Rheological, Mechanical, and Thermal Properties of Silane Grafted

Jun 11, 2018 - Surface Engineering and Tribology Division, Council of Scientific and Industrial Research-Central Mechanical Engineering Research Insti...
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Article Cite This: Ind. Eng. Chem. Res. 2018, 57, 8729−8739

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Rheological, Mechanical, and Thermal Properties of Silane Grafted Layered Double Hydroxide/Epoxy Composites Suman Chhetri,†,‡ Nitai Chandra Adak,†,‡ Pranab Samanta,†,‡ Naresh Chandra Murmu,*,†,‡ and Tapas Kuila*,†,‡ †

Ind. Eng. Chem. Res. 2018.57:8729-8739. Downloaded from pubs.acs.org by UNIV OF SOUTH DAKOTA on 08/31/18. For personal use only.

Surface Engineering and Tribology Division, Council of Scientific and Industrial Research-Central Mechanical Engineering Research Institute, Durgapur 713209, India ‡ Academy of Scientific and Innovative Research (AcSIR), CSIR-CMERI, Campus, Durgapur 713209, India ABSTRACT: Judicious surface design of a nanofiller is pertinent to exploit its reinforcing competency in polymer composites. Herein, sodium dodecyl sulfate intercalated layered double hydroxide (LDH) was functionalized using 3-aminopropyltriethoxysilane (APTES), which was then dispersed in the epoxy matrix. The grafting of APTES on the LDH surface was confirmed by Fourier transform infrared spectroscopy, X-ray diffraction, and thermogravimetric analysis. The robustness of the interface reflected the significant improvement of ∼104 and 90% in flexural strength and modulus at 0.25 wt % loading, while the tensile strength and Young̀s modulus were found to increase by ∼44 and 50.8%, respectively. The tangible improvement in interfacial interaction due to the functionlization was further reflected in the enhanced storage modulus and glass transition temperature, Tg, by ∼36% and ∼6 °C, respectively, at 0.25 wt % loading. A rheological study confirmed the Newtonian behavior of the composites in the shear rate range of 0.1−100 s−1.

1. INTRODUCTION Layered double hydroxide (LDH), also termed anionic clays, constitutes a class of 2D layered material akin to those of layered silicates, which exhibit higher capacity of anion intercalation.1−3 The chemical formula of LDH can be represented by [M 2+ 1−x M 3+ x (OH) 2 ] x+ ·[(A n‑ ) x/n ·yH 2 O] x‑, where M2+, M3+, and An‑ are divalent metal cations, trivalent metal cations, and interlayer anions, respectively.3,4 Over the years, considerable research interest has been devoted to LDH for a wide spectrum of applications such as in biomedical, flame retardant, catalyst, reinforcing filler, novel hybrid materials, etc.5−9 One of the reasons that made LDH attractive for scientific exploration is due to its highly tunable compositions. The presence of substitutable intercalated anions in LDH nanolayers facilitated the prospect to enlarge the interlayer space and to prepare exfoliated LDH-based materials. The advantages of tunable properties and multiple interactions have also made LDH an appealing nanofiller for polymer composites.10−14 However, akin to polymer composites reinforced by other types of reinforcing fillers, the macroscopic properties of LDH based polymer composites depend upon immobilization, exfoliation, and homogeneous dispersions of LDH in the polymer matrix. Thus, suitable interlamellar surface modification of LDH is of utmost importance to improve the compatibility of hydrophilic LDH nanolayers with the polymer matrix. Fortunately, higher anion exchange capacity and hydroxyl groups present at the edges have made it possible to modify the LDH layers, which subsequently increases the interlayer distance and facilitates the homogeneous dispersion of LDH in the host polymer © 2018 American Chemical Society

matrix. Over the years, a wide variety of anionic surfactants such as sulfonates, fatty acids and phosphates, and modifier like silane have been explored.15−19 Surfactant intercalations of LDH nanolayers contribute to the compatibility of LDH with the polymer matrix, thereby facilitating the dispersion and contributing to load sharing and transfer at the interface. Previous studies indicate that LDH modified with anionic surfactants have swelled well in organic solvents and polymers, resulting in robust compatibility and dispersion.6,15 In addition, the intercalated surfactants facilitated the penetration of prepolymers or polymers within the gallery of LDH, further enlarging the interlayer spacing. Thus, intercalated species have been found to assist the exfoliation of LDH nanolayers in the polymer matrix, which is beneficial for overall performance of the composites. Becker et al. used glycinate to prepare organo modified-LDH/epoxy composites and investigated their mechanical, thermal, and flame-retardant properties.13 The composites showed improved self-extinguishing at higher loading, and lower loading of LDH yielded the better mechanical performance. Recently, multimodifier approach has been executed to design multifunctional LDH for high performance polymer composites.10,11 The multimodifier system is a deliberate design to enlarge the interlayer spacing and to improve the chemical affinity of LDH with polymer matrix, by using the appropriate surfactants and modifier. Received: Revised: Accepted: Published: 8729

