Control of Stimuli Sensitivity in pH-Switchable LCST/UCST-Type Thermosensitive Dendrimers by Changing the Dendrimer Structure.

Stimuli-sensitive materials, such as pH- and temperature-responsive polymers, are useful as smart materials. Phenylalanine (Phe)-modified polyamidoamine (PAMAM) dendrimers with succinic acid termini, PAMAM-Phe-Suc, have been reported as unique pH-switchable lower critical solution temperature (LCST)-/upper critical solution temperature (UCST)-type thermosensitive polymers. Regulating the phase transition behavior of dendrimers is important for their applications. This study investigated the relationship between the dendrimer structure and stimuli sensitivity. Phe-modified PAMAM dendrimers with cyclohexanedicarboxylate termini (PAMAM-Phe-CHex) and sulfonate termini (PAMAM-Phe-SO3Na) were synthesized. The temperature-dependent transmittance of these aqueous dendrimer solutions was examined at various pH values. PAMAM-Phe-CHex with Phe at all termini (PAMAM-Phe64-CHex) demonstrated a broad UCST-like phase transition at pH 7.0 but lacked an LCST-type phase transition. PAMAM-Phe-CHex with ≤ 27 Phe residues showed both LCST- and UCST-like phase transitions at different pH values, but the phase transition was broad. PAMAM-Phe-SO3Na showed both LCST- and UCST-type phase transitions at different pH values, and the transition temperature increased as the bound Phe number decreased. Thus, the phase transition behavior of PAMAM-Phe-SO3Na dendrimers can be regulated by varying the Phe/PAMAM ratios.