Nanocomposite of Poly(Lactic Acid)/Cellulose Nanocrystals: Effect of CNC Content on the Polymer Crystallization Kinetics

Abstract

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...

Poly(Lactic Acid)/ZnO Bionanocomposite Films with Positively Charged ZnO as Potential Antimicrobial Food Packaging Materials

“…Generally, pure PLA has an orthorhombic crystal structure ( α -form) and consists of 103-helical chains [25]. The pure PLA film showed characteristic peaks located at 2θ = 12.5, 14.8, 16.8, and 19.1°, which were attributed to the (103), (100), (110)/(200), and (203) planes of the α -form crystal, respectively [26]. Pure ZnO NPs showed characteristic peaks located at 2θ = 31.6, 34.4, 36.2, 47.4, 56.7, 62.7, 66.2, 67.9, and 69.0°, which corresponded to (100), (002), (101), (102), (110), (103), (200), (112), and (201) planes, respectively, of the hexagonal crystal of ZnO NPs (JCPDS card No 36-1451) [13].…”

Sandwich-Structured, Hydrophobic, Nanocellulose-Reinforced Polyvinyl Alcohol as an Alternative Straw Material

“…Comparison of the XRD patterns of untreated and heat-treated PEG-PLA(70) + neat PLA films (1 h and 2 h) reveals that the heat-treated materials exhibit obvious PLA and PEG peaks at 16.8 • , 19.1 • , and 19.4 • , which correspond to the (1 1 0)/(2 0 0), (2 0 3), and (1 2 0) planes, respectively (E, F, G, and H in Figure 12a) [51][52][53], indicating that neat PLA undergoes significant crystallization at 90 • C and 125 • C. This observation agrees with the DSC results of the heat-treated materials, which exhibited relatively high PLA crystallinities (Table 4). 12a) [51,52]. Although the DSC measurements indicated an upward trend of the crystallinity of PLA with heat treatment (Table 4), no significant PLA peak was detected.…”

Nanocomposite of cellulose acetate reinforced with nanocrystals modified chemically: Modification with bifunctional reagent

“…Vestena et al has pointed out a decrease of the values of crystallinity when reinforcements from NCPs have been added to PLA. According to the authors, an increase in the content of NCs in the PLA matrix has a direct influence on the number of crystallization nuclei, which causes the impingement of spherulites to occur earlier and this affects the final size of the spherulite crystals.…”