March 31, 2018 June 6, 2018 June 11, 2018 June 11, 2018 DOI: 10.1021/acs.iecr.8b01399 Ind. Eng. Chem. Res. 2018, 57, 8729−8739

Article

Industrial & Engineering Chemistry Research

Figure 1. Schematic for the intercalation and functionalization of LDH and probable interaction of functionalized LDH with epoxy matrix.

Kalali et al. used hydroxy propyl-sulfobutyl-beta-cyclodextrinsodium (SCD) as a first modifier, sodium dodecyl benzenesulfonate (SDBS) as a second comodifier, and taurine (T) as the third comodifier for LDH and fabricated high performance fire retardant epoxy nanocomposites.10 The report revealed the significant reduction in peak heat release rate, total heat release, and total smoke production on incorporation of SCDDBS-T-LDH into epoxy resins. In another study, a multimodifier system was developed consisting of biobased phytic acid (Ph), functional cyclodextrin (sCD), and SDBS to fabricate epoxy nanocomposites.11 The nanocomposites showed improved flame retardant (reduction of the peak heat release rate over 70%) and anti-UV properties. Both of the studies were more concentrated on engineering the surface properties of LDH, basically to improve the flame retardant of the epoxy composites, where improvements in mechanical properties are not significant. Thus, there lies a scope on how intercalation and modification of LDH affect the mechanical and thermo-mechanical performance of epoxy composites. Further, viscoelastic properties of the polymer composites are sensitive to degree of dispersion and the extent of interfacial interaction between filler and polymer matrix. Hence, the addition level and the nature of the surface modification of LDH would affect the rheological behavior of LDH/epoxy composites considerably. Thus, there lies enough scope to study about how dispersion and distribution of meticulously functionalized LDH affects the rheological behavior of the epoxy composites. In this work, aiming to generate robust interface between LHD layers and epoxy matrix, an intercalation-modification approach is proposed. The modified LDH system consists of (SDS) and 3-aminopropyltriethoxysilane (APTES). As the interlayers of LDH are hydrophilic in nature, absorption of organic polymers on its surface is difficult due to the lack of compatibility. Further, the interlayer spacing of LDH is not big enough for polymer chain intercalation. Therefore, the modification of LDH layers by using suitable organic molecules is pertinent to improve its affinity toward the polymer matrix. Organo-modification of LDH not only

improves the compatibility of LDH with polymer but also enlarges the interlayer distance of LDH, which facilitates the intercalation of the polymer into the gallery. Thus, SDS, in this present study, is expected to perform a dual role: one, to enlarge the interlayer spacing of Mg−Al-LDH and, another, to improve the compatibility of LDH nanolayers with epoxy matrix. It is assumed that the intercalated SDS molecules would assemble themselves within the interlayer gallery in such a manner as to achieve optimum electrostatic interaction with the positive layer. On the other hand, APTES was used as the bridging moiety, to link LDH nanolayers chemically with the epoxy matrix. APTES was selected against other silane coupling agents like 3-glycidoxypropyl−trimethoxysilane. The free −NH2 groups of grafted APTES can react with the vulnerable oxirane rings of epoxy resin, rendering outstanding compatibility between the functionalized LDH and epoxy matrix. Thus, grafting of APTES on LDH was a deliberate approach to ensure the chemical interaction between the epoxy matrix and functionalized LDH. This interaction ensures a direct link between the two phases, thereby optimizing interfacial adhesion. Figure 1 shows the schematic of LDH modification and the probable interaction of functionalized LDH with epoxy matrix. The effect of functionalized LDH nanolayers on the mechanical, rheological, and thermal properties of the epoxy composites was studied.

