## 4 Results
### Isothermal crystallization kinetics  
As detailed previously, the isothermal crystallization of PEIT with varying isosorbide contents was studied by DSC with two different protocols. For low isosorbide contents (\(\leq\)15% molar of ethylene glycol replaced by isosorbide), the crystallization kinetics could be directly observed on the isothermal thermogram (see Figure 3). For isosorbide content higher than 15%, the crystallization process was too long to be precisely observed in the same way. Therefore, the crystallization kinetics was followed by measuring the melting enthalpy of the crystallized material (see Figure 4). Figure 3: Differential scanning calorimetry (DSC) signal recorded during the isothermal crystallization of PEI\({}_{4.9}\)T at different temperatures (from 130 \({}^{\circ}\)C to 200 \({}^{\circ}\)C). The color code for temperatures is embedded in the figure. Figure 3 presents DSC thermograms measured on amorphous PEI\({}_{4.9}\)T during isothermal treatments at different temperatures. These isothermal treatments were obtained after the complete melting of the material. It clearly appears on this figure that all curves presented an exothermic event taking place over a varying period of time depending on the isothermal temperature. This event was attributed to the crystallization of the PEI\({}_{4.9}\)T. It is to be noted that, for isothermal temperatures from 150 \({}^{\circ}\)C to 190 \({}^{\circ}\)C, the heat flow value recorded at the beginning of the thermogram was lower than that recorded at the end. This particular behavior could be attributed to the beginning of the crystallization process prior to the isothermal start. Actually, for those temperatures, the crystallization started during the cooling stage from the melt. The amount of material that crystallized before reaching the isothermal temperature could not be evaluated, but it was assumed to be small with respect to the whole thermograms. It is, thus, neglected in the rest of this paper. From Figure 3, it was possible to determine the crystallization progress at any time for the studied temperatures. Figure 4 presents DSC thermograms measured on PEI\({}_{17.1}\)T after different isothermal treatments at 170 \({}^{\circ}\)C. The duration of the isothermal period varies from 60 minutes to 600 minutes. It can be firstly observed on Figure 4 that a heat capacity (Cp) jump attributed to the glass transition of the material appeared at around 95 \({}^{\circ}\)C. The amplitude of this Cp jump decreased as isothermal period increased. A second endothermic event appeared between 180 \({}^{\circ}\)C and 210 \({}^{\circ}\)C, with the size of this event increasing as the isothermal period increased. It is to be noted that this event was itself composed of two sub-events. This endotherm was attributed to the melting of the crystalline part of the material obtained during the isothermal treatment. The opposite and concomitant evolution of both the amplitude of the Cp jump and the enthalpy of the peak confirmed this hypothesis. The presence of several peaks in the melting endotherm revealed that the melting of crystalline PEIT took place in different steps. Such a behavior was already highlighted on many polymers like PET [14], and it was attributed to the presence of several crystal types formed by primary or secondary crystallization. In the present study, the global melting endotherm was considered to evaluate the crystallinity of studied PEIT. From Figures 3 and 4, the evolution of the weight fraction crystallinity (X\({}_{\rm t}\)) could be determined according to Equations (2) and (3), respectively. The X\({}_{\rm t}\) evolution as a function of the isothermal crystallization time and temperature for PEI\({}_{4.9}\)T and PEI\({}_{17.1}\)T is presented in Figure 5a,b. In Figure 5c,d, the plot of Log(\(-\)ln(\(1-\)X\({}_{\rm t}\))) as a function of Log(t) is also given for the same experimental data.