Biomimetic, Strong, Tough, and Self-healing Composites Using

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Biomimetic, Strong, Tough, and Self-healing Composites Using Universal Sealant-Loaded, Porous Building Blocks Sung Hoon Hwang, Joseph Miller, and Rouzbeh Shahsavari ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b12532 • Publication Date (Web): 09 Oct 2017 Downloaded from http://pubs.acs.org on October 13, 2017

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ACS Applied Materials & Interfaces

Biomimetic, Strong, Tough, and Self-healing Composites Using Universal Sealant-Loaded, Porous Building Blocks

Sung Hoon Hwang§, Joseph B. Miller†, Rouzbeh Shahsavari*,§,§§,†

§

Department of Materials Science & Nanoengineering, Rice University, Houston, Texas 77005,

United States §§

Department of Civil and Environmental Engineering, Rice University, Houston, Texas 77005,

United States †

C-Crete Technologies, Stafford, Texas 77477, United States

* Corresponding author: [email protected]

Keywords: Calcium-silicate, Nanoparticles, Biomimetic, Self-healing, Nanoindentation, Mechanical properties.

ACS Paragon Plus Environment


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Abstract: Many natural materials – such as nacre, dentin - exhibit multifunctional mechanical properties via structural interplay between compliant and stiff constituents arranged in a particular architecture. Herein, we present, for the first time, a bottom-up synthesis and design of strong, tough, and self-healing composite using simple but universal spherical building blocks. Our composite system is comprised of calcium-silicate porous nanoparticles with unprecedented monodispersity over particle size, particle shape and pore size, which facilitate effective loading and unloading with organic sealants, resulting in 258% and 307% increase in indentation hardness and elastic modulus of the compacted composite. Furthermore, heating the damaged composite triggers controlled release of the nanoconfined sealant into the surrounding area, enabling moderate recovery in strength and toughness. The innovative concepts and strategies of this work open up an entirely new phase space for fabricating a novel class of biomimetic composites using low-cost spherical building blocks, potentially impacting diverse areas including bone-tissue engineering, insulation, refractory and cementitious materials, and ceramic matrix composites.


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INTRODUCTION

Among the 8.7 million eukaryotic species in nature,1 each species is endowed with the unique structural features tailored for its specific functions in ecosystem. The common denominator found in the structure of their bodies is subtle coordination between dissimilar materials. One renowned example is the periodic alternation of stiff inorganic constituents and soft organic constituents in biomaterials such as nacre. This bio-inspired design principle based on the structural coordination of organic and inorganic phases has been proven particularly effective in increasing toughness of brittle materials, encompassing nanoclays, alumina and hydroxyapatite.2-4 Nevertheless, this strategy has rarely been applied for calcium-silicate (CS) based materials, which are ubiquitous in diverse industries and have long suffered from inherent brittleness.

CS based materials serve as key building blocks in various applications including bonetissue engineering, drug-delivery, insulation, refractory and cementitious materials, owing to their exceptional mechanical strength, biodegradability, thermal stability and low-cost (Ca and Si are two of the most abundant elements on earth’s crust).5-15 However, they are susceptible to numerous forms of mechanical damage due to their high brittleness. As an example, in the case of bone-tissue engineering, high brittleness impedes the adaptation of mechanical properties to that of cortical bone.16 For repair and maintenance of cementitious infrastructures, $13.6 billions is spent annually in concrete bridges.17 Therefore, enhanced mechanical properties coupled with self-healing capability of CS based materials will benefit diverse fields of industry on both



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economical and environmental grounds.18-19 Increased mechanics allows “to do more with less” and increased durability (via self-healing) will reduce the cost for routine maintenance. Both of these factors decrease the production of certain precursor materials, such as cement which causes 5-10% carbon dioxide footprint.20-23

In the light of successes in applying structural interplay between organic and inorganic constituents to achieve high toughness and self-adaptability, the precise control over size,24 morphology,25 and chemical stoichiometry26 of CS nanomaterials has now enabled the application of similar bio-inspired strategies for CS based materials. Our group recently accomplished the comprehensive morphology control of calcium-silicate-hydrate (C-S-H) nanoparticles, the fundamental, strength-responsible building blocks of all cementitious materials and observed that cubic morphology benefits mechanical properties across different length scales.25 Although mesoporous CS nanomaterials have been synthesized and studied as drugcarriers,25, 27 there are key nanoscale features including uniform morphology, monodispersity and porous structures that have not been fully leveraged in the context of building a larger, assembled CS superstructure with enhanced mechanics and self-healing capability. Inspired by mechanically superior biomaterials such as nacre and bones, organic materials have been previously incorporated within the structure of CS based nanomaterials in order to improve their mechanical properties. Numerous CS/polymer composites have been developed using polyvinylpyrolidone,28 polydimethylacrylamide,28 polyvinyl alcohol,5,