2. EXPERIMENTAL SECTION 2.1. Materials. The low viscosity liquid modified Bisphenol-A epoxy resin (LAPOX*C-51) and low viscosity modified cycloaliphatic amine hardener (Lapox AH-428) were bought from Atul Limited (Gujarat, India) to make the matrix system in this study. Mg(NO3)2·6H2O, Al(NO3)3·9H2O (Merck, India), and SDS (SRL Pvt., Mumbai, India) were used as received. Tetrahydrofuran (THF) and toluene were purchased from SRL, Mumbai, India, while APTES was procured from Sigma-Aldrich. 2.2. Preparation of LDH and SDS Intercalated LDH. LDH was synthesized by coprecipitation and thermal crystallization technique, using mixed solutions of Mg8730

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Industrial & Engineering Chemistry Research (NO3)2·6H2O and Al(NO3)3·9H2O with a Mg/Al ratio of 3. In 100 mL of distilled water, Mg(NO3)2·6H2O (0.25 mol, 19.65 g) and Al(NO3)3·9H2O (0.75 mol, 9.25 g) were dissolved completely. The mixed metal solution was slowly added to an aqueous solution made up of 0.25 mol (2.65 g) of Na2CO3 and 0.2 mol (8 g) of NaOH while stirring. Throughout the addition, the pH of the solution was maintained at ∼10 by using 1 M NaOH solution. Afterward, the white precipitate was aged for 16 h at 70−75 °C and filtered using a 0.1 mm (pore size) cellulose acetate membrane, washed with deionized water until the pH reached 7, and dried at 80 °C for 24 h, inside a vacuum oven. In order to alter the hydrophilic properties, SDS was intercalated within the gallery of LDH nanolayers. For this, first, ∼2.5 g of LDH was calcinated for 6 h at 500 °C, and the finely ground powder was suspended in 100 mL of distilled water containing ∼2.5 g of SDS. The suspended solution was vigorously stirred at 70 °C for 12 h and then further refluxed for another 6 h to obtain a white powder of SDS intercalated LDH (DS-LDH). 2.3. Preparation of APTES Surface Modified DS-LDH. First, finely powdered DS-LDH was suspended in distilled water (75 mL) and ethanol (25 mL) in a two-necked flask under constant stirring at ∼50 °C for ∼30 min. The APTES solution was prepared in a separate beaker by dissolving 0.5 g of APTES in the mixture of ethanol (45 mL) and water (5 mL). The APTES solution was then slowly added to the DSLDH suspension under vigorous stirring, followed by the addition of acetic acid, until the pH reached ∼3. Then the temperature of the reaction was increased to 90 °C and kept for 24 h under nitrogen atmosphere. Finally, the white precipitate was filtered, repeatedly washed with distilled water, and dried under vacuum at 80 °C for 24 h. The APTES modified DS-LDH was designated as APTES-DS-LDH. 2.4. Preparation of APTES-DS-LDH/Epoxy Composites. For epoxy composite preparation, APTES-DS-LDH was dispersed in THF using water bath sonication for 1 h. The APTES-DS-LDH dispersion was added into the epoxy resin, and the composite mixture was further sonicated for 1 h followed by heating on a hot plate at ∼60 °C for 5 h under reduced pressure, to remove excess solvent. The composite mixture was kept inside a vacuum oven at ∼70 °C for ∼16 h to remove the residual solvent. The APTES-DS-LDH/epoxy composite was cooled to room temperature, and the desired amount of hardener was mixed by high-speed laboratory mechanical mixture at ∼1,800 rpm for 4 min. The composites were then placed inside a degassing chamber for ∼30 min to remove air-bubbles and residual solvent. The mixture was then poured into a silicon mold and cured at room temperature for 24 h followed by post curing at 80 and 100 °C for 2 h in each case. 2.5. Characterization. Fourier transform infrared (FT-IR) spectra were recorded with PerkinElmer RXI FT-IR in the frequency range of 4000−400 cm−1. X-ray diffraction (XRD) of LDH, DS-LDH, APTES-DS-LDH, and APTES-DS-LDH/ epoxy composites was carried out with PANalytical (model X pert PRO) at a scan rate of 0.106° s−1. Field emission scanning electron microscopy (FE-SEM) was carried out with ∑igma HD, Carl Zeiss, Germany. Transmission electron microscopy (TEM) was carried out using TEM 2100 (JEOL, Japan) at 200 kV. For TEM observation of APTES-DS-LDH/epoxy composites, an ultrathin sample of thickness ∼70 nm was cut using Leica Ultracut UCT (Leica EMFCS, U.S.A.) ultramicrotome at room temperature. Microtomed APTES-

