3.1 Membrane porosity properties
A. The Effect of Pulp DP
Table 2 show the high DP of pulp (Exp. Code A) result in higher porosity (low density) and water permeability of membrane. As it is well known that the molecular weight of membrane polymer has a great effect on the membrane performance while molecular weight is depend on Degree of Polymerization which indicate number of repeat unit (monomer) in chain molecules. Polymer with high degree of polymerization will have longer chain and larger molecule compare to a polymer with low degree of polymerization. This long and large chain make more entangle occur between polymer chain but less chain mobility which cause NMMO difficult to break the entangled cellulose hydrogen bond during dissolution process result in less homogenous casting solution. Moreover this long chain and less chain mobility avoid polymer chain to arrange closer each other during coagulation process induce more porous part on membrane. Therefore when membrane composed of mixed cellulose DP the porosity is decrease due to the low cellulose DP can reduce this mechanism, however this tend to induce a bigger mean pore size since the low homogeneity in casting solution.
B. The Effect of Cellulose Concentration in casting solution
To obtain membranes with good performance, the polymer concentration of the casting solution usually is the key factor affecting the permeation rate, rejection and mechanical properties of membranes. However this is restricted by process ability as well as production cost when higher concentration applied. In this work we use 11% as maximum concentration due to at 12% concentration the casting solution is too viscous so as during film casting process is difficult to obtain an even membrane. Data in Table 2.(Exp. B) show that with increasing cellulose concentration from 8% to 11 %, the porosity, pore size and water permeated (flux) decrease whereas increasing concentration from 10 % to 11% give similar porosity but the smaller pore size yield in lower flux. This can be explain by increasing polymer concentration is related to increase of polymer chain amount per unit area and yield in decrease of free space (pore) in membrane.
C. The Effect of NMMO in Coagulation Bath
Based on the data presented in Table 2 (Exp. Code C), it was found that the porosity is decrease from 31% to 19 % while mean pore size increases from 13.94 nm to 21.58 nm when the NMMO % wt content in coagulation bath temperature increases from 0% (water only) to 50% wt NMMO. There are two different mechanisms have to be considered as far as the liquid-liquid demixing of polymer solutions is concerned that is nucleation growth mechanism (NG) and spinodal mixing (SD). This membranes are believed to be the result of the NG mechanism for the slow solidification process and the uniform sponge-like pore structure. When the homogeneous solution is immersed into NMMO/water, liquid-liquid phase separation occurs. Entering the...