Critical fragility ratio governing anomalous component dynamics in hydrogen-bonded acrylic polymer/hindered phenol mixtures

Strong glass-forming polymers with a low fragility index (m) offer several practical benefits. Blending with a lower-fragility guest component is expected to reduce the fragility of the host polymer, while it usually comes at the expense of lowering the glass transition temperature. Moreover, achieving a fragility lower than that of the guest component remains challenging. Matching the fragility indices was found to be key to achieving a minimum fragility below that of the individual constituents, but the critical fragility ratio between the two components remains poorly understood. This work systematically explores this ratio by blending hindered phenols with poly(butyl acrylate)s or poly(butyl methacrylate)s that exhibit various side chain isomerizations, which provides a wide range of fragility ratios. An upper critical fragility ratio of ∼1.37-1.38 was identified, above which hydrogen bonding only leads to a negative fragility deviation from the linear mixing rule. Conversely, the lower critical fragility ratio is expected to be the reciprocal of the upper one, i.e., 0.72-0.73. Only when the fragility ratio lies between the upper and lower critical thresholds does a fragility minimum occur. These upper and lower ratios also well explain the presence or absence of a fragility minimum in several binary polymer blends and binary small-molecule blends. Our results may provide a quantitative basis for designing strong glass-forming polymers with high performance from individually fragile compounds.