PH-induced Self-assembly Of HPAM And HPAM/HEC In Aqueous Solution

At present, it is smart to prepare macromolecule micelles by using double hydrophilic, bio-compatible, environmental responsive and random copolymerized macromolecules. For this purpose, low-molecular-weight polyacrylamide (PAM) was firstly synthesized by the way of precipitation polymerization and solution polymerization, which following underwent hydrolysis reaction to give the partially hydrolyzed polyacrylamide (HPAM). And then the pH-induced self-assembly behaviors of HPAM and HPAM/HEC in aqueous solution were investigated.Low-molecular-weight PAM was synthesized by both the precipitation polymerization using mixed solvents of ethanol/acetone and the solution polymerization using isopropanol as chain transfer agent. HPAM with same molecular weight but different hydrolyzing degree (HD) was obtained by monitoring hydrolyzing conditions such as hydrolyzing time, hydrolyzing temperature and the concentration of sodium hydroxide. The pH-induced HPAM and HPAM/HEC micelles were prepared in aqueous solution by controlling the vary speed and scope of pH, in which the main driving force is recognized as hydrogen bonding complexation The molecular weight and molecular weight distribution of PAM was measured by high-performance gel filtration chromatography (HPGFC), and the molecular structure of PAM and HPAM was identified by Fourier transform infrared spectroscopy (FT-IR). Several means, for example, FT-IR, computational chemistry, zeta potential, dynamic laser light scattering (DLS) and transmission electron microscopy (TEM) were used to characterize the structure, morphology and properties of HPAM and HPAM/HEC micelles. The mechanism of macromolecule micelle formation was also discussed based on the above evidences. The influence of the HD of HPAM, the vary speed and scope of pH, the additional cationic surfactant cetyltrimethyl ammonium bromide (CTAB) and other factors on the morphology of micelles was investigated in detail.The main experimental results and some conclusions were summarized as follows:(1) PAM with molecular weight of (1.72~13.99)×104 was synthesized by precipitation polymerization by changing the ratio of acetone to ethanol, the more acetone the higher molecular weight of PAM. PAM with molecular weight of(2.53～6.60)×104 was obtained by solution polymerization using isopropanol as chain transfer agent, the higher temperature the higher molecular weight of PAM.(2) The influences of time, temperature and the concentration of NaOH on HD of HPAM were examined. The hydrolysis equilibrium was basically approached after 4 h and HD of HPAM linearly increased with the temperature increase. The rate of hydrolysis was lower when cNaOH<2.50 mol/L and higher when cNaOH>2.50 mol/L. The HD of HPAM could be controlled between 35%~53% as one need just by running the hydrolyzing reaction at 40~80 oC for 0.5~4h maintaining NaOH concentration between 1.25~5.00 mol/L.(3) HPAM micelles were obtained with different structures in alkaline or acidic conditions. HPAM36(HD=36%) would self-assemble into the large compound vesicles with the unique shape of asymmetric polygon sheet (350 nm×150 nm) and keeping vesicle diameter of 2~10 nm when slowly increasing acidity in its neutral solution, whereas spherical micelles when slowly increasing alkalinity in its neutral solution. HPAM46 would self-assemble into the "pearl necklace"-like micelles with average particle size of 125 nm, Ultrasonic wave made the size of micelles more uniform. The morphology of micelles was great influenced by adding CTAB before micellization.(4) HPAM/HEC micelles were obtained with different structures in alkaline or acidic conditions, too. Large composite vesicles wafers were observed under the conditions of both slowly increasing acidity in neutral solution and firstly slowly increasing acidity in neutral solution and then quickly increasing acidity to a designated pH. Ultrasonic wave made the diameter of the wafers sharply shrank from 920 nm to 350 nm. The HD of HPAM exerts some influence on the morphology of HPAM/HEC micelles. HPAM46/HEC formed micelles with irregular shape, while HPAM53/HEC assembled into "pearl necklace"-like micelles with the average diameter of 80 nm in acidic environment. There appeared a new type of micelle with hierarchical three-tier structure by adding CTAB in the HPAM (1 mg/ml)/HEC (1 mg/ml) system at pH 5.1.