Effect of heating rate on thermo-induced aggregation of poly(ethylene oxide)-b-poly(N-isopropylacrylamide) in aqueous solutions

• Published in: Chinese journal of polymer science 2010-05, Vol.28 (3), p.437-447
• Main Authors: Yan, Jing-jing, Ji, Wen-xi, Chen, Er-qiang, Li, Zi-chen, Liang, De-hai
• Format: Article
• Language: English
• Subjects: Characterization and Evaluation of Materials; Chemistry; Chemistry and Materials Science; Condensed Matter Physics; Industrial Chemistry/Chemical Engineering; Polymer Sciences
• ISSN: 0256-7679; 1439-6203
• DOI: 10.1007/s10118-010-9085-y

The effects of heating rate on the aggregate behavior of poly(ethylene oxide)- b -poly( N -isopropylacrylamide) in aqueous solutions were investigated in detail by laser light scattering and TEM. By employing two separate heating protocols, step-by-step heating at < 5 K/step and one-step jump, to heat the sample from 15°C to the selected temperature, we found that the heating rate only showed significant effect on the aggregates above the cloud point. The aggregate formed by step-by-step heating exhibited a much larger size and a broader size distribution than those formed by one-step jump heating. Moreover, neither of the aggregates were ideal micellar structures as indicated by the size and the R g / R h values. On the contrary, at temperatures below the cloud point where the block copolymer formed core-shelled micelles, the heating rate showed negligible effect on the size and size distribution of the micelles. Since the system underwent a phase separation above the cloud point, the heating rate effect could be reasonably explained by the phase separation mechanisms: the nucleation-and-growth mechanism in the metastable region and the spinodal decomposition mechanism in the unstable region.