Navid Namdari

Mechanical and physical properties of porous polymers are highly dependent on the arrangement of their internal pores, which once synthesized are widely considered static. However, here we introduce an unconventional dynamic porosity strategy in physically networked elastomer polymers, irrespective of their chemistry. This strategy allows for an omnidirectional and reversible reconfiguration of porosity in response to applied mechanical deformations, even at ambient conditions. In particular, the normal contact pressure between human fingers (just 0.62 MPa) applied on thin elastomer films results in a permanent reversion of the pores to a denser and more solid state. The porous-to-solid transition leads to a 3 order of magnitude reduction in pore density and up to a 22% relative volumetric shrinkage of the films, resulting in an opaque-to-transparent transition (OTT) that acts as a visual indication of porosity state (porous vs nonporous). It is shown that the pore reversion pressure onset is dependent on the average pore-to-pore distance that is controllable through process-specific parameters. Furthermore, the porosity transition is reversible for multiple cycles when the through-plane compression activation is coupled with an in-plane stretch (ε = 700%). A strain energy-mediated thermodynamic model is successfully implemented to confirm the effects of mechanical deformations on pore reversion and generation. Finally, applications of the newfound dynamic porosity concept are exploited for pressure indication, on-demand modulation of materials’ mechanical and thermal characteristics, and flexible photomasks.

A wide variety of coating methods and materials are available for different applications while the common purpose of all of them is protecting a part or structure exposed to mechanical or chemical damage. Decreased manufacturing cost is another important advantage of having a coating layer on substrate materials when there is a need for a high-performance and expensive material on the exposed surfaces of a part. In such cases, the part body may be manufactured out of less expensive materials and coated by the desired expensive material only at the exposed surfaces. Laser-assisted processes, such as laser cutting, laser welding, laser-assisted coating, etc. are a revolutionary set of processes in fabricating mechanical parts using a laser as the heat source. Laser-based processes have also been employed to create material deposition on selected areas to provide a protective layer. Depending on the deposited materials, these protective layers can serve to enhance corrosion and wear resistance, heat transfer, and mechanical behavior of substrates. To date, numerous laser-assisted processes have been introduced and investigated. Although each of these processes provides advantages, there are always drawbacks limiting their applications. In this article, laser-assisted coating methods are categorized, and a comparative study of these techniques is presented. It is likely to see higher qualities of protection by evolving more advanced techniques and materials in the future.

Shape memory polymers (SMP) have been extensively implemented for applications ranging from biomedical devices and soft robotics to deployable structures. However, these materials mostly rely on heat for shape recovery and programming. Here, we introduce a new class of smart SMPs with pressure-responsive characteristics based on dynamic porosity–an ability to configure a pore size within a solid– that allows for optical monitoring of the stimuli. Introducing pores in a structure reduces the relative density of the material and introduces interfaces resulting in a reduction in optical transparency of the bulk through light scattering. We show that utilizing the dynamic porosity, a macroporous film transitions instantly from an optically opaque (25% transmittance) to a transparent (96% transmittance) state by applying an out-of-plane contact pressure of 3.8 MPa at room temperature. The new SMP also presents an anomalous ‘cold’ shape recovery of the pressure-activated films through repeated in-plane stretching that renucleates the pores and transitions the material back to the initially opaque state. The presented concept ushers a new class of responsive materials that interlace optical sensitivity features with micro- and nanoscale material deformations, establishing new research opportunities in the field of SMPs.

Functional surfaces are of paramount engineering importance for various applications. The purpose of this review is to present counter-intuitive methods of fabrication based upon damage or instabilities for creating value-added surface functions.

The fabrication of micro-structured films with dynamic porosity is useful in various applications. This article presents fabrication of microcellular films exhibiting dynamic porosity upon applying an external pressure stimulus. Interestingly, we show that the dynamic porosity is in conjunction with an opaque to transparent transition (OTT) that imparts the material with a unique optomechanical behavior. The OTT experienced foams will have a transparency almost equal of the as-cast films. The foaming process used for this study is solid-state foming technique with CO₂ gas as physical blowing agent. A styrene-ethylene-butylene-styrene (SEBS) triblock copolymer was used as the foaming material. Polystyrene (PS) blocks in SEBS play a key role in this process since CO₂ reduces the T_g of PS and enable ethylenebutylene (EB) parts to swell during saturating stage while upon depressurization their T_g increases again that prevents the forming pores from collapse. A custom-made in-situ optomechanical setup was used to characterize the optical and mechanical behavior of the produced foams. Optomechanical tests at different strain rates and loads revealed that the films undergo a gradual OTT behavior indicating their ability to be used as optical pressure-sensitive films. Quenching temperature is of great importance in this process since the foams show various OTT behavior once the quenching temperature changes. Moreover, here we show after pressing the foams, the transparent films can be re-foamed by the same process for many cycles.

A wide variety of coating methods and materials are available for different coating applications with a common purpose of protecting a part or structure exposed to mechanical or chemical damage. A benefit of this protective function is to decrease manufacturing cost since fabrication of new parts is not needed. Available coating materials include hard and stiff metallic alloys, ceramics, bio-glasses, polymers, and engineered plastic materials, giving designers a variety freedom of choices for durable protection. To date, numerous processes such as physical/chemical vapor deposition, micro-arc oxidation, sol–gel, thermal spraying, and electrodeposition processes have been introduced and investigated. Although each of these processes provides advantages, there are always drawbacks limiting their application. However, there are many solutions to overcome deficiencies of coating techniques by using the benefits of each process in a multi-method coating. In this article, these coating methods are categorized, and compared. By developing more advanced coating techniques and materials it is possible to enhance the qualities of protection in the future.

In the precision thermoforming process, one of the main drawbacks occurs in the profile deviation of the produced part. In this study, the effect of different thermoforming process parameters on the mold replicability of a high impact polystyrene container produced by vacuum forming and drape forming processes has been experimentally and numerically investigated. According to experimental results, in the drape forming process, when the initial sheet thickness increases, the part will have higher mold replicability, whereas in the vacuum forming process, by increasing the initial sheet thickness, the mold replicability increases and reaches its peak, then decreases. The results also indicate that both temperature and vacuum pressure exhibit the most significant effect on mold replicability of the part. Furthermore, the finite element method is utilized by the implementation of a fully thermomechanically coupled hyperviscoelastic constitutive model in ABAQUS 6.13. By using this material model, it is possible to compare the sensitivity of the output (mold replicability of the part) to the changes in the range of the process parameters. The simulation results verified by the experimental data and the hyperviscoelastic model showed to be an outstanding and stable platform for the process simulation.