DS-LDH/epoxy samples were collected over copper grids for TEM study. Thermogravimetric analysis (TGA) was carried out with Jupiter STA 449 F1, Netzsch, Germany to study the thermal stability of the composites. The samples (∼5.56 mg) were heated from 30 to 750 °C at a heating rate of 5 °C min−1 under air atmosphere. The rheological properties of the APTES-DS-LDH/epoxy suspension (without hardener) were determined by using a stress-controlled rotational Rheometer MCR 501 (Anton Paar, Austria) set with a Peltier heating system and parallel plate−plate fixture of 25 mm. The gap size was determined in the range 0.7−1 mm depending on the APTES-DS-LDH content in epoxy resin. Steady shear measurements were conducted at shear rates in the range of 0.1−100 s−1 at 35 °C. Tensile measurements were carried out according to ASTM D-638 using a Tinius Olsen h50KS universal testing machine at 25 °C with a crosshead speed of 2 mm min−1. The dimension of the cured dumbbell shaped sample was 77 mm in length (working length of sample was 26 mm), 5 mm in width, and 2.5−3 mm in thickness. Five specimens of each composite were tested to obtain the standard deviation. The flexural measurements were carried out in a three-point bending test according to ASTM D790, with specimen dimensions of 128 mm × 13 mm × 4 mm, using a Tinius Olsen h50KS universal testing machine at 25 °C, with a crosshead speed of 2 mm min−1. Dynamic mechanical analysis (DMA) was carried out with DMA 8000 PerkinElmer in the temperature range of 30−200 °C, with a heating rate of 3 °C min−1 at a constant frequency of 1 Hz at a load strain of 0.10 mm. A PerkinElmer Pyris Diamond differential scanning calorimetry (DSC) was used for thermal characterization of the composites sample. The DSC measurements were carried out in a nitrogen atmosphere over the temperature range from 20 to 150 °C. A swelling test was performed by soaking a rectangular shaped pure epoxy and composites samples in toluene at room temperature for 3 days. The cross-linking density was calculated by a swelling test using toluene as the solvent.20,21 The cross-link density was measured using the values of Vr and Vp that were obtained, using the equation: cross‐link density =

−[ln(1 − Vp) + Vp + χVp2

(

DpV0 Vr −

Vp 2

)

(1)

where Vp = 1/(1 + Q), Q is the ratio of the weight of the solvent in the swollen polymer, (χ. Dp), and the weight of polymer, (χ. D0). Here Vp volume fraction of the polymer in the swollen polymer, Dp is the density of the polymer (g/cm3), V0 is the molar volume of the solvent (cm3/mol), and D0 is the density of the solvent (g/cm3).20 The Huggins epoxy−solvent interaction constant, χ, can be calculated from eq 2. χ = 0.431 − 0.311Vr − 0.036Vr 2

(2)

The volume fraction of the epoxy can be obtained from eq 3 Vr =

(0.56e−θ + 0.44)ω0ρs ω0ρs + ωsρr

(3)

where θ is the concentration (wt %) of LDH in the composites, ω0 is the weight (g) of the dry sample, ωs is the weight (g) of the absorbed solvent, and ρs and ρr are the densities of the solvent and epoxy, respectively.21,22 8731

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3. RESULTS AND DISCUSSION 3.1. Characterization of APTES-DS-LDH. FT-IR spectra were used to monitor the appearance or disappearance of chemical functional groups on LDH nanolayers. Figure 2

of CH3. Further, the two peaks appeared at 1215 and 1067 cm−1 are related to the −OSO3− stretching vibrations.18 All of these observations confirmed the intercalation of SDS on the LDH structure. The FT-IR spectrum of APTES-DS-LDH was found to be different from that of pristine LDH. Grafting of APTES onto the LDH layer brought a group of new absorption peak due to −NH2 and Si−O in the region of 1400−1600 and 950−1100 cm−1. The peaks at 1580 and 1476 cm−1 can be assigned to the −NH2 scissoring of the grafted APTES. The absorption peak at 2854 and 2928 cm−1 for APTES-SDS-LDH can be attributed to the C−H stretching vibrations of −CH3 and −CH2, respectively. The appearance of these peaks confirmed the organo-modification of LDH. The absorption bands that appeared at 1062 and 1382 cm−1 can be related to the typical Si−O−Si symmetric stretching and to −OSO3− stretching vibrations, respectively. The peak at 980 cm−1, which is related to the Si−O−M vibration, further confirmed the grafting of APTES molecules on the DS-LDH through condensation reaction between Si−OH and −OH of the LDH nanolayers.23 The emergence peaks related to −CH3, −CH2, and −OSO3− confirmed the grafting and intercalation of APTES and SDS on LDH layers. For comparison, the FT-IR spectra of DS-LDH were recorded and found to be almost similar to that of APTES-LDH except the peaks related to −OSO3−. XRD analysis is a useful method to assess the degree of intercalation and exfoliation of the nanoadditives having layered structure. Figure 3a,b shows the XRD pattern of MgAl-CO3-LDH, DS-LDH, APTES-LDH, and APTES-DSLDH in the 2θ range of 2−10° and 10−70°, respectively. MgAl-CO3-LDH exhibited a well-ordered layered structure with peaks at 2θ = 11.4°, 22.8°, and 33.5° corresponding to the typical (003), (006), and (009) reflection planes, respectively.24 The XRD pattern of MgAl-CO3-LDH did not exhibit

