Fig. 5a shows that neat PLA and PLA/NR blends undergo a sub- stantial decrease in average molecular weight (Mn) and the Mn almost reaches a constant value after 25 days. Compared to the Mn of neat PLA, the Mn of PLA/NR blends does not show observable difference. Two phases can be observed in the mass loss chart (Fig. 5b), which is much different from the changes of Mn. At the first 20 days hydrolysis, the mass loss almost does not happen. In the next 15 days, the mass decreases rapidly and lost about 20% at 35 days hydrolysis. Similarly, no visible difference of mass loss can be found between neat PLA and PLA/NR blends even though the moisture absorption are different from each other. The results imply that the degradation rates of the samples studied are inde- pendent of moisture absorption. Schmitt et al. suggest that only the
bound water can participate in the reaction of PLA hydrolysis [31]. In other words, the reactive water is constant and independent of total water content of the polymer. The analogous conclusion can also be found in the study of Wiggins et al. [29]. In our study, the extra water of PLA/NR blends than neat PLA is contained in the dispersed NR particles and it does not affect the content of the reactive water. So, the hydrolysis rate do not affect by NR. This further confirms that the introduction of NR has no effect on the hydrolysis of PLA, which is very useful for keeping the biodegrad- ability of PLA when toughened by NR and hence for applying the PLA/NR materials.
It is notable that when Mn of specimens lost to 20% of initial Mn at 20 days hydrolysis, the mass loss does still not occur, after 20