Importance of Sub-Nanosecond Fluctuations on the Toughness of Polycarbonate Glasses

This article explores the relationship between the nano- to picosecond dynamics, as quantified by elastic incoherent neutron scattering, in polycarbonate (PC) glasses with their mechanical toughness. We establish a robust correlation between mechanical toughness and these fast relaxations, suggesting that they are a critical pathway for dissipating energy under impact conditions. This understanding is developed across a series of PCs, where the impact strength is either systematically increased through the incorporation of 1,4-cyclohexyl linkages into the PC backbone or diminished through the addition of antiplasticizer molecules that embrittle the glass. Across this homologous series, a connection emerges between the level of anharmonicity in mean-square-atomic displacements of different PCs and their impact strength. We interpret the level anharmonicity in these fast dynamics as a metric of the collective, many-atom relaxations, suggesting that collective relaxation processes on the timescale of nano- to picosecond are critical for toughness in polymer glasses.