Jinben Wang

Self-assembly of a series of random copolymers bearing amphiphilic side chains.

A novel series of comb-like random copolymers were prepared by polymerization of amphiphilic macromonomers, 2-(acrylamido)-octane sulfonic acid (AMC(8)S), 2-(acrylamido)-dodecane sulfonic acid (AMC(12)S), and 2-(acrylamido)-hexadecane sulfonic acid (AMC(16)S), with 2-(acrylamido)-2-methylpropanesulfonic acid (AMPS) respectively. The synthesis of the polymers with the same contents of amphiphilic units as side chains, but different chain length, enabled us to study the chain length dependence of their association in salt solution. Steady-state fluorescence measurements with pyrene as a polarity probe, quasielastic light scattering techniques (QELS) and transmission electron micrograph (TEM) were employed to investigate the associative properties of the system. The above investigations showed that all kinds of side chains begin to assemble at certain polymer concentrations and the critical aggregation concentration (CAC) decrease dramatically with the increase in the length and content of alkyl. An interesting phenomenon is that the assembly tends more favorably to occur among different molecules rather than within single molecule when the number of carbon atoms in the alkyl groups or the polymer concentration increases, leading to the formation of larger multimolecular micelle-like aggregate. The aim of the present work is to establish the fundamental preconditions of intramolecular and intermolecular association fashions for the polymers, which is useful for the exploitation of functional groups and contributes to the development of amphiphilic random polymers.

Antibacterial Activity of Geminized Amphiphilic Cationic Homopolymers

The current study is aimed at investigating the effect of cationic charge density and hydrophobicity on the antibacterial and hemolytic activities. Two kinds of cationic surfmers, containing single or double hydrophobic tails (octyl chains or benzyl groups), and the corresponding homopolymers were synthesized. The antimicrobial activity of these candidate antibacterials was studied by microbial growth inhibition assays against Escherichia coli, and hemolysis activity was carried out using human red blood cells. It was interestingly found that the homopolymers were much more effective in antibacterial property than their corresponding monomers. Furthermore, the geminized homopolymers had significantly higher antibacterial activity than that of their counterparts but with single amphiphilic side chains in each repeated unit. Geminized homopolymers, with high positive charge density and moderate hydrophobicity (such as benzyl groups), combine both advantages of efficient antibacterial property and prominently high selectivity. To further explain the antibacterial performance of the novel polymer series, the molecular interaction mechanism is proposed according to experimental data which shows that these specimens are likely to kill microbes by disrupting bacterial membranes, leading them unlikely to induce resistance.

Interactions between colloidal particles in the presence of an ultrahighly charged amphiphilic polyelectrolyte.

A novel amphiphilic polyelectrolyte denoted as PAGC8 and a traditional amphiphilic polyelectrolyte denoted as PASC8 were prepared. PAGC8 consisted of gemini-type surfactant segment based on 1,3-bis (N,N-dimethyl-N-octylammonium)-2-propyl acrylate dibromide, while PASC8 incorporated acryloyloxyethyl-N,N-dimethyl-N-dodecylammonium bromide as single chain surfactant units within its repeat unit structure. Turbidity, stability, and zeta potential measurements were performed in the presence of PAGC8 and PASC8, respectively, to evaluate their effectiveness in inducing solid/liquid separations. It was found that the maximum transmittance was observed before the zeta potential values reached the isoelectric point, implying that not only charge neutralization but also charge-patch mechanism contributed to the separation process. Colloid probe atomic force microscopy technique was introduced to directly determine the interactions between surfaces in the presence of ultrahighly charged amphiphilic polyelectrolyte. On the basis of the AFM results, we have successfully interpreted the influence of the charge density of the polyelectrolytes on the phase stability. Electrostatic interaction played the dominant role in the flocculation processes, although both electrostatic interaction and hydrophobic effect provided contributions to the colloidal dispersions. The attractions upon surfaces approach in the case of PAGC8 were significantly larger than that of PASC8 due to the higher charge density. The strong peeling events upon retraction in the presence of PAGC8 implied that the hydrophobic effect was stronger than that of PASC8, which displayed the loose pulling events. A strong attraction was identified at shorter separation distances for both systems. However, these interactions cannot be successfully described by the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory of colloid stability due to the participation of charge-patch and strong hydrophobic effect. To account for the additional interactions, we proposed an extended DLVO empirical model to explain the non-DLVO forces in the systems. A reasonable physical model was also proposed to further describe the interactions between surfaces in the two amphiphilic polyelectrolyte systems.

