 to 8, and with chain contraction for alkaline metal ion concentrations from 0.01 to 0.1 M. The dependence of the hydrodynamic size of PMAA* chains on the counterion type followed the sequence: Li(+)>Na(+) approximately equal to Cs(+)>K(+). The dependence of translational diffusion on polyacid concentration was weak at the low concentration limit, but chain motions were significantly slower at higher polymer concentrations when PMAA chains overlapped. Finally, measurements of dynamics of PMAA* chains in "salt-free" solutions showed that self-diffusion of PMAA* chains significantly slowed down when PMAA concentration was increased, probably reflecting the sensitivity of PMAA* translational motions to the onset of interchain domain formation. These results illustrate the utility of the FCS technique for studying hydrodynamic sizes of polyelectrolyte coils in response to variation in solution pH or concentration of salt and polyelectrolytes. They also suggest that FCS will be a promising technique for selective observation of the dynamics of polyelectrolyte components in complex polymer mixtures.     

AFM single molecule experiments at the solid-liquid interface: in situ conformation of adsorbed flexible polyelectrolyte chains

This paper reports the in situ study of conformations of flexible synthetic polymer molecules of protonated poly(2-vinylpyridine) at the solid-liquid interface at different pH of aqueous solutions. The direct conformation study of such thin ( approximately 0.4 nm) single molecules under liquid is performed for the first time. The highly protonated poly(2-vinylpyridine) chains possess a conformation of 2D equilibrated random coil, while at low degree of protonation, molecules are in the conformation of a strongly compressed 3D coil. Molecules are immobile during the few hours of the AFM experiment.     

Complexation of poly(acrylic acid) and poly(ethylene oxide) investigated by enhanced Rayleigh scattering method

The method of enhanced Rayleigh scattering spectroscopy (ERS) was developed to investigate the complexation of poly(acrylic acid) (PAA) and poly(ethylene oxide) (PEO) in semidilute polymer solutions. Based on the Ornstein‐Zernike equation, the relationship between macromolecular static correlation length and ERS intensity was presented. Moreover, the ERS spectra were calculated by the moving window two‐dimensional (MW2D) correlation spectroscopy to get detailed information of the polymer complexation. The results indicated that the ERS spectroscopy characteristics of the polymer mixtures have similar trend, and the ERS intensity promptly increases as the macromolecular chains contract. The increase of ERS intensity showed that the degree of complexation between PAA and PEO increases when the pH value decreases. The complexation results from the collapse of macromolecular chains, which is induced by the PAA chains contracting and the enhanced association between PAA and PEO chains because of the hydrogen bond formation. In addition, the association resulting from the complexation of PAA and PEO in solution was demonstrated by the MW2D correlation spectroscopy. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1847–1852, 2010     

Molecular‐level structures at poly(4‐vinyl pyridine)/acid interfaces probed by nonlinear vibrational spectroscopy

The molecular structures of the interfaces between a solid poly(4‐vinyl pyridine) (P4VP) surface and poly(acrylic acid) (PAA) as well as hydrochloric acid (HCl) solutions were probed using sum frequency generation (SFG) vibrational spectroscopy in situ in real time. Spectroscopic results clearly reveal that the PAA molecules are adsorbed onto the P4VP surface via hydrogen bonding at the P4VP/PAA solution interface while the P4VP surface is protonated at the P4VP/HCl solution interface. Consequently, the water molecules near the interfaces are strongly perturbed by these two interactions, exhibiting different orderings at the two interfaces. This work clearly demonstrates the power of studying the interfacial molecular‐level structures via nonlinear vibrational spectroscopy when molecular adsorption happens at the solid–liquid interface and paves a way for our future study on tracing the adsorption dynamics of polymer chains onto solid surfaces. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 848–852     

Probing the effects of external species on poly(acrylate acid) chain dynamics by using cationic AIE-active fluorophore

