Interdiffusing core-shell nanofiber interleaved composites for excellent Mode I and Mode II delamination resistance

Abstract

Electrospun nanofibre interleaving has a great potential for toughening of composite laminates as an effective, safe and industrially relevant method. Although many studies showcase large increases in delamination resistance, these are typically obtained under either Mode I or Mode II loading and for a wide variety of nanofibres. Here, we present a more general approach towards simultaneous excellent Mode I and Mode II delamination resistance using a single nanofibre system without the need for additional chemical modification steps or speciality polymers. It is illustrated based on the concept of interdiffusion of polycaprolactone nanofibres during the curing process into the epoxy matrix resin for improved adhesion. The results show that for a simultaneous increase in Mode I and Mode II delamination resistance, the adhesion and the fibre morphology of the nanofibres are crucial. The methodology is then expanded to allow for industrial relevant working windows by core-shell structured polyamide/polycaprolactone nanofibres. This approach results in a of 650 ± 50 J m-2 (+ ca. 60% vs. virgin material) and a of 3160 ± 35 J m-2 (+ ca. 60% vs. virgin material).

Delamination resistant composites by interleaving bio-based long-chain polyamide nanofibers through optimal control of fiber diameter and fiber morphology

“…On the other hand our work on temperature stable nanofibers made from commercially available polyamide resins resulted in an excellent mode II IFT increase (þ80%) for polyamide 6, polyamide 6.6 and polyamide 6.9, but the mode I increase was limited, likely caused by a lack of matrix adhesion [5][6][7]14]. Although other researchers have booked more success using polyamide nanofibers, e.g.…”

“…PCL can be used for composite toughening through phase separation as well, but this decreases other mechanical properties such as elastic modulus and tensile strength as it is shown by Dadfar et al [17]. With the type of highly relevant high-end epoxy resin used in this manuscript, a pre-curing step would be required as has been thoroughly shown in our previous research [4,14,15].…”

“…Refs. [18,19], these results could not be reproduced by us without providing the polyamide nanofibers with a matrix-compatible shell made from polycaprolactone to increase its adhesion with the matrix [14,16]. The interplay of the nanofiber intrinsic mechanical properties and the adhesive interaction with the matrix material is thus highly important as pointed out by several research groups as well [5,6,[20][21][22][23].…”

“…Nanofiber toughening of composites has been studied for a variety of polymer types, e.g. PA46, PA6, PA66, PAN, PCL, PVA, PVB, PVDF [5][6][7][8][9][10][11][12][13][14]. Some of the most frequently occurring limitations of this toughening method stem directly from the choice of polymer.…”

“…It is clear nanofibrous veils are very promising toughening materials but a widespread use requires temperature-stable nanofibrous systems, that yet offer the appropriate intrinsic mechanical properties and the adhesive matrix interaction. Recently, we proposed a method to successfully achieve temperature-stable nanofibers with improved delamination resistance [14], based on dedicated coaxial nanofibers. However, application-wise, it would be more interesting if single component nanofibers are used made from commercially available polymers.…”