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Bilal Abdul Halim1 Sheikh Rasel1 Reza Rizvi1 Navid Namdari1 Emran Hossain1

1, Polymer and Inorganic Composites, Structures and Surfaces Lab, Toledo, Ohio, United States

Microcellular (1-30µm) and Nanocellular (<1µm) polymers have significantly lower densities relative to the base polymer resulting in improved specific mechanical properties as well as increased impact absorption and thermal insulation. Batch micro- and nano-cellular processes are generally well suited for low volume lab-based studies. The gas sorption within the polymer system tends to swell the polymer, resulting in increased free volume, higher chain mobility and a depressed glass transition temperature (Tg). These porous polymers acquire a static porosity, not quite as applicable or interesting as those with a dynamic porosity. Such a characteristic enables the polymer to mimic several stimuli activated porous systems found in nature, which exist at all length scales. For instance, the regulation of what enters and exits the cell (including oxygen, carbon dioxide, and other molecules and ions) cannot transpire if the cell membrane wasn’t permeable (porous). Another example is the ability of the cuttlefish to use its cuttlebone, which is not exactly a bone, but a calcium carbonate like material that is almost 90% porous. Disjoined from the fish, the cuttlebone floats on water, however, the cuttlefish is able to control the flow of water in and out of the cuttlebone which helps with its buoyancy and structure.
Synthetic attempts at systems with dynamic porosity have focused mainly at the molecular-scale and nano-scale. No proven examples of dynamic porosity exist at the micron-scale. This study introduces a fundamental examination of foams composed of the triblock copolymer styrene-ethylene-butylene-styrene (SEBS) which was prepared using batch foaming, where the polymer is put in a pressure chamber using CO2 as the blowing agent, and were then quenched at different temperatures (30C, 50C, 70C, 90C). These foams exhibit pores that shrink under stress and remain stable with an internal vacuum. Now, the reduced pore size fails to act as a light-scattering site, turning the polymer transparent and recovering back to an almost neat SEBS film. Different characterization techniques were done on foams before and after applying stress, using Scanning Electron Microscopy (SEM), Differential Scanning Calorimetry (DSC), and X-Ray Diffraction (XRD). The opaque to transparent transition (OTT) was studied using an in-situ optomechanical setup comprised of an LED and a photoresistor while under a dynamic load using a universal testing machine. Each foam at different quenching temperature undergoes OTT at different loads and remains transparent. However, collapsed pores can be re-foamed which was demonstrated through saturating the almost-recovered SEBS for many cycles whilst getting density measurements and electron microscopy images at each cycle. This behavior can be instrumental in making pressure sensitive films.

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