Effects of chain rigidity/flexibility of polyimides on morphological structures and thermal diffusivity of hBN-filled composites

Abstract Composite films composed of μm-sized hexagonal boron nitride (hBN) flakes and polyimides (PIs) having (a) rigid ( s BPDA-PPD, s BPPD) and (b) flexible ( s BPDA-ODA, s BPOD) molecular structures were fabricated to investigate the effects of PI chain structure on the morphological structures as well as in-plane and out-of-plane thermal diffusivity ( TD // and TD ⊥ ). SEM and WAXD measurements revealed that the two types of composites differ significantly in the void concentration ( ϕ v ) and the hBN/PI interfacial affinity. The s BPPD composites exhibited smaller ϕ v than s BPOD. However, delamination, which was rarely observed at the hBN/ s BPOD interface, was found at the hBN/ s BPPD interface. These differences are attributed to the structure of PI chains: rigid chains of s BPPD generate dense interchain and polymer-filler packing owning to their small free volume, but relatively high shrinkage rate during curing results in delamination. The s BPPD composites exhibited large anisotropy in TD // and TD ⊥ , reflecting the in-plane orientation of s BPPD chains. At higher hBN content, the s BPOD composites exhibited a smaller TD ⊥ increase due to an increase in ϕ v . In addition, it was demonstrated that the TD ⊥ is effectively increased by enhancing the relaxation of PI chains by removing the constraint from the substrate during thermal curing.