## 3 Results and Discussion  
### Parameters Optimization  
The results of the Taguchi experiments according to L9 orthogonal array were analyzed and the response values of mean particle size distribution obtained from particle size analyzer are shown in Table 2. All the responses with three repetitions from each experiment were evaluated individually. The results showed that each experiment exhibited a various range of particle sizes. The finest size of POA particles was obtained in experiment number nine with 0.09 (\(\pm\) 0.07) mm of mean particle size. Meanwhile, experiment number one obtained a greater size with 0.48 (\(\pm\) 0.06) mm of mean particle size. The TEM images of the POA nanoparticles obtained in nine experiments (Table 2) are shown from Figures 3-5. The particle size distribution histogram Figure 3A-C and TEM image of POA nanoparticles from experiment one to three (E1-E3) are shown in Figure 3. The TEM images only reveal the lateral dimension of the nanoparticle but no information on the nanoparticle height. Though, with the dimension of the width particles from the TEM images, the lateral particle size distribution can be obtained [22]. From the histogram, the size distribution is narrow, and the major peak is centered at 480, 410, and 230 nm. The TEM image of experiment one (E1) illustrated the identical of the POA shape which is spherical. However, The TEM image significantly reveals that the smaller the particle size distribution, POA tends to agglomerate, and identification of particle shape and size becomes more complicated. Figure 3: TEM image (E1–E3) and POA nanoparticle distribution (**A–C**) from experiment one, two, and three of the Taguchi optimization processes. Figure 4: TEM image (E4–E6) and POA nanoparticle distribution (**D**–**F**) from the experiment four, five, and six of the Taguchi optimization processes. Meanwhile, a distribution curve for particle size histograms in Figure 4D,E and TEM images from experiment four to six (E4-E6) are shown in Figure 4. It can be observed that the TEM images illustrated the irregular shape and size of POA nanoparticles with a wide range of distribution. The morphology showed the irregular shape of POA nanoparticles probably due to rigorous impact and forces during the ball milling process. For the particle size distribution according to the experiments, a major peak is centered at 280, 190, and 150 nm, respectively. The last three experiments of L9 orthogonal array with  
Figure 5: TEM image (E7–E9) and POA nanoparticle distribution (**H–I**) from experiments seven, eight, and nine of the Taguchi optimization processes. the TEM image and distribution of particle size histograms are shown in Figure 5. The TEM images from experiment seven to experiment nine (E7-E9) showed that the POA nanoparticles still retain the spherical morphology although through the different ball milling process parameters. This behaviour could increase the interest of POA utilization in a wide range of applications. In the meantime, the lower peak of distribution in experiment 7 Figure 5G may be due to the agglomeration of POA nanoparticles compared to experiment eight Figure 5H and nine Figure 5I. Furthermore, the smaller POA nanoparticles tend to adhere to each other due to growing of inter-particle bonding. It is significant to mention that there is a great possibility the aggregation of the nanoparticles occurred and leads to the declining volume of POA nanoparticles. Nevertheless, the distribution particle analysis still obtained the major peak at 110, 130, and 90 nm of each experiment, respectively, as presented in Figure 5.  
Although experiment number nine resulted in a higher distribution of POA nanoparticles, to determine the optimum parameters and most influential parameter, the signal to noise (S/N) needs to be calculated. In this experiment, the signal to noise ratio was used to identify the control factors that reduce the variability of the results by minimizing the effects of uncontrollable factors.