Figure 2. FT-IR spectra of Mg−Al LDH, DS-LDH, APTES-LDH, and APTES-DS-LDH.

shows the FT-IR spectra of MgAl-CO3-LDH, DS-LDH, and APTES-DS-LDH. The intense peaks at ∼3487 and 1643 cm−1 are ascribed to the stretching and bending vibrations of O−H groups of water molecules in the gallery of MgAl-LDH nanolayers. The peak at ∼1388 cm−1 is assigned to the stretching mode of the carbonate (CO3−2) anion. On intercalation of SDS, two intense peaks at 2852 and 2921 cm−1 were observed, which are attributed to the typical C−H stretching vibrations of −CH3 and −CH2. The strong peaks at 1468 and 1382 cm−1 are due to the C−H deformation vibration of CH2 and CH3 and the C−H symmetric vibration

Figure 3. XRD patterns of LDH, DS-LDH, and APTES-DS-LDH (a) small angle, (b) wide angle, and (c and d) small and wide angle XRD pattern of pure epoxy and APTES-DS-LDH/epoxy composites. 8732

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Industrial & Engineering Chemistry Research any peak at lower diffraction angles of 2θ = 2−10°. In contrast, the DS-LDH showed a set of peaks at lower diffraction angles. The shifting of peaks toward lower diffraction angles signifies an enlarged basal plane, which indicated that the anionic SDS has been successfully intercalated in between the LDH layers, similar to previously reported anionic SDS modified LDH.25 Further, it was found that the mother LDH structure is partly preserved, as suggested by the appearance of reflections of (003), (006), (009), and (110) planes. The diffraction pattern of APTES-LDH did not show any peaks at lower diffraction angles, which indicates that the grafting of APTES took place on the surface rather than the interlayer of LDH. The diffraction pattern of APTES-DS-LDH showed peaks at lower diffraction angles of 2θ ≈ 3.2°, suggesting that the grafting of APTES did not alter the DS-LDH gallery significantly. However, reflections of APTES-LDH and APTES-DS-LDH in the 2θ range of 10−30° were intensified. This indicates that the grafting of silane onto the surface hydroxyl groups of LDH has little influence on the long-range order of LDH during the modification procedure.26 XRD technique was also used to study the degree of dispersion and exfoliation of functionalized LDH in epoxy matrix and the structural changes if any. Figure 3c,d shows the small and wide angle XRD pattern of pure epoxy, and it is composite with APTES-DS-LDH. Pure epoxy showed a broad diffraction peak centered at 2θ ≈ 18° due to the scattering of cured epoxy molecules, which reflected its amorphous nature.27 The APTES-DS-LDH/epoxy composites also showed similar diffraction patterns as that of pure epoxy. The absence of the characteristic diffraction peak of the layered hydroxide indicated that these layered structures could have fully exfoliated and dispersed in the epoxy matrix. In addition, low concentration of APTES-DS-LDH in the epoxy composite could have resulted in segregation of layers leading to poor diffraction. This is probably due to the inclusion of epoxy polymer chains within the gallery of the LDH. It was also expected that the polar interaction between APTES-DS-LDH and epoxy might have played a crucial role in the insertion of polymer chains within the LDH gallery. The appearance of XRD patterns of these kinds, therefore, indicates the intercalated or partially exfoliated class of functionalized LDH/epoxy composites. In order to investigate the organic intercalation and grafting of APTES, the corresponding LDH derivatives were subjected to thermal degradation in air atmosphere from room temperature to 750 °C. Figure 4 shows the thermal degradation behavior of LDH, DS-LDH, and APTES-DSLDH. Thermal analysis of pristine LDH showed two-step weight loss, the first step from 28 to 265 °C with the total mass loss of ∼18%. The weight loss in this temperature range is attributed to the elimination of the physisorbed water molecule and CO3−2 ions and intercalated water molecules on the LDH gallery. The second step took place from 265 to 440 °C, which is ascribed to the decomposition of metal hydroxide layers. The range corresponding to absorbed water loss peak for DSLDH (28−217 °C) was found to be less than that of pristine LDH. The reduced decomposition peak suggested that the water molecules weakly interacted with the dodecyl sulfate. The weight loss in the temperature range of 217−430 °C could be ascribed to dehydroxylation. Thus, it can be said that intercalation of the SDS molecule did not affect the −OH group content in the interlayer of LDH. The decomposition peak at 400−504 °C could be assigned to the loss of alkyl

