Out-of-Plane Auxetic Behavior in Cellulose Nanofibril Films.

Area of Science:

• Materials Science
• Nanotechnology
• Mechanics of Materials

Background:

• Auxeticity, a property where materials expand in response to stretching, is known in paper products.
• Computational models predict auxetic behavior in cellulose structures.
• Experimental evidence for auxeticity in neat cellulose nanofibril (CNF) films was previously lacking.

Purpose of the Study:

• To experimentally confirm and characterize the auxetic response in neat CNF films.
• To investigate the influence of film density, microstructure, and sample geometry on auxeticity.
• To establish processing-structure-property relationships for auxetic CNF films.

Main Methods:

• CNF films were fabricated using vacuum filtration.
• Films were conditioned at 23 °C and 50% relative humidity.
• Varied CNF suspension volumes (90-450 mL) to control film density and aspect ratios (1.5-5) to study geometric effects.

Main Results:

• CNF films demonstrate an out-of-plane auxetic response.
• Film density, up to a plateau of 1.05 g/cm³, and aspect ratio significantly impact the magnitude of auxeticity.
• Films with 1.05 g/cm³ density and aspect ratio of 5 showed the highest auxetic response, with a Poisson's ratio of -5.3 and an instantaneous value of -7.99 at 0.4% strain.

Conclusions:

• This study experimentally validates auxetic behavior in CNF films.
• Processing parameters like density and geometry are crucial for tuning the auxetic properties of CNF films.
• CNF films hold potential as auxetic metamaterials for applications in sensing, protective gear, composites, and structural materials.