Nanodot-Loaded Clay Nanotubes as Green and Sustained Radical

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Nanodot-Loaded Clay Nanotubes as Green and Sustained Radical Scavengers for Elastomer Siwu Wu, Min Qiu, Baochun Guo, Liqun Zhang, and Yuri M. Lvov ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b02523 • Publication Date (Web): 11 Jan 2017 Downloaded from http://pubs.acs.org on January 13, 2017

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ACS Sustainable Chemistry & Engineering

Nanodot-Loaded Clay Nanotubes as Green and Sustained Radical Scavengers for Elastomer Siwu Wu,† Min Qiu,† Baochun Guo,*,† Liqun Zhang,‡ and Yuri Lvov*,§ †

Department of Polymer Materials and Engineering, South China University of Technology, 381

Wushan Rd., Guangzhou 510641, China. E-mail: [email protected]. ‡

State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical

Technology, 15 Chaoyang North Third Ring Rd., Beijing 100029, China. §

Institute for Micromanufacturing, Louisiana Tech University, 911 Hergot Ave., Ruston,

Louisiana 71272, United States. E-mail: [email protected]. KEYWORDS. Carbon nanodots, Clay nanotubes, Core-shell, Sustained release, Radicalscavenging

ABSTRACT: Radical-scavenging carbon nanodots (CDs) were loaded into the 50-nm diameter natural halloysite clay tubes to fabricate low toxic CD-delivery vehicles for elastomer composites with sustained antiaging functionality. 2-nm diameter carbon nanodots released from the halloysite lumens interact to reactive radicals, generated at the initial stage of oxidative processes, which significantly improved thermo-aging resistance of the elastomers. The

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antioxidative efficiency of these CD-delivery vehicles was further increased through the nanotubes’ surface modification with a thin grafted-silane shell, which allowed the slowing of the release rate, thus extending the protection. This nano-architectural design of the carbon dotloaded clay nanotubes doped at 9 wt. % into a rubber matrix allowed for a sustained radicalscavenging and provides a new strategy for long-lasting elastomer protection. Our antiaging rubber nanoformulation based on a natural tubule clay and biocompatible CDs can decrease environmental hazard of conventional petrol-derived antioxidants. Such clay core-shell systems may also be useful for biocompatible encapsulation of often-poisonous nanodots providing safe biomarkers.

Introduction Aging of organic polymer materials is induced by heat, especially at elevated temperatures, an excess of oxygen, ultraviolet radiation, and chemicals. The consequent changes deteriorate the performance and reliability of these materials and greatly limit their applications. One especially visible example is the aging of rubber tires. Dienic elastomers are very sensitive to thermooxidation because the unsaturated skeleton with allylic hydrogens can be easily activated, generating reactive radicals acting as initiators for the aging.1-2 In order to protect the elastomeric products from aging and to prolong their service time, antioxidants for radical inhibition and scavenging have been employed.3 Despite the fact that conventional antioxidants have exhibited a delay in the elastomers aging progress, these petrol-derived antioxidants are not satisfying because of their short working time, toxicity, and poor efficiency with direct admixing.4-5 These indicate the necessity of effective encapsulation of sustainable antioxidants in nano-containers


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doped into elastomers for sustained agents’ release during many months and years to ensure efficient protection. A number of nanomaterials have been introduced for the encapsulation and sustained release of additives.6-8 Halloysite nanotube (HNT) is a tubular alumina silicate clay material consisting of rolled kaolin sheets. The inner and outer surface chemistry of these tubes are different: a silica layer formed by tetrahedral SiO4 sheet is on the outside surface while the alumina layer consisting of an octahedral AlO6 sheet with internal aluminol groups is at the inner surface. Typically, halloysite has a hollow tubular structure with an inner lumen diameter of 10-15 nm, outer diameter 40-60 nm, and a length of 0.5-1 µm.9-10 This nanotube material is abundantly available in thousands of tons from natural deposits at a low price.11 We pioneered halloysite as a nanocontainer for sustained release of various chemical agents, from drugs to anticorrosion and antifouling substances, making self-healing and antibacterial composite polymeric coatings.12-16 For example, several corrosion inhibitors were loaded into these clay nanotubes for long-lasting anticorrosion metal paint coating. The paint doped with benzotriasole loaded halloysite provided sustained release of the inhibitor which protected metals from the electrolyte solution corrosion.13 The loading capacity of the nanotubes can be regulated by selective acid etching of the lumen while the release rate can be adjusted by additional encapsulation or tube-ends stoppers.12 Recently, we encapsulated a commercial antioxidant Nisopropyl-N’-phenyl-p-phenylenediamine (4010NA) into the halloysite lumen and then admixed with styrene-butadiene rubber. This procedure not only prevented the migration of antioxidant but also endowed the elastomer with long-lasting thermal-oxidative aging-resistance resulting from the sustained release of the agent.17 However, the usage of this petrol-derived antioxidant 4010NA is restricted by its unrenewable sources and toxicity to humans.4


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Fluorescent carbon nanodots (CDs) are prospective particles consisting of discrete quasispheres with diameter below 10 nm.18 Typically, CDs are comprised of an inner core with predominant sp2 hybridized carbon atoms and abundant functional groups such as carbonyl, hydroxyl, amino, amide and carboxylic acid on the surface.19 By virtue of the excellent electron donating/accepting ability, water solubility, chemical inertness and photoluminescence emission, low cell toxicity and cost, CDs have drawn attention in a variety of potential applications such as catalysis, bioimaging, biosensors, fluorescent probes, and optoelectronics. In our previous research, sustainable amine-passivated CDs with low toxicity were adopted as radical scavengers to resist the thermal-oxidative aging of elastomers. It turns out that the incorporation of the resultant CDs could significantly improve the aging-resistance of the dienic elastomers and suppress the oxidation process by wiping out the reactive radicals generated during thermaloxidative process.20 However, no sustained supply of the carbon nanodots to bulk elastomer was reached. In this study, we describe an ingenious architectural design with tubule halloysite clay as hollow nanocontainers for loading and sustained release of the antioxidative carbon nanodots. After admixing with rubber matrix, the CDs loaded halloysite provides long-lasting radicalscavenging activity suppressing the oxidative process of the organic matrix and preserving its physical and chemical properties after the severe thermo-aging for 20 days at 100 °C which corresponds to many months protection at lower service temperatures. Through a simple surface modification with silane bis-(γ-triethoxysilylpropyl)-tetrasulfide), an additional shell formed on the nanotubes slows down the release rate of the loaded carbon nanodots and further improved the aging-resistance properties of the rubber composites.


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Experimental Section Materials Critic acid (CA), 1,2-ethylenediamine (EDA), ferrous sulfate (FeSO4), salicylic acid (SA) and 2,2-diphenyl-1-picrylhydrazyl (DPPH) were purchased from Beijing InnoChem Science & Technology Co., Ltd., Beijing, China. All chemicals were used as received, without further purification. Halloysite (HNT) was mined from Hubei, China, followed with finely grinding and purifying according to a reported procedure. Used halloysite samples contained 99 wt.% of the nanotubes with minor admixture (

Nanodot-Loaded Clay Nanotubes as Green and Sustained Radical

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