ABSTRACT
In Fig. 3, differential Raman spectra of PMA aqueous solution (Fig. 3a) and PMA/PMB hydrogel (Fig. 3b) are shown. The curve fitting and resolution into each state of carboxyl groups was carried out and well done. Boxed numbers demonstrate the ratio of the peak areas attributed to the dimer of carboxyl groups (bright part of the figure) against the respective peak area of free carboxyl groups (dark part of the figure). In case of PMA aqueous solution, it was 1.1. In contrast, in the PMA/PMB hydrogel the ratio increased to 3.3. That is, dimerization of carboxyl groups proceeded by gel formation. It is well known that the carboxyl groups can form dimer through hydrogen bonding between them [24]. This result indicated that dimerization of carboxyl groups in MA unit of PMA through hydrogen bonding acts as physical cross-linking points of hydrogel. In the aqueous solution, PMB chains associate themselves and form hydrophobic domains [25, 26]. When the aqueous solutions of PMA and PMB mixed together, the PMA chains penetrated into hydrophobic domains of PMB. Since the permittivity, or polarity is lower in the hydrophobic domains, the dissociation of carboxyl groups is suppressed. Thus, the carboxylate anion becomes the free carboxyl group and dimerization of carboxyl groups through hydrogen bond formation is likely to be induced.
