The barrier and mechanical properties of polymer matrices are affected by the polymer crystallization kinetics and the incorporation of nanoparticles can modify the nucleation density as well as the rate and mechanism of crystallization. In this study, a nanocomposite of poly(lactic acid) (PLA) containing cellulose nanocrystals (CNC) was prepared in order to evaluate the effect of the CNC content on the crystallization kinetics of the polymer matrix. Dimethylformamide, a solvent of PLA, was used to replace the aqueous medium for the dispersion of the CNC, to facilitate the preparation of PLA films with dispersed CNC. Higher levels of CNC in the films induced an increase in the crystallization rate and reduced the degree of crystallinity without affecting the dimensions of the crystal lattice. In conclusion, the incorporation of CNC into PLA influences the crystallization kinetics, which significantly affects the PLA processing conditions. Keywords: cellulose nanocrystals, poly(lactic acid), crystallization kinetics, nanocomposites
IntroductionParticles with nanometric dimensions, such as carbon nanotubes, nanoclays and cellulose nanocrystals (CNC), have been used as reinforcing materials in polymer matrices. The addition of such particles alters the physicochemical characteristics of the matrix, which are related to specific properties associated with these materials.1 The polymer crystallization kinetics, for example, can be enhanced by adding nanoparticles, which also influences the crystalline morphology and rheological behavior.
2CNC are crystalline domains isolated from lignocellulosic fibers that are composed of cellulosic microfibrils surrounded by lignin and hemicellulose. A bleaching process is therefore required to remove the non-cellulosic components of the lignocellulosic fibers, followed by acid hydrolysis to isolate the CNC. These nanomaterials are of particular importance, considering that cellulose is one of the most abundant renewable resources and a biodegradable polymer. 1,3,4 In this context, environmental concerns have led to an increase in studies using biopolymer matrices, such as poly(lactic acid) (PLA), a linear aliphatic biodegradable polymer, derived from renewable resources. It can be produced by polycondensation from lactic acid or the ring-opening polymerization of lactides (cyclic dimer of lactic acid), resulting in low or high molar weight, respectively. The presence of a stereocenter in the PLA repeating unit results in the optically active enantiomers poly(L-lactic acid) (PLLA) and poly(D-lactic acid) (PDLA), or optically inactive poly(D,L-lactic acid) (PDLLA), as well as a racemic mixture. 1,5 PLLA can be crystallized in an α (orthorhombic), β (orthorhombic) or γ (pseudo-orthorhombic) system, and the α-structure is more stable.6 Commercial-grade PLA is made up of L-lactide and D,L-lactide copolymers. 5,7,8 The mechanical properties of PLA are comparable with other commodity thermoplastics, like polystyrene (PS) and poly(ethylene terephthalate) (PET). Besides presenting good pro...
