Aggregation Behaviors Of X-Shaped Block Polyethers And The Dispersing Ability Of Carbon Nanotubes

PEO-PPO block polyethers are typical nonionic polymeric surfactants, which have the characters of be rich in its designable speciality of structure and controllability of synthesis, the micellization of polyethers is temperature-dependent, as well as the diversity of solvent system. It greatly enriched the investigation of the aggregation behaviors at different interfaces and in various solvents. PEO-PPO-PEO block polyethers have many applications in the fields of enhanced oil recovery, cosmetics, wastewater treatment, fluid modification, controlled drug release, and synthesis of micro and nano materials, etc. Thus, the aggregation behaviors of block polyethers have attracted increasing attentions.The researches of the theory and application of linear block polyethers have been numerously reported; however, the studies on the block polyethers with branched structure are rarely published. The X-shaped block polyethers (named Tetronic), which have four-arms each one composed of EO and PO units bonded to a central ethylene diamine linker. The presence of a central ethylene diamine moiety in the molecular structure provides the polyethers with pH functionality. This character is used for the wide application in the biomedical and pharmaceutical fields, such as drug delivery and membrane biochemistry. However, the effect of different factors on the aggregation behaviors of X-shaped block polyethers is still lacking, such as relative size of the PEO and PPO blocks, temperature and the presence of cosolvents. Thus, the investigation of aggregation behaviors and the physicochemical properties of the X-shaped block polyethers are significative. In this paper, we present the effect of block sequence, temperature and cosolvents on the aggregation behavior of X-shaped block polyethers in aqueous solution. Meanwhile, we investigate the dispersion of carbon nanotubes and improvements on the rheology behavior of hydrophobic modified hydrolyzed polyacrylamide by X-shaped block polyethers. This thesis is divided into six parts:In the first section, the recent research advances of PEO-PPO-PPO aggregation behaviors in aqueous solution and the influence of structure, temperature and the presence of cosolvents are summarized. There is some information regarding the effect of pH and salts on the aggregation behaviors of X-shaped block polyethers. The applications in the fields of dispersion of carbon nanotubes, drug delivery and enhanced oil recovery are presentation.In the second section, the polyethers T1107and T90R4with a sequential and reverse architecture were studied. The effect of temperature on the aggregation behavior of T1107and T90R4at air/water surface is studied in detail by equilibrium surface tension, the surface dilational viscoelasticity and surface tension relaxation measurements. Due to the reverse block sequence, the conformations for T1107and T90R4are different at air/water surface. For T1107, the PPO segments on the surface are in a coiled manner, the hydrophilic PEO tails protrude into the aqueous solution, it have four branches, so the possible adsorption model is a "brush" model. For the PEO chains in T90R4, four chains are attached to the hydrophobic segments PPO at the air/water surface and ethylene diamine in the water, so T90R4molecule is in the shape of invert "umbrella". Obviously, the packing of "brush" is denser than "umbrella" model. So the surface excess concentration and dilational modulus of T1107is higher than that of T90R4. With the increasing temperature, the dilational modulus of T1107solutions decreases, and that of T90R4solutions increases. For T1107, a change in chain conformation of the poly ether molecules at the air/water surface takes place at an elevated temperature. The PEO chains become spread at the air/water surface. Increasing temperature results in expansion of monolayers spread at air/water surfaces, and the dilational modulus decreases. For T90R4, the hydrophobic PPO blocks are present at both ends. When the temperature is low, the PPO chains are located at the surface, the PEO segments fold around the PPO chains in the solution, which hinders the conformational transition of PEO segments at the surface. Thus the PEO segments are difficult to have a change in chain conformation. Therefore, more polyether molecules can fit at the surface, causing a thicker monolayer. Furthermore, the thermodynamic parameters of micellization at different temperatures were calculated. The enthalpy-entropy compensation phenomenon for the micellization of Tetronics in connection with the block sequence was discussed. In the third section, the aggregation behaviors of branched block polyether T1107at air/liquid surface were investigated in mixed solvents consisting of water and one of the following polar cosolvents:ethanol, n-propanol, ethylene glycol (EG) and glycerol (GLY). The polar cosolvents we have chosen to study resemble the monomers of the polyether blocks, as well as by practical interest. The addition of ethanol or n-propanol into water disfavors the micellization and the cmc shifts to higher concentration. The addition of ethanol or n-propanol into water results in better solvent conditions for T1107; they decrease the hydrophobic interaction and lead to an increase of cmc. GLY and EG affect the micellization to an opposite direction compared with the cases of ethanol and n-propanol. GLY-water and EG-water mixed solvents become poor solvents for T1107compared with pure water. T1107molecules tend to self assemble at lower concentrations. The octanol/water partition coefficient (log P) of cosolvent is used to correlate the effects, and it