3. Results and discussion
The changes of contact angle with contact time are presented in Fig. 1 and the static water contact angle values at 10 s contact time, averaged from 10 measurement values, are contained in the legend. As can be seen, the curves shift to higher values with increasing NR content and the values shown in the legend also present the trend that the contact angles increase with NR increase. This indicates that NR as a hydrophobic material compared to PLA dispersing on the PLA surface has led to an increase in hydrophobility of the PLA surface. In addition, the contact angles decrease continuously with contact time showing the behavior of contact angle relaxation. From the relaxation curves, three regimes can be clearly observed. At the first stage (0e2 s), contact angles decrease very fast. This is mainly due to the conformational change of PLA which could lead to a rapid decrease of contact angle. Compared to neat PLA whose contact angle reaches equilibrium in the second stage (2e10 s), a slow contact angle relaxation of PLA/NR blends occurs continuously
which is attributable to the viscoelastic dissipation of NR [26]. With water evaporation, during the third stage (10e60 s), contact angle relaxations are slower and have the same tendency for each sample