29

poly(1.8-octanediol

citrate)30 but in fact, only few of them led to improvement in mechanical properties. This is likely because the abovementioned composites lacked controlled intercalations of organic components. Furthermore, the synthetic procedures did not involve fine-tuning of size, shape and


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porosity of the constituent CS materials, which is a crucial part in developing a bio-inspired organic-inorganic composite.31-32 Herein, we demonstrate a novel bottom-up fabrication of self-healing CS materials with exceptional micromechanical properties. To the best of our knowledge, this is the first report on synthesis of bio-inspired materials using simple but universal spherical building blocks, illustrating the prospect of treating abundant and low-cost calcium silicates on equal footing with advanced materials in the current applications of biomimetic design to create strong, tough and self-healing composites. The smallest fundamental building blocks in our work are porous CS nanoparticles, which are loaded with organic sealants. Such cargo-loaded particles or capsules are typically incorporated into another matrix material to induce self-healing.33-36 In our case, they are employed as major building blocks of a composite material. Overall, our approach is ex post facto, using following sequential steps: We first prepare CS nanoparticles (CPNPs) with finely-controlled size, morphology and porosity. Then, the as-prepared CPNPs are separately loaded with two-part epoxy-based sealant system. The sealant-loaded CPNPs are then subjected to pressure-induced assembly to form a durable nanocomposite where the applied pressure induces release of the loaded sealant from pores of the particles into nanoscale interfacial regions. Upon damage, external heat stimulus activates the release of this nanoconfined sealant to the surrounding areas and enables self-healing therein. Overall, this sequential process enables more periodic and controlled incorporation of organic sealant within the arrays of CPNPs over a large scale, thus mimicking the hierarchical structure of natural organic-inorganic composites such as nacre, albeit nacre’ building blocks are platelets (versus spherical building blocks in this work). The proposed unique route towards building a mechanically-enhanced, self-healing organic-



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inorganic composite can be applied in a wide range of aforesaid industrial fields, where CS based materials are at the heart of mechanical support.

EXPERIMENTAL SECTION

Materials. Bisphenol A diglycidyl ether (DGEBA), Dimethylbenzylamine (DMBA), Hexadecyltrimethyl ammonium bromide (CTAB), tetraethyl orthosilicate (TEOS), calcium nitrate tetrahydrate, HPLC-grade water, and ammonium hydroxide were all purchased from Sigma-Aldrich. All chemicals were of reagent grade and used as purchased without further purification.

Preparation of CPNPs. CPNPs are produced based on the modified Stöber method with the incorporation of calcium nitrate tetrahydrate as the source of calcium ions.37-38 26.64 ml of ammonium hydroxide and 0.68 g of CTAB were dissolved in 1037 ml of distilled water. The resultant solution was stirred at 1000 rpm at 50°C for an hour. After the solution was cooled down to room temperature, 5.66 g of calcium nitrate tetrahydrate and 3.35 ml of TEOS were added. The mixture was then stirred at 1000 rpm at room temperature for additional 3 hours. The synthesized product was collected by centrifuging the final reaction mixture at 4000-5000 rpm. The product was washed with deionized water three times: 5 minutes of sonication followed by centrifugation at 4000-5000 rpm for 10 minutes per washing. The washed product was heated to 600°C and maintained at this temperature for 6 hours to eliminate residual surfactant.


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Morphology. The spherical morphology, size, and porous structure of our nanoparticles were confirmed by Scanning Electron Microscopy (SEM) using FEI Quanta 400 ESEM FEG and Transmission Electron Microscopy (TEM) using JEOL 2100 Field Emission Gun Transmission Electron Microscope. Energy Dispersive X-ray Spectroscopy (EDS) was also performed using FEI Quanta 400 ESEM FEG with an accelerating voltage of 15 kV. Before examination, a dilute suspension of the nanoparticles was obtained through ultrasonication and centrifugation and subsequently placed on carbon-coated copper grids. Nitrogen Adsorption/Desorption, Infrared Spectroscopy and Thermal Analysis. The pore characteristics of the free CPNPs and the loaded CPNPs were analyzed using nitrogen adsorption from Brunauer-Emmett-Teller (BET) analysis with a Quantachrome Autosorb-3b BET Surface Analyzer. Prior to use, pure CPNPs and sealant-loaded CPNPs were further dried at 400°C and 50°C, respectively, under vacuum for 12 hours to eliminate the dissolved gases in the powders. Fourier Transform Infrared Spectroscopy (FT-IR) was performed using Spectrum Two FT-IR Spectrometer from PerkinElmer. Thermogravimetric analysis (TGA) of the samples was performed using a Q-600 Simultaneous TGA/DSC from TA Instruments using a heating rate of 10°C/min under a nitrogen purge of 40 ml/min.

