Research Article Cite This: ACS Appl. Mater. Interfaces 2017, 9, 37055-37063
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Biomimetic, Strong, Tough, and Self-Healing Composites Using Universal Sealant-Loaded, Porous Building Blocks Sung Hoon Hwang,† Joseph B. Miller,§ and Rouzbeh Shahsavari*,†,‡,§ †
Department of Materials Science & Nanoengineering and ‡Department of Civil and Environmental Engineering, Rice University, Houston, Texas 77005, United States § C-Crete Technologies, Stafford, Texas 77477, United States
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ABSTRACT: Many natural materials, such as nacre and 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, the bottom-up synthesis and design of strong, tough, and self-healing composite using simple but universal spherical building blocks. Our composite system is composed 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% increases in the indentation hardness and elastic modulus of the compacted composite. Furthermore, heating the damaged composite triggers the controlled release of the nanoconfined sealant into the surrounding area, enabling moderate recovery in strength and toughness. This work paves the path towards fabricating a novel class of biomimetic composites using low-cost spherical building blocks, potentially impacting bone-tissue engineering, insulation, refractory and constructions materials, and ceramic matrix composites. KEYWORDS: calcium silicate, nanoparticles, biomimetic, self-healing, nanoindentation, mechanical properties
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is spent annually in concrete bridges.17 Therefore, enhanced mechanical properties coupled with the self-healing capability of CS-based materials will benefit diverse fields of industry on both economical and environmental grounds.18,19 Increased mechanics allows us “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 a 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 bioinspired 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 drug-carriers,25,27 there are key nanoscale features including uniform morphology, monodispersity, and porous
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 bioinspired 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 bone-tissue engineering, drug delivery, insulation, and 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 billion © 2017 American Chemical Society
Received: August 20, 2017 Accepted: October 9, 2017 Published: October 9, 2017 37055
DOI: 10.1021/acsami.7b12532 ACS Appl. Mater. Interfaces 2017, 9, 37055−37063
Research Article
ACS Applied Materials & Interfaces
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’s building blocks are platelets (versus spherical building blocks in this work). The proposed route toward building a mechanically enhanced, self-healing organic−inorganic composite can be applied in a wide range of aforesaid industrial fields, where CS-based materials are at the heart of mechanical support.
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 CSbased nanomaterials to improve their mechanical properties. Numerous CS−polymer composites have been developed using polyvinylpyrolidone,28 polydimethylacrylamide,28 poly(vinyl alcohol),5,29 and poly(1.8-octanediol citrate),30 but in fact, only a few of them led to improvement in mechanical properties. This is likely because the above-mentioned composites lacked controlled intercalations of organic components. Furthermore, the synthetic procedures did not involve fine-tuning of size, shape, and porosity of the constituent CS materials, which is a crucial part in developing a bioinspired 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 bioinspired 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. Our approach uses the following sequential steps. We first prepare CS nanoparticles (CPNPs) with finely controlled size, morphology, and porosity (Figure 1). Then, the as-
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EXPERIMENTAL SECTION
Materials. Bisphenol A diglycidyl ether (DGEBA), dimethylbenzylamine (DMBA), hexadecyltrimethylammonium bromide (CTAB), tetraethyl orthosilicate (TEOS), calcium nitrate tetrahydrate, highperformance liquid chromatography 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 A total of 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 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 h. The synthesized product was collected by centrifuging the final reaction mixture at 4000−5000 rpm. The product was washed with deionized water 3 times, with 5 min of sonication followed by centrifugation at 4000−5000 rpm for 10 min per washing. The washed product was heated to 600 °C and maintained at this temperature for 6 h to eliminate residual surfactant. Morphology. The spherical morphology, size, and porous structure of our nanoparticles were confirmed by scanning electron microscopy (SEM) using a FEI Quanta 400 ESEM FEG and transmission electron microscopy (TEM) using a JEOL 2100 field emission gun transmission electron microscope. Energy-dispersive Xray spectroscopy (EDS) was also performed using a 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 and 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 and 50 °C, respectively, under vacuum for 12 h to eliminate the dissolved gases in the powders. Fourier transform infrared spectroscopy (FT-IR) was performed using a 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 with pure sealant and unloaded CPNPs. The mass was calculated from the mass loss (ms) from 310 to 650 °C for DGEBA (monomer) and 140 to 650 °C for DMBA (initiator). The lower-bound temperatures of 310 and 140 °C were chosen because they correspond to the stage in which the residual surface sealant and absorbed contaminants such as water should be removed. This mass was converted to volume by dividing by the typical sealant density (ρs), assuming standard density at room temperature and pressure. This volume was then divided by the total pore volume (Vt) for pore
Figure 1. Illustration of a typical CPNP synthesis and loading process. (a) The formation of the seeded CPNP growth along with calcination to remove residual surfactant. (b) TEM image showing as-produced CPNPs and their internal pores (inset). (c) The process for loading the self-healing component into the CPNPs. (d) TEM image showing the loaded CPNPs with no visible internal pores (inset), providing evidence of successful loading.
