Polymer surfactants and their applications

Polymer surfactants refer to substances whose molecular weight reaches a certain level (ie, the molecular weight is generally 103 to 106) and has a certain surface activity [1-10]. From the structure can be divided into block copolymers, graft copolymers and so on. If polymer surfactants are classified by ion type, they can be divided into four categories: anionic, cationic, zwitterionic and non-ionic; according to the source, they can be divided into natural polymer surfactants and natural modified polymers Surfactants and synthetic polymer surfactants. Compared with low-molecular-weight surfactants, other main characteristics of polymer surface activity are: (1) It has a smaller ability to reduce surface tension and interfacial tension, and most polymer surfactants do not form micelles; (2) It has high molecular weight and weak penetration; (3) Poor foam formation ability, but the foams formed are relatively stable; (4) Good emulsifying power; (5) Excellent dispersion and cohesion; (6) Most Polymer surfactants are low-toxic.
The first polymer surfactants used are natural water-soluble polymer compounds such as starch, cellulose and their derivatives. Although they have certain emulsifying and dispersing capabilities, these polymer compounds have more hydrophilic groups. Group, so its surface activity is low. The development of polymer surfactants began in 1950. In 1951, Ceresa synthesized an amphiphilic block polymer-polyethylene oxide polypropylene oxide block polymer, and applied it to the surfactant industry. In the same year, Stauss synthesized polysoap. In 1954, the first commercial polymer surfactant came out. Since then, various synthetic polymer surfactants have been developed and applied in various fields. In 1954, the American Wyandotte Company published the Pluronic series of block copolymers of ethylene oxide and propylene oxide. Since then, many countries in the world have begun research work on polymer surfactants. In 1961, Strauss synthesized a polymer surfactant called polysoap. Subsequently, ethylene oxide and propylene oxide block well polymers [18] were used as non-ionic surfactants to achieve industrial production. Compared with commonly used low-molecular-weight surfactants, the ability to reduce surface tension is poor, and the cost is relatively high, and it has not been able to occupy the dominant position in the field of surfactants. In the past ten years, due to the needs of energy industry (enhanced oil recovery, fuel emulsification, oil/coal emulsification), coating industry (soap-free polymerization, high-concentration latex), membrane science (bionic membrane, LB membrane), the surface activity of polymer New progress has been made in the agent, and good performance ethylene oxide-siloxane copolymers, ethylene imine copolymers, vinyl ether copolymers, alkylphenol-formaldehyde condensation polymers-ethylene oxide copolymers and other varieties have been obtained.
For a long time, in monographs on surfactants, only polyoxypropylene and polyoxyethylene copolymers were classified as polymer surfactants, while other polymers were not included. The reason is that other water-soluble polymers do not greatly reduce the surface tension of the solution. However, a large number of recent studies have shown that these polymers have strong adsorption on the interface, especially on the solid-liquid interface, indicating that they have extremely strong interfacial activity. Therefore, in the past ten years, people have researched and developed a series of soluble polymers that produce various effects through interface absorption as polymer surfactants. Compared with low-molecular-weight surfactants, polymer surface-active agents have the advantages of high solution viscosity and good film-forming properties. They are a type of polymer material with great application prospects in the oil exploration and coating industry. It is used in bionic membranes. It also has a wide range of applications, and has become the subject of cross-over research in chemistry, chemical engineering, petroleum, medicine, materials, and life sciences.
1.2 Characteristics and functions of polymer surfactants
1.2.1 Surface tension
Because the hydrophilic and hydrophobic segments of polymer surfactants have a certain orientation on the surface or interface, they have the ability to reduce surface tension and interfacial tension, but they are often less active than low-molecular surface active agents.
The ability of high molecular surfactants to reduce surface tension is not as good as low molecular surfactants, and the surface activity decreases sharply as the molecular weight increases.
Starting from the molecular mechanism of surface activity, Xu Jian analyzed the relationship between polymer chemical structure, solution molecular morphology, and surface activity, and proposed that the formation of complete monomolecular and multimolecular micelles by polymer surfactants leads to poor surface activity. The main reason for this is to suppress the association of the hydrophobic components of the polymer, which will effectively increase its surface activity.
 
1.2.2 Emulsification and dispersion function
Although the molecular weight is relatively high, there are many polymers with surface activity that can form micelles in the dispersed phase, and they have CMC value and play an emulsifying function. Due to their amphiphilic structure, part of their molecules can be adsorbed on the surface of the particles, and the other parts are dissolved. In the continuous phase dispersion medium, when the molecular weight of the polymer is not too high, it has a steric hindrance effect, creating obstacles on the surface of the monomer droplets or polymer particles, preventing them from associating and causing agglomeration.
 
1.2.3 Cohesion function
When the molecular weight of the polymer surfactant is very high, it will be adsorbed on many particles, creating a bridge between the particles, forming flocs, and acting as a flocculant.
 
1.2.4 Other functions
Many polymer surfactants have poor foaming power, but have strong water retention and excellent foam stability. Because polymer surfactants have high molecular weight, they have excellent properties such as film formation and adhesion. .
 
