Abstract
Thermoplastic elastomer foams with tunable cellular structures and excellent mechanical properties prepared by supercritical physical foaming are highly promising. As society continues to advance, there is a growing demand for the customization of elastomer foams with features such as ultra-low density, enhanced resilience, and multifunctional capabilities. This trend is driving the need for ongoing innovation in both the design of materials and cellular structures. Nevertheless, there are still many challenges waiting for elastomer foams, such as the post-shrinkage behavior after foaming and the lack of knowledge of heterogeneous cell structures. In this study, the environmentally friendly supercritical carbon dioxide (scCO2) foaming method, cooperated with other interdisciplinary technologies, was utilized to regulate the shrinkage behavior of elastomer foams and construct diverse structures inside elastomer foams (bimodal cellular structures and folded cellular structures) to satisfy customized demands. Moreover, the formation mechanisms of the corresponding structures were studied, and the potential applications of elastomer foams in advanced fields were also explored. The main research contents are as follows:(1) Micro-crosslinked olefin block copolymer (OBC) foams with shrinkable and recoverable functions were successfully prepared via supercritical CO₂ foaming, and the underlying shrinkage and recovery mechanisms were systematically studied. The elastic shrinkage and the shrinkage induced by gas diffusion were independently observed and discussed. It was found that the reformed crystalline regions formed after foaming at the melting temperature significantly suppressed elastic shrinkage, enabling the foams to achieve close to 100% recovery after shrinkage. By regulating the gas exchange rate, OBC foam volume can be reduced by up to 78.5% and maintained through vacuum sealing, while the foams can be quickly and fully recovered using an N₂-assisted recovery process, reducing recovery time by 80%. Ultra-low-density OBC foams, with a density as low as 0.028 g/cm³, were successfully prepared. These design strategies significantly reduce foam volume during transportation and storage while improving recovery efficiency and hold promise for application in other thermoplastic elastomer systems.
(2) A strategy of pre-setting cells into elastomers before supercritical CO2 foaming was introduced to customize the cellular structures. By regulating the configuration of the pre-set cells, OBC foams with bimodal, sandwich, and gradient multi-modal cellular structures were prepared successfully, in which the pre-set cells would further grow into large cells, while the newly nucleated cells would grow into small cells. The formation process of pre-set cells and the interaction mechanism between the growth of pre-set cells and the growth of newly nucleated cells were systematically investigated. The bimodal cellular structures can improve the expansion ratio and elasticity of OBC foams. Furthermore, the feasibility of this strategy in other elastomer foams (like POE, EPDM, and EVA foams) was also verified, which was expected to promote the development of high-performance elastomer foams.
(3) A folded cellular structure was incorporated into polyolefin elastomers/olefin block copolymers/carbon nanostructure (POE/OBC/CNS) foams through supercritical CO2 foaming combined with vacuum treatment to deliver comprehensive electromagnetic interference (EMI) shielding protection. The volume of the foams could be stored and released through the conversion between conventional cellular structure and folded cellular structure, which could not only manipulate the EMI shielding and mechanical performance but also provide an excellent sealing effect to avoid wave leakage and water penetration. The EMI shielding effectiveness enhanced from 35.8 dB to 53.6 dB after releasing the stored volume. Meanwhile, the expanded foam could effectively seal both regular and irregular components, preventing water penetration even when subjected to a mass of water 100 times greater than that of the foam, while also suppressing gas penetration under CO2 exposure. The mechanism behind the folding and opening of the cells has been systematically analyzed. Due to the unique folded cell structure, the foam exhibited commendable traits in terms of flexibility and resilience, providing cushioning protection. With the above merits, the folded foams are promising for the development of advanced multifunctional EMI shields providing comprehensive protection.
| Date of Award | 15 Aug 2025 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Xiaoling Liu (Supervisor) & Wenge Zheng (Supervisor) |