Measurement of Specific surface area of porous materials determination Essay

most frequent method for determination of specific surface area of porous materials. This measurement is based on physical adsorption of gas on the surface of sample. BET method was used for determination of specific surface areas of nanofibers or for observation of changes that appear in the structure during post preparation modification of inorganic and organic nanofibers (Imaizumi et al., 2009 Bazargan et al., 2009). BET method was used for characterization of nanofibers with internal porosity or in investigation of the effect of preparation conditions on the subsequent structure of nanofibers (Zhang et al., 2009 Rye et al., 2003).
Degree of crystallinity
To measure the degree of crystallinity, a typical method is X-ray diffraction (Park et al., 2010 Garvey et al., 2005 He et al., 2008). The origin of the cellulose obviously affects the degree of crystallinity and crystallite size. The degree of crystallinity can be around 70% of MFC obtained from wheat and straw soy (Alemdar and Sain, 2009) while a much lower value, 30%, was reported for the MFC obtained from beet pulp (Heux et al., 1999). Iwamoto et al. (2007) reported the degree of crystallinity decrease with an increase in the number of passes because hornification of cellulose nanofiber under high shear forces, regardless of the origin of the cellulose. Moreover, the grinding process causes more damage to the crystalline structure of cellulose than homogenizer process and blender treatment (Agoda-Tandjawa et al., 2010 Uetani and Yano, 2011).
Degree of polymerization
For measuring the degree of polymerization of MFC, researchers normally calculated the average DP from viscosity by using the Mark-Houwink equation (Iwamoto et al., 2007 Zimmermann et al., 2010). The DP of sulfite pulp is around 1200-1400, while the DP of MFC produced from sulfite pulp is 825 (Zimmermann et al., 2010), which normally is about 30-50% lower (Henriksson et al., 2007). Degree of polymerization is highly correlated with the aspect ratio of the nanofibers. High cellulose DP is desirable for MFC since this is correlated with increased tensile strength of the nanofiber (Henriksson et al., 2007). Elastic modulus of 145 GPa for a single TEMPO-oxidized MFC nanofiber was reported by Iwamoto (2008), while the elastic modulus of a perfect crystal of native cellulose was found to be in the range of 130-250 GPa (Zimmermann et al., 2004).
Suspension properties
Rotating rheometer is used for collecting rheological data. The rheological property of MFC suspension is quite different from normal pulp suspension. It shows pseudoplastic and shear-thinning behavior (Herrick et al., 1993 Pkko et al., 2007). Herrick et al. (1983) reported the viscosity of a 2% MFC suspension are 17400, 264, 136 MPa at 10, 1000, 5000 s-1, respectively.
Pkk et al. (2007 ) reported the MFC suspension displayed a gel-like behavior at a concentration of 0.125-5.9 wt%. The authors also observed that the storage modulus value of MFC is very high, 10000 Pa for a 3 wt% MFC suspension. Moreover, the storage modulus is almost 10 times higher than the loss moduli regardless of the suspension concentration.