No abstract

Some precision electronics such as signal transmitters need to not only emit effective signal but also be protected from the external electromagnetic (EM) waves. Thus, directional electromagnetic interference (EMI) shielding materials (i.e., when the EM wave is incident from different sides of the sample, the EMI shielding effectiveness (SE) is rather different) are strongly required; unfortunately, no comprehensive literature report is available on this research field. Herein, Ni-coated melamine foams (Ni@MF) were obtained by a facile electroless plating process, and multiwalled carbon nanotube (CNT) papers were prepared via a simple vacuum-assisted self-assembly approach. Then, step-wise asymmetric poly(butylene adipate-co-terephthalate) (PBAT) composites consisting of loose Ni@MF layer and compact CNT layer were successfully fabricated via a facile solution encapsulation approach. The step-wise asymmetric structures and electrical conductivity endow the Ni@MF/CNT/PBAT composites with unprecedented directional EMI shielding performances. When the EM wave is incident from Ni@MF layer or CNT layer, Ni@MF-5/CNT-75/PBAT exhibits the total EMI SE (SET) of 38.3 and 29.5 dB, respectively, which illustrates the ΔSET of 8.8 dB. This work opens a new research window for directional EMI shielding composites with step-wise asymmetric structures, which has promising applications in portable electronics and next-generation communication technologies.

A series of microporous open-cell poly­(vinyl formal) (PVF) foams were obtained by cross-linking poly­(vinyl alcohol) (PVA) with different contents of formaldehyde during the first acetalization process. In this reaction, water acted not only as the solvent of PVA but also as the pore-forming agent, which made this method more concise and environment-friendly. However, the PVF foam walls were not strong enough to maintain the initial pore shapes and sizes. The second acetalization process was used to increase the acetalization degree to enhance the strength of the foam walls. The poly­(vinyl formal) foams obtained by the two-step acetalization method (TPVF) were able to keep the pretty microporous structures during the drying process. The materials with highly microporous structures exhibited a good ability to absorb normally incident sound. Therefore, the microporous open-cell TPVF foams have great potential in the application of sound absorption materials.