Hybrid Effects in the Fracture Toughness of Polyvinyl Butyral‑Based Nanocomposites

This document is downloaded from DR‑NTU (https://dr.ntu.edu.sg) Nanyang Technological University, Singapore. Hybrid effects in the fracture toughness of polyvinyl butyral‑based nanocomposites Shemer, Eldad; Hu, Xiao; Marom, Gad; Stern, Nadya; Dyamant, Itay 2018 Stern, N., Dyamant, I., Shemer, E., Hu, X., & Marom, G. (2018). Hybrid effects in the fracture toughness of polyvinyl butyral‑based nanocomposites. Nanocomposites, 4(1), 1‑9. doi:10.1080/20550324.2018.1447827 https://hdl.handle.net/10356/107018 https://doi.org/10.1080/20550324.2018.1447827 © 2018 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricteduse, distribution, and reproduction in any medium, provided the original work is properly cited. Downloaded on 01 Oct 2021 05:17:18 SGT NANOCOMPOSITES, 2018 VOL. 4, NO. 1, 1–9 https://doi.org/10.1080/20550324.2018.1447827 RESEARCH ARTICLE OPEN ACCESS Hybrid effects in the fracture toughness of polyvinyl butyral-based nanocomposites Nadya Sterna, Itay Dyamantb, Eldad Shemerb, Xiao Huc and Gad Maroma aThe Casali Center of Applied Chemistry and the Center for Nanoscience and Nanotechnology, The Institute of Chemistry, The Hebrew University of Jerusalem, Jerusalem, Israel; bOSG Armor R&D, Oran Safety Glass Ltd, Kibbutz Tzuba, Israel; cSchool of Materials Science and Engineering, Nanyang Technological University, Singapore, Singapore ABSTRACT ARTICLE HISTORY Hybrid polyvinyl butyral (PVB) nanocomposite films comprising surface-treated carbon Received 3 January 2018 nanotubes (CNT-COOH) and nanoclays (NC) were tested for their “trouser-leg” fracture Accepted 27 February 2018 toughness in comparison to the values of the respective CNT and NC parent composites and to KEYWORDS that of the pristine PVB film. Relative to the fracture toughness of the pristine PVB, the parent Carbon nanotubes (CNT); composites PVB/CNT-COOH-0.2% and PVB/NC-2% and the PVB/CNT-COOH-0.2%/NC-2% hybrid fracture toughness; exhibited 105, 118, and 181% improvements, respectively. These were both impressive fracture hybrid effect; mechanical toughness improvements and a significant hybrid effect – the latter being only slightly lower properties; nanoclay (NC); than the nominal maximum effect of 223% based on the combined improvements of the parent nanocomposites; polymer- composites. Part of the samples was tested qualitatively by ball impact test of ballistic glass matrix composites (PMCs) samples with relevant films. Among these samples, the hybrid film presented the best result. A significant linear correlation between fracture surface energy and roughness values was taken to reflect a fracture resistance mechanism of crack front slowdown by its interactions with nanoparticles. 1. Introduction the literature, entailing different constituents and different mechanical properties, with emphasis on modulus Hybridization with more than one type of reinforcement strength and fracture toughness, as presented in a variety has been shown classically in fiber-reinforced compos- of papers.1–6 ites to generate synergism, which can potentially pro- The re-discovery of carbon nanostructures in the duce positive hybrid effects in the mechanical properties. 1990s7 – and eventually of a host of other nanoparticles – Positive hybrid effects (for two types of reinforcement, generated immediate interest in the prospects of e.g. carbon and aramid fibers) are generally defined harnessing nanoparticles to ultra-high mechanical prop- either as a positive deviation from the rule of mixtures erty composite materials, termed nanocomposites. It was behavior (considering the weighted contributions of the their nominal mechanical properties, being orders of reinforcements and the matrix) or, more generally, as a magnitude higher than those of classical reinforcement hybrid property that is higher from those of any of the fibers, which made nanoparticles attractive as reinforce- constituents alone. A large number of examples exist in ment constituents, wherein low concentrations would be CONTACT Nadya Stern [email protected] © 2018 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. .N. STERN ET AL 2 potentially required to match and surpass properties of toughness can be obtained by introducing nanoparti- classical composites of high concentrations.8 It is impor- cles such as CNT, graphene, nanoclays, and nanosilica, tant to note, however, that a number of requirements where optimized toughness can be achieved by incor- ought to be fulfilled for nanoreinforcement to perform poration of a proper size, type and amount of nanopar- up to its theoretical potential, which include a uniform ticles, which in turn determine the energy dissipating dispersion of individual (exfoliated) nanoparticles, good mechanisms.17–19 interfacial bonding to the polymer matrix and a high The fact that fracture toughness