📄 chemistry

Rheology of concentrated CNF suspensions following PEG and NaOH modification

This study establishes a quantitative rheological framework for concentrated cellulose nanofibril (CNF) suspensions, revealing that while polyethylene glycol (PEG) reduces viscosity through surface-site-limited interactions, sodium hydroxide (NaOH) induces stronger network disruption via hydroxide-driven swelling, with their combined use creating a highly deformable yet partially stabilized fibrillar network.

Original authors: Shadi Ghaebi Mehmandoust Olya, Thomas Rainey

Published 2026-10-12

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Shadi Ghaebi Mehmandoust Olya, Thomas Rainey

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine a world where the materials we use to build, wrap, and protect our lives are made from the same stuff as cotton shirts and wooden trees. This is the promise of cellulose nanofibrils, tiny, thread-like strands of plant fiber that are so thin they are measured in billionths of a meter. When these strands are mixed with water, they do not simply dissolve; instead, they tangle together to form a thick, gel-like substance. This gel is the starting point for making everything from biodegradable packaging to medical scaffolds. However, turning this gel into a useful product is a delicate balancing act. If the mixture is too thick, it clogs pipes and resists being spread into a thin film. If it is too thin, it cannot hold its shape once applied. The challenge for engineers is to understand exactly how to control the thickness and flow of this plant-based gel without breaking the delicate network of fibers that gives it strength.

Researchers at the Queensland University of Technology set out to solve this puzzle by studying how concentrated suspensions of these plant fibers behave when mixed with common chemicals. They focused on two specific additives: polyethylene glycol, a slippery polymer often used in lotions and industrial coatings, and sodium hydroxide, a strong base known as lye that is used to swell and soften plant fibers. The team wanted to see how these chemicals changed the way the fiber network held together, flowed, and snapped back when stretched. By testing mixtures with different amounts of fibers and varying levels of these additives, they aimed to create a clear guide for manufacturers on how to tune the flow of these materials for large-scale production.

The researchers began by observing the natural behavior of the fiber mixtures. They found that as the concentration of fibers increased, the mixture became dramatically thicker and more resistant to flow. A suspension with just a small amount of fiber flowed relatively easily, but as the amount of fiber grew to seven percent by weight, the mixture transformed into a stiff, gel-like structure that required significant force to move. This happened because the fibers, which are long and thin, began to bump into and tangle with one another, creating a dense web that spanned the entire container. This web acted like a temporary scaffold, holding its shape until enough force was applied to break the connections and allow the material to flow.

When the team added polyethylene glycol to the mix, they observed a distinct change. The mixture became easier to stir and less resistant to flow, but the effect happened quickly and then stopped. Adding a small amount of the polymer reduced the thickness significantly, but adding more did not make a further difference. This suggested that the polymer molecules were attaching themselves to the surface of the fibers, acting like a lubricant that prevented the fibers from sticking together too tightly. Once all the available spots on the fiber surfaces were covered, the system reached a limit, and the flow properties stabilized. This behavior indicated that the polymer was not tearing the network apart but was simply making the individual fibers slide past each other more smoothly.

In contrast, the addition of sodium hydroxide produced a much more dramatic and continuous effect. As the researchers increased the amount of this strong base, the mixture became progressively thinner and more fluid, with no sign of reaching a limit. The chemical action of the base caused the fibers to swell and lose the strong bonds that held them together, effectively dismantling the rigid network. Unlike the polymer, which merely coated the fibers, the base changed the structure of the fibers themselves, making the entire system much more deformable. When the researchers combined the polymer and the base, the result was a material that was very fluid and easy to deform, yet it retained a tiny bit more stiffness than the base alone. This suggested that even in a highly disrupted state, the polymer provided a small amount of extra connection between the fibers, preventing the network from collapsing completely.

The study also looked at how the material responded to being stretched and squeezed, measuring its ability to store energy like a spring or dissipate it like a liquid. The results showed that the fiber network behaved like a weak gel, meaning it was mostly solid but could flow under pressure. Increasing the fiber concentration made this gel stiffer and better at storing energy, while the chemical additives weakened this ability. The base was particularly effective at breaking down the gel structure, allowing the material to stretch much further before breaking, whereas the polymer mainly reduced the overall strength without changing how the material stretched.

These findings offer a clear picture of how to control the flow of plant-based gels for manufacturing. The research suggests that if a manufacturer needs a material that flows easily but still holds its shape after being applied, they might use a small amount of polymer to lubricate the fibers. If they need a material that is extremely fluid for pumping or mixing, a strong base would be the better choice, though they would need to accept that the final product might be softer and less rigid. By understanding these specific mechanisms, engineers can now design better processes for creating sustainable materials, ensuring that these plant-based gels can be processed efficiently without losing the structural integrity needed for their final use. The work confirms that the behavior of these complex mixtures is governed by a balance between how crowded the fibers are and how strongly they stick together, a balance that can be precisely tuned with the right chemical tools.

Technical Summary: Rheology of Concentrated CNF Suspensions Following PEG and NaOH Modification

Problem Statement
Cellulose nanofibril (CNF) suspensions are critical for scalable manufacturing processes such as coating, casting, and film formation. While the rheology of dilute and semi-dilute CNF suspensions is well-documented, the behavior of concentrated suspensions (relevant to industrial processing) remains insufficiently understood, particularly regarding the combined influence of CNF concentration, polymer additives, and alkaline conditions. Existing literature often examines additives like polyethylene glycol (PEG) and sodium hydroxide (NaOH) indirectly through dispersion quality or chemical pretreatment rather than through systematic rheological characterization of concentrated networks. This study addresses the gap in understanding how PEG and NaOH individually and collectively modify the network structure, deformation behavior, and flow resistance of concentrated CNF suspensions (2–7 wt%).

