## 3 Results
### Analysis of Chemical Structures  
We measured FT-IR to confirm the progress of the reaction. As shown in Figure 3a, in PEDOT:PSS the C-S vibration peaks due to thiophene structure appear at 830 and 972 cm\({}^{-1}\)[20], and a small band due to the alkyl group appears at 2928 cm\({}^{-1}\). However, in the case of the products, the absorption band of the alkyl group becomes larger. Basically, since the thiophene structure of PEDOT is not affected by the reaction, we can monitor the progress of the reaction by comparing the absorbances of the alkyl groups with those of the vibration peak caused by C-S. As indicated in Figure 3, the greater the length of the alkyl group, the larger it becomes. This result clearly indicates that the alkyl group was introduced by the reaction. Figure 4 shows the thermal properties for the pristine PEDOT:PSS and the products. As described in Figure 4a, all six samples show a slight mass loss at around 100 \({}^{\circ}\)C, which is attributed to the loss of water and adsorbed solvent. The remaining weight at 800 \({}^{\circ}\)C decreases as the length of alkyl chain of epoxy alkane increases due to the fact that the alkyl groups easily decompose. As for pristine PEDOT:PSS, similar profile with published reports for a PEDOT:PSS film [21; 22; 23] was observed. In dTGA (Figure 4b), which can observe the change in the decomposition pattern, PEDOT:PSS showed second and third degradation temperature at around 350 and 400 \({}^{\circ}\)C, which are due to the pyrolysis of sulfonic acid of PSS and polyethylene-type main chain of PSS, respectively [21; 22]. Aromatics of PSS and thiophene groups of PEDOT are known to slowly decompose over 435 \({}^{\circ}\)C [24]. On the other hand, when reacted with epoxyalkanes, decomposition of an alkyl group will appear at the third degradation temperature shown in PEDOT:PSS since the alkyl group has the same chemical structure as the polyethylene group. In fact, in products that reacted  
Figure 3: FT-IR spectra of PEDOT:PSS and derivatives. with epoxyalkanes, decomposition peaks around 400 \({}^{\circ}\)C are noticeable, and the peaks become more pronounced as the length of the alkyl group increases. These results strongly indicate that the alkyl groups are successfully introduced into PEDOT: PSS by the reaction with epoxyalkanes. We also studied the chemical structure of the products by XPS. The detailed deconvoluted patterns of S, C, and O atoms of three representative products are summarized in Figure 5. As can be seen in Figure 5a, S\({}_{2\text{p}}\) core-level lines show two major peaks, which can be assigned to sulfur atom in PEDOT and PSS, respectively, and no significant change was observed by the reaction with epoxyalkanes. However, the C\({}_{18}\) spectra are strongly affected by the reaction with epoxyalkanes. For PEDOT:PSS, C\({}_{18}\) signals can be divided into two major contributions with binding energies at about 284.2 and 285.9 eV, which are attributed to C--C/C=C and C--O species, respectively [25,26]. The product (PEDOT:PSS-C4) that has reacted with epoxybutane also displays the same profile; however, the peaks corresponding to C-O turn out to be larger since the reaction with sulfonic acid in PSS and epoxide generate a new hydroxyl group. In fact, the area of C-O relative to that of C--C/C=C peak is 0.48, whereas the relative areas is 0.34 for the pristine. The earlier study [18] on the reaction with PEDOT:PSS with epoxide compounds also demonstrated an increase in the peaks corresponding to C-O. On the other hand, for the reactions with epoxyalkanes with longer alkyl chains such as C-12 and C-16, the area of C-O relative to that of C-C/C=C peak shrank into 0.29 and 0.10, respectively, since the C-C peak increased. Reaction between PEDOT:PSS and epoxyalkanes was also confirmed from O\({}_{18}\) signals.