## 4 Results
### Isothermal crystallization kinetics
The embedded equation with a regression coefficient \(R^{2}\) almost equal to 1 confirmed this effect of isosorbide. Figure 8 shows the effect of the isosorbide content on the melting behavior of PEIT (melting temperature and melting enthalpy). It actually appears from Figure 8 that the increase in isosorbide content led to a decrease in both melting temperature and melting enthalpy. In the isosorbide content range studied (0% to 20%), the melting temperature decreased by more than 40 \({}^{\circ}\)C (from 249 \({}^{\circ}\)C to 207 \({}^{\circ}\)C). In the meantime, the maximum melting enthalpy was reduced by around 50% (from 54.7 to 28.4 J/g). Figure 6: Evolution of t\({}_{1/2}\) with temperature for isothermal crystallization of PEIT with varying isosorbide contents. Figure 7: Evolution of the half crystallization time as a function of the isosorbide molar content of PEIT for isothermal temperature of 170 \({}^{\circ}\)C.  
The decrease in maximum melting enthalpy described the reducing capability of the material to crystallize when adding isosorbide. As seen in Table 1, replacing ethylene glycol by isosorbide induced a decrease in both the ET sequence length in the polymer and the percentage of those sequences. At the same time, the molar fraction of ITI sequences increased but with an almost constant average sequence length (between 1 and 1.4). As mentioned by [PERSON] et al. [15], the minimum sequence length for possible crystallization is about five. This means that ITI sequences are too small to crystallize. Therefore, the crystallinity of PEIT should only come from the ETE sequences. To validate this hypothesis, X-ray diffraction analyses were carried out on all crystallized samples prepared in this study. Pure PET and pure PIT were used as reference samples. Firstly, contrary to what is commonly admitted in the literature [1,16], PIT was found to be subjected to crystallization when precipitated in cold methanol from a polymer solution in chloroform. As shown in Figure 9, this polymer displayed a multiple melting peak with a first endotherm around 194 \({}^{\circ}\)C (10.4 J/g) and a second at 241 \({}^{\circ}\)C (5.5 J/g). The annealing of that polymer at 200 \({}^{\circ}\)C for 7 h induced changes in the melting profile which led to modifications of the crystals. Two melting peaks at 206 \({}^{\circ}\)C (9.4 J/g) and 234 \({}^{\circ}\)C (4.2 J/g) were recorded. As anticipated, these endotherms disappeared on the second heating scan due to the low ability of the polymer to organize for crystallization. This sample was, thus, not considered in this thermal crystallization study. Figure 8: Evolution of the melting temperature and melting enthalpy as a function of the molar content of isosorbide. Melting enthalpy was measured after complete crystallization at 170 \({}^{\circ}\)C.  
Figure 9: DSC thermograms of PIT crystallized from solution and after annealing treatment of 7 h at 200 \({}^{\circ}\)C.  
Figure 10 presents X-ray diffractograms obtained on semi-crystalline PET, PEI5T, PEI9.5T, and PEI19.5T crystallized from melt, as well as that of PIT measured on a polymer crystallized from solution. For PEIT, the peak intensity decreased with increasing isosorbide levels. This confirmed the decrease in the degree of crystallinity measured by the melting enthalpy in DSC. The diffraction spectra observed for PEITs were close to those of PET. As suggested by [PERSON] (2017), this indicates that the crystalline parts of the PEITs are related to PET patterns. However, we cannot rule out the fact that the presence of isosorbide units can disrupt the crystallization of the PET units, with this disturbance reflected by the observed peak offsets as a function of the isosorbide level. Finally, the spectrum recorded on pure crystalline PIT had diffraction peaks very different from those of PET and PEIT. This led to confirming the previous hypothesis according to which, for the isosorbide contents studied here (up to 20%), the PIT patterns present in the PEIT samples were not crystallized.