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20 pages, 4628 KiB

Open AccessArticle

Achieving 3-D Structural Uniformity in Cellulose Gel Beads via Salt Screening

by Matthew T. Garnett, Seyed Armin Seyed Esfahani, Andrew P. Yingst, Luke T. May and Symone L. M. Alexander

Polymers 2024, 16(24), 3519; https://doi.org/10.3390/polym16243519 - 18 Dec 2024

Viewed by 1108

Abstract

Cellulose microgel beads fabricated using the dropping technique suffer from structural irregularity and mechanical variability. This limits their translation to biomedical applications that are sensitive to variations in material properties. Ionic salts are often uncontrolled by-products of this technique, despite the known effects of ionic salts on cellulose assembly. In this study, the coagulation behavior of cellulose/salt solutions was explored as a way to combat these challenges. An ionic salt (NaCl) was added to a cellulose solution (cellulose/NaOH/urea) prior to coagulation in a hydrochloric acid bath. Quantification of the bead geometry and characterization of the pore architecture revealed that balancing the introduction of salt with the resultant solution viscosity is more effective at reducing structural variability and diffusion limitations than other pre-gelling techniques like thermal gelation. Three-dimensional visualization of the internal pore structure of neat cellulose, thermo-gel, and salt-gel beads revealed that adding salt to the solution is the most effective way to achieve 3-D structural uniformity throughout the bead. Coupled with nanoindentation, we confirmed that the salt produced during coagulation plays a critical role in mechanical variability, and that adding salt to the solution before dropping into the coagulation bath completely screens this effect, producing uniform microgel beads with reproducible mechanical properties. Full article

(This article belongs to the Special Issue Advanced Preparation and Application of Cellulose: 2nd Edition)

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Cellulose microgel beads are formed using a dropping technique in a 2M HCl coagulation bath. Full article ">Figure 2
(a) Computational heat maps were used to predict thermal gradients present in cellulose solutions over time as a function of bath temperature. A 25 °C water bath resulted in the lowest thermal gradient as a function of radial distance and time, indicating the least amount of thermal stress. (b) Experimental temperature vs. time data were used to validate the model (r = 0.00 m) and determine the time required to reach thermal equilibrium for water bath temperatures of 25, 45, and 65 °C. (c–e) Solutions were visually examined for signs of thermal stress (transition from colorless to yellow) and precipitation (transition from transparent to opaque). Signs of thermal stress increased as a function of bath temperature, with the 25 °C water bath (c) showing the least signs of thermal stress after 30 min compared to the 45 °C (d) and 65 °C (e) water baths. Full article ">Figure 3
Electrophoretic mobility and dynamic viscosity were compared for a CNF solution and CNF suspensions. The presence of NaOH in solution leads to opposite trends for CNF suspensions. In DI water, the CNFs display a net negative charge that decreases as the salt concentration increases. In a NaOH/Urea solution, the CNFs display a net positive charge that becomes more negative as the salt concentration increases. Dissolved CNFs in a NaOH solution displayed an electrophoretic mobility near zero that became more negative as the salt concentration increased. Both of the CNF suspensions displayed negligible changes in viscosity as the salt concentration increased compared to the CNF solution. The electrophoretic mobility and viscosity of the suspensions and solutions were measured at 20 °C. Full article ">Figure 4
(a) Neat cellulose, thermo-gel, and salt-gel beads were examined for tail formation using ImageJ. The red square indicates a bead with tail formation. Salt-gel beads had the lowest degree of tail formation compared to neat cellulose and thermo-gel beads. (b,c) The circularity of the thermo-gel beads slightly increased compared to that of the neat cellulose beads and had lower variability. The diameter of the thermo-gel beads was smaller than that of the neat cellulose beads. (d,e) Salt-gel beads with a salt concentration of 0.5 wt% did not improve bead geometry, while 1 and 2 wt% salt increased circularity and decreased variability compared to both neat cellulose and thermo-gel beads. A t-test was used to determine if there were significant differences between the diameter and circularity of neat and thermo-gel cellulose beads. A one-way ANOVA was used to determine if the means of the diameter and circularity were significantly different between the neat and salt-gel beads. *** p < 0.001, **** p < 0.0001, and no * = not significant. Full article ">Figure 5
(a–c) Cross-sectional SEM images were taken of (a) neat (no pre-gel), (b) thermo-gel, and (c) salt-gel beads. SEM revealed that neat and thermo-gel beads had a tighter cellulose network that limited the diffusion of salt, while salt-gel beads had a larger network that allowed salt to diffuse and improve uniformity throughout the network. Neat cellulose and thermo-gel beads displayed a porosity of 54.62% and 53.09%, while the salt-gel beads displayed a porosity of 75.80%. All scale bars = 80 µm, corresponding to 2000X magnification. (d–f) 3-D nano-CT scans were taken of (d) neat (no pre-gel) cellulose, (e) thermo-gel, and (f) salt-gel beads fabricated in a 2M HCl coagulation bath. Nano-CT revealed that adding salt into the solution generated a uniform network structure throughout the bead. Access to 360° rotations for each bead are available in the <a href="#app1-polymers-16-03519" class="html-app">Supplementary Information</a>. All scale bars = 500 µm. Full article ">Figure 6
Increasing the concentration of salt into solution prior to coagulation effectively screens the salt produced via the neutralization reaction and provides elasticity to the cellulose network. Neat and thermo-gel solutions have salt concentrations that cause competition between cellulose chains during aggregation due to the lack of salt screening effects. In contrast, introducing salt into solution helps avoid this problem and leads to more uniform beads with less structural variability. Full article ">Figure 7
Nanoindentation was performed on (a) neat cellulose, (b) thermo-gel, and (c) salt-gel beads in a control (water), simulated gastric fluid (SGF), and simulated intestinal fluid (SIF) environment to compare the changes in Young’s moduli. The Young’s moduli of the neat cellulose, thermo-gel, and salt-gel beads were also compared to see if there were significant differences in the beads when subjected to (d) water, (e) SGF, and (f) SIF. Overall, salt-gel beads displayed lowest variability and lowest Young’s modulus compared to both thermo-gel and neat cellulose beads. (a–c) A one-way ANOVA was used to determine if the means of the three groups tested were significantly different. (d–f) A two-way ANOVA was used to determine if the means of the bead type and environment were statically significant. * p < 0.05, *** p < 0.001, **** p < 0.0001, and no * = not significant. Full article ">

