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Multifunctional polymer composites with anisotropic properties are attracting interest as they fulfil the growing demand of multitasking materials. In this work, anisotropic polymer composites have been fabricated by combining the layer-by-layer (LBL) filtration method with the alternative assembling of carbon nanotubes (CNTs) and hexagonal boron nitride flakes (hBN) on natural rubber latex particles (NR). The layered composites exhibit anisotropic thermal and electrical conductivities, which are tailored through the layer formulations. The best composite consists of four layers of NR modified with 8 phr (parts per Hundred Rubber) CNTs (∼7.4 wt%) and four alternate layers with 12 phr hBN (∼10.7 wt%). The composites exhibit an electromagnetic interference (EMI) shielding effectiveness of 22.41 ± 0.14 dB mm -1 at 10.3 GHz and a thermal conductivity equal to 0.25 W m -1 K -1 . Furthermore, when the layered composite is used as an electrical thermal heater the surface reaches a stable temperature of ∼103 °C in approx. 2 min, with an input bias of 2.5 V.

Anisotropic aerogel-foam composites were developed by embedding a reduced graphene oxide (rGO)/chitosan aerogel directly into an open-cell polyurethane foam through an in situ bidirectional freeze-drying process. The resulting aerogel-foam composites possess both excellent compression-resilience performance and stable piezo-resistive properties due, respectively, to the excellent mechanical properties of polyurethane foams and to the presence of a chitosan-based aerogel loaded with rGO. The latter, indeed, provides outstanding electrical properties due to its conductive and parallel flat lamellar structure. It has been proven that both mechanical and piezo-resistive properties are stable even after 1000 loading/unloading cycles and a reduction of the electrical resistance of about 86% is observed upon the application of a 60% strain. The high sensitivity, long cycling life, and reliable performance over a wide strain range make this unique anisotropic aerogel-foam composite a highly promising candidate for the production of wearable sensors and healthcare monitoring devices.

Natural rubber/carbon nanotubes composite foams (F-NR/CNTs) with high electrical conductivity and excellent electromagnetic interference (EMI) performance were developed through a multi-step process including: (a) CNTs assembled on natural rubber latex particles, (b) pre-crosslinking of natural rubber, (c) supercritical carbon dioxide foaming of pre-crosslinked composite samples and (d) post-crosslinking of foamed composite samples. A closed-cell porous structure and a segregated CNT network are clearly observed in the resulting foams. Due to this morphology, F-NR/CNTs exhibit low density, good mechanical properties, and high electrical conductivity. Owing to the multiple radiation reflections and scattering between the cell-matrix interfaces, the composite foams presented an excellent specific shielding effectiveness (SSE) of 312.69 dB cm2 g-1 for F-NR/CNTs containing 6.4 wt% of CNTs, which is significantly higher than those already published for rubber composites containing comparable filler content. Furthermore, the analysis of EMI SE highlights that absorption efficiency is more significant than reflection efficiency, implying that most of the incident electromagnetic radiation is dissipated in the form of heat. This work provides the fundamentals for the design of innovative light weight and efficient EMI shielding foams characterized by a three-dimensional segregated CNT network with huge potential for use in the electronics and aerospace industries.

Chitosan (CS) is one of the most promising polymers due to its biocompatibility, biodegradability, and natural abundance. However, its poor mechanical and barrier properties make it difficult to satisfy a wide range of applications. Herein, borate ions, originating from the hydrolysis of sodium tetraborate decahydrate (borax), have been used to crosslink chitosan and graphene oxide (GO) nanocomposites. Chitosan films consisting of 1.0 wt% boron and 1.0 wt% GO exhibit a significant improvement in both the toughness and oxygen barrier properties compared to pristine chitosan. In particular the tensile strength of the samples after thermal treatment increases by ∼160% compared to pristine chitosan, whereas their oxygen permeability reduces by ∼90%. This is ascribed to the chemical crosslinking between chitosan and GO nanoplatelets through borate ions, as well as the formation of a layered morphology with graphene nanoplatelets oriented parallel to the sample surface. The exceptional robust and high gas barrier film has promising application in the packaging industry. The borate-crosslinking chemistry represents the potential strategy for improving properties of other polymer nanocomposites.