The effect of cellulose nanofibers on the crystallinity and nanostructure of poly(lactic acid) composites

Poly(lactic acid) (PLA) is a semi-crystalline polymer with good transparency, mechanical strength, and melt processability. However, the use of PLA in applications like food packaging is limited by its low heat resistance and slow crystallization rate. The addition of cellulose nanofibers can be an efficient solution to enhance PLA crystallinity and stiffness, maintaining the transparency and biodegradability, which are mandatory for packaging applications. The combined effect of annealing and cellulose nanofibers (surface treated-SCN or untreated-CN) on the crystalline structure of PLA was thoroughly studied at both micro- and nano-scales by differential scanning calorimetry (DSC), mechanical tests, X-ray diffraction (XRD), scanning electron microscopy (SEM), and atomic force microscopy (AFM), Peak Force QNM (Quantitative Nanomechanical Mapping) mode. The ability of CN to act as nucleating agents was highlighted by DSC and XRD, both methods pointing out similar trends of crystallinity variation. Higher crystallinity was observed after annealing, 59 % instead of 40 % for PLA, 67 % instead of 48 % for PLA-CN and 57 % instead of 39 % for PLA-SCN, respectively. A mixture of alpha and alpha' crystals was detected in all the samples after annealing but the concentration of the more disordered alpha' form was higher in PLA-CN than in PLA-SCN. The transformation of the grained structure specific to amorphous PLA into a crystalline lamellar structure (average lamellar thickness of about 22 nm) after annealing was detected by Peak Force QNM. Nanoindentation tests showed an improvement of the surface modulus and hardness for the composites compared to PLA, after annealing. Thus, modulus increased from 5.9 GPa for PLA to 6.2 GPa for PLA-CN and 6.6 GPa for PLA-SCN. The understanding of PLA nanostructure modification by annealing and CN addition may contribute to adjust post-processing treatments, thus making PLA more suitable for packaging and other applications.