## 3 Results and Discussion
### PLLA Crystal Phase  
The improvement of crystallinity in annealed samples is also proved by FTIR spectroscopy. Figure 2a,b show the FTIR spectroscopy of electrospun PLLA nanofibers without and with thermal treatment in different spectral regions. According to many spectroscopic studies on the polymorphism of PLLA, it has been proven that the band at 956 cm\({}^{-1}\) can be assigned to the amorphous fraction, whereas the band at 921 cm\({}^{-1}\) is ascribed to the PLLA crystal phases, \(\alpha\) and \(\alpha^{\prime}\)[37, 38]. The appearance of the absorption band at 921 cm\({}^{-1}\) as well as the decrease in absorbance at 956 cm\({}^{-1}\) in all annealed samples with respect to those of as-spun samples are supporting evidence of the improvement of the crystalline content in the heat-treated samples. Investigation of FTIR spectroscopy also revealed information regarding crystalline phase present in the electrospun fibers. According to [PERSON] et al. [39], the FTIR spectrum of the \(\alpha\) crystalline phase of PLA presents multiple band-splitting in the spectral region of 750-650 cm\({}^{-1}\), whereas the spectral patterns of the \(\alpha^{\prime}\) crystalline phase in the same region are much less complex. In this regard, the absence of band splitting in the 750-650 cm\({}^{-1}\) region for all annealed nanofibers indicates that the crystalline forms generated here are dominantly \(\alpha^{\prime}\). Additional evidence includes the presence of a broad band around 1750 cm\({}^{-1}\), corresponding to the C=O stretching vibration region, in all FTIR spectra of annealed samples. Indeed, the \(\alpha^{\prime}\) crystalline form is described as slightly different than the \(\alpha\) form with a distorted 10\({}_{3}\) helical packing chain, resulting in weaker interactions and coupling between carbonyl groups [40]. Consequently, the carbonyl band of \(\alpha^{\prime}\) displays a single broad band, as seen in Figure 2a.  
Figure 1: Differential scanning calorimetry (DSC) curves of electrospun poly(L-lactic acid) (PLLA) nanofibers: as-spun (1) annealed at 90 \({}^{\circ}\)C (2), 100 \({}^{\circ}\)C (3), and 120 \({}^{\circ}\)C (4) Exothermic upward.  
In addition to FTIR spectroscopy, WAXS experiments were performed to study the crystallinity as well as the structure of electrospun PLLA fibers. Figure 2c shows the integrated intensity profile of electrospun fibers, as-prepared and heat-treated samples. Annealed samples show two sharp diffraction peaks at 20 = 16.6\({}^{\circ}\) and 19.0\({}^{\circ}\) corresponding to the (200/110) and (203) planes respectively and attributed to the PLLA \(\alpha^{\prime}\)-phase [41]. On the contrary, the broad signal of as-spun PLLA fiber is attributed to the intrinsic amorphous nature of the material. In short, all those results clearly show that the crystallinity of the nanofibers can be significantly improved by annealing above the polymer glass transition. Due to the fact that annealed nanofibers present the same crystalline content and only one crystalline phase (\(\alpha^{\prime}\)) occurred in all heat-treated samples, the sample annealed at 100 \({}^{\circ}\)C is considered to be representative of all annealed samples and will be investigated as the annealed nanofibers in the following sections.