Poly(L-lactic acid) crystallization in a confined space containing graphene oxide nanosheets

The semicrystalline polymer incorporated with nanofillers frequently exhibits complicated crystallization behavior, which is probably attributed to the nanofiller-constructed complex crystalline circumstance, especially a confined space. In the present work, in order to have a thorough understanding of biodegradable poly(L-lactic acid) (PLLA) crystallization behavior on the dependence of graphene oxide nanosheet (GONS) loadings, in particular the relatively high GONS loading, a set of GONS/PLLA nanocomposites with different GONS loadings ranging from 0 to 4.0 wt % were investigated in terms of isothermal crystallization behavior by differential scanning calorimetry and time-resolved Fourier-transform infrared spectroscopy techniques. The results indicated that GONSs not only served as heterogeneous nucleating agents for PLLA crystallization but also restricted the mobility and diffusion of PLLA chains. At low GONS concentrations of 0.25 and 0.5 wt %, GONSs acted as a temple for PLLA chains to land on due to extremely high specific surface area, thus promoting the conformational ordering and reducing the nucleating barrier. The nucleation effect of GONSs was dominant to achieve accelerated overall crystallization kinetics. As the GONS concentration rose up to 1.0 wt %, the GONS network was formed in the PLLA matrix, which was verified by solid-like rheological behavior at low frequencies in rheological measurement. The nanofiller network significantly constrained the mobility and diffusion of PLLA chains and offset the nucleation effect of GONSs, giving rise to a turning point in crystallization rate from promotion to restriction. Furthermore, a severely confined space was constructed by the more crowded and denser GONS networks at a higher GONS concentration of 4.0 wt %, compelling PLLA lamellae to grow in a two-dimensional mode. The unusual crystallization behavior of PLLA from promotion to restriction was also understood by the four-region model, in which the semiquantitative description of crystalline circumstance was provided. These results pave an effective way to further reveal the crystallization behavior of polymer at a relatively high nanofiller loading.