Diffusion and equilibrium amount of polyethylene glycol in wood cell walls

Diffusion and equilibrium amounts of polyethylene glycol (PEG) in the cell walls (CWs) of red pine sapwood were studied as a function of molecular weight (MW) of PEG, wood moisture content, and solvent properties. Relative amounts of PEG in the CWs were determined by Raman microscopy, and by extraction of PEG-treated wood with toluene. The study confirmed penetration of high MW (PEG 20000) into the CWs by scanning transmission electron microscopy coupled with energy dispersive X-ray analyzer (STEM-EDXA) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). A trend to higher concentrations of PEG in the middle lamellae was observed. ToF-SIMS was shown to be more appropriate for determining penetration of high MW PEG. Solvent properties, such as wood swelling ability and affinity for PEG were shown to be important factors determining PEG penetration into the CWs. Samples treated with PEG from chloroform solution adsorbed less PEG into the CWs than samples treated with PEG-water solution. These results were in agreement with the relative amounts of PEG measured by Raman microscope, which was shown to be a useful tool for quick and sample-preparation-free estimation of PEG in the CWs. Increase in ambient relative humidity caused movement of PEG from cell lumens to cell walls.;Samples treated with lower MW PEG showed better CW impregnation, both when treated with nominal PEG MWs and with mixtures of PEG MWs. Preferential extraction of PEG from the cell lumens by toluene was shown to be an unacceptable approach for measuring PEG amounts in the CWs. Rates of diffusion of different PEG MWs into CWs were estimated by applying Fickian diffusion equations for plane and cylindrical surfaces. PEG Diffusion coefficients were in the order of 10-9 cm2/s for PEG MW 1000 and 10 -10 cm2/s for PEG 2000 and 4000 for both plane and cylindrical geometries. The high CW penetration rates suggested vacuum impregnation with PEG solutions as an efficient means of bulking wood.