We report here our results on the investigation of the chain dynamics of poly(acrylic acid) in aqueous solution. The concentration of poly(acrylic acid) was approximately 3.8×10~(-4) mol/L, two orders of magnitude higher than that reported in the literature. The p H value of the solution was 3.9, and the hydrogen bonds between the intrinsic and ionized carboxylic acid groups formed dynamic networks, which captured aggregation-induced emission-active molecules(a tetra-quaternary ammonium modified tetraphenylethene derivative) inside the polymer coils and induced fluorescence emission. The hydrogen bonds can be classified as intra- or intermolecular; both can be probed based on the emission change of the tetra-quaternary ammonium modified tetraphenylethene probes. The effects of different external stimuli on the polymer chain dynamics were investigated using different metal cations(including Na~+, Li~+, Zn~(2+), Ni~(2+), Ca~(2+), and Co~(2+)), different cation concentrations(1×10~(-6) to 4×10~(-4) mol/L), different poly(acrylic acid) molecular weights(5, 240, and 450 k Da), and different copolymers. The experimental results indicate that the long poly(acrylic acid) chains(high molecular weight) tend to form dense globular coils and exclude the probe molecules outside, which are robust and unsusceptible to water-soluble metal cations. However, the shorter poly(acrylic acid) chains tend to form intermolecular hydrogen bonds, which are helpful in capturing more probe molecules inside the networks, thus inducing stronger emission. Because of the dual functions of forming hydrogen bonds with carboxylic groups and acting as an acceptor of protons from the carboxylic acid group to form cationic species, copolymerization with acrylate amide [poly(acrylic acid)-co-poly(acrylamide)] can greatly affect the chain dynamics of poly(acrylic acid) segments, which is reflected by the drastically decreased emission intensity from the fluorescent probes.     

How does solvent molecular size affect the microscopic structure in polymer solutions?

Monte Carlo simulation has been used to investigate the effects of linear solvent molecular size on polymer chain conformation in solutions. Increasing the solvent molecular size leads to shrinkage of the polymer chains and increase of the critical overlap concentrations. The root-mean-square radius of gyration of polymer chains (R(g)) is less sensitive to the variation of polymer concentration in solutions of larger solvent molecules. In addition, the dependency of R(g) on polymer concentration under normal solvent conditions and solvent molecular size is in good agreement with scaling laws. When the solvent molecular size approaches the ideal end-to-end distance of the polymer chain, an extra aggregation of polymer chains occurs, and the solvent becomes the so-called medium-sized solvent. When the size of solvent molecules is smaller than the medium size, the polymer chains are swollen or partially swollen. However, when the size of solvent molecules is larger than the medium size, the polymer coils shrink and segregate, enwrapped by the large solvent molecules.     

Interactions between poly(acrylic acid) and sodium dodecyl sulfate: isothermal titration calorimetric and surfactant ion-selective electrode studies

Interaction between a monodispersed poly(acrylic acid) (PAA) (M(W) = 5670 g/mol, M(w)/M(n) = 1.02) with sodium dodecyl sulfate (SDS) was investigated using isothermal titration calorimetry (ITC), ion-selective electrode (ISE), and dynamic light scattering measurements. Contrary to previous studies, we report for the first time evidence of interaction between SDS and PAA when the degree of neutralization (alpha) of PAA is lower than 0.2. Hydrocarbon chains of SDS cooperatively bind to apolar segments of PAA driven by hydrophobic interaction. The interaction is both enthalpy and entropy favored (deltaH is negative but deltaS is positive). In 0.05 wt % PAA solution, the SDS concentration corresponding to the onset of binding (i.e., CAC) is approximately 2.4 mM and the saturation concentration (i.e., C(S)) is approximately 13.3 mM when alpha = 0. When PAA was neutralized and ionized, the binding was hindered by the enhanced electrostatic repulsion between negatively charged SDS and PAA chains and improved solubility of the polymer. With increasing alpha to 0.2, CAC increases to approximately 6.2 mM, C(S) drops to 8.6 mM, and the interaction is significantly weakened where the amount of bound SDS on PAA is reduced considerably. The values of CAC and C(S) derived from different techniques are in good agreement. The binding results in the formation of mixed micelles on apolar PAA coils, which then expands and dissociates into single PAA chains. The majority of unneutralized PAA molecules exist as single polymer chains stabilized by bound SDS micelles in solution after the saturation concentration.     

