# Optimizing CNT Loading in Antimicrobial Composites for Urinary Tract Application
## 3 Results and Discussion  
### CNTs Characterization  
The characterization of ball-milled CNT samples is of particular importance since it is well known that particle morphology influences the porosity and the microstructure of the final composites [27]. The textural properties of CNT samples were evaluated by the N\({}_{2}\) adsorption-desorption isotherms for original and modified CNT samples (Figure 1). N\({}_{2}\) adsorption isotherms of original and ball-milled CNT samples were type IV isotherms (characteristic of carbon materials with some mesoporous), according to IUPAC [37]. Despite the similarity between these isotherms, it is possible to notice a difference in the amount of N\({}_{2}\) adsorbed at high relative pressure, with lower amounts of N\({}_{2}\) adsorption for higher exposure periods to BM technique. These results are in accordance with previous studies, where the BM treatment by itself (4 h at a constant vibration frequency of 15 vibrations s\({}^{-1}\)) induced a decrease of about 40% of N\({}_{2}\) adsorption at p/p\({}_{0}\) = 0.95 [38]. The results of N\({}_{2}\) adsorption-desorption isotherms are reflected in the specific surface area (S\({}_{\text{BET}}\)) and total pore volume (V\({}_{\text{p}}\)) of original and modified CNTs, represented in Table 1.  
Figure 1: N\({}_{2}\) adsorption–desorption isotherms at \(-\)196 \({}^{\circ}\)C for pristine and modified carbon nanotubes (CNTs). From the results, it is possible to observe that BM treatment of CNTs resulted in an increase in the specific surface area compared to CNT-O. This phenomenon is already well-characterized and can be explained by the increase in the content of broken nanotubes and sidewall openings in the milled samples [39], which is in concordance with high-resolution transmission electron microscopy results reported elsewhere [40]. Taking into consideration that \(\mathrm{S}_{\mathrm{BET}}\) of CNT samples tends to be similar for milling times up to 1 h and to decrease for BM periods greater than 4 h (probably due to CNT compaction and twisting that limits the access of N\({}_{2}\) molecules to their inner part) [26], this work was limited to the study of CNT samples milled during 1, 2, 3 and 4 h. Although the \(\mathrm{S}_{\mathrm{BET}}\) of CNT-BM 4 h was higher than that of CNT-BM 1 h (227 and 215 m\({}^{2}\) g\({}^{-1}\), respectively), no relevant changes were observed with increasing exposure periods of CNTs to BM treatment (with differences within the experimental error). This is in accordance with N\({}_{2}\) adsorption isotherms at low relative pressure, where no major differences between samples were observed. In addition to the changes in \(\mathrm{S}_{\mathrm{BET}}\), a decrease in the \(\mathrm{V}_{\mathrm{p}}\) of ball-milled CNT samples was observed in comparison with CNT-O. This result can be explained by the differences in the degree of entanglement of CNTs, as the major part of the pore volume results from the space in CNT bundles [38]. During BM treatment, CNTs tend to compact and form larger agglomerates due to the collision of CNTs with the balls [26; 41], thus decreasing the space between the tubes and, therefore, \(\mathrm{V}_{\mathrm{p}}\). In fact, with increasing BM periods, a decrease was observed in the \(\mathrm{V}_{\mathrm{p}}\), and the lowest value was obtained for 4 h ball-milled samples. According to previous studies, after this optimum value, the sample tends to present an opposite behaviour [26]. The pore size distribution (PSD), which is a measure of the distribution of pore volume concerning pore size (Figure 2), was also determined. The results obtained for ball-milled CNT samples showed significant changes in PSD compared to the original CNT sample. For the ball-milled samples, the number of pores with a half pore width higher than 80 A is notably lower than the original sample. At the same time, new contributions were observed in the PSD of ball-milled CNTs for pore radius lower than 80 A.