The synthesis and properties of isocyanate-based polyimide foam composites containing MWCNTs of various contents and diameters

Polyimide foams (PIFs) were synthesized using in situ polymerization from poly((phenyl isocyanate)-co-formaldehyde) (PAPI), pyromellitic dianhydride (PMDA), and multi-walled carbon nanotubes (MWCNTs) (0.05, 0.1, 0.2, 0.4, and 0.6 wt%) functionalized with –OH; the diameters were 10–20 nm, 20–30 nm, and >50 nm. The morphology, mechanical properties, and flame retardancy of the composites made from MWCNTs with different contents and diameters were studied. The effects of different contents of MWCNT on the properties of composites were compared. SEM results show that the pore morphology of PIF was not damaged when the content of the MWCNTs was low due to crosslinking between MWCNTs and amide bonds. When the content of the MWCNTs was high, the vacuoles of PIF became large and uneven. Compared to pure PIF, mwCNT-1 (0.2% MWCNT content) significantly increased the compressive strength (330%) and compression modulus (210%) of PI. Due to the significant thermal stability of PIF/MWCNTs, the degradation temperature of PIF/MWCNT-1 (0.2% MWCNT content) was increased from 302 °C to 321.5 °C upon addition of MWCNTs. The effects of different diameters of MWCNTs on the morphology and properties of the PIF/MWCNT composites were also compared. The morphology, thermal stability, and mechanical properties of the composites containing smaller MWCNTs were higher than those of composites containing larger MWCNTs. This is because MWCNTs act as nucleating agents to promote the formation and growth of bubbles. Smaller diameters of MWCNTs lead to higher MWCNT contents in the unit volume and more nucleation points of MWCNTs in the PIF. An increasing MWCNT diameter leads to a gradually decreasing number of bubbling nucleation centers. The LOI of PIF/MWCNTS increased with increasing MWCNT due to the nitrogen heterocyclic interaction between the PIF and MWCNTS. The diameter of MWCNTS had only a minor effect on the flame retardancy.