Tuning the Dynamic Fragility of Acrylic Polymers by Small Molecules: The Interplay of Hydrogen Bonding Strength

High-<i>T</i>_g polymers exhibit high fragilities in general. Here, we report for the first time that small molecules with double phenolic end groups are effective to independently mediate the dynamic fragility (<i>m</i>) and glass transition temperature (<i>T</i>_g) of acrylic polymers. Broad band dielectric spectrometer (BDS), Fourier transform infrared spectroscopy (FTIR), and differential scanning calorimeter (DSC) measurements showed that the addition of small molecules with a concentration lower than 30 wt % leads to a narrower relaxation time distribution of the intermolecular cooperative rearrangement motion because of the formation of hydrogen bonding networks. These acrylic polymers exhibited a significant decrease in <i>m</i> while showing a linear increase in <i>T</i>_g by changing the loading of the small molecules. Further experimental results demonstrated that <i>m</i> decreases monotonically with intermolecular hydrogen bonding strength for a given host polymer matrix. The <i>m</i>/<i>T</i>_g value diminishes with increasing small molecule content, whereas the value remains slightly changed by the copolymerization of different amounts of styrene on the acrylate chains. These results demonstrate that the compelling opposite change in <i>m</i> and <i>T</i>_g in the small molecule-loaded system is dominated by enthalpic intermolecular interactions. A distinct reduction of <i>m</i> in relation with small molecules was observed in poly­(butyl methacrylate), where a methyl group attached to the same C atom of the hydrogen bonding ester group. The impact difference of size, number, and steric hindrance of phenolic groups in small molecules, as well as the chemical structure of polymers, on the mixture’s fragility and <i>T</i>_g was discussed based on the generalized entropy theory of glass formation