Microstructural and property changes in high pressure treated carbon nanotube/polybutadiene composites

Abstract

In a comprehensive investigation of carbon nanotube (CNT) filled liquid and solid polybutadienes of molecular weights 2600 and 100000, respectively, we report results of thermal conductivity (κ), glass transition temperature (Tg), interfacial interaction and microstructure before and after simultaneous high-pressure and high-temperature (HP&HT) treatment. The HP&HT treatment changed polybutadiene from a liquid or solid to a highly cross-linked, ebonite-like, state. Concurrently, the microstructure changed from randomly dispersed CNTs to a web-like structure of coated and/or wrapped CNTs, with a permanent shift in their D*-band by as much as ∼16 cm−1. Moreover, κ of the recovered state of a 2.9 wt% –COOH functionalized multi-wall carbon nanotube (MWCNT) composite increased by ∼34% predominantly due to an irreversible densification and a consequentially increased phonon velocity. Results prior to treatment show that single-wall carbon nanotube (SWCNT) fillers promote κ better (17%/wt%) than –SH functionalized MWCNT fillers (8%/wt%), which is accounted for by their higher aspect ratio, whereas their about twice as high κ appears to be unimportant. The SWCNTs also raise Tg slightly more than MWCNTs and, in particular, under the most densified conditions and for the high molecular weight polybutadiene, which may be due to more favorable conditions for coating/wrapping.