Aggregation properties of a novel class of amphiphilic cationic polyelectrolytes containing gemini surfactant segments.

A novel class of amphiphilic cationic polyelectrolytes, poly(A-co-G)s, comprising of gemini type surfactant segment 1,3-bis(N,N-dimethyl-N-dodecylammonium)-2-propylacrylate dibromide (G) and acryloyloxyethyl trimethyl ammonium chloride (A), were synthesized. Their aggregation properties were investigated by employing fluorescence spectroscopy, dynamic light scattering, transmission electron microscopy, and ζ-potential measurements. For comparison, a series of polyelectrolytes containing a traditional single alkyl chain surfactant unit (acryloyloxyethyl-N,N-dimethyl-N-dodecylammonium bromide (D)), poly(A-co-D)s, were also synthesized and investigated. It was found that the critical aggregation concentration (cac) of poly(A-co-G)s is much lower than that of poly(A-co-D)s. The huge interpolymer aggregates (with a hydrodynamic radius of >450 nm) occur in poly(A-co-G)s aqueous solution, and the size of aggregates increases with the increase of the molar content of the gemini-type surfmer segment and the concentration of the copolymer. The size of aggregates in poly(A-co-D)s aqueous solution is much smaller than poly(A-co-G)s, which also increases with the increase of the molar content of the single alkyl chain surfmer segment and the concentration of the copolymer. The results of aggregation number and charge density of aggregate in poly(A-co-G)s and poly(A-co-D)s indicate that the copolymers have a strong tendency toward interpolymer aggregation and the aggregates in poly(A-co-G)s are much more compact than those of poly(A-co-D)s. These results are interpreted in terms of the synergistic effects of double hydrophobic chains on the gemini surfactant unit.

Rheologic Properties and Molecular Configuration of Polymers in Salt‐Alkali‐Surfactant Mixed Solutions

With the vacuum sublimation freezing‐drying and rheological techniques, the salt/alkali/surfactant effects on the solution properties and the aggregate conformations of the polymers, synthesized hydrophobically modified poly (acrylamide) (HMPAM), were investigated. The experimental methods included viscosity measurements, the photomicroscopy, scan electron microscopy, and the freeze fracture transmission electron microscopy. The variety of conformations of the ultramolecular aggregates, the hydrophobical region, and the spatial network structures resulted from the hydrophobical interaction were studied. Combined results confirmed that HMPAM could form several of interesting three‐dimensional network structures either in distilled water or NaCl solution. The results have revealed the mechanism of the salt/alkali/surfactant effects on the peculiar rheological properties of the solution.

Synthesis and Emulsification Properties of an Amphiphilic Polymer for Enhanced Oil Recovery

A novel amphiphilic polymer, APAM, for enhanced oil recovery (EOR), was synthesized by radical copolymerization of acrylamide, dodecyl polyoxyethylene acrylate (DPEA), and N-(1,1,3,3-tetramethyl butyl) acrylamide (TBA), using potassium persulfate-sodium bisulfite as initiator–activator. The aggregation morphological observation of APAM in aqueous and NaCl solutions was performed using scanning electron microscope. The ability of APAM to solubilize and emulsify n-heptane and crude oil was investigated by visual methods, microscopy, and particle size analyses. The results show that APAM forms supramolecular aggregates in aqueous and NaCl solutions and has strong emulsification ability. As a result, the emulsion consisting of APAM aqueous solution and n-heptane (or crude oil) is much more stable than that of HPAM (partially hydrolyzed polyacrylamide, widely used in EOR), indicating a important function of APAM to emulsify oil component.