Role of sequential adsorption of polymer/surfactant mixtures and their conformation in dispersion/flocculation of alumina

The role of sequential adsorption of a non-interacting pair of polymer (polyacrylic acidPAA) and surfactant (sodium dodecyl sulfate-SDS) on alumina particles and in determining the stability of their suspensions was studied with particular attention to the conformation of the polymer. It was found that the sequence of addition is of critical importance in determining the dispersion/flocculation of this system. When SDS was added first, both PAA conformation and suspension stability varied with SDS concentration. Whereas when PAA was added first, the subsequent adsorption of SDS molecules had no effect on either the polymer conformation or the suspension stability. This is attributed to masking of SDS species by the larger polymer chains. SDS can not however be completely masked by PAA chains once the PAA concentration is decreased below the optimal concentration for flocculation. When the stirring duration was increased, the mode of addition showed no effect suggesting that the system then reaches equilibrium. Interestingly, a small amount of pre-adsorbed PAA was found to facilitate the adsorption of SDS.     

Effect of solution pH on the stability of mixed silica -alumina suspension in the presence of polyacrylic acid (PAA) with different molecular weights

The influence of solution pH (in the range 3–9) on mixed silica-alumina suspension in the absence and presence of polyacrylic acid (PAA) was studied. The composition of the adsorbent was SiO₂ (97%) and Al₂O₃ (3%). The turbidimetry method was applied to record changes in the stability of the investigated systems as a function of time. It was shown that the suspension without the polymer is less stable at pH 3, whereas at pH 6 and 9, the systems were stable. PAA with molecular weights 100 000 and 240 000 at pH 3 (improvement of system stability conditions) and PAA 2 000 at pH 6 (deterioration of suspension stability) have a great effect on the silica-alumina suspension stability. The stabilization-flocculation properties of polyacrylic acid are a result of a specific conformation of its chains on the solid surface where it depends on the solution pH and the polymer molecular weight.     

活性炭复合改善聚丙烯酸凝胶耐盐性及其溶胀机理

在活性炭(Ac)存在的情况下通过自由基溶液聚合,以过硫酸铵为引发剂,N,N′-亚甲基双丙烯酰胺为交联剂,制备了活性炭复合聚丙烯酸凝胶(PAA/AC)。 考察了凝胶在蒸馏水、生理盐水和不同pH值缓冲溶液中的平衡溶胀比以及溶胀动力学,结果表明,活性炭能有效提高凝胶的平衡溶胀比,在实验设计的pH值范围内复合凝胶具有比PAA凝胶更高的平衡溶胀比,蒸馏水和生理盐水中PAA/AC凝胶的平衡溶胀比分别可达到303和60 g/g,约为PAA凝胶的2.4倍。 讨论了凝胶的溶胀机理,结果表明,活性炭成分的介入破坏了聚合物链段之间的聚集态结构,减弱了聚合物链段之间的相互作用,提高了凝胶的溶胀能力。 示差扫描量热仪测定复合前后凝胶的玻璃化转变温度,扫描电子显微镜观察了复合前后凝胶的断面网络结构,结果进一步表明活性炭复合后聚合物链段之间的作用力减弱。     

Size and pH effect on electrical and conformational behavior of poly(acrylic acid): Simulation and experiment

Monte Carlo simulations, experimental titrations and fluorescence correlation spectroscopy experiments were used to investigate the conformational and electrical properties of polyacrylic acids (PAA). On the one hand, titration curves were calculated to get an insight into the role of pH on the degree of ionization and conformation of PAA chains. On the other hand, experimental potentiometric titrations of PAA were also achieved for different PAA molecular weights and compared to the calculated titration curves obtained by Monte Carlo coarse grained simulations. It was found that for a large range at intermediate PAA ionizations, a good correlation is obtained between experimental and simulations data thanks to the prominence of electrostatic interactions in this domain. The effect of ionic concentration and PAA molecular weight on the titration curves was also investigated. In order to get a better understanding of PAA conformational behavior, we also investigated PAA diffusion properties in aqueous solutions as a function of pH and ionic strength by fluorescence correlation spectroscopy (FCS), thanks to its high sensitivity to measure diffusion coefficients of tracer solutes. Good qualitative agreements were observed between experimental diffusivities and polymer properties calculated from MC simulations. It was shown that the high molecular weight PAA chains display more significant changes in diffusivity in agreement with the ionization degrees and conformational changes observed in the simulations.     

