# Toughening Behavior of Carbon/Epoxy Laminates Interleaved by PSF/PVDF Composite Nanofibers
## 1 Introduction  
Carbon-fiber-reinforced polymer composites (CFRP) are applied widely in various industries, such as electronics, construction, and aeronautics. Among different resins, epoxy is the most frequently used because of its good mechanical properties, suitable fatigue resistance, and low shrinkage while curing. On the other side, its highly crosslinked structure leads to brittleness and thus to poor resistance to crack propagation [1, 2]. Among the several methods that have been presented during the years to increase the fracture toughness of carbon/epoxy laminates [3, 4, 5, 6], interleaving polymers [7, 8, 9], in the form of particles, films, or nanofibrous mats [10, 11, 12, 13, 14, 15, 16], has proved to be one of the most effective. In particular, nanofibrous mats have been found to be a suitable choice because of their high porosity (which lead to rapid penetration of epoxy) and the strengthening effects they are able to provide.  
Literature reviews on nanofibers reinforcing composites are wide, and many polymers, such as polyvinylidene fluoride (PVDF) [17, 18, 19, 20], polyvinyl butyl (PVB) [21, 22, 23], polysulfone (PSF) [10, 11, 24], Nylon [25, 26, 27, 28, 29, 30, 31, 32], phenoxy [33, 34], and carbon [35, 36, 37] nanofibers, have been used to enhance composites' mechanical properties. [PERSON] et al. [20] Showed that PVDF nanofibers can increase Mode-I fracture toughness by about 43%, while another study [19] in this field had completely reverse outcomes. The considerations showed that the main reason was a non-suitable curing process and the highthickness of the nano-mat in the second study. As seen, some limited study was also conducted regarding the effect of PSF nanofibers on fracture behavior of nanomodified laminates. For instance, [PERSON] et al. [10] used PSF nanofibers and PSF/carbon nanotube (CNT) hybrid nanofibers for increasing Mode-II energy release rate (GIIC) of carbon/epoxy laminate. According to results, PSF and the best combination of hybrid nanofibers (PSF + 10%wt of CNT) improved GIIC by about 11% and 50%, respectively.  
The interesting matter in this regard is the toughening mechanisms that lead to improved properties when polymeric nanofibers are interleaved. (1) Fiber bridging: When nanofibers do not melt during curing cycle, they bridge the two layers they are interleaved between, thus hindering fracture propagation [38]. (2) Phase separation: Some nanofibers, such as polycaprolactone (PCL), due to the heat provided during the curing process, change shape to spherical particles and distribute in the matrix during curing, increasing fracture toughness due to crack deflections [11]. (3) Some other thermoplastic polymers, such as PVDF, melt and mixed with epoxy during curing, due to high porosity of the mat, and a plastic zone is produced in front of crack tip, capable of absorbing energy during loading [17].  
Interleaving nanofibers that can act different toughening mechanisms is an interesting topic, and this is what this paper means to present. Recently, [PERSON] et al. [39] used a combination of nylon nanofibers and PCL film as interleave to increase the interlaminar fracture energy of carbon/epoxy laminates. The results demonstrated a synergistic effect; for instance, Mode-I fracture tests proved that fracture toughness for the laminates interleaved by nylon and PCL, separately, were enhanced by 30% and 50%, respectively, while a remarkable increase of 110% occurred for the laminates interleaved by nylon/PCL. In the present study, the effect of mixing two other mechanisms, i.e., phase separation and plastic zone, is considered. For this aim, electrospun PSF, PVDF, and PSF/PVDF nanofibers were produced separately and interleaved between carbon/epoxy laminate. Then, Mode-I and Mode-II fracture tests were conducted to investigate their effect. For deeper investigation, SEM pictures were also taken to find out toughening mechanism.