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Formation of a Segregated Electrically Conductive Network Structure in a Low-Melt-Viscosity Polymer for Highly Efficient Electromagnetic Interference Shielding
A segregated electrically conductive network structure has been well demonstrated to efficiently improve electrical and electromagnetic interference (EMI) shielding performance owing to the controllable assembling of conductive additives in polymer matrices; however, up to now, the polymer matrices are mainly limited to high-melt-viscosity polymers (e.g., ultrahigh molecular weight or cross-linked polymers). In the current work, we proposed a strategy to form a typical segregated structure in a low-melt-viscosity polymer, i.e., poly(lactic acid) (PLA), making use of the melting temperature difference of crystallites. Poly(l-lactide) (PLLA) and poly(d-lactide) (PDLA) were first melt mixed, then crystallized, herein the resultant blend consisted of homocrystallites (hc) and stereocomplex crystallites (sc), and, interestingly, spontaneously granulated into 100 μm particles during melt mixing and crystallizing. The mixture of PLA crystallite granules and carbon nanotubes (CNTs) was compression molded at a temperature between the melting temperatures of hc and sc, where the survived sc, still in solid state, acts as the physical cross-linking points to confine PLA chain motion, forcing CNTs to localize only at the interfaces of PLA domains. The morphological observation indicates the successful formation of the typical segregated structure, resulting in an ultralow percolation threshold of 0.040 vol % CNT. The segregated CNT/PLA composite with only 0.60 vol % (1.0 wt %) of CNT loading achieved high electrical conductivity of 12.0 S/m and outstanding EMI shielding effectiveness of 35.5 dB. This special structure provides numerous interfaces to reflect, scatter, and absorb the incident microwaves many times, endowing an absorption dominated EMI shielding mechanism. Our work reveals a major breakthrough in creating a segregated conductive network structure in low-melt-viscosity polymers, further developing economical, environmentally friendly, and highly efficient EMI shielding composites.