Design and Precise Synthesis of Thermoresponsive Polyacrylamides by Keita Fuchise

Author:Keita Fuchise
Language: eng
Format: epub
Publisher: Springer Japan, Tokyo

2.3.3 Aggregation State of Urea End-Functionalized PNIPAM in Water

In the previous section, the robust and efficient hydrogen bonding was revealed to be actually present even in water for the series of urea end-functionalized PNIPAMs. Given that hydrogen bonding of the urea groups realizes a continuous and hierarchical intermolecular interaction, the polymers should self-assemble in water even at a temperature below the cloud point. Thus, the 1H NMR measurements of 4f and g were carried out in D2O at 25 °C to elucidate the aggregation behavior of the polymers, as shown in Fig. 2.5. No significant difference was observed in the signals originated from the main chain protons of 4f and g. Moreover, a direct investigation of the urea protons was unavailable because of the rapid exchange with the deuterium atoms of D2O. An obvious difference was observed in the signals due to the aromatic protons at the chain end, which appeared between 6.8 and 8.0 ppm. The signals due to the aromatic protons of 4f were broad, while those of 4g were observed as sharp peaks. Therefore, the peak broadening was found to closely correlate with the hydrogen bonding ability of the urea moiety. These results indicate that the mobility of the chain ends for 4f was probably restricted due to the strong hydrogen bonding ability of terminal urea group, implying that the polymer chains assemble via intermolecular hydrogen bonding even at a temperature below the cloud point.

Fig. 2.5 1H NMR spectra of (a) 4f and (b) 4g in D2O (2.0 mg mL −1) at 25 °C

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