In situ identification of the molecular-scale interactions of phenol-formaldehyde resin and wood cell walls using infrared nanospectroscopy
Atomic force microscope infrared spectroscopy (AFM-IR), contact resonance AFM (CR-AFM) measurement, and nanoindentation were combined to identify the interactions between wood cell wall and phenol-formaldehyde resin (PF) on the nanoscale. Significant differences in both chemical structure and mechanics were observed among the cell wall, resin, and interphase regions, indicating that PF resin had diffused to the cell wall effectively. In particular, the maximum penetration depth of the resin in the glueline reached approximately 3 μm, showing that PF resin was able to penetrate into the wood's secondary cell wall. The penetrating resin molecules not only dispersed within the cell wall but also reacted with cell-wall polymers, resulting in an increase in the elastic modulus and hardness of the wood cell wall. Nanoscale mechanical interlocks also formed between the resin and the wood cell wall in the interphase region, which may improve adhesion performance in wood-based composites.