壳聚糖大分子链的形态调控及其在微悬浮聚合中的分散稳定作用

以易去除可回用的壳聚糖(CS)为分散剂,通过微悬浮聚合制备微米级的软硬质聚合物胶粒.考察不同酸碱度下水相介质中CS大分子链的质子化程度、亲疏水性和形态结构,及其对油水界面处CS存在形态的影响,进而评估其对剪切均质化所制单体液滴的分散稳定作用.发现通过调节体系pH值可较容易地控制CS大分子链的质子化程度、亲疏水性以及在单体液滴表面的吸附效率和铺展程度,进而可在弱酸性环境下调控微悬浮聚合体系中CS的分散能力和稳定效果.特别是当pH值在6.0左右时,CS大分子链质子化程度和亲疏水性适中,链内易形成具有一定内聚密度的高分子链收缩构象、链间易形成由多根CS链缠结而成的疏松聚集状态.在此状态下的CS链对苯丙单体液滴具有较强的分散能力和稳定作用,因而通过微悬浮聚合可制得形态结构规整、分散状态良好的聚合物粒子.进一步与微悬浮聚合常用的无机粉末类和高分子类分散剂进行应用效果比较,发现CS具有形态调控性好、分散稳定效率高、易去除能回用、特别适合制备软质微米胶粒等优点,是一种有别于无机粉末类和高分子类分散剂的新型微悬浮聚合分散剂.     

Effect of the Side‐Chain‐Distribution Density on the Single‐Conjugated‐Polymer‐Chain Conformation

The spatial arrangement of the side chains of conjugated polymer backbones has critical effects on the morphology and electronic and photophysical properties of the corresponding bulk films. The effect of the side‐chain‐distribution density on the conformation at the isolated single‐polymer‐chain level was investigated with regiorandom (rra‐) poly(3‐hexylthiophene) (P3HT) and poly(3‐hexyl‐2,5‐thienylene vinylene) (P3HTV). Although pure P3HTV films are known to have low fluorescence quantum efficiencies, we observed a considerable increase in fluorescence intensity by dispersing P3HTV in poly(methyl methacrylate) (PMMA), which enabled a single‐molecule spectroscopy investigation. With single‐molecule fluorescence excitation polarization spectroscopy, we found that rra‐P3HTV single molecules form highly ordered conformations. In contrast, rra‐P3HT single molecules, display a wide variety of different conformations from isotropic to highly ordered, were observed. The experimental results are supported by extensive molecular dynamics simulations, which reveal that the reduced side‐chain‐distribution density, that is, the spaced‐out side‐chain substitution pattern, in rra‐P3HTV favors more ordered conformations compared to rra‐P3HT. Our results demonstrate that the distribution of side chains strongly affects the polymer‐chain conformation, even at the single‐molecule level, an aspect that has important implications when interpreting the macroscopic interchain packing structure exhibited by bulk polymer films.     

Self-aggregation of dendritic poly(benzyl ether)–poly(acrylic acid), an amphiphilic block copolymer, studied by 1H NMR

The dynamic properties of the micelles of a novel synthesized amphiphilic block copolymer, dendritic poly(benzyl ether)–poly(acrylic acid) (Dendr.PBE-PAA), formed in aqueous solutions were studied by the ¹H self-diffusion coefficient, relaxation measurements and 2D nuclear Overhauser enhancement spectroscopy. The experimental results show that Dendr.PBE-PAA molecules self-aggregate in aqueous solution. The dynamic properties of the Dendr.PBE-PAA micelles vary with their total concentration in the solution. The motion of the molecules in the micelles of a concentrated solution is more restricted than that in a less concentrated one. The main chains of PAA are densely packed in the surface layer of the hydrophobic core with the carboxyl side chain pointing to the aqueous medium and the hydrophobic phenoxy rings stay in the interior. The self-aggregate becomes larger as the degree of polymerization of PAA increases. However the phenoxy rings situated in the interior of the hydrophobic core become more loosely packed. n-Hexadecane is solubilized in the micelles. The optimal position of n-hexadecane is between the phenoxy rings next to the PAA chains. Received: 25 January 2001 Accepted: 18 July 2001     

pH-induced reversible conformational and morphological regulation of polyleucine grafted polyallylamine assembly in solution

