Cellulose nanocrystal interactions probed by thin film swelling to predict dispersibility

Abstract

The production of well-dispersed reinforced polymer nanocomposites has been limited due to poor understanding of the interactions between components. Measuring the cohesive particle-particle interactions and the adhesive particlepolymer interactions is challenging due to nanoscale dimensions and poor colloidal stability of nanoparticles in many solvents. We demonstrate a new cohesive interaction measurement method using cellulose nanocrystals (CNCs) as a model system; CNCs have recently gained attention in the composites community due to their mechanical strength and renewable nature. Multi-wavelength surface plasmon resonance spectroscopy (SPR) was used to monitor the swelling of CNC thin films to elucidate the primary forces between CNCs. This was achieved by measuring swelling in situ in water, acetone, methanol, acetonitrile, isopropanol, and ethanol and relating the degree of swelling to solvent properties. Films swelled the most in water where we estimate 1.2-1.6 nm spacings between CNCs (or 4-6 molecular layers of water).Furthermore, a correlation was found between film swelling and the solvent's Hildebrand solubility parameter (R 2 =0.9068). The hydrogen bonding component of the solubility parameters was more closely linked to swelling than the polar or dispersive components. The films remained intact in all solvents, and using DLVO theory we have identified van der Waals forces as the main cohesive interaction between CNCs. The trends observed suggest that solvents (and polymers) alone are not sufficient to overcome CNC-CNC cohesion and that external energy is required to break CNC agglomerates. This work not only demonstrates that SPR can be used as a tool to measure cohesive particle-particle interactions but additionally advances our fundamental understanding of CNC interactions which is necessary for the design of cellulose nanocomposites.

Mechanically adaptive thermoplastic polyurethane/cellulose nanocrystal composites: Process‐driven structure–property relationships

“…The TPU/CNC film and fiber sample absorbed significantly more water compared to the pure TPU samples within the same 24 h period. This result indicates that the hydrophilic nature of the CNCs is increasing the water absorptivity when incorporated into the TPU matrix material . In addition, processing approach did not significantly impact the absorptivity of the composite as the percentage volume change between the TPU/CNC cast film (11.1%) and TPU/CNC fiber (9.0%) is negligible within experimental error (± 1.5%).…”

Benchmarking Cellulose Nanocrystals: From the Laboratory to Industrial Production

“…103 Exposing CNCs to unsuitable solvents force CNCs to strongly aggregate and increase cohesive particle-particle interactions. 63 Potentially sonication following Soxhlet extraction was insufficient to completely redisperse CNCs resulting in CNCs with larger apparent sizes. Although DLS provides an assessment of the relative particle size and colloidal stability, particle dimensions are more appropriately measured by microscopy, specifically transmission electron microscopy (TEM) or AFM (or a combination of DLS and microscopy 102 ).…”

“…No significant change in crystallinity following Soxhlet extraction was observed for CNCs containing solely cellulose I which is to be expected since cellulose I does not swell or dissolve in ethanol. 63 Conversely, FPL CNCs show a significant change in polymorph composition with the percentage of cellulose I nearly three times greater than cellulose II after Soxhlet extraction and suggesting that a portion of loosely bound material, which potentially contains cellulose II was lost. …”

“…This has included characterization of chemical, physical, mechanical and selfassembly properties, 34,52,[60][61][62] as well as the development of new methods to predict and assess CNC dispersion. 63,64 Although CNCs have been extracted from a wide variety of natural cellulose sources using numerous methods, 65 CNCs obtained via sulfuric acid hydrolysis from cotton or wood have been the primary focus of both academia and industry. Commonly, CNCs are extracted by exposing cellulose to strong sulfuric acid (~64 wt%), which favorably hydrolyzes accessible disordered regions leaving highly ordered cellulose in the form of rod-shaped particles.…”

Cross‐Linked Nanocellulosic Materials and Their Applications

“…The strength of the junctions between NC chains is the key to transfer mechanical loads among chains to achieve mechanically strong materials . The strong interactions between CNC particles are dominated by hydrogen bonding and van der Waals forces . Under elevated ambient humidity or direct contact with water, water molecules can penetrate the junctions between the NC units and compete for interfibrillar hydrogen bonding which results in higher motion of chains and hence drastically decreases their mechanical strength properties .…”