Designing carbon fiber composite interfaces using a ‘graft-to’ approach: Surface grafting density versus interphase penetration

Abstract

This paper examines the effect on interfacial shear strength (IFSS) when grafting polyethyleneoxide (PEO) polymers of various molecular weights to a carbon fiber surface. Using copper-azide-alkyne cycloaddition click chemistry, PEO polymers of 1 kDa, 2 kDa, 5 kDa, and 10 kDa were tethered to the fiber surface without causing degradation of the fiber surface. The resulting IFSS increases were maximised (130% and 160%) for the 1 kDa and 10 kDa surface modified fibers, respectively. These data suggest that increases in IFSS are the result of an interplay between the density of surface modification versus the penetration of the grafted polymer into the matrix interphase. The trade-off between interphase penetration and surface grafting density is highlighted for the 2 and 5 kDa PEO chains on the fiber surface which display smaller IFSS increases (85% and 117%, respectively). Measuring the mobility of the PEO polymers by ¹H NMR found an order of magnitude decrease in diffusion coefficient for each successive increase in molecular weight, supporting the hypothesis that grafting density decreases with molecular weight. Molecular dynamics simulations of the carbon fiber-matrix interface further supports these observations. These results will inform the design of complementary interfaces for various materials in a range of supporting media.

Introduction

Carbon fiber reinforced plastics (CFRPs) are rapidly becoming a viable replacement for traditional materials, such as metals, for light weight applications. They serve as a means to provide substantial savings in fuel consumption over the lifetime of a vehicle, and are critical to achieve reduced CO₂ emission targets. The promise of CFRPs is substantial though they possess an inherent weakness in the fiber-to-matrix interface, typically poor interfacial adhesion. The critical juncture where the resin and fiber meet in a composite dictates to a large extent the overall performance of the material. Often, due to the mismatch in surface chemistry between fiber and resin, fiber debonding occurs at relatively low shear forces leading to premature failure of the overall composite [[1], [2], [3]]. This is a function of both the fiber undergoing breakage and the void left in the polymeric phase from where the fiber was removed.

During manufacture the fiber is subjected to an oxidative chemical bath, typically post carbonisation, which is thought to assist in removing loosely bound carbonaceous species. Additionally, both roughening the surface and introducing polar functional groups on the periphery of the fiber are also thought to be occurring at this time, which are believed to assist in enhancing fiber-to-matrix adhesion. As a finishing step, a thin sizing layer is applied to the surface treated fibers, which typically consists of a low molecular weight polymer, emulsifying agents, anti-static agents, and other proprietary constituents. The role of the sizing is often debated, from a practical perspective it causes the fibers to stick together which enables them to undergo weaving and other processes to generate a usable fabric. Another functional role of carbon fiber sizing is to serve as an intermediary layer between the fiber and resin, enhancing fiber wet out and interfacial adhesion. Indeed, it is common when examining sizing materials that a resin ‘compatibility’ is provided, though what ‘compatibility’ is and how it is quantified is not commonly disclosed.

Other approaches to enhance interfacial shear strength have included ex situ surface treatments including plasma treatment, chemical functionalisation, incorporation of nanomaterials, and oxidation [[4], [5], [6], [7], [8], [9], [10], [11], [12], [13]]. These have seen various levels of success and typically include post functionalisation reactions to introduce the functional group of choice, such as amines, carboxylic acids, etc. In these cases, the user must be careful not to overdo oxidation processes as these can sometimes result in concomitant defect generation, or ‘pitting’ on the fiber surface which can severely lower the tensile strength and Young's modulus of the fiber.

In our previous work, and that reported by others, both thermal and electrochemical means have been used to functionalise carbon fiber towards the goal of enhancing interfacial shear strength [[14], [15], [16], [17], [18], [19], [20], [21]]. Our approach, using reductive electrochemistry of aryldiazonium salts, has typically revolved around the use of carbon fibers bearing a pendant amine group (in the form of an anilinic nitrogen) which is capable of cross-linking to an epoxy resin. Using this covalent cross-linking approach we have shown increases of interfacial shear strength (IFSS) of up to 220% [21]. Interestingly, analysis of the interfacial interactions in these systems by molecular dynamics indicates that the majority of the IFSS increases result from penetration of the molecules into the polymer phase [18,20,[22], [23], [24]]. This raised the question of how the penetration, and non-covalent interconnection, of compounds tethered to the fiber surface into the polymeric interphase affects the IFSS. The generation of complementary interfaces has important ramifications in several areas as it is critical to the dispersion of nanomaterials within a specific medium. Typically, controlling the surface chemistry [[25], [26], [27]] (and thus interface) of graphene and polymers [[28], [29], [30]], metals [31,32], and within biological systems [[33], [34], [35]] are critical to their successful implementation in modern materials science. Thus, the motivation for this work was to investigate the how the carbon-polymer interface is affected using controlled molecular weight polymers. These insights will have ramifications within various fields incorporating carbonaceous materials. The use of carbon fibers in this case was due to our previous experience in this area, which gives the observations in this work context for interpretation.

With this in mind, the focus of this work was to graft polyethylene oxide (PEO) polymers, due to their high wettability and polarity, of sequentially increasing length (c.f. molecular weight) and determine their effect on IFSS (Fig. 1). Of additional interest in this study is the comparison of ‘soft’ van der Waals interactions versus ‘hard’ covalent crosslinking interactions previously reported [16,22,24,36]. This is a key point of differentiation from our previous work in which a small (3 repeating unit) ethylene oxide diamine was tethered to the surface of carbon fibers using an in situ diazonium salt generation. In that work the presence of the pendant amine cross-linking with the epoxy resin can result in a brittle interface leading to premature failure and poor toughness. Thus, the use of sequentially increasing PEO polymers in this work serves to examine polymer interpenetration effects at the fiber-matrix interface. This serves to enlighten and guide the potential exploitation of interpenetrating networks to optimise composite performance at the macroscale [35].

Herein, we report this effect and show that there is a trade-off between the molecular weight of the polymer (and thus interphase penetration) and grafting density of these compounds on the fiber surface to affect a large IFSS increase. Molecular modelling of the PEO groups on the surface of the carbon fiber suggests that the coiled nature of these PEO groups supresses the potential gains associated with the molecular weight.