Results and discussion
Table 1 showed the properties of PLLA synthesized in the TSE and the TSMR. It was found that the molecular weight and OP of PLLA polymers in both reactors increased firstly and then decreased with the increasing polymerization time, and the properties of PLLA synthesized in TSMR were better than those in TSE within the corresponding polymerization time [9]. This may be due to the higher mixing and heat removal efficiency of the static mixers in TSMR. Compared with L- lactide, the OP of PLLA in the TSMR decreased slightly, indicating that little racemization occurred. This result was similar to the reported experiments [12]. By comparison, the TSMR is more suitable than the TSE for preparation of PLLA with better properties. In order to investigate the performance of these two reactors, the kinetic parameters were chosen to reflect the energy consumption of polymerization reactor. The apparent activation energy (△Ea) and the pre-exponential factor (A) for ROP of L-lactide in two reactors were investigated. Here, a higher value of A reflected a higher collision frequency among molecules during the polymerization. The results showed that the polymerization of L-lactide is first-order in monomer and catalyst concentration for both two cases. Moreover, the (△Ea)s for ROP of L-lactide in both the TSMR(58.0 kJ mol−1) and the TSE(58.4 kJ mol−1) were smaller than that in the conventional batch apparatus (such as Haake Rheocord mixer(91.1 kJ mol−1) [13]), in- dicating that less energy was needed for synthesizing PLLA using the TSMR and the TSE. Consequently, these two reactors were both energy-saving polymerization reactors. It was also found that the A
value in the case of TSMR(4.58E+9) was greater than that in the case of TSE(4.47E+9), which was attributed to the excellent mixing perfor- mance of the TSMR consisting of four identical corrugated plate-type static mixers. The mixing performance of static mixer is usually evaluated by striation thickness [14], which is closely related to the number(N) of striations. The larger the number of striations is, the better the homogeneity of the materials in the mixer will be. According to equation (1) [14], the number of striations of one corrugated plate- type static mixer was up to 1.6E+20, indicating an excellent mixing performance of TSMR. Highly uniform mixing of reactant mixture led to obtain PLLA with a smaller PDI. Nmm = (2 ) n−1 (1) where m is the number of corrugated plates and n is the number of mixer elements. Besides the striation thickness, the mixing performance of static mixer is also related to its inner structure. The corrugated plates-type static mixers have many V-shaped channels, which can provide strong transversal flow(see Fig. 2). It forces the materials to flow along the intersecting channels from the center to the wall of the mixer or vice versa. Consequently, this flow reduces the inhomogeneity of tempera- ture of materials in static mixers, which will avoid the local overheating and improve the homogeneity of materials even for high-viscosity polymers in the laminar flow region. In addition, Fig. 3 showed the shear rates of the reactants in the TSE and the TSMR during polymerization, the calculation methods of which were described in literatures [15–17]. Here, γ̇sm was the shear rate of reactants during polymerization in the TSMR. For the poly- merization in TSE, γ̇ ε, γ̇ α andγ̇ h represented the shear rates of reactants in the nip zone, in the zone from the kneading block tip to the barrel wall and in the zone from block root to the barrel wall, respectively, whileγ̇se was the shear rate of reactants in the normal screw element. It could be found that the reactants in the TSE experienced higher shear rates than in the TSMR. Particularly, the γ̇ ε was over 14 times greater than the γ̇sm. Since high shear rates could cause the local overheating and lead to the degradation of resultant polymers, the molecular weight of polymers synthesized in the TSE was small and its distribution was broadened. In contrast, the reactants under lower shear rate in the TSMR had a good uniformity, which resulted in the polymer products with better properties.