(Top) Scanning electron micrograph image of the crazed mound formed when a projectile is arrested by a polystyrene thin film and (bottom) a corresponding schematic.
(Top) Scanning electron micrograph image of the crazed mound formed when a projectile is arrested by a polystyrene thin film and (bottom) a corresponding schematic.
(Top) Scanning electron microscope image of the perforation and melted rim region around the hole formed by the projectile and (bottom) a corresponding schematic.
(Top) Scanning electron microscope image of the perforation and melted rim region around the hole formed by the projectile and (bottom) a corresponding schematic.

Glassy polymers like polystyrene are brittle and absorb little energy when they deform. But now researchers have found that very thin films of polystyrene don’t behave as expected and show very surprising deformation behavior [Hyon et al., Materials Today (2018), https://doi.org/10.1016/j.mattod.2018.07.014].

“We found record-setting specific energy absorption,” says Edwin L. Thomas of Rice University.

Along with colleagues from the University of Wisconsin-Madison, Institute for Soldier Nanotechnologies at Massachusetts Institute of Technology, UES, Inc., and Air Force Research Laboratory at Wright Patterson Air Force Base, the team carried out small-scale ballistic impact testing of thin films of the polymer.

Polystyrene is usually a stiff, transparent solid plastic or a rigid but brittle foam, widely used as packaging material for consumer and commercial products like electronics or car parts. But with a glass transition temperature of just over 100°C, where polystyrene transforms from a glassy, brittle material to a more viscous one, its individual chain-like polymer molecules are not very mobile at room temperature. At this temperature, the polymer chains cannot respond fast enough to an impact and absorb little energy.

But polymer chains near the surface of thin films of polystyrene, however, are more mobile. When projectiles are fired at freestanding polystyrene films with a thickness comparable to the end-to-end length polymer chain, a dense network of surface deformation features known as crazes forms and plastic deformation takes place, leading to localized heating. This heating melts the polymer, enabling it to absorb larger amounts of energy because of its high viscosity and extensibility.

In practice, when a projectile hits a thin film of polystyrene, instead of immediate brittle fracture, the polymer deforms and stretches, leaving a volcano-like crater and flaps of stretched, melted polystyrene.

“Polymer toughness, that is the ability to dissipate energy, nominally decreases with increasing deformation rate. This general relationship was thought to apply to all polymers,” explains Thomas. “We observed the exact opposite for thin polystyrene films – toughness increased with increasing deformation rate.”

The effect is substantial, say the researchers. Once the energy dissipated is normalized for mass, polystyrene is twice as good as absorbing energy as other leading materials such as graphene and could compete with Kevlar and steel.

“To our knowledge, [this is] the first report of such behavior in any polymer,” says Thomas. “It provides a new way to think about designing energy absorption materials and structures.”

Multiple thin films of polymers could be stacked together to provide greater energy absorption upon impact or damage than a single film of comparable thickness. Many other technologies, from automobile safety to armor protection, could also benefit from a better understanding of how polymer thin films deform at extremely high rates.