## 4 Conclusions  
The effect of an annealing process on the crystallinity of PLLA electrospun nanofibers was studied by DSC, FTIR, and XRD techniques while the enhancement in nanomechanical and piezoelectric properties was revealed by AFM-based methods. The annealing process strongly increases the crystallinity of the nanofibers, from dominantly amorphous into a material with 50% of crystallinity. The investigation by ImAFM revealed that, for PLLA nanofibers, elastic response was dominant over the viscous response and the DMT model is then an appropriate model to extract the values of the contact modulus proven by the fact that the Maugis parameter \(\lambda\) remains small. As shown  
Figure 7: Out-of-plane PFM amplitude and phase hysteresis loops of (**a**) as-spun and (**b**) annealed PLLA nanofibers. All measurements were recorded in remnant mode to avoid electrostatic contributions.  
by both PFQNM and ImAFM results, annealed nanofibers possess an improved modulus in the direction normal to fiber axis. Both amorphous and annealed semi-crystalline nanofibers present a ferroelectric and piezoelectric response at the local scale. Because of their higher crystallinity, the annealed nanofibers also show better out-of-plane PFM responses which links to a better piezoelectric property. In conclusion, by enhancing the piezoelectricity and mechanical properties of the nanofibers, the post-annealing process could play an important role for PLLA electrospun fibers in the context of energy harvesting applications, such as self-powered wearable devices.  
The following are available online at [[http://www.mdpi.com/2076-3417/10/2/652/s1](http://www.mdpi.com/2076-3417/10/2/652/s1)]([http://www.mdpi.com/2076-3417/10/2/652/s1](http://www.mdpi.com/2076-3417/10/2/652/s1)), Figure S1: Tapping AFM height images of several pristine PLLA nanofibers. Figure S2: Tapping AFM height images of several annealed PLLA nanofibers. Figure S3: Tapping AFM height images of large area of as-spun nanofibers. Figure S4: peakforce QNM height images corresponding to different applied forces; Figure S5: Typical force-separation curve obtained by peakforce QNM. Figure S6: ImAFM results of PS-LDPE reference sample. Figure S7: Triangular voltage bias signals applied to the nanofibers.  
Conceptualization of the experimental research, P.L. and S.B.; TMAM, PFQNM, ImAFM experiments and discussions, N.T.C. and P.L.; Electrospinning, N.T.C., M.D., and N.T.; FTIR and XRD analysis, S.B., M.D. and N.T.; PFM experiments, A.D.C. and A.F. All authors discussed the results and implications and commented on the manuscript at all stages, R.L., G.-M.R. All authors have read and agreed to the published version of the manuscript.  
This work was done under the framework of BIOHARV (BIObased Energy HARvesting Solutions Using poly (L-Lactide) INTERREG V FWVL Project. Scanning probe research activities in Mons were supported by the FRS-FNRS PDR Project \"Hybrid Organic/Inorganic Nanomaterials for Energy COnversion and STOrage Devices on FLEXible and Stretchable Substrates\" (ECOSTOFLEX). The authors thank the \"Region Hauts-de-France\", the \"Fonds Europeen de Developpement Regional\" (FEDER) and the Chevreul Institute (FR 2638) for funding the X-ray diffusion/diffraction equipment and the electrospinning system. Chevreul Institute is supported by the \"Ministere de l'Enseignement Superieur, de la Recherche de de l'innovation\", the \"Region Nord-Pas de-Calais\" and the \"Fonds Europeen de Developpement des Regions\". Region Hauts-de-France and Fonds Europeen de Developpement Regional (FEDER) are also gratefully acknowledged for funding the MFP-3D microscope under the Program, \"Chemistry and Materials for a Sustainable Growth\". The authors thanks [PERSON] for the WAXS experiments. [PERSON] and [PERSON] are Senior Research Associates of the FRS-FNRS and Research Associate of the FRS-FNRS (Belgium), respectively.  
The authors declare no conflict of interest.