Studies On The Structure And Rheological Behaviors Of Block Copolymers In Selective Solvents

The structures of micellar aggregation formed by block copolymers are not only dependent on the composition and structure of the block copolymer molecules, but also on the ambient consitions, such as solvent solubility and temperature. For the charged copolymer, i.e., polyelectrolytes, solution pH value and added ion concentration will both influence the structures of micellar aggregation and the corresponding rheological properties. Recently, the rheological properties of block copolymer have become the attractive interest because of the challenging physical problems and the increasing technological applications behind. Rheological measurements are used to demonstrate the relationship between micellar structural variety and the macroscopical properties of the micellar systems to control rigorously the structure and obtain anticipant properties of materials.In first part of this thesis, the relationship between dynamic rheological response and the micellar morphologies formed by the poly(styrene-b-isoprene-b-styrene) tricopolymers (SIS) in the selective solvent n-heptane(C₇) with different copolymer concentration(C_p), temperature and composition in the nondilute solution regime is studied.The micellar samples need to experience heat equilibrium treatment before any test to decrease the defects in the micellar system and ensure the same rheological test condition.Micellar aggregation in nondilute solutions of triblock copolymer polystyrene-polyisoprene-polystyrene (SIS) in the slightly selective solvent, n-heptane, with different copolymer concentrations (C_p) has been studied using atomic force microscopy (AFM) and rheology. Aggregation as a function of C_p has been investigated for range concentrations 0.1 mg·mL^-1≤C_p≤30 mg·mL^-1. For 0.1 mg·mL^-1≤C_p≤1.6 mg·mL^-1, the zero shear rate viscosity (η₀) keeps constant with increasing C_p and the rheological response is Newtonian liquid like. In this concentration range, AFM observation shows that the morphologies of the micelles exhibit a mixture of rod-like cylinder and spheres, and interconnection between micelles is insignificant. Aggregation of micelles becomes obvious and connected worm-like micelles form with increasing C_p. For 2.0 mg·mL^-1≤C_p≤5 mg·mL^-1,η₀ increases slowly with increasing C_p. The rheological characteristics of solutions in this concentration range exhibit a non-Newtonian fluid behavior with two Newtonian plateau and shear-thinning behavior. The single relaxation time Maxwell model always used in the similar systems becomes invalid and the linear Jeffrey model is found to fit well with the experimental data. For 6 mg·mL^-1≤C_p≤30 mg·mL^-1,η₀ increases rapidly with increasing C_p. The single relaxation time Maxwell model is found to fit well with the experimental data. The function between critical shear strain,γ_c and C_p, and the departure from the Cox-Merz rule at different frequency range also partition solutions in nondilute concentration range into regions corresponding to different micellar interconnection states.The effect of temperature on the morphology and rheological properties of system of SIS1#/C₇ is studied. Results show that the cylinders change into vesicles at T=50℃. On the other hand, the solvent solubility decreases when temperature increases and the micellar core is swollened by the solvent, resulting in the change of micellar structure, i.e., the micellar aggregation number decreasing and the micellar number density increasing. Hence, the rheological properties of the system have corresponding changes. The micellar structure change due to the changes of temperature is an unreversible process according to the unsuperposition curves of the G'-T, G"-T,η*|-T and tanδ-T. The heat and cooling process also have effet on the rheological properties.The effect of composition of the block copolymer is also studied. The sample SIS2# has the same molecular weight of PS block and shorter chain length of PI block as compared with the sample SIS1#. Due to the shorter PI block chain length, PI block could hardly play a role of "bridge", connecting the different micelles, so that the rheological properties are mainly characterized by viscosity and could not form network structure even at very high copolymer concentration. Also, the effect of temperature is different from the system of SIS1#/C₇. In the second part of this thesis, the relationships between solution parameters including concentration of polyeletrolyte and potassium bromide (KBr), pH value of solution and the aggregation behavior for the micelles formed by the amphiphilic diblock polyeletrolyte, poly(styrene-b-acrylate acid) (PS-b-PAA) were studied. Moreover, the effect of the micellar aggregation behavior on the rheological properties of the micellar latex was examined. The results reveal that increasing the copolymer concentration and salt concentration leads to formation of greater irregular structure in polyelectrolyte micelles due to the aggregation of them, while the effect of pH value can be negligible due to the higher composition of PS block in the copolymer composed of a long hydrophobic block (PS) and a comparable short amphiphilic block (PAA). All of the micellar latex exhibit shear thinning behavior, while the apparent viscosity increases with the increasing copolymer concentration at lower shear rate, and approaches a constant value (0.01 Pa·2s) of ten times of magnitude greater than that of water. Furthermore, the presence of potassium bromide remarkably increases the latex viscosity, comparable with that without added salt, and Newtonian plateaus at low shear rates appeares when the salt concentration reached 4.31 g/L. This may be due to the effect of corona expansion resulting from the charges-shielding by the added ion. The obvious effect of pH value on the latex viscosity could hardly be observed due to the comparable short amhpihpilic block.