prepared CPNPs are separately loaded with a two-part epoxybased sealant system. The sealant-loaded CPNPs are then subjected to pressure-induced assembly to form a durable nanocomposite in which the applied pressure induces the 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 37056
DOI: 10.1021/acsami.7b12532 ACS Appl. Mater. Interfaces 2017, 9, 37055−37063
Research Article
ACS Applied Materials & Interfaces
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 To prepare a nanocomposite tablet, the die body was placed on the base and the anvil in the die chamber, and the preweighed CPNP powder was placed inside of 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 U.S. tons, corresponding to the external pressure of ∼335 MPa, it was maintained for 5 min. Each of the five mechanical tests involved precracking of the as-produced reference tablet and sealant-loaded tablet, followed by heating at 120 °C for 4 h 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 the initial crack formation, a three-point bending load was applied at the center of the sample tablet (Figure 5a), with the crosshead approach speed of 0.03 mm/min, 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 on the basis of analysis of previous tests, in which 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√m. The work-of-fracture is the area under the stress−strain curve, which is deeply affected by gradual, graceful fracture, whereas the “fracture toughness” does not incorporate this entire process.42,43 SEM Images. Figures 2a,c, 3c, 4d, and 5a,b,e were sputtered with a ∼4.5 nm layer of gold to enhance image contrast and resolution. The
Figure 2. Characterization of unloaded and loaded CPNPs. (a) SEM micrograph of the as-synthesized, unloaded CPNPs demonstrating the monodisperse, spherical nature of the nanoparticles. Inset shows the nanoparticle size histogram. (b) EDS spectrum showing the peaks of oxygen, silicon, and calcium with a sample Ca-to-Si atomic ratio of 0.56. (c) SEM micrograph of the DGEBA-loaded CPNPs showing no change in nanoparticle morphology. For panels a and c, the scale bar indicates 1 μm. (d) Nitrogen adsorption BET isotherms of our unloaded and loaded CPNPs, clearly showing the decrease in pore volume for both DGEBA- and DMBA-loaded CPNPs. The inset is volume pore-size distribution showing the narrow distribution spread and main pore diameter of 31 Å or 3.1 nm. (e) TGA curves showing the weight loss of the pure sealant, pure CPNPs, and loaded CPNPs along with the corresponding pore volume filled (inset). (f) FT-IR spectra for pure CPNPs, sealant-loaded CPNPs, and pure sealants. For panels d−f, the labels “S” and “D” indicate the solvent-assist and direct impregnation techniques, respectively. sizes less than 234 Å (23.4 nm), which represents the overwhelming majority of pores, as seen in Figure 2c: m Vf = s ρs ·Vt (1) Nanoindentation. A 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 and Parr method:39
H=
E=
Pmax AC
Figure 3. Illustration of CPNPs’ self-healing ability and nanoindentation results. (a) Proposed mechanism for how our CPNP hybrid system could be utilized for self-healing within a matrix material. (b) The process utilized in this study to simulate a selfhealing process similar to that in panel a. Blue and green balls indicate DGEBA- and DMBA-loaded nanoparticles, respectively. (c) SEM micrograph of the surface of a compacted CPNP tablet along with a closer look at a typical indent (inset). Nanoindentation was performed on 36 different points that can be seen as the pyramidal indents of the Berkovich tip. (d) Typical nanoindention curves of a loaded (orange) and unloaded (blue) CPNP tablet. (e) The average nanoindentation hardness and modulus of 36 points for both the unloaded and the loaded compacted CPNP tablet (inset).
(2)
0.5πS / A C 1 − v2
(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 37057
DOI: 10.1021/acsami.7b12532 ACS Appl. Mater. Interfaces 2017, 9, 37055−37063
Research Article
ACS Applied Materials & Interfaces
Figure 4. Self-healing of the bulk CPNP tablet. (a) Creating initial microcracks in the sample to simulate real-world material stress and damage was done through a small-scale 3-point bending setup. An initial flexural stress near ∼0.4 MPa introduced microcracks, verified through SEM (graph background). (b) Sealant bonding between loaded CPNPs at three different length scales. (c) The average compressive strengths after heating the precracked samples to 120 °C for 4 h. A substantial increase in compressive strength is observed for the sealant-loaded CPNPs. (d) SEM micrographs illustrating the fracture surface from compressive testing, clearly verifying the sealant release upon heating and polymerization therein.
silicate-hydrate seed, which is patterned around the surface of the templating surfactant. Because our synthesis mixture is slightly below twice the critical micelle concentration (CMC), most of our micelles are in spherical form, although 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. 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, in which 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 h 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
Figure 5. Sequential stages of heat-responsive self-healing. (a) Mixture of sealant-loaded CPNPs. (b) The topmost surface of the sealantloaded tablet created using pressure-induced assembly. (c) Nanocrack created under flexural mechanical loading. (d) The release of liquid DGEBA onto the topmost surface at the initial stage of heating. (e) Polymerized DGEBA on the topmost surface.
