Synthesis Of ABA Type Of Amphiphilic Polyurethanes And Study On The Rheological Properties Of Their Aqueous Dispersions

Synthesis, self-assembly behavior and application of amphiphilic polymers attract worldwide polymer scientists in recent years. ABA type of amphiphilic polyurethane is the amphiphilic polymer containing urethane groups, which is composed of two hydrophobic block ends and hydrophilic middle block. It has wide applications in such fields as associative thickener, waterproof and moisture permeable materials, medical dressing and polymer surfactant, especially as associative thickener to improve workability and film forming property of coating obviously. The end capping ratio of ABA type of amphiphilic polyurethane was increased by optimizing reaction conditions based on the investigation of the main factors affecting its hydrophobic block end capping ratio in this thesis. The thickening mechanism of amphiphilic polyurethanes was investigated by studying the relationship between their structure and their micellization behavior in aqueous medium or the rheological properties of their aqueous dispersions. The results will be of theoretical and practical significance for understanding the relationship between the structure and properties of amphiphilic polyurethanes, and improve the properties of the existing associative thickeners. The main research work and obtained results are as follows:(1) A series of ABA type of amphililic polyurethanes with different molecular weight from 10000～60000 were synthesized by two-step solution polymerization in toluene using isophorone diisocyanate, poly (ethylene glycol) of molecular weight 6000 and cetyl alcohol as raw materials, and dibutyl tin dilaurate as catalyst. Their structure was characterized by Fourier transformation infrared spectroscopy (FTIR) and nuclear magnetic resonance (NMR). The hydrophobic end capping ratios and molecular weights of ABA type of amphiphilic polyurethanes were calculated by a series of equations which were established on the basis of the relative integral area of suitable ¹H NMR peaks. Their intrinsic viscosity numbers were measured in tetrahydrofuran at 30℃, and their Mark-Houwink constant K was 5.3×10^-2dL/g and a was 0.20. The suitable reaction conditions were as follow: reaction temperature was 110℃, reaction time was 4h and added mass fraction of the catalyst was 0.3%.(2) The critical micellization concentration (CMC) of ABA type of amphiphilic polyurethanes was measured by fluorescence spectroscopy and ultraviolet-visible spectroscopy. It was found that they can form micelles by self-assembly in the aqueous solution of very low concentrations (1×10^-4～5×10^-3g/mL). The CMC values obtained by these two methods were very closed, which decreased with the increase of the hydrophobic block content within amphiphilic polyurethane chain.(3) All ABA type of amphiphilic polyurethane dispersions of low concentration behaved as Newton's fluid. When their concentrations exceeded the critical association concentration (CAC) of the amphiphilic polyurethanes, they behaved as Newton's fluid under low shear rates and showed shear thinning phenomena under high shear rates. The dispersion of the ABA type of amphiphilic polyurethanes with high hydrophobic blocks content exhibited shear thickening behavior under medium shear rate. Their storage modulus G' and loss modulus G" increased with the oscillation frequency co, and G' had a higher increase rate than G". The viscoelastic property of the dispersion of the ABA type of amphiphilic polyurethanes with high hydrophobic blocks content was predominated by elastic property of solid, while the viscoelastic property of the dispersion of the ABA type of amphiphilic polyurethanes with low hydrophobic blocks content was predominated by viscous property of liquid. The values of CAC measured by steady rheological test and dynamic rheological test were basically in accordance with each other. As the hydrophobic block content was increased, the zero shear viscosity and infinite storage modulus were gradually enhanced, and the CAC at which the micellar network was formed gradually decreased.