Fiber reinforced polymer composite laminates are the material of choice for applications requiring high stiffness and high strength with minimal density. Hence, their wide use in wind turbine blades, airplanes and many other applications over the last decades. Composite laminates are often composed of several layers of glass or carbon fibers, surrounded by an epoxy matrix. However, due to their laminated structure, delamination between reinforcing plies and brittle matrix fracture are a major concern for fiber reinforced composites and are among the most frequent types of failure encountered in service. Traditionally proposed solutions for this problem often involve mixing tough thermoplastic materials, reactive rubbers, or stiff nanoparticles into the epoxy matrix. However, these systems significantly increase the viscosity of the uncured resin and thus have a detrimental effect on the resin flow which is crucial to obtain high quality laminates. Furthermore, a homogeneous dispersion of the toughening phases throughout the composite is not easily obtained. Both a reduced resin flow as well as an inhomogeneous distribution of the toughening phases can result in a reduction of the overall mechanical properties of the laminate.
Throughout this PhD, the use of tough electrospun nanofibrous veils is investigated as a more viable method to increase the resistance against delamination (interlaminar fracture toughness). Nanofibrous veils are relatively new materials composed of very fine fibers into a non-woven textile. A nanofibrous veil is much like a traditional nonwoven material in the sense that it is a relatively strong and dimensionally stable macroscopic structure which is easy to handle. Nevertheless, the nanoscale diameters of these nanofibers lead to very interesting properties such as small pore sizes (about one order of magnitude larger than fiber diameter), large specific surface area (typically around 20 m2.g-1), high porosity (typically around 90 %) and good inter-pore connectivity. The unique properties of electrospun nanofibers are very useful for interlaminar toughening of composite materials. The basic concept is simple and straightforward. Nanofibrous veils can act as a “pre-shaped co-continuous thermoplastic phase” which is then impregnated with epoxy resin. The resulting nanofiber toughened epoxy has a morphology which is similar to the co-continuous morphology in traditional toughening systems using thermoplastic materials. However, by using the nanofibrous structure as a “pre-shaped” morphology, there is no need for a complicated and difficult to control phase separation process and the resin viscosity remains unaffected.
The concept of interlaminar toughening using electrospun nanofibers was introduced by Dzenis and Reneker and further detailed by recent work of ourselves as well as other research groups during the course of this PhD. However the results reported so far in open literature appear to be quite scattered. In order to explain these different results and design novel toughened composites, a better understanding of the toughening (micro) mechanisms which are present in nanofiber interleaved laminates is necessary. This PhD aims to provide a generic understanding of the toughening mechanism involved, which can open up the true potential of electrospun thermoplastic nanofibers.
Following a condensed literature review in chapter 1 and an overview of the materials and methods used throughout this PhD in chapter 2, chapter 3 presents the general micromechanical fracture mechanisms of electrospun nanofiber toughened composites. These mechanisms were unraveled by analyzing the nanofiber toughening effect on three different levels, coinciding with the hierarchical nature of laminated composite materials: the nanotoughened epoxy, the nanotoughened interlaminar region and the nanotoughened laminate. Experimental analysis of the nanotoughened epoxy has shown that yielding of nanofibers in the fracture processing zone as well as nanofiber bridging increases the fracture toughness of the epoxy substantially.
On the level of the nanotoughened interlaminar region, careful analysis of the delamination path was shown to be crucial for understanding and optimizing the interlaminar toughening effect of electrospun nanofiber. More specifically it was found that under the right conditions, crossings of the interlaminar region can occur. These interlaminar crossings are one of the main contributors to the increase in interlaminar fracture toughness as nanofiber bridging zones develop in them. In the subsequent chapters, the nanotoughened interlaminar region was extensively studied. Several parameters were identified that influence the interlaminar fracture toughness, either directly or by changing the interlaminar crack path.
The first parameter under investigation was the electrospun morphology (chapter 4), since it is well known that the phase separated morphology in traditional thermoplastic and rubber toughened epoxies has a major influence on the final fracture toughness of the epoxy. Throughout chapter 4, five different electrospun morphologies (i.e. nanofibers, microfibers, microspheres, films and spray-coated PCL) were introduced in the interlaminar region of glass epoxy laminates. It was shown that only porous interleave structures, having a fine distribution of PCL phases in a surrounding epoxy phase allow for significant improvements in both Mode I and Mode II interlaminar fracture toughness of the resulting composites. From all the tested porous structures, the nanofiber interleaved structures show the best performance.
The next parameter under investigation was the interleaving method (chapter 5). It was shown that a double layer deposited configuration (DLD), in which the nanofibers were directly electrospun on both sides of the glass fiber mats facing the interlaminar region, gives rise to the highest Mode I interlaminar fracture toughness values. As opposed to a single layer deposited configuration in which nanofibers were spun on only one side of the glass fiber mat or an interleaved configuration in which a standalone nanofiber membrane was placed between the glass fiber mats, a DLD configuration improved the formation of interlaminar crossings.
Chapter 6 and chapter 7 report on the effect of the mechanical properties of the electrospun fibers on the interlaminar fracture toughness. Chapter 6 focuses on the development of electrospun SBS fibers with tunable mechanical properties. First, a novel electrospinning solution with relatively low toxicity was formulated, after which a post modification strategy based on triazolinedione click chemistry was developed. Although the main goal of chapter 6 was to obtain mechanically tunable SBS fibers, this post modification strategy can be applied to introduce a wide variety of functional groups onto SBS fibers. The mechanical tunability was achieved by using MDI-TAD (a bi-functional TAD cross-linker). Cross-linking with MDI-TAD allowed tuning the elongation at break of the SBS fibers from about 90 to 700 %, whereas the modulus of the fibers covers a range from 11 MPa to over 130 MPa by adjusting the extent of modification.
The mechanical properties of these tunable fibers are subsequently linked to the interlaminar toughness of the composite laminates in chapter 7. The variation in the mechanical properties of the SBS fibers had a significant effect on both the Mode I as well as the Mode II interlaminar fracture toughness of the glass epoxy laminates. Electrospun fibers with high elongations at break and low E-moduli are shown to be less suitable for interlaminar toughening. Although these fibers have a high work of rupture, the energy absorbed when strained up to small elongations is nearly zero. SBS fibers with a lower elongation at break and a higher E-modulus resulted in an increased amount of absorbed energy in the low strain region. This significantly affects the toughness of the interlayer as well as the interlaminar crack path, both in Mode I and Mode II loadings.
Overall it can be concluded that important insights were gained throughout this PhD, which will contribute to the advancement of the research on these novel materials and help in optimizing and designing highly toughened advanced composite applications. In addition it was clearly shown that using these insights one can produce nanofiber toughened laminates by resin transfer molding, which have a superior delamination resistance, both under Mode I as well as Mode II loading conditions and this without resulting in a decrease in in-plane properties.