Methodology
The study utilized wood-extracted CNF pulp (Celova M250R-P) to prepare suspensions at 2, 4, 6, and 7 wt% (dry weight). A baseline 6 wt% suspension was used for additive studies.

Additives: PEG (8,000 g/mol) was added at 1, 3, 5, and 10 wt%; NaOH was added at 1, 5, 9, and 10 wt%. Combined effects were tested using 1 wt% PEG and 9 wt% NaOH. pH-controlled experiments were conducted using phosphate buffers (pH 3, 5, 8, 10) to isolate pH effects from alkaline concentration.
Characterization: Rheological tests were performed using an Anton Paar MCR 302 rheometer at 25°C.
- Steady Shear: Apparent viscosity and shear stress were measured (0.1–1000 s⁻¹) and fitted to the Herschel–Bulkley (HB) model to determine yield stress ( $\tau_y$ ), consistency index ( $k$ ), and flow index ( $n$ ).
- Oscillatory Shear: Frequency sweeps (0.1–628 rad/s) evaluated viscoelastic moduli ( $G'$ , $G''$ ), crossover frequency, and relaxation time. Amplitude sweeps (0.01–100% strain) determined the linear viscoelastic region (LVER), yield strain, and energy-based descriptors (recoverable elastic energy $U_C^*$ and dissipated energy $W_d$ ).
Analysis: Statistical significance was evaluated using one-way ANOVA ( $p < 0.05$ ). The fibril aspect ratio was estimated via sedimentation-based gel point analysis.

Key Contributions and Results

Concentration Dependence:
- Increasing CNF concentration from 2 to 7 wt% resulted in a substantial increase in viscosity (nearly two orders of magnitude at low shear), yield stress (from ~3 Pa to ~71 Pa), and viscoelastic stiffness ( $G'$ increased from ~0.05 kPa to ~2.3 kPa).
- The crowding factor ( $N$ ) for all studied concentrations exceeded the percolation threshold ( $N \approx 60$ ), confirming the formation of fully percolated fibrillar networks.
- Yield stress and oscillatory yield stress followed power-law relationships with the crowding factor ( $\tau_y \propto N^{2.54}$ and $\tau_f^* \propto N^{2.64}$ ), indicating that flow initiation and nonlinear yielding are governed by a common connectivity-controlled mechanism.
Effect of PEG (Surface-Site-Limited Interaction):
- PEG addition reduced viscosity, yield stress, and network stiffness. However, the most significant changes occurred at the lowest addition (1 wt%), with diminishing returns at higher concentrations (3–10 wt%).
- This "saturation-type" behavior suggests a surface-site-limited interaction mechanism where PEG modifies inter-fibrillar surface interactions (e.g., via steric interference or hydration layer modification) without fundamentally disrupting the percolated network structure.
- PEG did not significantly alter the frequency-scaling exponents or the strain-sensitivity index, indicating that the underlying relaxation behavior and deformation mode remained largely unchanged.
Effect of NaOH (Structural Disruption via Swelling):
- NaOH induced substantially stronger rheological changes than PEG or pH adjustment alone. Viscosity dropped by ~84% at 10 wt% NaOH, and yield stress decreased to ~9 Pa.
- Unlike PEG, NaOH significantly altered all rheological parameters, including relaxation times and scaling exponents.
- The authors attribute this to hydroxide-driven swelling and the disruption of hydrogen-bonded inter-fibrillar contacts, leading to a more compliant and weakly connected network. At high NaOH concentrations, the network became highly deformable, sustaining larger strains before yielding.
Combined PEG and NaOH Effects:
- In combined formulations (1 wt% PEG + 9 wt% NaOH), the rheological response was dominated by the alkaline effect, exhibiting low yield stress and stiffness similar to NaOH-only suspensions.
- However, the combined suspension retained slightly higher stiffness and energy storage capacity ( $U_C^*$ ) than equivalent NaOH-only suspensions. This suggests that PEG partially moderates alkali-induced network disruption, likely through residual fibril interactions that persist even under strong swelling conditions.
pH vs. Alkalinity:
- Adjusting pH via buffers produced moderate changes in viscosity and yield stress compared to direct NaOH addition. While acidic conditions increased yield stress (likely due to protonation-induced aggregation) and alkaline conditions reduced it (via electrostatic repulsion), the magnitude of change was far less than that caused by high NaOH concentrations.
- This indicates that in concentrated systems, fibril crowding and physical entanglement dominate over electrostatic regulation, and pH primarily modulates interaction strength rather than network architecture.

Significance and Claims
The paper claims to establish a quantitative rheological framework for concentrated CNF suspensions, demonstrating that:

Network Connectivity is Paramount: Rheological properties in concentrated regimes are governed by the balance between fibril connectivity, interaction strength, and swelling.
Distinct Modification Mechanisms: PEG and NaOH operate via fundamentally different mechanisms. PEG acts as a surface-interaction modifier with a saturation effect, suitable for tuning flowability without destroying structural integrity. NaOH acts as a structural disruptor via swelling, significantly altering network connectivity and deformability.
Processing Implications: The findings provide a basis for formulating CNF suspensions for specific manufacturing needs. PEG may be advantageous for applications requiring improved flowability (coating, printing) while maintaining post-deposition shape retention. NaOH facilitates pumping and mixing but may compromise shape retention due to network weakening.
Limitations: The authors modestly note that the proposed interaction mechanisms are inferred from rheological behavior and would benefit from complementary microstructural and spectroscopic characterization. Future work is suggested to link these rheological findings directly to processing behavior and the final properties of cellulose-based materials.

Technical Summary: Rheology of Concentrated CNF Suspensions Following PEG and NaOH Modification

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