Effect of Hydrogen Bonding on Relaxation Time of Polymers for Ballistic Impact Resistance

The toughness of some polymers such as polycarbonate (PC) and poly(dicyclopentadiene) has led to their use in ballistic impact resistance applications. However, the origin of this toughness and how it relates to the structure and dynamics of the polymer is still not well-understood. Previously quasielastic neutron scattering (QENS) experiments on a series of PCs were carried out to investigate the molecular dynamic origin of toughness. It was found that the activation of a (1 to 3 ps) relaxation process in QENS is critical for realizing the toughness in these PCs. In this presentation we extend these QENS experiments to a series of copolymers of 5-ethylidene-2-norbornene (ENB) and 5-methanol-2-norbornene (NBOH). The NBOH concentration was systematically varied to determine the effect of hydrogen bonding on relaxation times and toughness.  These studies show that as hydrogen bonding increases the elasticity of the system increases and slows down this quasielastic relaxation process that was critical for toughness in the PCs by a factor of five, from approximately 0.6 ps to 3 ps. This is accompanied by a suppression in the amplitude of the quasielastic peak, indicating number of atom participating in the relaxation also decreases. We show that toughness of these materials, quantified in terms of the critical stress intensity factor (K1C), is strongly coupled with these relaxations; K1C decreases as the amplitude of the relaxation process decreases and the time scale of the relaxation processes slows down.  This confirms our previous finding in PCs that relaxation processes on the time scale of a ps are critical for dissipating energy and realizing toughness in polymer glasses.