Interface engineering strategies to induce multifunctionality in conventional carbon fiber/epoxy composites

Abstract

Carbon fiber reinforced plastics composites (CFRPs) are widely used in structural and space applications due to their superior properties such as high specific strength, high specific stiffness, good corrosion resistance, and low maintenance cost. However, the anisotropy in their average mechanical, electrical, and thermal properties poses a difficult challenge in designing CFRPs for next-generation advanced applications. Moreover, in most advanced structural applications, composites are no longer required as load-bearing members alone. For example, they should be able to resist lightning strikes and dissipate thermal energy in various applications. Thus, the CFRPs are expected to perform more functions than just bearing the structural loads. This necessitates inducing multifunctional characteristics in CFRPs. The key to achieving multifunctionality is to improve the electrical and thermal conductivities while maintaining the mechanical properties of conventional CFRPs. The average mechanical, thermal, and electrical properties of CFRPs depend on their constituents i.e. carbon fiber and polymer matrix (epoxy here). Since, carbon fibers are usually good conductors, the poor electrical and thermal properties of CFRPs are primarily due to the insulating nature of the epoxy matrix and interfacial thermal and electrical resistance at the fiber/matrix interface. Among several possible strategies, chemical modification of fiber surface is a well-proved approach to improve the interfacial properties in CFRPs. Molecular dynamics (MD) is a suitable tool to probe the interface and hence qualitatively as well as quantitatively reveal the molecular level mechanisms that operate at the fiber/matrix interface. This will help in identifying suitable strategies to optimally functionalize the carbon fiber/epoxy interface. The present thesis aims to propose suitable design and fabrication routes to process multifunctional CFRPs for advanced structural applications. To achieve the objectives, MD simulations are performed to investigate the effect of functionalization on the interfacial thermal conductivity of carbon fiber/epoxy composites. Various functional groups, including small ones like carboxylic and amine as well as larger ones like aniline, single-chain para-amine surface grafted molecules (SGM), and double-chain meta amine SGM are attached to the carbon fiber surface. The degree of functionalization is varied to gain insights into the underlying mechanisms that govern interfacial thermal conductivity. MD simulations show that suitable functionalization increases the interfacial thermal conductivity (𝐾i f) by up to ~ 25 times. A relatively higher improvement in interfacial thermal conductivity is observed with larger functional groups. This is due to the fact that larger molecules exhibit more vigorous fluctuations in their end-to-end distances. Also, 𝐾i f does not seem to depend strongly on the degree of functionalization (f) beyond 5% f in most cases. An exception is, single-chain para-amine SGM which shows increasing improvement with f. Next, MD simulations are carried out to explore whether same functional group would be effective in increasing interfacial shear response in carbon fiber/epoxy system. To this end, microbond test is modelled in MD simulations. MD simulations predict that functionalization of carbon fiber may lead to ~ 12 times improvement in interfacial shear strength (IFSS) as compared to unfunctionalized carbon fiber/epoxy composite. Moreover, large functional groups are more effective in improving the IFSS. The interfacial failure mechanics seem to depend on the length of the functional group. The failure in carbon fiber with an aniline functionalized carbon fiber/epoxy system is confined to epoxy molecules only. Breaking of bonds within the functional groups is recorded in case of single-chain para-amine and double-chain meta-amine SGM functionalized carbon fiber/epoxy system. In the next part, an experimental investigation is carried out to find the electrical percolation threshold of multi-walled carbon nanotubes (CNTs) in the CNT modified epoxy (CNT/Ep) nanocomposites. Varying concentration of CNTs filler (0.5 – 3 wt.%) is used to fabricate the CNT/Ep nanocomposites. The percolation threshold is found to be 0.1 wt.%. A sudden jump in the electrical conductivity by factor of ~ 109 at 0.1 wt.% CNT as compared to neat epoxy (Ep) suggests the formation of a percolating network of CNTs within the epoxy. Fracture toughness tests reveal that the maximum improvement is obtained at 1 wt. % CNTs which is 61% higher as compared to neat epoxy. This suggests that 1 wt.% CNTs are optimum to improve the electrical and mechanical properties of CNT/Ep nanocomposites. To verify the predictions from MD simulations, an optimum aniline functional group that allows only cohesive failure in epoxy is synthesized. Later, carbon fiber surface is functionalized with the aniline. Microbond tests are performed to evaluate the interfacial shear strength (IFSS) of the fiber/matrix interface as a function of fiber (unfunctionalized and functionalized) and matrix (Ep and CNT/Ep) combinations. Functionalization of carbon fiber with anilines shows IFSS of 37.6 ± 3.3 MPa which is 149% higher as compared to unfunctionalized carbon fiber (15.1 ± 1.7 MPa). Moreover, the IFSS of the aniline functionalized carbon fiber/CNT/Ep system is found to be higher than the aniline functionalized carbon fiber/Ep composites. Fractography analysis of tested samples shows that different failure mechanisms operate in unfunctionalized and functionalized carbon fiber/epoxy composites. Moreover, the strengthening of epoxy due to the addition of CNTs helps in improving the IFSS in CNT/Ep composites. It is envisaged that harnessing the synergistic effects of carbon fiber functionalization and CNTs modified epoxy should provide multifunctionality to conventional carbon fiber reinforced plastics (CFRPs) with enhanced thermal, electrical, and mechanical properties for structural applications.