Objective  In order to further analyze the interaction between wood cell wall and water, the changes of physical environment (pores) and the binding state of water of wood cell wall before and after furfuryl alcohol modification were investigated.

  Method  The fast-growing poplar (Populus cathayana) was taken as the research object, and the physical environment of water in the wood cell wall was modified by furfuryl alcohol. The microscopic morphology, modifier distribution, functional groups and pore structure of the modified wood were characterized by scanning electron microscopy, confocal laser scanning microscope, Fourier transform infrared spectrometer and nitrogen adsorption under oven-dry state. Besides, differential scanning calorimetry thermoporosimetry and two dimensional low field nuclear magnetic resonance were used to analyze the changes of physical environment and the binding of water in the cell wall before and after modification under low humidity, air-dry, high humidity and fiber saturation state.

  Result  The mass percent gain and bulk percent gain of wood modified by furfuryl alcohol were 35.1% and 12.6%, respectively, accompanied by cell wall thickening. After modification, the specific surface area and pore volume of wood cell wall were reduced by 29.9% and 35.3%, respectively, and furfuryl alcohol resin blocked part of the pores in wood cell wall. From the low humidity state to the fiber saturation state, the pore volume of both untreated and furfuryl alcohol modified wood showed an increasing trend, and the maximum distribution of cell wall pore size of untreated wood ranged from 3.41 to 5.65 mm³/g, at an increase of 65.7%. The maximum cell wall pore size distribution of furfuryl alcohol modified wood rose from 2.99 to 4.63 mm³/g, increased by 54.9%. Under different water conditions, the pore volume of furfuryl alcohol modified wood was all lower than that of untreated wood. Moreover, at high relative humidity, the effect of water on the pore volume of wood cell walls was more pronounced. With the increase of relative humidity, the moisture content of both untreated wood and furfuryl alcohol modified wood became greater, while the moisture content of furfuryl alcohol modified wood was lower compared with that of untreated wood under the same conditions, suggesting a reduction in hygroscopicity. As the moisture content increased, the T₁/T₂ value of cell wall water in untreated and furfuryl alcohol modified wood decreased, and the water mobility increased. The T₁/T₂ values of two types of cell wall water in furfuryl alcohol modified wood were much higher than those in untreated wood, which further indicated that furfuryl alcohol modification changed the physical environment of cell wall, and the mobility of water molecules weakened with the increasing bound of confining space.

  Conclusion  After modification by furfuryl alcohol, furfuryl alcohol resin enters the cell wall of wood and polymerized in situ, resulting in lower cell wall pore volume of modified wood than that of untreated wood under oven dry, low humidity, air-dry, high humidity and fiber saturation state. Besides, in the high humidity state, the pore volume shows a greater growth rate. The variations in the physical environment give rise to a decrease in the space of wood cell walls to hold water, and at the same time, the physical binding of water molecules by the cell wall increases, leading to a decrease in mobility.