CPNP Pore Volume Filled. Calculation of the CPNP pore volume filled was done by first determining the mass of the sealant within the pores. To do this, TGA was performed (Figure 2e) and the loaded CPNPs were compared to pure sealant and unloaded CPNPs. The mass was calculated from the mass loss ( ) from 310°C to 650°C for DGEBA (monomer) and 140°C to 650°C for DMBA (initiator). The lower-bound temperatures of 310°C and 140°C were chosen as they correspond to the stage, where the residual surface sealant and absorbed contaminants such



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as water should be removed. This mass was converted to volume by dividing by the typical sealant density ( ), assuming standard density at room temperature and pressure. This volume was then divided by the total pore volume ( ) for pore sizes less than 234 Å (23.4 nm), which represents the overwhelming majority of pores as seen in Figure 2c:

=

. ⋅

(1)

Nanoindentation. Berkovich indenter tip with the pyramidal shape was used for loading and 120 mN/min was selected for the rate. The maximum load was set at 60 mN. The nanoindentation hardness (H) and modulus (E) were calculated using the following equations based on the Oliver & Parr method39:

(2)

0.5/√ 1 −

(3)

=

=

where Pmax is the maximum load for a given indentation, AC is the projected area of the contact surface between the sample and the indenter tip, S is the slope of the unloading curve, and v is the Poisson’s ratio of the material.

Compression. The unloaded and loaded CPNPs were compacted in a cylindrical pellet press consisting of a hardened stainless steel base equipped with a vacuum outlet, the main die block with a 13 mm cylinder, a plunger, anvils and a pellet extractor.25 In order to prepare a nanocomposite tablet, the die body was placed on the base and the anvil in the die chamber and


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the pre-weighed CPNP powder was placed inside the chamber. The resultant assembly was brought to a hydraulic press machine, which was used to apply external pressure on the powder. When the pressure reached 5 US tons, that corresponds to the external pressure of ~335 MPa, it was maintained for five minutes. Each of the five mechanical tests involved pre-cracking of the as-produced reference tablet and sealant-loaded tablet, followed by heating at 120 °C for 4 hours and final compressive testing. For each test, the amount of CPNP powder was selected such that the difference in the average thickness of the unloaded CPNP (3.19 mm) and sealant-loaded CPNP (3.06 mm) tablets was kept within 0.13 mm. Between the tests however, the thickness of the tablet varied to a greater degree but the maximum difference between the thickest and the thinnest tablet was 0.8 mm.

For initial crack formation, a 3-point bending load was applied at the center of the sample tablet (Figure 5a), with the crosshead approach speed of 0.03 mm/minute and it was manually interrupted after the load reached about 0.035 kN. This end-load value 0.035 kN for initial loading was selected based on analysis of previous tests, where the load value of approximately 0.045 kN induced the flexural failure in most of the samples with the same size.

For final compressive strength testing, an Instron 4505 machine with a 100 kN load cell was used to measure the compressive properties. The loading was performed at the rate of 1 mm/min. Toughness was defined as the amount of energy a material absorbs before failure (representing the work-of-fracture),40-42 which is different from the classical “fracture toughness” with the unit of Pa√. The work-of-fracture is the area under the stress–strain curve, which is deeply affected



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by gradual, graceful fracture, whereas the “fracture toughness” does not incorporate this entire process.42-43

SEM images Figure 2a, 2c, 3c, 4d, 5a, 5b and 5e were sputtered with a ~4.5 nm layer of gold to enhance image contrast and resolution. The nanoindentation indents of Figure 5a were done at 200 mN max force purely to enhance image contrast for visualization. However, all data and results from this paper were given by using a 60 mN max force.

