The role of nanocrystalline cellulose during crystallization of poly(ε-caprolactone) composites: Nucleation agent or not?

Poly(ε-caprolactone) (PCL) composites with pristine nanocrystalline cellulose (NCC) and acetylated nanocrystalline cellulose (aNCC) (with 1.85±0.15 degree of substitution) were prepared. Different roles of NCC and aNCC to the crystallization of PCL were explored. NCC acts as the nucleation agent, promoting the formation of thickened PCL lamellae. Thus, PCL-NCC sample shows higher melting point (Tm) and degree of crystallinity (Xc) than neat PCL. However, aNCC plays the role of antinucleation agent, decreasing Xc and crystallization temperature (Tc) of PCL. This is because the relaxation of PCL chain segments is restrained by the presence of aNCC through hydrogen bonding between two phases. As a result, the looser fold surfaces of lamellae form in PCL-aNCC sample, with decreased lamellar thickness. The ring-banded morphology is therefore observed on this sample because of good compatibility between aNCC and PCL and higher level of surface stress of PCL lamellae, which is not observed on PCL-NCC one.