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.
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RESULTS AND DISCUSSION Our refined synthesis of the as-produced calcium silicate porous nanoparticles (CPNPs) begins with the nucleation of a calcium 37058
DOI: 10.1021/acsami.7b12532 ACS Appl. Mater. Interfaces 2017, 9, 37055−37063
Research Article
ACS Applied Materials & Interfaces
similar patterns to those of the unloaded CPNPs, indicating that the loaded DGEBA only serves to fill the pores and has no effect on the integrity of the nanoparticles, as confirmed by TEM and SEM in Figures 1 and 2. In addition, the nitrogen isotherm branches exhibit a broadened and reduced step near P/P0 = 0.4, further confirming the successful filling of the pores by DGEBA. After solvent-assist loading, the pore volume decreased from 0.336 to 0.184 cm3/g, and the surface area decreased from 337.14 to 179.25 m2/g. The direct impregnation of DMBA (initiator) into CPNPs also led to the successful filling of the pores as confirmed by the BET data. As with DGEBA-loaded CPNPs prepared using the solvent-assist method, the DMBA-loaded particles exhibit reduced volume adsorbed at each relative pressure. Furthermore, the pore volume decreased from 0.336 to 0.063 cm3/g, and the surface area decreased from 337.14 to 70.26 m2/g, confirming the successful infiltration. In contrast to DGEBA-loaded (solvent-assist) and DMBAloaded CPNPs, the nitrogen isotherm obtained from direct impregnation of DGEBA remained around 0 until a relative pressure near 1, where interparticle voids become relevant. Most likely, this is due to residual DGEBA on the loaded CPNP surface that may block the pathway of nitrogen into the interior pores. Nevertheless, analysis of the TGA results (Figure 2e) clearly shows weight loss at higher temperatures than is required to remove pure sealant for both the direct and solventassist loading methods. This in turn indicates that, despite the presence of residual sealant, which exists at the exterior of the particles, internal pore loading has also been achieved successfully. In the TGA graph of Figure 2e, the dashed lines represent the lower-bound temperature at which further weight loss can be attributed to pore-loaded sealant (versus the residual surface sealant or absorbed contaminants such as water) and was used to calculate the pore volume filled. For this purpose, 310 °C was selected for DGEBA-loaded CPNPs based on the clear slope change observed during the TGA measurements of direct-loaded CPNPs (Figure 2e), and 140 °C was selected for DMBA-loaded CPNPs because it is the temperature by which pure DMBA evaporates completely during TGA measurements. For the loading of DGEBA, the solvent-assist method filled 40.5% of the pore volume (matching closely to the BET analysis, 0.336−0.184 cm3/g/ 0.336 cm3/g = 45.2%), while the direct method achieved 58.3%. The direct impregnation of DMBA filled 43.0% of the pore volume. Thus, we have successfully demonstrated the synthesis and loading of our unique spherical CPNP building blocks. FT-IR was also performed to further investigate structural features of intact and loaded CPNPs (Figure 2f). The IR spectrum for intact CPNPs shows a very similar pattern of peak positions to tobermorite, a mineral analogue of calcium silicate hydrate (C− S−H), which is a major building block of all cementitous materials.52,53 The spectrum shows an intense peak at 1020 and 780 cm−1, both of which arise from the Si−O−Si stretching vibrations.54,55 Furthermore, the asymmetric stretching bands of CO32− appearing at 1410−1480 cm−1 confirm the presence of calcium carbonate, which is likely to have formed from contamination of our CPNPs with carbon dioxide from atmosphere. The spectrum for pure DGEBA exhibits sharp peaks at 2860−2980 cm−1, arising from stretching vibrations of C−H bonds in its aliphatic and aromatic groups, and also the C−O and C−O−C stretching bands at 915 and 831 cm−1, respectively, because it contains epoxide groups. DMBA also
by sonicating for 5 min 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 h. The 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 showing that the internal pores have been loaded with DGEBA in comparison with the unloaded CPNPs in Figure 1b. Direct impregnation was also adopted for the loading of DMBA (initiator). The SEM image in Figure 2a confirms the universal spherical morphology of our monodisperse nanoparticles 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 is unique for CS nanomaterials, particularly in comparison with 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-to-Si) atomic ratio is 0.56. In our study, most CPNPs had a Ca-to-Si ratio between ∼0.4 and 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 (Table S1), and TGA was used to evaluate which carrier-to-DGEBA ratio leads to the maximum uptake. The optimum ratio was found to be 2:3 and was used for the following sections. Interestingly, Figure 2c shows that the CPNPs maintain their original shape and size after sealant (DGEBA) loading and are not aggregated. The nitrogen adsorption isotherms from BET analysis are illustrated in Figure 2d. Both the adsorption and the 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 isotherms according to the IUPAC classification.49 The welldefined 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 nm).50 The BET surface area of 337.14 m2/g, calculated from the range for P/P0 of 0−0.29, along with the total pore volume of 0.336 cm3/g, calculated at P/P0 = 0.91 (pores of