19 pages, 2034 KiB

Open AccessArticle

Activated Nanocellulose from Corn Husk: Application to As and Pb Adsorption Kinetics in Batch Wastewater

by Aydeé M. Solano-Reynoso, Ruth Fany Quispe-Quispe, Yudith Choque-Quispe, Fredy Taipe-Pardo, Yovana Flores-Ccorisapra, Celia R. Yauris-Silvera, Diego E. Peralta-Guevara, Yakov Felipe Carhuarupay-Molleda, Liliana Rodriguez-Cardenas, David Choque-Quispe and Carlos A. Ligarda-Samanez

Polymers 2024, 16(24), 3515; https://doi.org/10.3390/polym16243515 - 18 Dec 2024

Viewed by 931

Abstract

The aim of this study was to evaluate the removal of Pb and As from an aqueous solution using corn residue cellulose nanocrystals (NCCs). The corn husk was subjected to alkaline digestion, followed by bleaching and esterification with 3% citric acid to obtain NCCs. A 10 ppm multimetal solution of Pb and As was prepared. The adsorption process was evaluated by adjusting the pH and NCC dosage, optimized through the nonlinear regression of empirical mathematical models. Based on the optimal parameters, the kinetics were evaluated using the PFO and PSO models. The NCCs displayed nanometer-level characteristics with a particle size less than 383.7 nm, a ζ potential in the range of −28–70 mV, pH_ZCP with an acidic tendency, a porous crystal structure as evaluated through SEM images, and the presence of functional groups with a high chelating capacity, as identified via FTIR. Optimum values of pH 8.0 and 20 mg/L of the NCC dose were found, from which it was observed that the PFO, PSO, and Elovich kinetics showed R² > 0.974, with an adsorption capacity in the order Pb > As. The adsorbent-formulated NCCs presented a good capacity to remove heavy metals from aqueous media. Full article

(This article belongs to the Special Issue Advanced Preparation and Application of Cellulose: 2nd Edition)

