Minimum fragility below individual components in associating acrylic polymer/hindered phenol hybrids: The critical role of their fragility proximity

Achieving a significant reduction in the segmental fragility of a host polymer is critical for optimizing binary blend applications such as pharmaceutical stabilization. However, attaining a fragility lower than that of the individual components remains challenging, as segmental dynamics are complexly affected by molecular rigidity and cohesive energy. Interestingly, certain acrylic polymer/hindered phenol blends with strong intermolecular hydrogen bonding (inter-HB) exhibit such behaviors. In this work, we systematically investigate the role of molecular rigidity by varying either the host polymer or the hindered phenol type while maintaining constant inter-HB strength. Results show that a large fragility disparity between components leads only to a negative deviation from the linear additivity rule in blend fragility. In contrast, a minimum fragility emerges with matched component fragility, and this minimum becomes more pronounced as the fragilities of the components converge. These findings indicate that while hydrogen bonding governs the negative fragility deviation, matching the fragility indices-rather than the intrinsic rigidity of either component-is key to achieving a minimum fragility below that of the individual constituents.