Study On The Behavior And Properties Of Hydrophobically Modified Poly Acrylic Acid In Aqueous Solution

Water-soluble polymers have been widely used in enhanced oil recovery, cosmetics products, pharmaceutical synthesis and food industry in recent years, and hydrophobically modified water-soluble polymers have shown more important applications potential. In this work, both experimental methods and molecular simulation were conducted to study the molecular behavior and properties of hydrophobically modified poly acrylic acid (HMPAA) both in bulk solution and at oil/water interface as well as the effect of these properties on emulsion stability. Combining experimental methods including bulk viscosity, confocal laser fluorescence microscopy (CLFM) and interfacial rheology with dissipative particle dynamics (DPD) simulation, the relationship between molecular behavior and emulsion stability properties of macroscopic solution was built.1. Properties of HMPAA solution and the molecular behavior of polymerThe effect of pH and ionic strength on bulk viscosity and conformation of aggregation formed by polymer molecules were explored by bulk rheology and CLFM, while DPD was used to simulate the change of solution conditions by varying molecular structure, the ratio of COOH and COO- was adjusted to reflect the variation of pH in simulation. When pH is low, the viscosity is low, because HMPAA chains shrink into nubby aggregation due to the strong hydrogen bonds between the plentiful COOHs while the association between different polymer chains is rare. As pH increases, COOHs are neutralized into COO-s and HMPAA chains expand owing to the electrostatic repulsion between COO-s of the same molecule. As a result, polymer chains associate together and the viscosity becomes higher. However, the spread can be restrained by screening the electrostatic repulsion between COO-s as the ionic strength increases, which lead to a lower viscosity. In the meaning time, DPD also shows that gel network can be more easily to be formed as grafting degree of HMPAA increases and high viscosity is appeared because of the association of polymer chains, which is well agree with experimental results. 2. Properties of oil/HMPAA solution interfaceProperties of oil/HMPAA solution interface were investigated by interfacial rheology. The main content is how pH and ionic strength affect interfacial tension and viscoelasticity. The results reveal that polymer could absorb at oil/water interface, which causes the decrease of interfacial tension and increase of the modulus gradually. However, the extent of interfacial tension decreasing and modulus increment is much lower than normal surfactants. The solution viscosity increases as pH increases, at the same time, the diffusion and adsorption of polymer from bulk phase to interface are slowed down. The increment of ionic strength leads to the decreasing of viscosity of polymer solution and accelerates the absorption of polymer molecules at oil/water interface. Comparing with the non-modified polymers, the absorption tendency of hydrophobically modified polymer is increased and is affected by pH and ionic strength more weakly. Moreover, this part also includes the investigation of effect of non-ionic surfactants on oil/water interfacial tension of polymer solution. It is found that non-ionic surfactants decrease the interfacial tension dramatically. Though the properties of oil/water interface is mainly determined by the surfactants, there are still a small amount of HMPAA molecules obviously adsorbing at the interface, which leads to higher interfacial tension than the solution only containing surfactant.3. Emulsion properties stabilized by HMPAAThe former two parts have shown that the various effects of pH, ionic strength and surfactants on the viscosity and interfacial tension of the polymer solution, which provides us typical systems for the emulsion stability mechanism study. The results show that in the absence of surfactants, the effect of pH and ionic strength on the stability of emulsions stabilized by HMPAA solution is consistent well with the bulk viscosity. The bulk viscosity plays a more important role in stabilizing emulsion than oil/water interface properties, which do not perform the same tendency with emulsion stability as pH and ionic strength change. Hydrophobically modified polymer can stabilize emulsions by kinetic factors, because the network is easily to form and resulting with high bulk solution viscosity. The HMPAA polymer molecular adsorbed at oil/water interface could prevent emulsified droplets destroying by both steric resistance and electrostatic repulsion. This work also revealed that hydrophobically modified polymer with stronger interface absorption ability has higher emulsion stability capacity. The oil/water interfacial tension will sharply decrease when surfactants are added into the system; the size of emulsified droplet is small, which can enhance the emulsion stability by thermodynamic mechanism. In surfactant/polymer mixed system, the above two mechanisms could run together.The molecular behavior of HMPAA in aqueous solution and the effect of molecular structure on the aggregation conformation have been investigated by connecting molecular simulation with experimental methods in this work. What is more, the effect of bulk phase and interfacial properties on the stability of emulsions stabilized by HMPAA is also been studied. Those results expand the mechanism of emulsion stabilization and could be helpful for formula design of emulsion and structural design of polymers as the emulsifiers.Innovations in the thesis are as follows1. Combining molecular simulation with experimental methods, the conformation of HMPAA and the effect of environmental conditions on the aggregation of HMPAA in aqueous solution were explored.2. Interfacial activity of HMPAA and its effect on properties on interface layer were investigated. And the influences of various environmental conditions on bulk and interfacial properties provide information to set up typical systems for the research of emulsion stabilization mechanism.3. The effect of bulk and interfacial properties of HMPAA solutions on emulsion stability were also investigated. The stability mechanism of emulsion stabilized by surfactants hydrophobically modified polymers was discussed.