depends on the aspect ratio.9,10 The requirement of high aspect ratio is specific interactions between the crack front and the elaborated as follows. The general definition of nanorein- nanoparticles, as determined by their particular prop- forcement refers to particles with at least one nanometric erties,20–22 calls for the use of two or more types of nan- dimension, thus dividing the family of nanoparticles to oparticles to form hybrid nanocomposites. And so, the three subfamilies as shown in Figure 1. Of them, only motivation for the present study was to explore the con- two subfamilies, the 1-D and 2-D nanoparticles are ditions that may produce positive hybrid effects in the suitable for reinforcement, as they may present a high fracture toughness of nanocomposites based on nano- geometrical aspect ratio, which can be above the theo- clay (NC) and treated carbon nanotubes (CNT-COOH), retical critical aspect ratio.11 which represent an arbitrary case, where the two types As shown recently in an extensive review of the of nanoparticles differ in both their chemical nature and mechanical behavior, based on a survey of a large geometry. number of scientific articles on a wide range of nanocomposite materials, the strength and modulus fail to 2. Experimental section fulfill their expected values and the experimental values, except for low nanoparticle concentrations, do not nec- 2.1. Materials essarily exhibit a linear increase with the nanoparticle Poly vinyl butyral (PVB) films of Willing®ArchitexTM 12–15 concentration (a rule of mixtures behavior). A num- with density of 1.065 g/cm3 and thickness of 0.76 mm ber of reasons have been listed to explain the observa- were supplied by OSG, Israel. Multi-walled CNTs tion that the reinforcing potential of nanoparticles is not functionalized with COOH groups, small concentra- fully realized, including weak interfacial interactions, tions of OH groups, length of 1–2 μm, tube diameter poor dispersion, and degradation of the nanoparticles of 10–30 nm, and purity above 95% were provided by during the preparation processes, undermining an Cheap tubes, USA. Nanoclay, Nanomer-1.30E, mont- effective stress transfer mechanism, which is essential morillonite clay surface modified with 25–30% octa- for the reinforcing process. If fact, it was claimed that decylamine, bulk density of 0.41 g/cc, thickness of nanocomposites should not be expected to act like a 8 nm, and particle size below 20 μm were produced true composite and that they rather be regarded as a from Nanocor, and supplied by Sigma Aldrich, Israel. molecular mixture with nanoparticles, for which the Chloroform with purity rank of CP, was provided by micromechanical models of strength and modulus do Bio-Lab Ltd, Israel. not apply.16 In contrast, it has been shown that the fracture toughness of polymers can be enhanced significantly 2.2. Sample preparation by the introduction of nanoparticles to form nanocom- Polyvinyl butyral was dissolved in chloroform and posites. This is because fracture toughness of nanocom- stirred at room temperature for 48 h. The solutions were posites – unlike the static properties – depends on the divided in two and each half underwent the following mechanisms of energy dissipation by crack front inter- process: 30 ml of the solution were sonicated either at actions with the nanoparticles, such as pinning of the 100 or 700 W (which are commonly acceptable son- crack front and particle pull-out, and not on a stress ication energies for nanocomposite9) with half of the transfer mechanism. Thus, the enhancement of fracture nanofiller for 20 min, 70 more ml were added for an additional sonication period of 20 min, and then the remaining 150 ml were added for a concluding sonication of 20 min. Finally, the two solutions were combined and mixed together, cast into a Teflon template and allowed to dry slowly over 5 days to obtain ~500 μm thick nanocomposite films with different weight per- centages (wt.%) of fillers (CNT-COOH and NC). An ultrasonic cell crusher (economy) JY88-11N of Ningbo Figure 1. Classification of the particles according to their Sklon Lab Instrument was used for 100 W sonication geometries into three groups: nanoparticles, nanofibers, and and Syclon Ningbo Haishu Sklon Electronic Instrument nanoplatelets. The aspect ratio is the ratio between the long was used for 700 W sonication. and the short dimensions, as noted. NANOCOMPOSITES 3 2.3. Mechanical characterization PVB film between two glass layers of 0.38 mm thickness and 300 × 300 mm area. Fracture surface energy was tested by the Mode III trouser-leg tear test using Instron 3345 universal testing machine with load cell of 500 N. The specimens were 2.4. Morphological characterization 5-cm long and 2-cm wide with an initial cut length of The quality of dispersion was analyzed by STEM, a 3 cm.

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