Epoxy‐Polypyrrole‐Cotton Tertiary Composite with Electro‐Tunable Stiffness

Abstract

In this work, a tertiary epoxy composite reinforced with polypyrrole (PPy) coated cotton fabric layers exhibiting electrical voltage induced thermally tunable stiffness is reported. The thin coating of PPy over cotton fabric is accomplished via oxidative vapor phase polymerization that allows creation of an active thin layer over the fibers without affecting their mechanical properties. Six such functional layers are stacked within an epoxy matrix to prepare the composite that shows in‐plane electrical insulator behavior (volume resistivity > 109 Ω cm) but considerably reduced resistivity by an order of 103 across the cross‐sections. The presence of conductive layers enables the composite to heat via Joule's effect when an electrical voltage is applied across two ends. This causes softening of matrix near the matrix‐reinforcement interface and thereby changing composite's stiffness. On application of variable voltage, a non‐linear decrease of 91% in composite stiffness is observed (6371.2 N m−1 at 0 V to 566.4 N m−1 at 63 V). A stable and tunable mechanical performance of the composite is further demonstrated by cyclic changes in stiffness due to voltage change with recovery up to 95% of original stiffness after 14 continuous cycles.