Aggregation behaviour of a novel series of polyamidoamine-based dendrimers in aqueous solution

Dendrimers composed of G1 polyamidoamine branched with poly(propylene oxide) (PPO)-block-poly(ethylene oxide) (PEO) were synthesised for the first time. Surface tension results showed an increase in the critical aggregation concentration (cac) with PPO-b-PEO chain lengthening. The thermodynamic parameters associated with aggregate formation were determined by isothermal titration microcalorimetry, indicating that both the cac and the Gibbs free energy for aggregation (ΔG°) ascended as the amphiphilic chain length increased. The gain of entropy (ΔS°) was the main contribution to aggregation due to TΔS° being larger than the enthalpy (ΔH°). Aggregate morphology and size distribution were studied through transmission electron microscope and dynamic light scattering measurements. Furthermore, the relationship between PEO chain length and aggregation behaviour was discussed.

Self-assembly of brush-like amphiphilic statistical polymers: the effect of salt stimulus on the molecular associative mode

The solution properties of amphiphilic statistical polymers, originating from the nanostructures of macromolecular assemblies, strongly depend on the associative behaviours of the polymers. In this paper, we report the effect of salt stimulus on the associative mode of the 2-(acrylamido)-2-methylpropanesulphonic acid–2-(acrylamido)-dodecane sulphonic acid brush-like amphiphilic statistical polymers, employing steady-state fluorescence, dynamic light scattering and transmission electron micrograph methods. It was found that the stimulus effects work more intensely for the polymers with less hydrophobic side chains. As the salt concentration increases, the salt valence state increases or the salt structure adds an organic group, the polymer molecules associate at a lower critical concentration. Meanwhile the molecules undergo an intramolecular associative mode in preference to an intermolecular associative mode, forming hundreds of times smaller unimers rather than giant multipolymer aggregates.

pH and salt effects on the aggregation behaviour of star polymer with G1 polyamidoamine core and terminal amphiphilic blocks

In this study, the pH and salt effects on the aggregation behaviour of star polymer, with G1 polyamidoamine as the core and poly(propylene oxide)-block-poly(ethylene oxide) as the terminal branches, were investigated. The results from pH and turbidimetric titration experiments showed that with the pH value increasing from 2.0 to 10.0, the solution transforms from transparent to turbid at pK a = 5.5 and eventually presents phase separation at the pH value above 7.5. As revealed by dynamic light scattering and transmission electron microscopy measurements, the size of the aggregates of star polymer becomes bigger with the increase in the pH value or in the presence of salt. It was found that the extent of the salt effect on the aggregation behaviour follows the sequence: C6H5COONa>Na2SO4 > NaCl.

Molecular interactions between DOPA and surfaces with different functional groups: a chemical force microscopy study

The adhesion of mussel foot proteins (Mfps) to a variety of surfaces has been widely investigated, but the mechanisms behind the mussel adhesion to surfaces with different properties are far from being understood. Most of Mfps contain a significant amount of 3,4-dihydroxyphenylalanine (DOPA) which is considered to be responsible for the strong wet adhesion. In the present work, self-assembled monolayers (SAMs) were prepared as a series of model surfaces with variable functional groups. DOPA-surface interactions were investigated using chemical force microscopy (CFM) for the first time, in which an atomic force microscope (AFM) tip was chemically modified with DOPA terminated groups. The ability of DOPA to adhere to different surfaces with variable wettability was compared, showing that DOPA behaves with the strongest and weakest adhesion to C6H5- and OH-terminated surfaces, respectively. The interaction strength of DOPA at different surfaces does not always increase with the increase of surface wettability, because the hydrophobic interaction does not play a decisive role in DOPA adhering to surfaces. By the use of classical and extended Derjaguin–Landau–Verwey–Overbeek (DLVO) theories, the contribution of non-DLVO forces was isolated. We found out DOPA can adhere to each surface functional group, since DOPA residues containing o-hydroxy or aromatic rings alone can control the adhesion process, and the aromatic ring is oriented perpendicularly or parallel to the surface. This study served as a basis for understanding the relationship between DOPA adhesion mechanisms and different wet surfaces, representing important concepts for the design of bioadhesive materials and anti-adhesion surfaces.