| Used as a renewable biomass nano-materials with excellent properties,cellulose microfibril(CMF)has extensive application value and market prospect.However,a large amount of mechanical energy would be used to make the fibers disintegrated during the preparation of CMF,and mechanical dissociation also has a great effect on the structures and properties of CMF products.Therefore,the bleached wheat straw fibers were used to prepare CMF,treated by cellulase pretreatment combined with high pressure homogenization in this paper.The influences of three different types of cellulase(complex cellulase,endo-cellulase and exo-cellulase)on the morphology and pore structure of fiber cell wall were respectively studied to clarify the mechanism of the cellulase hydrolysis.On this basis,the action effects of different cellulase pretreatment on subsequent homogeneous treatment and the influences of the treatment condition on the structures and properties of CMF were explored minutely.Then,the aforesaid CMF was used to prepare CMF film and the related properties of CMF film were further studied.The purpose of the above studies was to realize the efficient preparation and application development of CMF product.The influences of three types of cellulase pretreatment on the morphology and pore structures of fiber were respectively studied.The results showed that the outer surface of the fiber became rough,and the outside layer of the fiber cell wall was gradually removed.The transverse grooves and longitudinal cracks,even cutting off,appeared in the direction of the fiber cell wall after cellulase pretreatment.This was because that hydrolysis could occur gradually from the outside layer of the fiber wall to the inside layer,and the fiber wall structure was curled during cellulase pretreatment.Meanwhile,when the cellulase dosage was less,the tiny fiber components contained more porosity were degraded firstly by three types of cellulase hydrolysis respectively,causing that the total entranceand average pore diameter of the fiber decreased.With the increase of the dosage,the inside layer of the fiber wall started to be hydrolyzed gradually by cellulase,which resulted in the formation of new pores and destruction of original pores,and there was an increasing tendency to the above pore parameters as a whole.The above pretreated fibers(at a yield of 75%)by three kinds of cellulase respectively were circulated for 30 cycles though homogenizer to prepare CMF.The results showed that CMF still had the basic chemical structure and crystalline structure of natural cellulose.Compared with raw materials,CMF suspension had a great dispersion stability,and the thermal stability increased to some extent.Compared with the CMF without cellulase pretrentment,the average diameter of CMF with cellulase pretrentment decreased significantly from 64.2nm to about 28.5nm,and the length was roughly between a few microns to ten microns,resulting in a greater ratio(length/diameter).What’s more,the crystallinity of CMF increased by about 18%,and the DP of CMF decreased by 48%,and the cationic demand increased by about 22%,and the WRV increased by about 37%.Compared with other cellulases,the pore structure parameters of the fibers treated by complex cellulase was bigger so that the pretreated fibers were easier to be disintegrated,and the product with lower cost had higher crystallinity and DP.Therefore,cellulase pretreatment could promote significantly the effect of subsequent homogenization treatment,and compound cellulase were better than other cellulases.The influences of homogenization cycles on the structures and properties of CMF products were investigated after the fibers were pretreated by compound cellulase.The results showed that with the increase of homogenization cycle,the dimension of the separated microfibrils gradually became more fine and uniform,along with the decrease of DP and the increase of cationic demand.The stability of the CMF suspension improved gradually,and the crystallinity increased first and then decreased.After 20 cycles,the tendency to decrease the diameter and DP of CMF started to slow down,so was to the tendency to increase the cationic demand.In this case,the properties of CMF products could not be significant improved though the increase of homogenization cycles,but instead this would increase the mechanical energy consumption.Therefor,the selection of 20 cycles could be a very good choice.The influences of cellulase pretreatment and homogenization cycles on the mechanical properties and optical properties of CMF film were further studied after the aforesaid CMF was prepared CMF film.The results showed that with the increase of cellulase dosage or homogenization cycle,the tensile strength and elongation at break of CMF film showed a tendency to increase first and then decrease,while the transmittance of CMF film showed an increasing trend.At the cellulase dosage of 15 FPU/g and 20 cycles,the tensile strength and elongation at break of CMF film(thickness at 54μm)reached to the maximum of30.65 MPa and 1.61%,and its transmittance could reach to 70% obviously. |
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