Figure 4. TGA and DTG curves of LDH, DS-LDH, and APTES-DSLDH.

chains of SDS and residual CO3−2 ions. The APTES grafted LDH showed a thermal degradation profile different from that of the unmodified LDH and DS-LDH. The mass loss peak corresponding to the physisorbed and interlayer water molecule disappeared completely, signifying water consumption during grafting of APTES with LDH. The most striking difference is that a small percentage of mass loss was recorded in the process of dehydroxylation as compared to those for unmodified LDH and DS-LDH. The diminished content of −OH groups indicated the consumption of −OH groups during the condensation reaction with −Si−OH to form −Si− O−M. The thermal decomposition profile of APTES-DS-LDH corroborated well with FT-IR spectra confirming the grafting of APTES moiety onto the surface of LDH. 3.2. Rheological Behaviors of APTES-SD-LDH/Epoxy Suspension. The rheological analysis of APTES-DS-LDH filled epoxy suspensions was carried out to investigate the variations in flow behavior and absolute viscosity, as it was observed that the incorporation of functionalized LDH on the epoxy matrix affected the properties of epoxy composites in the solid state. Thus, the effect of functionalized LDH and its content on the rheological properties in the molten states were studied, and the corresponding results are discussed in terms of viscosity, shear stress variation, and loss modulus (G″). It is expected that the rigid filler influenced the molecular dynamics and chain movement of the polymer matrix, which impedes the motion of the polymer chains. Further, rheological study is relevant to understand the degree of interaction between the filler and polymer matrix. The variations in viscosity and shear stress are generally expressed as a function of shear strain rate. As depicted in Figure 5a, the shear stress of the composites almost increased linearly with the shear rate, and the values of the generated shear stresses of the LDH modified epoxy suspension were found to be more than that of pure epoxy resin. It confirmed that the interfacial bonding between the APTES-DS-LDH and epoxy resin made the composite stronger. The change in absolute viscosity of the APTES-DSLDH/epoxy composites is shown in Figure 5b. The inset illustrates the magnified portion of Figure 5b at higher shear rate (15−100 s−1). Appearance of a horizontal line in the viscosity versus shear rate curve indicated Newtonian fluidic nature and the decrease in viscosity with increasing the shear rate is termed as shear thinning behavior.28 The viscosities of all of the composites, except 1 wt % APTES-DS-LDH loading, increased with increasing shear rate, especially at high shearing 8733

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Figure 5. (a) Shear stress vs shear rate, (b) viscosity as a function of shear rate, the inset illustrates the magnified portion of viscosity within the shear rate of 15−100 s−1, (c) loss modulus (G″) vs frequency of pure epoxy and APTES-DS-LDH/epoxy composites, and (d) variation of viscosity against shear rate of 1 wt % APTES-DS-LDH/epoxy composites. The inset shows the shear stress as a function of shear rate.

Figure 6. (a) Stress−strain cure of pure epoxy and APTES-DS-LDH/epoxy composites and (b) variations of Young’s modulus against weight fraction of APTES-DS-LDH.

velocity. Thus, the composites showed Newtonian behavior in the shear rate range of 0.1−100 s−1. The increase in viscosity of the epoxy-APTES-DS-LDH mixture is accounted to the network structure formed by APTES-DS-LDH with the epoxy matrix. At lower loading, due to homogeneous dispersion of APTES-DS-LDH, the contacting area between APTES-DS-LDH and epoxy matrix might have improved, leading to robust network formation and, hence, improved viscosity. The decreased in viscosity at 1 wt % APTES-DSLDH loading can be attributed to the increased free volume, resulting from residual solvent that was used to disperse the APTES-DS-LDH in the epoxy matrix prior to rheological analysis. In order to completely disperse the particles in the epoxy matrix, a comparatively higher amount of solvent was required for 1 wt % APTES-DS-LDH/epoxy suspension; hence, there is a chance that solvents might not have

