Study On The Residual Stress And Non-normal Deformation Processes Of Gelatinous Layer In Tension Wood

In order to explore the paradoxical properties of tension wood,adsorption isotherms of both normal wood and tension wood have been measured on six tropical species,and also the anisotropic strains (longitudinal and tangential) and weight loss were measured to clarify the effect of the solvent on the mechanical behaviour in tension wood of chestnut,simarouba and poplar.The specific surface area and pore size distribution of tension wood have been described by nitrogen adsorption method.This method provides new insights into the understanding of gelatinous layer(G-layer) since it confirms that major differences between species can be observed in tension wood fibre secondary wall.Form experimental and theoretical aspects,the effect of the release of internal stresses in G/S₂ layer on the swelling properties of tension wood have been studied.These results are discussed and explained by the role of stress state at the cell wall level.The main conclusion from both pore structure and internal stresses in tension wood were summarized as follows:(1).All these species are able to produce high level of mechanical tensile stress(ranging from -2408 to -1485μstrain) to control the orientation of the growth axis,but their anatomy and nanostructure can differ widely,from thick G-layer to no microcosmic difference compare to normal wood can be observed.Even within the tension wood with G-layer,remarkable difference exist in the appearance,thickness and distribution of G-layer.As already shown earlier,general relations between tensile stress level in tension wood and macroscopic anatomical variations are not visible while observation at ultrastructural level allows to see some common feature in cellulose organisation.The characterisation of microstructural features of different woods and their correlation with mechanical parameters are a first step towards an assessment of the possible mechanisms.Then we can hypothesize the mechanisms to differ from species to species and in some cases not to be directly linked to mesoporosity.An alternative hypothesis would be a common mechanism,but apparent mesoporosity would be a residual state of the maturation process later hidden in some species.(2).Based on the nitrogen adsorption isotherm at 77K,the specific surface area and pore size distribution among tension wood fibers have been measured by BET(Brunauer,Emmett and Teller) and BJH(Berrett,Joyner and Halenda) methods,respectively.It appears that mesoporosity can always be detected in never-dried wood samples.The amount of pores and the structures of the pore networks can be very different between species and are also different between tension wood and normal wood within one species.Tension wood that does not develop G-layer does not contain a mesoporosity significantly higher than the normal wood.Tension wood developing G-layer can be separated in two classes:thin G-layer presenting little mesoporosity(sometimes lower than that of corresponding normal wood) and thick G-layer showing a high mesoporosity.In wood with thick G-layer,the high amount of mesopores can be easily attributed to G-layer itself and provides indications about the nature of pores.Not depending on the amount of pores,the pore sizes distribution in tension wood are always centred around 6-12 nm.(3).Then specimens of normal wood and tension wood after ethanol substitution keep the dimension stably in both longitudinal and tangential direction,and ethanol was used as the preferred solvent during CO₂ supercritical drying.Compared with CO₂ supercritical drying,freeze drying and normal drying bring significant shrinkage deformation to specimens in both longitudinal and tangential directions.And supercritical drying can release the porous texture intact by avoiding the pore collapse phenomenon induced by interface tension during freeze drying and normal drying.CO₂ supercritical drying has been suggested as the best pre-treatment drying method to obtain the pore characteristic and the pore size distribution of the wet materials in the solid state.(4).Based on longitudinal deformation induced by the ethanol substitution and microfibril angle (MFA),chestnut tension wood can be divided into two parts which coincides the classification of tension wood and normal(opposite) wood,tension wood formation on the upper side and normal wood on the lower side.And three types of wood for simarouba:normal wood,opposite wood and tension wood(without clear G-layer),although the MFA was not that expected in tension wood,simarouba formed tension wood on the lower side of the branch in contrary to chestnut.The longitudinal shrinkage of tension wood of simarouba,even if not high as in chestnut tension wood,is three times the one of normal wood.During ethanol substitution,normal wood/opposite wood and tension wood expand in tangential direction.The largest expansion value appeared around 70%ethanol solution.At higher concentration,the expansion was partly recovered.With the increased concentration of ethanol solution both normal wood and opposite wood expand,at different rates in the longitudinal direction.In contrast, tension wood tends to contract,regardless of the presence or absence of G-fibres.The exchange between solvent and water molecular induced by the release of internal tensile stress in tension wood is the main reason to produce longitudinal shrinkage in tension wood.Whereas in oven-drying,the water evaporation,the release of internal tensile stress and the collapse of gelatinous structure in G-layer are the mainly factors to induce the highly longitudinal shrinkage in tension wood.(5).Both normal wood and tension wood undergo a notable weight reduction after serial organic substitution by an amount depending on the polarity of the solvent.Mean values of weight loss around 14%for strong polar solvents and 22%for less polar solvents,which fits quite well the theoretical values.During serial increasing concentration solvent substitution,at 70%the system was in most disorder state,then the volume and tangential dimension increased to the maximum.When the concentration reached to 100%,the system tended to stable and both volume and tangential dimension were back to the original state(less than±0.03%).Normal wood swelled during solvent substitution, whereas tension wood shrank in longitudinal direction.And the shrinkage extent in tension wood was far bigger than the extent of swelling in normal wood.The amount of shrinkage in tension depended on the polarity of the solvent.The less polar the solvent,the greater the strain and weight loss observed.(6).During the organic solvent substitution,whatever the exchange procedure,when the final concentration of solution was the same,then the specimens can have the similar weight loss and dimension deformation.But more than three times substitution were needed when pure solution substitution was used.Then the weight loss and dimension deformation of specimen tended to be stable. Solvent exchange does not affect the cell wall structure,as confirmed by nitrogen adsorption measurement.The internal stress release of tension wood in organic substitution can be divided into two parts:the remarkable decrease of stress at the beginning stage and then slowly release in a long time. The greater partition coefficient,the greater molecular mobility in amorphous region of cell wall and the greater stress release at the beginning stage of organic substitution.Serial solvent,direct immersion in pure ethanol or cycling sorption,is only the different way to increase the molecular mobility within the amorphous substance of the cell wall,and then induced the release of internal stresses in the cell wall. That is,during solvent substitution the macro-strains of tension wood in longitudinal direction are controlled by the release of internal stress.