How Water Alters the Structure and Dielectric Behavior of Microcrystalline Cellulose

Background

Cellulose, a renewable, eco‑friendly resource derived from plant biomass, is the basis for producing microcrystalline cellulose (MCC). MCC’s crystalline particles are of great interest for applications in pharmaceuticals, cosmetics, composites, electronics, and laser optics. Its ability to absorb moisture significantly affects structural, electrical, and thermophysical properties, making the study of its moisture‑dependent behaviour essential.

Methods

The Samples

Commercial MCC (Cellets‑100, Shin‑Etsu, Japan) was used. Samples were dried at 115 °C for 3 days to remove free water, then vacuum‑sealed. Moisture‑laden samples were exposed to saturated water vapor for varying durations to achieve different water contents.

Equipment

Structural analysis employed a DRON‑3M X‑ray diffractometer (λ = 1.54178 Å). Thermogravimetric and differential thermal analyses were performed on a Q‑1500D derivatograph from 20 °C to 250 °C at 5 °C min⁻¹. Dielectric measurements were carried out on MCC pellets compressed between stainless‑steel electrodes at 120 kg cm⁻², using a four‑electrode cell with an air‑dielectric capacitor for thickness control. Capacitance and loss factor were recorded at 5, 10, 20, and 50 kHz from –180 °C to 120 °C using an AC bridge (P5083).

Results and Discussion

Thermogravimetric Investigations

The mass loss curves of MCC at different moisture levels reveal three distinct peaks: (i) evaporation of physically bound water in micropores, (ii) desorption from multimolecular hydration layers, and (iii) thermal degradation of the cellulose matrix. Gaussian fitting of the derivative mass loss data (dm/dT) confirms this tri‑peak behaviour. The proportion of physically bound water increases with overall moisture content, while the hydration‑layer water fraction saturates around 12.9 % of the total mass.

Temperature dependence of the relative mass change of MCC with varying water content.

Derivative of mass change for MCC samples at different moisture levels.

Activation energies and pre‑exponential factors for the second peak (hydration‑layer desorption) were extracted using non‑isothermal kinetic analysis. The activation energy rises with increasing water content, reaching a plateau at 12.9 % moisture—consistent with the formation of a continuous hydrate shell comprising roughly eight hydrogen bonds per crystallite surface node.

Linear plots of ln[(–dQ/dt)/Qⁿ] versus 1/T for different reaction orders; the best fit occurs at n = 2.

Activation energy, pre‑exponential factor, and estimated hydrogen‑bond count versus moisture content.

X‑ray Diffraction Analysis

Diffraction patterns of MCC across 5–45° 2θ show characteristic peaks that shift slightly to lower angles as moisture increases, indicating lattice expansion. Calculated crystallinity (Ck) and transverse crystallite dimensions (Bhlk) both increase with water content, with Bhlk growing by ~0.4 nm. These changes suggest that water molecules in the hydration shell promote better ordering of cellulose chains at the crystallite surface.

Diffraction intensity versus 2θ for MCC with different water contents.

Transverse crystallite dimensions and their growth with humidity.

Dielectric Properties of MCC

Temperature‑dependent dielectric measurements at 5–50 kHz reveal two distinct processes. The low‑temperature β‑relaxation, associated with tg → tt conformational changes of surface methylol groups, shifts to lower temperatures as water content rises. The high‑temperature transition, linked to water desorption and re‑condensation within pores, intensifies with moisture and subsequently diminishes upon further heating.

Δε′ versus temperature for MCC + 0.3 % H₂O at 5–50 kHz.

ε″ versus temperature for MCC + 0.3 % H₂O at 5–50 kHz.

Δε′ for MCC + 2.8 % H₂O at 5–50 kHz.

ε″ for MCC + 2.8 % H₂O at 5–50 kHz.

Δε′ versus temperature at 10 kHz for MCC samples with varying moisture.

Fitting the Δε′ data with a single‑relaxation‑time model yields a constant energy difference (V) but an increasing concentration of active methylol groups (N) with water content, indicating that hydration enhances the number of sites contributing to dielectric relaxation.

ε″ versus temperature at 10 kHz for MCC samples with different moisture levels.

Analysis of the relaxation peak maxima (ωτ = 1) provides entropy (ΔS/k) and activation energy (U) values that rise with moisture and then plateau. This saturation reflects the formation of a solid hydrate shell around MCC crystallites once the surface water threshold (~13 %) is reached.

Entropy and activation energy of the relaxation process versus water concentration.

Conclusions

This study demonstrates that water profoundly alters MCC’s structure and dielectric behaviour. The low‑temperature relaxation, linked to tg → tt methylol‑group reorientation, shifts to lower temperatures as the hydration shell thickens. A continuous hydrate shell forms around MCC crystallites once the moisture exceeds ~13 %, restructuring the surface layer and raising crystallinity and transverse crystallite dimensions.