Insertion of a Cardo Structure into a Polyurethane Framework Using a pH Indicator and Its Impact on Tensile and Shape Memory Properties

Abstract

Perpendicular cardo structural units were inserted into a polyurethane (PU) framework from the pH indicator (phenolphthalein (PP), phenol red (PR), or bromocresol green (BG)) sharing a triphenylmethyl group. The thermal transitions, mechanical properties, pH-sensitive color change, and low temperature flexibility of the PU modified with the cardo structural units were compared with those of the ordinary PU. Slight cross-linking between PU chains was performed because the insertion of the indicator moieties deteriorated the mechanical properties. The breaking tensile stress and shape recovery capability at 0 °C of the BG-PU series surpassed those of the PP-PU and PR-PU series. The inserted pH indicator moieties did not show the color change under different pH conditions because the isomerization under basic conditions was blocked. Among the three series of PUs, the BG-PU demonstrated the best low temperature flexibility compared with that of the PP-PU and PR-PU due to the functional groups on the phenyl rings of BG. Insertion of a cardo structural unit from the pH indicator reduced the molecular interaction between PU chains.