Positrons as microprobes to study water-dependent free volume of wood cell walls: a preliminary study

Positron annihilation lifetime spectroscopy (PALS) was employed to study the water-dependent free volume characteristics of wood cell walls in earlywood and latewood of loblolly pine (Pinus taeda). Measurements were conducted across relative humidity levels (1–80% RH) at room temperature, demonstrating good reproducibility and consistency with polymer science principles. The Tao-Eldrup model was applied to the measured ortho-positronium lifetimes to estimate the mean sizes of free volume elements in wood cell walls. At low relative humidity (below ~ 11%), water absorption resulted in antiplasticization, evidenced by a decrease in mean free volume element sizes. As relative humidity increased, water started acting as a plasticizer, causing the free volume elements to expand. While dry cell walls showed no significant differences in free volume element sizes between earlywood and latewood, earlywood exhibited larger mean free volume element sizes at all higher relative humidity levels. At higher relative humidity levels (above ~ 70% RH), the ortho-positronium lifetimes of cell wall free volume elements overlapped with those of liquid water, indicating PALS cannot provide reliable free volume information at higher cell wall moisture contents. Interpreting the intensity of ortho-positronium annihilation was complicated by the possibility of water inhibiting ortho-positronium formation in wood cell walls.