Freeze-dried Cellulose Nanocrystals Research Articles

Ultrasonication-induced re-dispersion of freeze-dried cellulose nanoparticles for improving strength and durability of PAN polymer matrix

Cellulose nanocrystals (CNCs) have acquired considerable attention owing to their noteworthy properties, including high crystallinity, biodegradability, high specific surface area, and excellent mechanical characteristics that alter the inherent features of the polymer matrix upon reinforcement. Typically, CNCs undergo freeze-drying or spray-drying to facilitate handling and dispersion in a hydrophobic polymer medium. Unfortunately, these drying procedures compromise the intrinsic nano-scale properties of CNCs. Therefore, achieving a uniform re-dispersion of dried CNCs is imperative to attain the desired characteristics in the polymer composite. This study specifically focuses on achieving a homogeneous re-dispersion of freeze-dried CNCs within a waste polyacrylonitrile (PAN) matrix through a simple ultrasonication treatment. The enhanced mechanical properties of the PAN polymer matrix validate the impact of ultrasound energy on re-dispersing CNCs. A noticeable improvement in the tensile properties of the composite polymer film is observed when CNC particles are well-dispersed in the PAN matrix. This observation is substantiated through SEM imaging, XRD, and FTIR data of both neat PAN and PAN–CNC composite films. The results affirm that particle dispersion plays a crucial role in augmenting the tensile properties of PAN composite films.

Preparation of composites based in poly(3-hexylthiophene) and freeze-dried cellulose nanocrystals by a simple method, and their characterization

Poly(3-hexylthiophene) (P3HT)/cellulose nanocrystals (CNC) composites were prepared by a simple mixing method. The influence of the CNC incorporation on the properties of the composite was investigated. CNC were obtained from filter paper and purified by an acid hydrolysis process. P3HT was synthesized by direct oxidation of 3HT monomer using ferric chloride as oxidant. CNC were freeze-dried before mixing with P3HT. P3HT/CNC composites were prepared at different concentrations (P3HT/CNC weight ratio: 75/25, 50/50, 25/75, 5/95). P3HT, freeze-dried CNC and composites were characterized by FT-IR, TGA, UV–vis, XRD, cyclic voltammetry, and FE-SEM. Composites showed greater thermal stability than the freeze-dried CNC up to a 75 wt% CNC concentration. The increased ordering of P3HT chains in the composites was confirmed by XRD and UV–vis. Incorporating up to 50 wt% CNC in P3HT improved electrochemical stability. Charge storage in the composites increased with the P3HT content. P3HT/CNC composites exhibit interesting physicochemical properties that could be useful for optoelectronic device applications.

Thermal characterization and lifetime prediction of the PHBV/nanocellulose biocomposites using different kinetic approaches

In the present study, biocomposite films from cellulose nanocrystals (CNCs) were obtained by the solution casting method. CNCs were isolated from pineapple crown using chemical treatments followed by sulfuric acid hydrolysis and added into poly (3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) matrix. The effect of freeze-dried CNC content (1, 3, and 5 wt%) on the structural, crystallization, thermal degradation lifetime prediction, and thermogravimetric simulation was investigated. An irreversible agglomeration observed after freeze-dried provided changes in the morphology and size of CNCs. Addition up to 3 wt% of CNCs increased the thermal stability, crystallization rate, and crystallinity index of PHBV, as showed by thermal and crystallinity analysis, respectively. The kinetic degradation study by thermogravimetric analysis (TGA) was done using the F-test method by statistically comparing degradation mechanisms in the solid-state. The most probable degradation mechanism was the autocatalytic reaction model for all samples (represented by Cn and Bna-types) with a suitable adjustment of the simulated curves. Lifetime prediction showed to be successfully applied based on the kinetic analysis, and PHBV reinforced with 3 wt% of CNCs presents the highest results for the isothermal temperature of 180 °C.

Influence of Nanoparticle Pretreatment on the Thermal, Rheological and Mechanical Properties of PLA-PBSA Nanocomposites Incorporating Cellulose Nanocrystals or Montmorillonite.

Nanoparticles based on cellulose nanocrystals (CNC) and montmorillonite clay (MMT) were prepared using spray freeze-drying. The nanoparticles were then used as reinforcement to prepare nanocomposites with poly(lactic acid) (PLA) as the polymer matrix. The effect of spray freeze-dried CNC (SFD-CNC) and spray freeze-dried MMT (SFD-MMT) on the rheological and mechanical properties of PLA and its blends with poly[(butylene succinate)-co-adipate)] (PBSA) were investigated. An epoxy chain extender was used during preparation of the blends and nanocomposites to enhance the mechanical properties of the products. Different methods such as scanning electron microscopy, X-ray diffraction and adsorption/desorption analyses were used to characterize the prepared nanoparticles and their localization in the blends. Dynamic oscillatory shear behavior, elongational viscosity and mechanical characteristics of the nanocomposites of PLA and the blends were evaluated. The results obtained for nanocomposites filled with unmodified SFD-MMT were compared with those obtained when the filler was a commercial organically modified montmorillonite nanoclay (methyl-tallow-bis(2-hydroxyeethyl) quaternary ammonium chloride) (C30B), which was not spray freeze-dried.