1.2.5 Solution performance
The behavior of molecular surfactants in selective solvents: polymer surfactants are mostly amphiphilic block and graft copolymers. In selective solvents, their solution performance is more complex than that of small molecules or homopolymers. The structural characteristics, the length ratio of the amphiphilic segments, the composition of the components and the properties of the solvent all have a greater impact on its solution morphology. The amphiphilic polymer is the same as the low-molecular surfactant, the hydrophobic group adsorbs on the surface to reduce the surface tension, and at the same time associates into micelles in the solution. Merrett used electron microscopy to prove the formation of copolymer multi-molecular micelles for the first time. Subsequently, a large number of documents proved the existence of multi-molecular micelles and critical micelle concentration [26]. The driving force for the formation of micelles is the interaction between the hydrophobic group and water, and the insoluble repulsive force of polymer chains is also an important factor. The solution of micelles is usually bluish milky white, containing a large number of micelle particles of high molecular weight, high segment density and small size. It is generally considered that multi-molecular micelles are spherical with a narrow size distribution, the center of the sphere is an insoluble core, and the periphery is a soluble block or grafted part. Due to the diversity of polymer surfactants, there are also reports of the formation of ellipsoidal, rod-shaped, worm-shaped and other morphological micelles.
Different from low-molecular-weight surfactants, polymer surface activity may form single-molecule micelles in solvents at lower concentrations, but many reported results are very different, so far there is no conclusion. Sadron first put forward the hypothesis of monomolecular micelles. He believed that the different solubility of the chain segments and their mutual incompatibility promote the formation of micelles from monomolecules of polymer surfactants in dilute solutions. The characteristics are shown under the condition of constant molecular weight. The viscosity number and the radius of rotation have obviously decreased, and the surface tension isotherm has a double break point phenomenon. Block copolymers generate single-molecule and multi-molecular micelle structures in solution. At the same time, although polymer surfactants have critical micelle concentration like low-molecular surfactants, their surface tension isotherm shapes are different. On the one hand, there may be multiple breakpoints. On the other hand, after the critical micelle concentration, as the surfactant concentration increases, the surface tension will continue to decrease, but the magnitude of the decrease becomes smaller and smaller. This is because the arrangement of the hydrophobic segments of the macromolecules on the surface is much lower than that of the low-molecular surfactants. As the concentration of the macromolecules in the solution further rises, the macromolecular segments on the surface are further compressed and the arrangement density of the hydrophobic segments is increased. As a result, the surface tension is further reduced. Using static, dynamic light scattering, small-angle X-ray and neutron scattering, GPC, sedimentation analysis, viscosity measurement, permeation measurement, fluorescent probe method, electron microscope and 1H, 13C NMR and other methods can be used to study amphiphilic polymers in dilute solutions Formation and size of micelles in.
 
1.3 Application of polymer surfactants
    Polymer surfactants are widely used in a variety of fields due to their unique superior performance. At present, many hotspots are studied, such as nanomaterials (LB film), medical polymer materials, tertiary oil recovery and adhesives and emulsions in the chemical industry. Aggregation and other aspects are related to its density.
 
1.3.1 Application of polymer surfactants in tertiary oil recovery
Some water-soluble amphiphilic block polymers have better salt and temperature resistance. When it dissolves in water, the hydrophobic part will associate with it in order to minimize contact with water. Therefore, the apparent molecular weight of the polymer will also increase with the association, showing that the apparent viscosity of the solution increases accordingly. The viscosity of a polymer solution with the same molecular weight and the same concentration increases with the increase of the hydrophobic base. Experiments have shown that adding electrolyte or increasing the temperature will facilitate hydrophobic association, so the viscosity of the polymer solution increases with the increase of the electrolyte concentration within a certain range, and increases with the increase of temperature. Therefore, this water-soluble amphiphilic block polymer exhibits excellent salt resistance and temperature resistance, and has bright prospects in large-scale industrial applications.
 
1.3.2 Application of polymer surfactants in emulsion polymerization
    Emulsion polymerization is becoming an important field of polymer science and technology, and one of the important methods for synthesizing polymers. In emulsion polymerization, the stability, number, size and polymerization speed of latex particles are directly related to the molecular weight of the final product, so the surface of the latex particles can make the latex particles very stable. However, since these emulsifiers are generally low-molecular compounds, the lipophilic end is connected to the surface of the polymer colloidal particles through physical adsorption, which makes the latex not resistant to electrolytes and easy to flocculate, and the presence of small molecular compounds affects the latex and its The performance and other characteristics of the membrane. As an emulsifier, polymers can overcome the above shortcomings. Because of its high molecular weight, and the lipophilic end structure can be selected from the same substance or the like as the polymer to be prepared, according to the principle of similar compatibility, the lipophilic end of this polymer surfactant is anchored to the colloidal particles , Not just adsorption, which can greatly improve the binding fastness. As a stabilizer, polymer surfactants stabilize the emulsifier mainly through steric hindrance, thereby increasing the electrolyte resistance of the latex and improving the performance of the latex and its membrane. Therefore, the emulsion polymerization of polymer surfactants as emulsifiers is of great significance.
 
1.3.3 Applications in other areas
With the deepening of the research on polymer surfactants, the application of polymer surfactants has become increasingly widespread. As a medical material, it has the advantage of good anticoagulant properties. Use it to make artificial organs or coat them on artificial organs to obtain good anticoagulant properties. In addition, polymer surfactants can also be used as flocculants for the treatment of reactive dye wastewater, preparation of sewage treatment agents, soap-free hydrosol materials, and soap-free emulsion polymerization.