(639e) Ice Engineered Cellular Plastics

Cellular plastics with excellent mechanical, thermal, and acoustic properties are widely used in areas such as transportation, aerospace, civil engineering and personal safety. In order to meet various future engineering requirements, their performance needs to be significantly improved by mainly controlling their porous structure or composition distribution. For example, polymeric and metallic foams can be engineered with graded pores for outstanding energy absorption or enhanced acoustic capabilities. Higher strength and more efficient mass transfer can be realized in foams with oriented pores. However, in the sense of structure and composition distribution complexity, current cellular plastics are still far behind their natural counterparts such as spongy bone and wood, which achieve high specific strength and modulus by developing sophisticated hierarchical architectures. This is mainly because that conventional approaches to fabricate cellular plastics still have limitations in either microstructure and composition distribution complexity (e.g., foaming) or scalable production (e.g., 3D printing). Therefore, it is highly demanded to develop new approaches to engineer cellular plastics with complex structure and composition distribution without compromising their scalability.

Herein, we demonstrate that ice can be used as a template to produce cellular plastics with complex structure and composition distribution. In contrast to the conventional toxic and expensive foaming agents which are increasingly prohibited, ice is an environmentally friendly template. Our facile approach involves controllable freezing of a monomer/water emulsion, followed by cryo-polymerization and room temperature thawing. The assembly process of the emulsion droplets was studied through both in situ observation and mechanism analysis. By controlling the freezing dynamics, ice crystals with various morphologies can be readily realized to engineer cellular plastics into a wide spectrum of porous structures, including wood-mimetic, multi-layered, orthogonally and radially aligned pores. Specifically, the cellular plastic with radially aligned structure shows a negative Poisson’s ratio under compression. Due to its high modulus, our cellular plastic can resist a much higher impact force and dissipate much more energy. Additionally, with its simplicity and scalability, we believe that our approach holds great promise for making high-performance cellular plastics with well-defined porous structure and composition distribution.

As a proof of concept, cellular polymethyl methacrylate (PMMA) was fabricated for its wide applications in transportation and automobile industry (Figure a). Liquid MMA monomer containing a redox initiating system (benzoyl peroxide (BPO) and N, N-Dimethylaniline (DMA), which generates free radicals at low temperature and initiate the polymerization.) and crosslinker (Ethylene glycol dimethacrylate, EGDMA) was emulsified in water with reactive emulsifier (ADEKA REASOAP series, ER-10). This emulsion was then subjected to a programmable temperature gradient for controllable freezing. Upon cooling, the emulsion droplets were gradually expelled and assembled by the growing ice crystals. The entirely frozen sample was further maintained at -15 to allow full coalescence of the monomer droplets and cryo-polymerization with the existence of the ice template. Note that the polymerization started right after the emulsion was prepared. However, it took at least 6 hours to complete polymerization, while freezing of emulsion only took around 10 minutes. Therefore, we believe that the emulsion droplets were still liquid after the sample was entirely frozen. After polymerization, the cellular plastic became strong enough to avoid capillary-induced collapse during drying. Therefore, room temperature thawing instead of freeze-drying was utilized in the final step for the removal of the ice template which is beneficial for both simplicity and energy saving. A wood-mimetic architecture plastic was firstly demonstrated by freezing at a directional temperature gradient (Figure b, c).

In the past decade, the freezing technique (also known as freeze-casting or ice-templating) has been extensively used to assemble various building blocks including ceramics, metals, and polymers into functional porous materials. However, on the polymer side, this technique was only limited to hydrophilic starting materials (monomers, polymers, and hydrogels), due to the inherent requirements of the freezing process for dissolvability and dispersibility. Most of the widely used engineering polymers are therefore ruled out from this processing technique for their hydrophobicity. Our approach solves the dissolvability issue by using monomer emulsion instead of polymer, making it compatible with a variety of polymer resins.