Ultrasonication of spray- and freeze-dried cellulose nanocrystals in water

The structural and rheological properties of aqueous suspensions of spray-dried cellulose nanocrystals (CNCs) were investigated and compared to those of freeze-dried. The cellulose nanocrystals were obtained from sulfuric acid hydrolysis of wood pulp. Ultrasonication was used to disperse cellulose nanocrystals in Milli-Q water and the power applied during ultrasonication was shown to be the controlling parameter for their dispersion, more than total energy. Dynamic light scattering measurements showed a decrease of the average hydrodynamic diameter down to the same limiting value, i.e. ∼75 nm, for both spray and freeze-dried cellulose nanocrystals. Since the same maximum dispersion state was reached for both CNC types, it indicated that the spray drying process did not limit dispersion, provided that sufficient ultrasonication was provided. Moreover, no desulfation occurred during ultrasonication at ambient temperature. Strong ultrasonication also caused a decrease of intrinsic viscosity, along with an increase in maximum packing concentration. These properties were correlated to agglomerates break-up, which released both ions and water in suspension. The ionic strength increase may lead to a thinner electrostatic double layer surrounding the cellulose nanocrystals, reducing their apparent concentration.

Novel Polyvinyl Alcohol/Starch Electrospun Fibers as a Strategy to Disperse Cellulose Nanocrystals into Poly(lactic acid).

In this work, electrospun fibers of polyvinyl alcohol (PV) and starch (ST) were obtained to improve dispersion of cellulose nanocrystals (CNC) within a poly(lactic acid) (PLA) matrix with the aim of enhancing mechanical and barrier properties. The development and characterization of electrospun fibers with and without CNC, followed by their incorporation in PLA at three concentrations (0.5%, 1% and 3% with respect to CNC) were investigated. Morphological, structural, thermal, mechanical and barrier properties of these nanocomposites were studied. The purpose of this study was not only to compare the properties of PLA nanocomposites with CNC embedded into electrospun fibers and nanocomposites with freeze-dried CNC, but also to study the effect of electrospinning process and the incorporation of CNC on the PV and starch properties. SEM micrographs confirmed the homogenous dispersion of fibers through PLA matrix. X-ray analysis revealed that the electrospinning process decreased the crystallinity of PV and starch. The presence of CNC enhanced the thermal stability of electrospun fibers. Electrospun fibers showed an interesting nucleating effect since crystallinity of PLA was strongly increased. Nanocomposites with electrospun fibers containing CNC presented slightly higher flexibility and ductility without decreasing barrier properties.

Cellulose nanocrystals prepared in H3PW12O40-acetic acid system

Cellulose nanocrystals (CNC) were prepared by destruction of cotton cellulose using phosphotungstic acid in acetic acid medium. The study shows the influence of the heteropolyacid concentration, pre-activation, and hydrogen peroxide addition on the size of CNC particles. CNC were characterized using the methods of dynamic light scattering, FTIR-spectroscopy, thermogravimetric analysis, differential scanning calorimetry, X-ray diffraction, scanning and transmission electron microscopy, and potentiometric titration. CNC particles have highly-crystalline structure and rod-like morphology. Freeze-dried CNC hydrosols form lamellar or fibrous agglomerates, depending on the initial concentration of the CNC sol. Potentiometric titration results show increase in surface activity index and offset of pKa values for CNC active centers in comparison to initial cellulose material.

Concentration driven cocrystallisation and percolation in all-cellulose nanocomposites

All-cellulose nanocomposites reinforced by cellulose nanocrystals (CNC) were produced using a solvent consisting of 1-butyl-3-methylimidazolium chloride and dimethyl sulfoxide. Microcrystalline cellulose (MCC) was pre-dissolved at high temperature in the solvent. Freeze-dried CNC were then added to the slurry at room temperature, thereby avoiding complete CNC dissolution. Solid all-cellulose composite films were obtained by film casting, solvent exchange and drying. The MCC to CNC ratio was kept constant while the solvent content was incremented. The short-range and long-range cellulose–cellulose interactions in the solid materials were respectively assessed by Fourier-transform infrared spectroscopy and X-ray diffraction. The CNC used in this work contained both cellulose I and cellulose II. The cellulose concentration in the mixture drastically changed the overall crystallinity as well as the cellulose I to cellulose II ratio in the ACC. Cellulose II was formed by recrystallisation of the dissolved fractions. These fractions include the pre-dissolved MCC and the cellulose II portion of the CNC. Cocrystallisation with the cellulose I CNC acting as a template was also evidenced. This phenomenon was controlled by the initial solvent content. The correlation between the hygromechanical properties and the nanostructure features of the ACC was investigated by humidity-controlled dynamic mechanical analysis (RH-DMA). The introduction of the cocrystallisation and percolation concepts provided a thorough explanation for the humidity dependency of the storage modulus.

Effect of cellulose nanocrystals (CNC) particle morphology on dispersion and rheological and mechanical properties of polypropylene/CNC nanocomposites.

Polypropylene (PP) nanocomposites containing spray-dried cellulose nanocrystals (CNC), freeze-dried CNC, and spray-freeze-dried CNC (CNCSFD) were prepared via melt mixing in an internal batch mixer. Polarized light, scanning electron, and atomic force microscopy showed significantly better dispersion of CNCSFD in PP/CNC nanocomposites compared with the spray-dried and freeze-dried CNCs. Rheological measurements, including linear and nonlinear viscoelastic tests, were performed on PP/CNC samples. The microscopy results were supported by small-amplitude oscillatory shear tests, which showed substantial rises in the magnitudes of key rheological parameters of PP samples containing CNCSFD. Steady-shear results revealed a strong shear thinning behavior of PP samples containing CNCSFD. Moreover, PP melts containing CNCSFD exhibited a yield stress. The magnitude of the yield stress and the degree of shear thinning behavior increased with CNCSFD concentration. It was found that CNCSFD agglomerates with a weblike structure were more effective in modifying the rheological properties. This effect was attributed to better dispersion of the agglomerates with the weblike structure. Dynamic mechanical analysis showed considerable improvement in the modulus of samples containing CNCSFD agglomerates. The percolation mechanical model with modified volume percolation threshold and filler network strength values and the Halpin-Kardos model were used to fit the experimental results.