Modeling the Mechanical Behavior of Transgenic Aspen with Altered Lignin Content and Composition

Recent advances in tree genetics permit modification of lignin content and structure. However, the consequences of lignin modifications on many wood properties need to be explored. The purpose of this dissertation was to measure, analyze, and model the effect of genetic modification of lignin on selected wood mechanical properties.;In this dissertation, young (1- to 2.5-year-old) genetically modified quaking aspen (Populus tremuloides Michx.) trees with reduced lignin content and/or increased syringyl to guaiacyl (S/G) ratio were investigated and compared to the wild type. The results of this investigation are presented as follows:;The modulus of elasticity (MOE) and ultimate compression strength (UCS) of wild-type and genetically-modified quaking aspen wood were measured at the green condition using modified micro-mechanical tests. The results indicated that the transgenic trees with reduced lignin content had severe reductions in modulus of elasticity and ultimate compression strength, while the transgenic trees with increased S/G ratio had only slight decrease in these properties compared to the wild type.;The applicability of a dynamic mechanical analyzer (DMA) in static bending mode was investigated to determine the MOE of 2.5-year-old transgenic aspen submerged in water and in ethylene glycol. The MOE values measured by DMA were compared to dynamic MOE by a nondestructive evaluation and static MOE by micromechanical testing. Results showed that DMA measurements were able to detect significant differences between the genetic groups. However, the measured values were lower than those from the static mechanical test and the nondestructive method test.;A calibration model was constructed using transmittance near-infrared (NIR) spectroscopy to predict the measured mechanical properties of one- and two-year-old transgenic and wild type aspen at the green condition. Ultimate compression strength had a strong correlation (R2=0.91) and MOE had a good correlation (R2=0.78) with the spectra, thus showing that NIR spectroscopy is an effective tool for predicting the mechanical properties of transgenic aspen.;Raman microscopy was used to obtain information on the spatial distribution of major wood polymers (lignin and carbohydrates) in the cell wall of young transgenic aspen with reduced lignin content and/or changed structure. The lignin content of the cell wall and compound middle lamella were reduced by the genetic modification but the amount of carbohydrate did not change significantly. The higher amount of water in the cell wall of transgenic aspen compared to the wild type aspen indicated that there was an increase in the hydrophilicity of the cell wall.;Numerical and three-dimensional finite element models were developed to improve our understanding of the major factors affecting the hygro-mechancial properties of wood. The models revealed that a reduction in lignin content and a corresponding increase in cellulose content increased the mechanical properties of the transgenic aspen. However, when other factors such as decreased crystallinity, increased microfibril angle and increased hydrophilicity were considered, a severe reduction in the Young's modulus of the cell wall was observed. The model successfully predicted the elastic modulus of the microfibrils and cell wall; however, it underestimated the measured macro-mechanical properties, indicating that the mechanical properties of wood not only depend on the chemical, physical, and structural properties but also on the morphological characteristic of the wood. To further advance our understanding of the major factors affecting the hygromechanical properties of wood, fully characterized materials with wide range of chemical and physical properties are required.