## 2 Results and Discussion
### Effect of Acrylate and Ammonium Oligomer Molar Mass
The films generated from both oligomers **(1)** revealed to have an activity in comparison to the control surface (polystyrene). For the three pathogenic species tested (PA, SE, SA), there were fewer cells on the acrylate-based surfaces than on the control (Figure 2). It seems that the chain length had no influence in the case of thick films, because in the case of bacteria SE and SA the shorter chain was more effective, but it was the contrary for PA. In the suspension, the smaller oligomers were always effective in reducing the number of bacteria in comparison to the initial concentration, while the oligomers with higher molecular weight were not. Figure 1: Structures of natural rubber derived oligomers and molecules used in this work. The oligomers **(5)** bearing one ammonium group (Figure 1) were also synthesized with the two different molecular weights and, as they have only one acrylate bond, the relative films were formed by linear polymers. As already observed, the presence of the ammonium group in the structure increased in every case the activity of the polymer surface in comparison to the acrylate surfaces alone and to the control; the same effect was noticed in the suspension (Figure 2). The longer ammonium oligomers produced also the most active surfaces in comparison to the shorter ones and this effect was observed as well in the suspension medium. A similar effect of a higher microbiological activity from longer polymer chains was found by [PERSON] _et al._[19]. They covalently grafted _N_-hexylated, _N_-methylated-polyethyleneimine (PEI) chains to amino functionalized glass slides and incubated them with airborne _Staphylococcus aureus_. It was found that chains of MW 750 and 25 kD were almost completely lethal to the microbes, while the 2 and 0.8 kD ones had a negligible activity, similar to the grafted analogous monomer diaminoethane. The authors tried to estimate the PEI chains lengths and compared them to the dimensions of the bacteria; they found that the 750 and 25 kD chains could be able to completely or partially penetrate into the cell wall and cell membrane and disrupt it, provoking bacterial death. On the other hand, the 2 and 0.8 kD chains were not long enough to considerably damage the cell structure. It is possible that an analogous mechanism takes place in the case of the  
Figure 2: Effect of acrylate **(1)** and ammonium oligomer **(5)** chain lengths. _Pseudomonas aeruginosa_ (**left**), _Staphylococcus aureus_ (**middle**), _Staphylococcus epidermidis_ (**right**). (**a**), (**c**), (**e**) = bacteria suspended in the supernatant medium; **(b**), (**d**), (**f**) = bacteria attached to the control (polystyrene) or polymer surface. *, difference in values significant at \(p\leq 0.05\), ** difference in values significant at \(p\leq 0.01\). ammonium groups of the oligoisoprenes, which could explain their antimicrobial effect. However they have a linear structure while the PEI used by [PERSON] _et al._[19] had a branched structure, with QA all along the polymer backbone. The results pointed out that in future work the study of the influence of the oligoisoprene chain length should be pursued, whatever biocide group is attached, in order to assess the optimal molecular weight to use to enhance the antibacterial activity. It was already demonstrated [16] that formulations can be composed by a mixture of acrylate oligomers **(1)** and ammonium oligomers in different percentages, and that an active surface can be obtained using a minimum amount of ammonium oligomers. The new indication is that, in future formulations, the MW of the acrylate oligomers has no influence but the global activity can be improved playing with the parameter of the chain length of the oligomer containing the biocide.