evaporated completely. As can be observed from the inset of Figure 5b, the difference was not very significant. The viscosity study of 1 wt % APTES-DS-LDH loaded epoxy composites (without hardener) was carried out to investigate the effect of residual solvent. First, the required amount of APTES-DSLDH particles was finely ground into a fine powder and immersed in epoxy resin for around 36 h without any external disturbance for the proper wetting of the epoxy resin. The mixture was then heated with vigorous stirring at ∼400 rpm for ∼1 h. The mixture was heated to lower the viscosity of the epoxy resin and to improve the diffusion rate of the epoxy resin into the galleries of the LDH nanolayers. To ensure the fine dispersion, the suspension was further mixed using a highspeed laboratory mechanical mixture at ∼3000 rpm, for 10 min. The variation in absolute viscosity of the APTES-DSLDH/epoxy suspension (1 wt %) is shown in Figure 5d, where 8734

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Figure 7. (a) Flexural stress vs strain response of pure epoxy and APTES-DS-LDH/epoxy composites and (b) effect of APTES-DS-LDH content on the flexural modulus of APTES-DS-LDH/epoxy composites.

Figure 8. FE-SEM images of the tensile fractured surface of neat epoxy (a and b) and APTES-DS-LDH/epoxy composites (c and d), respectively.

properties were investigated. The typical stress−strain curves of pure epoxy and APTES-DS-LDH/epoxy composites are shown in Figure 6a. The calculated values of Young’s modulus are presented in Figure 6b. At a loading of 0.25 wt %, APTESDS-LDH/epoxy composites exhibited ∼44 and 50.8% improvement in ultimate tensile strength and Young’s modulus, respectively, which is significantly higher than the tensile properties reported in prior studies.10,11,13 The interfacial interaction between epoxy matrix and functionalized LDH was anticipated to be robust due to the presence of amine functionalities on the APTES-DS-LDH surface, which can chemically interact with the epoxy network. Thus, the improved strength of the composites could be accounted to the formation of a strong interface between the epoxy matrix and APTES-DS-LDH endures enough to share and transfer the applied load. Further, an increase in APTES-DS-LDH loading

the inset depicts the shear stress of the composites against the shear rate. It was found that the viscosity of the APTES-DSLDH/epoxy suspension (1 wt %) was found to be slightly higher than the neat epoxy. The slightest increase in viscosity against neat epoxy could be due to the improper distribution of functionalized LDH in the epoxy matrix and the relatively weaker epoxy−LDH interaction. The above observation established the impact of residue solvent on viscosity of the APTES-DS-LDH/epoxy composites. Figure 5d shows the variation of G″ against frequency. G″ quantifies the energy dissipation of the suspensions, which is related to interfriction of the materials. The improved G″ can be accounted to the interfacial friction between APTES-DS-LDH and epoxy matrix. 3.3. Mechanical Properties. In order to understand the effects of chemically functionalized LDH on the mechanical properties of the final composites, tensile and flexural 8735

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Figure 9. TEM images of (a) 0.25, (b) 0.5, and (c) 1 wt % APTES-DS-LDH/epoxy composites.

sites appeared rough and coarse with numerous hackles and crazing (Figure 8c,d). The rough morphology revealed the generation of a much newer fracture surface area as a result of crack obstruction by embedded functionalized LDH particles. The firmly entrenched particles impeded the proliferation of cracks to follow a normal path, thus driving the cracks in other new routes leading to surplus absorption of energy. It is observed that, at lower loading, the fracture surface appeared with less dimples; however, it is rougher than that of the pure epoxy. 3.5. Exfoliation and Dispersion of APTES-DS-LDH in Epoxy Matrix. In order to validate the exfoliation and dispersion of functionalized LDH into the epoxy matrix, TEMs of the microtome composite samples were carried out. Figure 9a−c shows the TEM images of 0.25, 0.5, and 1 wt % APTESDS-LDH/epoxy composites. The bright white region can be attributed to the epoxy matrix, while the black region represents the presence of embedded particles. The micrograph corresponding to the 0.25 wt % epoxy composite (Figure 9a) showed relatively uniform dispersion of APTES-DS-LDH. It appeared that the LDH lamellar were firmly entrenched in the epoxy matrix, which could have resulted from the strong interfacial interaction between meticulously functionalized LDH and the epoxy matrix. On the contrary, dense black regions were observed for 0.25, 0.5, and 1 wt % APTES-DSLDH/epoxy composites. The appearance of black spots could be due to the aggregation or stacking of LDH layers, which must have defied the transmission of beams of electrons. When the content of APTES-DS-LDH was increased to 1 wt %, the proportion of dark spot was found to increase further (Figure 9c). Thus, it can be inferred that, with the increase in APTESDS-LDH weight fractions, the aggregation becomes more prominent. The observation derived from TEM micrographs corroborated very well with the achieved properties.

