Temperature-dependent structural changes of crosslinked cellulose nanocrystal film

Cellulose nanocrystals (CNCs) are promising sustainable alternatives to fossil-based plastics due to their biodegradability, recyclability, high mechanical strength, and thermal stability. However, CNC films are typically brittle and lack structural stability under high humidity. To address these limitations, plasticizers and crosslinkers are commonly used to improve flexibility and moisture resistance of CNC film. However, their incorporation alters not only the chemical bonding but also the internal structure of the film, which in turn dictates the film properties. In this study, malonic acid and maltitol were used as a crosslinker and plasticizer, respectively, with the aim of understanding how malonic acid crosslinking affects the structure of CNC film. The formation of ester bonds is confirmed by dynamic Fourier transform infrared (FTIR) analysis, with crosslinking initiated at 120 °C. In-situ small-angle X-ray scattering (SAXS) and cross-sectional scanning electron microscopy (SEM) were used to explore CNC film structure during crosslinking, proving crosslinking initially led to a reduction in interdomain distances (~49±1 nm) compared to the control sample (~62±1 nm). As the reaction progressed, the dense alignment was disrupted, leading to the formation of micropores and locally dense regions. The impact of structural change alongside crosslinking on the film’s mechanical properties and permeability is also investigated. The crosslinked film exhibits reduced mechanical integrity, suggesting that although some hydrogen bonds between CNCs are transformed into stronger covalent bonds, the increase in bonding strength is less than offset by the reduction in bonded area due to pore formation within the structure. The film’s gas permeability also increases due to pore formation. These results underscore the importance of understanding and balancing chemical modifications and structural alteration in CNC films, providing insights for designing advanced materials with tailored properties for functional applications.