Supramolecular Cellulose Crystalline Organisation and Its Relationship with Density, Thermal Stability, and Micromechanical Behaviour in Native Wood

Understanding the extent to which the supramolecular crystalline organisation of native wood is associated with its macroscopic, thermal and micromechanical properties remains an active area of inquiry in biomass characterisation. In this study, the crystalline, thermal, and micromechanical characteristics of Oriental spruce (Picea orientalis (L.) Peterm.), Scots elm (Ulmus glabra Huds.), Canadian poplar (Populus × canadensis Moench), Austrian pine (Pinus nigra J.F. Arnold), Turkish red pine (Pinus brutia Ten.), and black locust (Robinia pseudoacacia L.) were comparatively investigated using X-ray diffraction (XRD), thermogravimetric–derivative thermogravimetric (TGA-DTG) analysis, Vickers microhardness, and density measurements. Microhardness generally aligned with density trends among the sampled wood specimens; black locust exhibited the highest density (0.727 g/cm3) and, alongside Austrian pine, recorded the highest mean parallel-to-grain hardness (18.0 kgf/mm2). In the perpendicular-to-grain direction, Turkish red pine reached the maximum value (9.9 kgf/mm2). Conversely, Canadian poplar yielded the lowest values across all three evaluated parameters (0.371 g/cm3 density, 6.7 kgf/mm2 parallel, and 3.1 kgf/mm2 perpendicular). XRD profiles confirmed the characteristic cellulose I structure in all examined specimens, with the (200) reflection occurring at 22.06–22.31° and crystallinity index (CrI) values ranging from 55.91% (Scots elm) to 60.07% (Turkish red pine). Apparent crystallite sizes (L200) ranged from 2.68 nm (Scots elm) to 2.94 nm (black locust), indicating modest variation in the measured crystalline parameters. TGA-DTG analysis revealed variation in the observed thermal degradation profiles, with maximum degradation temperatures of 333.0–366.7 °C and total mass losses of 52.3–65.3%. Exploratory comparisons indicated that these nanoscale crystallographic parameters do not present a simple linear relationship with macroscopic thermal mass loss or hardness. Overall, these findings indicate that cellulose crystalline organisation, thermal degradation, density, and micromechanical behaviour are interconnected yet non-equivalent properties. Integrating these multiparametric evaluations provides further insight into the relationships between nanoscale cellulose organisation and the higher-level properties of wood.