Cellulose Nanocrystals Influence Polyamide 6 Crystal Structure, Spherulite Uniformity, and Mechanical Performance of Nanocomposite Films

Abstract

Cellulose nanocrystals (CNCs) are an ideal reinforcing agent for polymer nanocomposites. CNCs can form hydrogen bonds with polyamide 6 (PA6); however, the direct effects of unmodified CNCs on PA6 morphology and crystal structure have not been fully elucidated. This work investigated the influence of CNCs on the mechanical performance and physicochemical properties of spin-coated CNC−PA6 films through quantitative analysis using techniques that probe multiple length scales. CNCs interacted with PA6 to induce the γ (chiral) allomorph over the α allomorph at low CNC loadings (≤1 wt %) and nucleated a high density of small uniform spherulites, leading to stiffer nanocomposites. Higher loadings caused CNC aggregation and crystalline, non-spherulitic features. Overall, we hypothesize that the reinforcement mechanism of CNCs in PA6 is dominated by morphological changes in the matrix, not percolation. Understanding CNC−polymer interactions and morphology (on films prepared without thermal processing or surface modification of CNCs) offers "design rules" for how to incorporate CNCs into nanocomposites for optimized material performance in various applications, for example, membranes, coatings, and packaging.

A predictive model towards understanding the effect of reinforcement agglomeration on the stiffness of nanocomposites

“…For instance, the conventional models predict a linear or exponential increase in elastic modulus of composites with respect to the concentration of fillers; however, experimental observations indicate that the elastic modulus of nanocomposites reaches a plateau value or decreases with increasing loading. [19][20][21][22][23] It is often observed that conventional models predict the elastic modulus at very low concentrations and often diverge from experimental findings as nanofiller concentration increases. 19,[24][25][26] Consideration of the individual state of agglomerates is necessary for accurate predictions at high concentrations, which can be achieved by combining the continuum-based models and statistical approaches.…”

Influence of Dispersion and Orientation on Polyamide-6 Cellulose Nanocomposites Manufactured through Liquid-Assisted Extrusion

“…The addition of MMT (4.7 wt.%) resulted in an increase in the tensile strength of these composites from 69 to 97 MPa and in their tensile modulus from 1.1 to 1.9 GPa. The use of cellulose nanocrystals (CNCs) [ 8 , 9 , 10 , 11 , 12 ] and cellulose nanofibers (CNFs) [ 8 , 13 , 14 , 15 ] as reinforcements for polymer composites have also been studied in PA6.…”

“…The main challenge in their processing is associated with an effective dispersion and distribution of the cellulose nanomaterials in the polymer matrix, owing to their tendency to agglomerate and their thermal instability at temperatures above 200 °C [ 16 ]. To overcome these challenges, high-energy processes [ 11 , 13 ], and chemical modifications [ 12 , 15 , 16 ] have been used. For example, ball-milling and cryo-milling [ 11 ] were used to produce the polymer and cellulose in powder form to facilitate the mixing and dispersion of cellulose with PA6.…”

“…Acetylated cellulose (10 wt.%) was fibrillated during the extrusion process, resulting in improved mechanical properties; however, a chemical modification requires solvent exchange and the use of harsh chemicals. Solvent casting [ 12 , 16 ] is another process used for the preparation of nanocomposites; through it, a good dispersion can be achieved, but both the polymer and nanocellulose (NC) must be dissolved and dispersed in the same solvent, followed by a casting and evaporation of the solvents used. This type of process is useful on a laboratory scale but is not environmentally friendly or scalable.…”

A hydrophobic coating on cellulose nanocrystals improves the mechanical properties of polyamide-6 nanocomposites

“…Spin-coating has been used to produce modified CNC reinforced polylactic acid (PLA) thin films. 24 In the case of PA6, our previous study is the only research in which this technique was used to produce CNC reinforced PA6 thin films 25 ; however, spin-coated modified CNC reinforced PA6 films have not been investigated in the literature. Numerous pathways have been followed to modify CNC.…”