could capture the effect of cosolvents on the cmc qualitatively. In the presence of ethanol or n-propanol, dilational elasticity reduces monotonously, which is coinciding with the change of Γmax-Ethanol and n-propanol are both considered as water structure breakers, their presence disrupt the hydrogen-bond network of water, which results in a less dense coiled packing of these molecules. Furthermore, GLY and EG interact favorably with water and strengthen the H-bond network of the mixed solvent. Thus, dilational viscoelasticity increases. The fluorescence probe technique of pyrene was employed to determine the polarity of microenvironment. When the concentration of T1107is lower than cmc, the added alcohols can effectively reduce the micropolarity, while the concentration of T1107is larger than cmc, the micropolarity of T1107is not changed by adding alcohols. It indicates that the addition of alcohols only influence the solvent character without effecting the micellar structure.In the fourth section, the effects of alcohols such as ethanol, n-propanol, EG or GLY on the dispersion of carbon nanotubes (CNTs) in the aqueous solutions of T1107have been investigated by UV-vis-NIR absorbance spectra, Raman spectra and the high-resolution transmission electron microscopy (HRTEM). The dispersion limit of CNTs (Climit), optimal polyether concentration (Copt) and the defect density of CNTs (ID/IG) are used to demonstrate the dispersion ability of CNTs in mixed solvents, and they can be changed greatly by the variation of alcohols. The polyether in good solvent conditions have better dispersibility of CNTs. Ethanol and n-propanol are known to be good solvents for both PEO and PPO blocks of polyethers, so the addition of ethanol or n-propanol into water dramatically increases the Climit, while GLY-water and EG-water mixed solvent become poor solvent for T1107compared to pure water. The value of Copt is in the order of n-propanol-water> ethanol-water> EG-water≈GLY-water mixtures, which is almost consistent with the order of cmc of T1107in different solvents without CNTs. The ID/IG values in T1107solutions with EG or GLY are lower than that in T1107aqueous solutions. Since each EG and GLY molecule has more than one donor and acceptor sites, there is a high probability that all molecules are interlinked by a number of hydrogen bonds. EG and GLY interacts favorably with water and strengthens the H-bond network of the mixed solvent. This may be the result of an increase in water structuring which makes better protection of CNTs; consequently, the ID/IG decreases. We believe that our results will be greatly useful for the selection of solvents in the practical applications of CNTs.In the fifth section, the block polyethers LPE (linear) and TPE (X-shaped) are synthesized by anionic polymerization. The ratio of EO/PO and molecular weight of block polyethers are the same. The aggregation behaviors at the air/water and n-heptane/water interfaces were systematically studied. The molecular construction can influence the efficiency and effectiveness of block polyethers in decreasing surface tension. LPE has better efficient ability to decrease surface tension of water and n-heptane than that of TPE. The dynamic interfacial tension curves indicate that the lag-time of the adsorption of block polyethers at n-heptane/water interface is smaller than that at air/water surface. This may be because the PPO groups have a higher affinity to the oil molecules than the air molecules. Therefore, the PPO groups immerse more energetic favorably into the oil phase than the air phase. The dynamic parameters n represents the diffusion speed of the polyether molecules from bulky solution to the subsurface, which is obtained according to the equation proposed by Rosen. At n-heptane/water interface, the diffusion speed of block polyethers is faster than that at the air/water surface. The oil molecules can insert into the adsorption layer, the hydrophobic interaction between oil molecules and PPO moieties leads to a relatively ordered arrangement of adsorbed polyether molecules. The dilational elasticity at the n-heptane/water interface is much lower than that at the air/water surface. Because n-heptane is a better solvent for the hydrophobic chains of the adsorbed polyether molecules than the air. Thus the interposition of n-heptane molecules will weaken the interactions between the hydrophobic groups. The dilational elasticity of TPE is much lower than that of LPE at same condition. It is conjectured that the linear LPE has a stronger intermolecular interaction at the interface than the X-shaped TPE and it will be hard for the branched chains of TPE to adopt the loop and tail structure. Therefore, more relaxation processes will take place for LPE molecules to resume the equilibrium when the interface is perturbed.In the sixth section, the effects of block poly ethers LPE (linear) and TPE (X-shaped) on the rheology characteristic of hydrophobic modified hydrolyzed polyacrylamide (HMPAM) are investigated. As increase of polyether concentration, the viscosity of HMPAM/LPE and HMPAM/TPE systems increase firstly and then decrease. When the concentration of polyethers is lower, the hydrogen bonding interactions between polyether and HMPAM occurs, the network between the molecules enhanced, thus the viscosity increase. When the concentration increases, the hydrophobic interactions between PPO and HMPAM occurs, the hydrophobic parts aggregate, the network structure is destroyed which caused the reduction of viscosity. The apparent viscosity, storage modulus and loss modulus of HMPAM/TPE are higher than those of HMPAM/LPE, indicating that network structure of HMPAM/TPE system is more firm and can resist higher shear.