Research Article pubs.acs.org/journal/ascecg
Nanodot-Loaded Clay Nanotubes as Green and Sustained Radical Scavengers for Elastomer Siwu Wu,† Min Qiu,† Baochun Guo,*,† Liqun Zhang,‡ and Yuri Lvov*,§ †
Downloaded via KAOHSIUNG MEDICAL UNIV on June 30, 2018 at 17:43:11 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
Department of Polymer Materials and Engineering, South China University of Technology, 381 Wushan Road, Guangzhou 510641, China ‡ State Key Laboratory of Organic−Inorganic Composites, Beijing University of Chemical Technology, 15 Chaoyang North Third Ring Road, Beijing 100029, China § Institute for Micromanufacturing, Louisiana Tech University, 911 Hergot Avenue, Ruston, Louisiana 71272, United States S Supporting Information *
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. Then, 2 nm diameter carbon nanodots released from the halloysite lumens interacted with reactive radicals, generated at the initial stage of oxidative processes, which significantly improved thermo-aging resistance of the elastomers. The 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 nanoarchitectural design of the carbon dotloaded clay nanotubes doped at 9 wt % into a rubber matrix allowed for sustained radical-scavenging 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 the environmental hazards of conventional petrol-derived antioxidants. Such clay core−shell systems may also be useful for biocompatible encapsulation of often-poisonous nanodots providing safe biomarkers. KEYWORDS: Carbon nanodots, Clay nanotubes, Core−shell, Sustained release, Radical-scavenging
■
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 thermo-oxidation 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 nanocontainers 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 A halloysite © 2017 American Chemical Society
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 Received: October 18, 2016 Revised: January 9, 2017 Published: January 11, 2017 1775
DOI: 10.1021/acssuschemeng.6b02523 ACS Sustainable Chem. Eng. 2017, 5, 1775−1783
Research Article
ACS Sustainable Chemistry & Engineering
heated for 3 min in a microwave oven at 700 W, during which the water was evaporated and red-brown solid was obtained. After cooling down to room temperature, the red-brown resultant was dissolved in distilled water and then precipitated and washed with excess anhydrous ethanol repeatedly for three times, ensuring removal of the impurities. The purified CDs were finally dried in vacuum at 50 °C for further use. Nanotube Loading Procedures. Vacuum pouring technique was utilized for loading the CDs into the lumen of halloysite.12,13 First, a CDs solution of 30 mg/mL in a water−acetone solvent was prepared. Then, powdered clay nanotubes were added to the CD solution under stirring. The suspension was sonicated for 30 min. Halloysite was added stepwise three times to minimize aggregation. Subsequently, the suspension was transferred to a vacuum chamber under a vacuum at 100 Torr for 20 min, which deaerated halloysite lumen and pulled the CDs into the tubes. Vacuuming was repeated three times at 20 min intervals and atmospheric pressure between each process. Finally, the suspension was collected by centrifugation, and briefly washed with distilled water to remove residual CDs on the outside surface of halloysite. The resultant CD loaded halloysite (loaded-HNTs) was dried at 50 °C overnight and milled to a fine powder. This dried nanotube formulation could be stored for a long time. Carbon Nanodots Release Kinetics. The CD release experiment was conducted in distilled water with 6.5 pH at 25 °C. The nanotube suspension was stirred during the whole experiment in order to accelerate CD release in equilibrium conditions. The samples for analysis were taken from the suspension by centrifugation at 8000 rpm for 2 min. The concentration of CDs in the resultant supernatant was measured by monitoring the variation of absorption peak of CDs at 360 nm with UV−vis spectrophotometry. To evaluate the overall loadage of CDs, all samples were subjected to vigorous sonication for 1 h at the end of each release experiment. Modification of the Loaded Halloysite Nanotubes. Silane bis(γ-triethoxysilylpropyl)-tetrasulfide) (TESPT) was adopted to modify the surface of loaded-HNTs. Briefly, 0.375 g of TESPT was dissolved in 150 mL of ethanol (95 vol %) and the pH was adjusted to 4 with acetic acid. To hydrolyze TESPT, the solution was stirred and heated at 40 °C for 2 h. Then, 5 g of loaded hallosyite was added and the suspension was sonicated for 30 min, followed by stirring and heating at 40 °C for 1 h. Finally, the TESPT-modified CD-loaded halloysite (modified loaded-HNTs) was separated from the suspension by centrifugation, washed with anhydrous ethanol three times, and then dried at 50 °C. Preparation of Rubber Composites. First, loaded halloysite was mixed with styrene−butadiene rubber (SBR) in an open two-roll mill at room temperature, followed by the addition of curing agents. The resultant compounds were then hot-pressed at 150 °C for the optimum vulcanizing time which was measured by a U-CAN UR-2030 vulcameter. The compositions of all SBR composites are given in Table 1 expressed as parts per hundreds of rubber (phr). Characterization. High-resolution transmission electron microscopy (HRTEM) was acquired by using a Philips Tecnai G2 F30 STwin TEM microscope with an accelerating voltage at 300 kV and all samples were supported by a carbon-coated copper grid. Scanning
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 N-isopropyl-N′-phenyl-pphenylenediamine (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 Fluorescent carbon nanodots (CDs) are prospective particles consisting of discrete quasi-spheres with diameters 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 their excellent electron donating/accepting ability, water solubility, chemical inertness, 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 radical-scavenging activity suppressing the oxidative process of the organic matrix and preserving its physical and chemical properties after severe thermo-aging for 20 days at 100 °C which corresponds to many months of 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.
■
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 (