To further investigate the mechanism of our approach, the emulsion droplets during ice-templated assembly and cryo-polymerization processes were observed in situ under an optical microscope with the emulsion film sandwiched by two glass slides. Upon freezing, the emulsion droplets are expelledand assembled in between the ice crystals, where the droplets start to coalesce. The coalescence of emulsion droplets starts immediately during freezing as indicated by the disappearance of droplet boundary (Figure d). However, the coalescence is only partial when the whole sample is frozen. Further observation implies that coalescence and deformation of the assembled monomer phase continues during cryo-polymerization (Figure e). This can be attributed to the volume shrinkage of the monomer phase during polymerization. Enlarged images show that the boundary of the monomer phase gradually becomes smoother before it is completely solidified. Obviously, full coalescence of emulsion droplets is crucial for the mechanical properties of the resulted cellular plastics.

Besides single component polymer, our cellular plastic can be made by copolymerization of different monomers. The mechanical performance of the poly(MMA-co-BA) cellular plastic can be easily tuned by the ratio of two monomers (BA and MMA). Additionally, inorganic fillers can also be incorporated to yield multifunctionality. A multi-layered cellular plastic with alternating compositions was fabricated by sequentially adding MMA and BA emulsions in a single directional freezing process (Figure f, g), which is challenging for conventional foaming technique. The PBA layer was doped with silicon carbide (SiC) to demonstrate the potential of achieving multifunctionality and to help probe the difference in composition between layers. Such a multi-layered structure with alternating modulus may be beneficial for energy dissipation during damping due to stress mismatch across the layers. Beyond mechanical damping, dielectric and conducting fillers can be incorporated into alternative layers to enhance microwave absorption by forming a supercapacitor network. Taking advantage of the rich designability of temperature gradient (e.g., direction and magnitude), ice crystals with various morphologies can be realized to generate complex porous structure. Layered cellular plastic with orthogonal channels, which is demanded for impact resistance, can be constructed by freezing under orthogonal temperature gradient (Figure h-j). Obviously, the orienting angle of the channels between adjacent layers can be adjusted by the temperature gradients to optimize the impact resistant performance.

Specifically, we have fabricated a cellular plastic with radially aligned porous structure by immersing the emulsion into a cold bath to yield simultaneous freezing from all directions (Figure k). Interestingly, the cellular plastic shows a negative Poisson’s ratio under compressive loading (Figure l). During compression, longitudinal strain gradually reaches -25% while transverse strain monotonically decreases to -4.4%, indicating a negative Poisson’s ratio at the lowest of -0.176 (Figure m). From the stress-strain curve, the energy dissipated in a single compression cycle can be calculated by integrating the area between the loading-unloading curves (7.5 mJ/cm³). Materials with negative Poisson’s ratio have wide applications for their outstanding performance in impact energy absorption. It has been intensively studied that materials with negative Poisson’s ratio is advantageous than those with positive Poisson’s ratio in energy absorption during compression. This is mainly because their density increases rapidly with lateral contraction, resulting a higher modulus. While previous cellular plastics with auxetic structure fabricated by conventional foaming methods or freeze casting usually show low modulus, our cellular plastic can resist a much larger impact force due to its higher modulus. Comparing to other elastic materials with negative Poisson’s ratio, our rigid plastic shows much higher energy dissipation capability at similar strains (Figure n). It should be noted that our rigid cellular plastic was designed to dissipate impact energy for specific applications such as crash cushion that requires to dissipate impact energy at a small, safe deformation distance. Although elastic foams can be reused for many times, it dissipates less energy at a similar strain comparing. More importantly, we believe that our approach is applicable to fabricate cellular materials with various properties for distinct designing requirements.

In summary, we have demonstrated an approach of using ice crystals as a template to fabricate cellular plastics, consisting three steps including controllable freezing of monomer/water emulsion, cryo-polymerization and room temperature thawing. Our approach is environmentally friendly comparing to conventional foaming technique, and capable of generating complex porous structures by harnessing the rich morphologies of ice crystals. Cellular plastics with wood-mimetic, multi-layered, orthogonally and radially aligned microstructures were successfully fabricated, which display many intriguing properties such as low density, high strength and negative Poisson’s ratio. Specifically, comparing to other materials with similar negative Poisson’s ratio, our rigid plastic shows much higher energy dissipation capability. Our strategy provides new possibilities of engineering high-performance cellular plastics with complex porous structure and composition distribution, which may find wide applications in areas like damping, microwave absorption, impact resistance, and so on.