The importance of sub-nanosecond relaxations on the ballistic impact resistance of cross-linked thermoset network polymers.
basic_science · Level V
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- Record sourced from PubMed, PMID 42429030.
- Also identified by DOI 10.1039/d6sm00313c.
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Abstract
This manuscript explores the importance of the collective, many atom relaxations that occur in cross-linked glassy polymer networks on the picosecond to nanosecond time scale on the mechanical toughness of these materials under ballistic rates of deformation. The relevant strain rates that are generated during ballistic impact events can readily exceed 10<sup>6</sup> s<sup>-1</sup> and our premise is that a strong population of relaxation mechanisms faster than the time scale of deformation are important to dissipate energy during impact. Our study focuses on a broad range of cross-linked networks, including several different epoxy resins with variations in their network chemistries and a norbornyl-based network cross-linked by ring opening methathesis polymerization. The glass transition temperatures across this series of networks varies by nearly 200 K, leading to wide variations in the materials ballistic impact resistance. The ballistic impact resistance of these cross-linked networks in the glassy state are characterized by variable temperature ballistic gas gun tests while the temperature dependent relaxation dynamics in the nanosecond to picosecond regime are quantified by inelastic and quasielastic neutron scattering. We observe striking similarities between temperature dependent ductile-to-brittle transitions in these materials from the ballistic impact tests and the thermal activation of the collective relaxations on a timescale of 1 to 2 ps, suggesting that activating these relaxations are important for ballistic resistance. Furthermore, we explore the ratio of the collective relaxations on the picosecond time scale to all of the sub-picosecond collective vibrations, that includes the Boson Peak, seems to indicate that the ability of a glass to parlay soft, collective many-atom vibrations into collective relaxations is a key indicator of ballistic impact resistance. The nature and origins of these dynamics correlations are discussed in detail, including some of their potential limitations.