# Multifunctional Modifying Systems Based on Ionic Liquids for Epoxy Resin Systems and Composites
## 3 Results and Discussion
### Carbon Fiber Reinforced Composites' Properties  
DMA  
Dynamic Mechanical Analysis was used to investigate the properties of carbon fiber-reinforced epoxy composites produced using ionic liquid systems. Table 5 summarizes the results of the determined glass transition temperatures for the composites, using data on storage modulus (EURUR), tan delta values (tan \(\delta\)), and loss modulus (EUR). The value of the T\({}_{\mathrm{g}}\), which actually represents a certain range of temperatures, was presented as a specific numerical value in order to evaluate the performance of the obtained materials. The results obtained in the DMA analysis were compared with the T\({}_{\mathrm{g}}\) result determined during the DSC analysis of the resin systems themselves without carbon fiber reinforcement. The results show how much influence the reinforcement has on the properties of the final material. Moreover, for all the systems modified with ionic liquids, a significant increase in the glass transition temperature value was recorded compared to the reference. Systems A, B, and C show an increase in T\({}_{\mathrm{g}}\) values of nearly 50%, while for system E the increase was 75%. To assess the effect of the modification on the polymer network inside the material, the crosslinking density was determined, the value of which for each system translates into an increase in the glass transition temperature, which is the expected result. Crosslinking density was determined by a mathematical equation presented in several other works [49, 50]. A denser network inside the material reduces the potential of the polymer chain to stress and rotate by making the final material stiffer and stronger. An explanation for this phenomenon may be that ionic liquids supplied to the system build into the polymer structure, and the presence of long chains stabilizes bond rotation. Interestingly, in the case of resin systems without carbon fiber reinforcement, the trend in the change in glass transition temperature values is opposite to that observed for fabricated composite structures. This primarily demonstrates the influence of factors such as reinforcement, fibers, sample size, or, most importantly, the technique of determining the parameter, on the value of glass transition temperatures.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Sample** & **T\({}_{5\%}\)** & **T\({}_{50\%}\)** & **dTG\({}_{\max}\)** & **T\({}_{\mathrm{dTGmax}}\)** \\  & [\({}^{\circ}\)C] & [\({}^{\circ}\)C] & [\%/\({}^{\circ}\)C] & [\({}^{\circ}\)C] \\ \hline R & 401 & 443 & 3.1 & 442 \\ A & 322 & 403 & 1.7 & 401 \\ B & 285 & 396 & 1.4 & 395 \\ C & 324 & 402 & 1.9 & 401 \\ D & 325 & 418 & 2.2 & 418 \\ E & 329 & 436 & 2.0 & 438 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Thermogravimetric analysis results of the reference and samples modified with ionic liquids.  
Complementing the T\({}_{\text{g}}\) analysis is the effect of temperature on the E' value and the intensity of the shift of this value toward lower values depending on the modification system. The results presented in Table 6 indicate that the sample modified with the D system retains the most similar storage modulus parameters at 28 \({}^{\circ}\)C compared to the reference. In the case of the other systems, a shift towards more elastic rather than rigid materials is observed. On the other hand, the situation changes dramatically if we consider the rate at which the value of the parameter E' changes with increasing temperature. In this comparison, the most stable in relation to the reference is the E system, while the fastest change is observed in the D sample, which, as noted earlier, at a temperature of 28 \({}^{\circ}\)C retained the value of E' closest to the reference, while at temperature 75 \({}^{\circ}\)C this value decreased by almost half in relation to the reference. We suppose that the modifying system D influences the elasticity of the composite material the most among all those tested, however, systems A, C, and E also show noticeable activity in this direction.