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17 pages, 71463 KiB

Open AccessArticle

Nonlinear Dynamic Mechanical and Impact Performance Assessment of Epoxy and Microcrystalline Cellulose-Reinforced Epoxy

by Mertol Tüfekci

Polymers 2024, 16(23), 3284; https://doi.org/10.3390/polym16233284 - 25 Nov 2024

Viewed by 711

Abstract

This study focusses on imrpoving the mechanical performance of epoxy resin by reinforcing it with microcrystalline cellulose (MCC). Epoxy composites with varying MCC mass fractions (0.5%, 1%, 1.5%, and 2%) are prepared and characterised to assess the influence of MCC on strain-rate-dependent flexural properties, impact resistance, and nonlinear viscoelastic behaviour. Three-point bending tests at different strain rates reveal that MCC notably increases the flexural strength and leads to nonlinear mechanical behaviour. It is shown that stiffness, strength and elongation at break increase with rising MCC content. Charpy impact tests show improved energy absorption and toughness, while Dynamic Mechanical Analysis (DMA) demonstrates that the materials prepared exhibit increased storage modulus and improved damping across a frequency range. These results indicate that MCC serves as an effective bio-based reinforcement, significantly boosting the strength and toughness of epoxy composites. The findings contribute to the development of sustainable, high-performance materials for advanced engineering applications. Full article

(This article belongs to the Special Issue Advanced Preparation and Application of Cellulose: 2nd Edition)

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DMA test setup with dual cantilever fixture and specimen dimensions. Full article ">Figure 2
Three-point bending test setup and specimen dimensions. Full article ">Figure 3
Charpy impact test setup. Full article ">Figure 4
Storage modulus of epoxy samples with various MCC mass fractions (<math display="inline"><semantics> <msub> <mi>m</mi> <mi>f</mi> </msub> </semantics></math>). Full article ">Figure 5
Loss modulus of epoxy samples with various MCC mass fractions (<math display="inline"><semantics> <msub> <mi>m</mi> <mi>f</mi> </msub> </semantics></math>). Full article ">Figure 6
tan(<math display="inline"><semantics> <mi>δ</mi> </semantics></math>) of epoxy samples with various MCC mass fractions (<math display="inline"><semantics> <msub> <mi>m</mi> <mi>f</mi> </msub> </semantics></math>). Full article ">Figure 7
Surface plots showing the dependence of modulus of elasticity, flexural strength, and elongation at break on MCC mass fraction and strain rate. Full article ">Figure 8
Charpy impact test results. Full article ">

17 pages, 4622 KiB

Open AccessArticle

Utilization of Forest Residues for Cellulose Extraction from Timber Species in the High Montane Forest of Chimborazo, Ecuador

by Dennis Renato Manzano Vela, Cristina Nataly Villegas Freire, Rolando Fabian Zabala Vizuete and Ana Carola Flores Mancheno

Polymers 2024, 16(19), 2713; https://doi.org/10.3390/polym16192713 - 25 Sep 2024

Viewed by 1009

Abstract

The present study explored the extraction of cellulose from forest residues of four timber species, namely Cedrela montana Moritz ex Turcz, Buddleja incana Ruiz & Pav, Vallea stipularis L. f. and Myrsine andina (Mez) Pipoly, in the high montane forest of Chimborazo province, Ecuador, for the sustainable utilization of leaves, branches, and flowers. An alkaline extraction method was used on the residues without the need for prior degreasing. An ANOVA analysis was applied to evaluate significant differences in cellulose extraction yields among the species’ residues. The characterization techniques used were Fourier transform infrared spectroscopy (FTIR) and polarized light optical microscopy, which confirmed the successful extraction of cellulose with characteristics comparable to standard cotton cellulose and other traditional species. The results showed significant variations in cellulose yield among the species, with Vallea stipularis L. f achieving the highest yield of 80.83%. The crystallinity of the samples was clearly evidenced by the polarity of the light in the samples during microscopy, demonstrating that the residues can be a viable and sustainable source of cellulose, contributing to a circular economy and reducing the environmental impact of forest waste. Full article

(This article belongs to the Special Issue Advanced Preparation and Application of Cellulose: 2nd Edition)

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