RESULTS AND DISCUSSION

Our refined synthesis of the as-produced calcium-silicate porous nanoparticles (CPNPs) begins with the nucleation of a calcium-silicate-hydrate seed, which is patterned around the surface of the templating surfactant. Since our synthesis mixture is slightly below twice the critical micelle concentration (CMC), most of our micelles are in spherical form though there may be a lower percentage of elongated cylinders.44 These seeded micelles begin to assemble into groups under the influence of van der Waals forces, and eventually cluster until the free energy is minimized, which depends on reaction temperature and pH along with reactant and surfactant concentration (Figure 1a).45 This process results in a spherical nanoparticle that, upon further calcination to remove residual surfactant, produces the CPNPs as viewed in Figure 1b. The TEM image in the inset of Figure 1b clearly shows the CPNP’s internal pores, which are needed for subsequent sealant loading.


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The loading procedure is crucial in obtaining nanoparticles with sufficient sealant to enable a self-healing bulk material. For this work, diglycidyl ether of bisphenol A (DGEBA), in combination with dimethylbenzylamine (DMBA), were utilized as the dual-system sealant materials. DGEBA is an epoxy monomer with a molecular weight of 340.42 g mol-1. Unmodified, pure DGEBA often shows a transition from liquid to solid due to crystallization and thus, direct impregnation, where DGEBA enters the nanoparticles by diffusion, must be performed at an elevated temperature. During direct impregnation, the nanoparticles were directly soaked in excessive DGEBA at 90°C for 11 hours with constant stirring at 1000 rpm. The CPNPs were retrieved from the excessive DGEBA by allowing the mixture to cool and then washing with isopropanol by sonicating for 5 minutes and then performing centrifugation. The washing process was repeated three times.

A second similar loading method, the solvent-assist method, which is the widely established technique for loading organic drugs into porous drug-carriers,46 was also employed for infiltrating DGEBA into CPNPs (Table S1). The excessive DGEBA was removed using the washing process adopted in our direct impregnation method, and the loaded nanoparticles were dried again at 50°C for around 12 hours. The overall loading procedure is illustrated in Figure 1c and the resultant particles are shown in Figure 1d. The inset in Figure 1d is a TEM image clearly showing that the internal pores have been loaded with DGEBA when compared to the unloaded CPNPs in Figure 1b. Direct impregnation was also adopted for the loading of DMBA (initiator), but unlike the direct impregnation for DGEBA, room temperature was used since DMBA is a low-viscosity liquid under ambient conditions.



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SEM image in Figure 2a confirms the universal spherical morphology of our monodisperse nanoparticles (with standard deviation of σ = 6.9%) with a controlled mean diameter of 243 nm. Although the synthesis of spherical, porous CS nanoparticles has been reported elsewhere,47 the precise control of size while maintaining uniform morphology with such a small standard deviation is unique for CS nanomaterials, particularly when compared to spherical silica-based counterparts.48 This is one of the significant results of this paper. This simple but uniform morphology and size will play crucial parts in self-healing applications that require self-assembly and controlled packing (akin to nature where it employs simple but universal building blocks). Figure 2b is a representative energy dispersive spectroscopy (EDAX) graph for which the calcium to silicon (Ca/Si) atomic ratio is 0.56. In our study, most CPNPs had a Ca/Si ratio between ~0.4-0.7. Figure 2c shows the CPNPs after performing the solvent-assist loading procedure, which was the preferred method due to less sealant being needed for the loading procedure as well as a reduction in surface-adhered sealant. Thus, this procedure was used in the following sections unless stated otherwise. In addition, a variety of different carrier:DGEBA ratios were attempted for the solvent-assist method (SI Table 1) and thermogravimetric analysis (TGA) was used to evaluate which carrier:DGEBA ratio leads to the maximum uptake. The optimum ratio was found to be 2:3 and used for the following sections.

Interestingly, Figure 2c shows that the CPNPs maintain their original shape and size (σ = 7.1%, mean diameter = 240 nm) after sealant (DGEBA) loading and are not aggregated. The nitrogen adsorption isotherms from BET analysis are illustrated in Figure 2d. Both adsorption/desorption isotherms displayed very similar curves, and thus, only the adsorption branch is shown for clarity. The isotherms obtained for pure CPNPs were typical type IV


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isotherms according to the IUPAC classification.49 The well-defined step of capillary condensation near P/P0=0.4 indicates the existence of mesopores with the size of around 3 nm. The inset in Figure 2d demonstrates the tight distribution of pore sizes achieved, with the peak centered on 31 Å (3.1 nm). The pore sizes appear to be intimately linked to the micelle size of the surfactant (CTAB spherical micelle diameter in water = ~3 nm50). The BET surface area of 337.14 m2/g, calculated from the range of P/P0=0-0.29, along with the total pore volume of 0.336 cm3/g, calculated at P/P0=0.91 (pores

Biomimetic, Strong, Tough, and Self-healing Composites Using

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