Exploiting carbon nanotubes (CNTs) to modulate the energy absorption and viscoelastic properties of polymeric foams

Abstract

Polymeric foams are used in many engineering applications including cushioning/padding, packaging, helmet inner liner, sandwich structures, tissue engineering etc. One key issue which limits the application of polymer foams in structural applications is their relatively low mechanical strength. The average mechanical properties of polymer foams depend on their microstructure and base bulk material. While the foam microstructure has been designed through subtle variations in processing routes, the use of reinforcing fillers has been reported to enhance the mechanical properties of polymer foams. However, our understanding of how the addition of nano-fillers affects the microstructure, energy absorbing capacity, relaxation spectra and deformation modes of polymer foams is still rather limited. Therefore, answering these questions by establishing structure – property relationship in reinforced polymer foams through a detailed experimental analysis is the primary objective of the present thesis. This work aims to understand the mechanics of deformation in nano-composite polymer foams under quasi-static, dynamic and impact loading. Based on the insights gained from well-designed experiments, some possible ways have been proposed to enhance the average mechanical properties of polymer foams for structural applications. To achieve the objectives, experimental investigations are carried out on two classes of polymeric foams namely polyvinyl alcohol (PVA) and polyurethane (PU) foams. While the average properties of PVA foams are suited for biomedical applications, the PU foams are preferred for structural applications. Both types of foams are reinforced with different variant of carbon nanotubes (CNTs) filler (as-grown, oxidized and purified) with varying concentration to characterize the effect of filler incorporation on the cell wall, cell size, cell geometry and foam density. The elastic modulus and mechanical strength coupled with complex processing routes found to be unsuitable for structural applications of PVA foams. On the other hand, PU foams are relatively easy to fabricate and have sufficiently high mechanical strength. Thus, CNTs are used to modulate the average mechanical properties and energy absorbing capability of PU foams for helmet inner liner (HIL). To this end, polyurethane foams are reinforced with as-grown and oxidized CNTs at varying carbon nanotube concentrations. It is observed that the inclusion of CNTs up to a threshold concentration decreases the density of polyurethane foams. Uniaxial and cyclic compression testing of foam samples reveals that while energy dissipation is higher in neat polyurethane foams, carbon nanotube reinforced foams show better recovery when compressed beyond elastic limit due to their stiffer foam cell walls. The PU foams reinforced with oxidized CNTs have better sound attenuation capabilities in the frequency range of 3000 - 4000 Hz. The SEM analysis of deformed foam samples reveals that cell shearing; cell bending and fracture at nodes are the predominant mode of deformation in all types of foam samples. Next, a comparative analysis of PU foams fabricated at room temperature (RT) and -5 oC with varying concertation of CNTs is performed. It is observed that PU foam processed at -5 oC and reinforced with 1.6 wt. % of oxidized CNTs is suitable for HIL as it shows 40 % higher specific elastic modulus, 12 % more efficiency and 11 % better recovery in comparison to commercially used expanded polystyrene (EPS) helmet foam (HF). Moreover, it absorbs 97 % more energy per unit volume in comparison to HF sample under low velocity drop weight impact tests. The 34 % higher thermal conductivity of optimized PU foam implies that it will provide better comfort by efficiently dissipating the heat generated in the helmet. The superior combination of properties makes CNT reinforced PU foams a better alternative for HIL applications in comparison to EPS foam. As polymers are typically viscoelastic materials, it is important to understand the effect of CNT addition and processing temperature on the viscoelastic (VE) behavior of PU foams. While, dynamic mechanical thermal analysis is performed to construct the master curve, stress relaxation test is carried out to characterize the effect of processing condition and CNT addition on the relaxation spectra of PU foams. The experimental data obtained from these tests is used to predict the response of PU foams in a broad range of frequency and strain rates using frequency temperature superposition principle (fTSP) and Prony series analysis respectively. The VE analysis shows that PU foam fabricated at -5 oC and reinforced with 1.6 wt. % oxidized CNTs are stiffer, have higher storage modulus and relatively more rate sensitive in comparison to other variants of PU foams studied in this thesis. Based on findings of present work, it is concluded that modifying processing conditions and adding optimum wt. % of oxidized CNTs is an efficient strategy to tailor the average mechanical and viscoelastic properties of polyurethane foams. It is envisaged that present study is a significant step ahead to design helmet inner liner for safety and comfort of helmet wearer.