Investigation of the temperature responsive behaviors of novel polyaspartamide derivatives bearing alkyl ether-type pendants
Previous works reported that some temperature responsive polyaspartamide derivatives exhibited pH responsiveness, although pH responsive functional groups or pH-cleavable linkages were not introduced during the preparing process, which may be related to the hydroxyl groups of their pendant alkanolamide moieties. In order to investigate whether pH will also affect the temperature responsive behavior of temperature responsive polyaspartamide derivatives without hydroxyl groups, four linear alkyl ether-type amine compounds with one ether oxygen atom and 3 to 6 carbon atoms, denoted as OCx (x represents the number of carbon atoms in alkyl ether-type amine compounds), were selected for the aminolysis reaction of poly(succinimide) to design and synthesize the polyaspartamide derivatives bearing alkyl ether pendants, named PASP-OCx. The chemical structures and molecular weights of the obtained polymers were characterized and confirmed by FTIR, 1 H NMR and SEC-MALLS. The water-solubilities, surface tensions and transmittances of PASP-OCx aqueous solutions were also investigated. Results showed that PASP-OCx can be dissolved in deionized water when x is less than or equal to 5. The surface tensions of PASP-OC4 and PASP-OC5 solutions exhibited obvious turning points. Interestingly, transmittances results showed that PASP-OC5 exhibited temperature responsive behaviors under acidic, neutral or alkaline conditions (pH ≤ 10.0); PASP-OC4 only exhibited temperature responsive behaviors under acidic condition (pH ≤ 3.0); while PASP-OC3 did not exhibit temperature responsive behavior. The LCST value of PASP-OC5 and PASP-OC4 can be turned by adjusting the pH value of their solutions, and decreases with decreasing the pH value. Additionally, the influence of pH on the LCST values of PASP-OC4 and PASP-OC5 was investigated by zeta potential, and the results showed that the decrease of LCST values may be due to the reducing of electrostatic repulsion with pH decreasing. Therefore, results showed that pH can also affect the temperature responsive behavior of temperature-responsive polyaspartamide derivatives that do not contain hydroxyl groups.
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