One of the essential parts in the molecular mechanism of biological properties is the structural changes of proteins induced by stimuli. An amphiphilic copolymer, poly(L-leucine) grafted polyallylamine as a simple model of proteins, has been prepared by NCA polymerization with free amino groups of polyallylamine as an initiator. Here, we report the pH-induced reversible conformational and morphological regulation of the amphiphilic copolymer, whose hydrophobic peptide graft chains have no pH-sensitive groups, in an aqueous solution containing 50 vol % trifluoroethanol. The conformation of the poly(L-leucine) graft chain was found to be strongly pH dependent. Under acidic conditions, where electrostatic repulsion existed between the neighboring protonated amine moieties of the polyallylamine main chain, the rapid aggregation of the poly(l-leucine) graft chains was disturbed, and the peptide graft chains formed a beta-sheet structure owing to the intramolecular hydrogen bonding among the graft chains. Under this condition, the amphiphilic polymer formed amyloid-like fibrils, and then the fibrils grew into a planer plate composed of staked beta-sheets. On the other hand, under basic conditions, the poly(L-leucine) graft chains showed conformational transitions from a beta-sheet structure to an alpha-helical conformation owing to a distortion of the regular arrangement of the peptide graft chains by the conformational change of the polyallylamine main chain, whose amino groups were deprotonated. The conformational transition resulted in a disturbance of the regular sheet assembly of the amphiphilic copolymer and induced morphological changes to the amorphous globular aggregates. The pH-induced conformational and morphological changes of the poly(L-leucine) graft polyallylamine were reversible and synchronized with the protonation of the polyallylamine main chain.     

Unique fluorescence behavior of perylenetetracarboxydiimides in polyimide systems

Perylenetetracarboxydiimide (PEDI) molecularly dispersed in polyamic acid (PAA) and polyimide (PI) films has unique fluorescence properties. An originally strong fluorescence of PEDI is efficiently quenched in the PAA films. The systematic variation of the chain structure of the PAA matrices revealed that the aromatic amide groups in the PAA chains function as a quencher. When a PAA derived from 3,4,3′4′-biphenyltetracarboxylic dianhydride (BPDA) and p-phenylenediamine (PDA), BPDA/PDA, was used as a matrix polymer, the fluorescence of the dye dispersed in the film increased abruptly as imidization of the matrix proceeds. But annealing at temperatures higher than 320°C in the step-heating process caused a gradual decrease in the fluorescence intensity. The decreased intensity results from the dye–PDA units interactions intensified by the denser molecular packing of the matrix polymer chains. PEDI shows significant dependence of the fluorescence intensity on the chain structure of the PI matrices. In the various PI films containing a fixed diamine component, the dye fluorescence intensity reduces linearly with an increase in the intramolecular charge transfer ability of the PI matrices. From the result, we propose a fluorescence quenching mechanism through multistep electron transfer processes. The BPDA/PDA polyimide matrix leads to a strong PEDI fluorescence whereas the pyromellitic dianhydride (PMDA)-based PI matrices do not. For the blends composed of these PIs, the fluorescence of PEDI bound into the main chains provides a valuable indicator of the miscibility on the molecular level. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 827–840, 1998     

Effects of solute-solvent proton exchange on polypeptide chain dynamics: a constant-pH molecular dynamics study

A method for performing implicit-solvent molecular dynamics simulations at constant pH was applied to a pentapeptide acetyl-Ala-Asp-Ala-Lys-Ala-amide at pH 4. As a reference, molecular dynamics simulations were done for the same peptide with two variants of its fixed protonation patterns expected to dominate at pH 4, i.e., with a protonated and a deprotonated side chain of the Asp residue and the protonated Lys residue in both cases. The dynamic trajectories of the peptide were used to discuss the problem of the significance of the solute-solvent proton exchange phenomena for the dynamics and structural distributions of the polypeptide chain. The Asp-Lys distance was used as a probe of the overall molecular structure of the investigated pentapeptide. To characterize the dynamics, distributions of the "waiting" times for a transition from a "short" distance conformation to a "long" distance conformation were constructed, based on the generated molecular dynamics trajectories. We show that the relaxation time for the transitions, derived from the constant-pH simulations, is very close to the relaxation time characterizing a permanently protonated molecule, although the average protonation probability of the short-distance conformation is close to zero. However, the distribution of the Asp-Lys distances obtained from constant-pH simulations cannot be reproduced as a linear combination of the distributions resulting from the simulations with fixed protonation states.