Nature and interrelations of fast dynamic properties in a coarse-grained glass-forming polymer melt

Recent experimental studies have demonstrated that the dynamics of glass-forming liquids on fast picosecond timescales are directly relevant to developing effective formulations for protein drug preservation. In these systems, short-time dynamic properties, including the Debye-Waller factor, non-ergodicity parameter, and the 'fast beta' relaxation time, are 'tuned' by the addition of antiplasticizer additives, with associated improvements in protein preservation. We employ molecular dynamics simulations of a coarse-grained polymer melt with and without the addition of an antiplasticizer additive to characterize how these fast dynamic properties interrelate and how they are altered by the addition of antiplasticizer additives. We find that the Gaussian approximation for the van Hove correlation function holds to an excellent approximation at short times, facilitating the establishment of precise interrelations between these properties and the characterization of their temperature and composition dependence. Following previous studies, we first fit the fast relaxation process to an empirical 'stretched Gaussian' form (exp[-(t/tau)(beta)] with 1 < beta < 2) and find that beta equals 3/2 over a wide range of temperature for the post-inertial relaxation process. We then consider a more theoretically motivated description of fast beta relaxation inspired by an earlier model for the relaxation of a fractal network of elastic clusters in glass-forming colloids, and we find that this description provides an excellent fit to the entire short time relaxation over a large range of temperatures with physically meaningful parameters. Finally, we show that it is possible to tune the fast beta relaxation process with antiplasticizer additives, resulting in effects that rationally enhance protein preservation in glassy sugar matrices.