MWCNTs (Aldrich) were functionalized with carboxyl groups and hydroxyl groups by nitric acid treatment [5] and filtered with deionized (DI) water until the pH reached to 7. The functionalized MWCNTs (6 mg) were dispersed in DMAc (10 g) by soniation in ultrasonic bath (Fisher, FS30H) for 10 min. CDI (120 mg) was dissolved in DMAc (10 g). The CDI and MWCNTs solutions were mixed and sonicated in the ultrasonic bath at 60 ◦C for 12 h. The reaction converted carboxyl groups on MWCNTs to imidazolide. By reacting imidazolide–MWCNTs solution with the cellulose solution at 60 ◦C for 18 h, MWCNTs were covalently grafted to cellulose chains [18]. The final solution was spin-coated on a silicon wafer and cured in isopropyl alcohol (IPA), DI water/IPA mixture (the ratio of 4:6) and DI water, in a sequence. This slow curing process can prevent instant aggregation of MWCNTs and eliminate remnant Li+(DMAc)x macrocations as well as solvents [21]. To align covalently bonded MWCNTs with cellulose chains, the wet M/C
films were mechanically stretched with 80% strain rate. During the stretching process, the films were dried with a near infra-red heater.