slightly diminished the tensile strength and Young’s modulus, which could be due to the generation of stress concentration region at higher loading, inducing early failure of composites. The enhancement in Young’s modulus can be ascribed to the immobilization of the epoxy network caused by the finely dispersed LDH lamellar. However, the elongation at break, εb, value of the epoxy matrix was found to diminish on addition of the APTES-DS-LDH. Figure 7a,b shows the typical flexural stress−strain curve and flexural modulus of the APTES-DS-LDH/epoxy composites against varied amounts of APTES-DS-LDH. The flexural modulus was derived from the slope of stress−strain curve. It is remarkable to note that the flexural strength and modulus of the composites improved significantly. With the addition of 0.25 wt % of APTES-DS-LDH, the flexural strength and modulus increased by ∼104 and 90%, respectively. The highly dispersed APTES-DS-LDH firmly interacted with the epoxy network resulting in a strong interface that facilitated efficient load sharing and transfer, leading to significant improvement in the flexural strength. The chemical bonding between the epoxy matrix and APTES-DS-LDH at the interface could have hindered mobility in the polymer segment, which in turn leads to improve flexural modulus. 3.4. Morphological Investigation Tensile Fracture Sample. FE-SEM images of the tensile fractured sample were recorded to study the variations in the micromorphology of fracture surface of pure epoxy and it is composite with APTES-DS-LDH. Figure 8a,b shows smooth and fragile fracture surfaces of pure epoxy, which is the characteristics of brittle materials. Smooth fracture morphology indicated poor absorption of energy and, hence, low impact resistance of the matrix. The area between the cleavages of flat plane appeared featureless (Figure 8b), which is the evidence for rapid proliferation of cracks in a linear manner. In contrast, the fracture morphology of the APTES-DS-LDH/epoxy compo8736

DOI: 10.1021/acs.iecr.8b01399 Ind. Eng. Chem. Res. 2018, 57, 8729−8739

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Industrial & Engineering Chemistry Research 3.6. Differential Scanning Calorimetry Analysis of APTES-DS-LDH/Epoxy Composites. The DSC curves for the neat epoxy and the composites with APTES-DS-LDH are shown in Figure 10. DSC experiments of all epoxy composite

which could be possibly due to the agglomeration associated with higher weight content. 3.8. Thermo-Mechanical Analysis. In order to understand the impact of meticulously designed LDH on the properties of epoxy composites, dynamic mechanical analyses were carried out, as, dynamic properties of the composites are sensitive to interfacial interaction between filler and polymer matrix. Figure 11a,b shows the dynamic mechanical properties in terms of storage modulus (E′), damping parameter, tan delta, etc. As it is discernible from the modulus diagrams, the composites showed improved modulus in the glassy and rubbery region, i.e., before and after glass transition temperature (Tg), as compared to that for the pure epoxy. In order to compare the modulus quantitatively, the storage modulus of pure epoxy and composites at 30, 60, 120, and 180 °C were considered and are summarized in Table 2. The E′ value of the composites increased from ∼497.52 MPa for neat epoxy to ∼677.16 MPa at 0.25 wt %, which indicated an improvement of ∼36%. The improvement in E′ within the rubbery region substantiates the robust interaction between the host matrix and filler, manifesting thermomechanical stability of the composites at high temperature. The improvement of storage modulus across the entire temperature range corroborates the fact that functionalized LDH has successfully reinforced the epoxy network. It is expected that the amine functionalities on the surface of functionalized LDH assisted the LDH nanolayers to chemically interact with epoxy network, which led to a robust interface. Further, due to intercalation and functionlization of LDH, the lamellar structure of LDH is more exposed, which rendered optimum contact with the epoxy matrix, thereby restricting the movement of epoxy network. Thus, considering the lamellar structure of LDH and the robust interaction between the functionalized LDH and epoxy network, the enhancement in the storage modulus of the composites across the studied temperature is justified. The tan δ curve of the pure epoxy and its composites against temperature is shown in Figure 10b, and the temperature at damping peak represented the Tg value of the composites. The epoxy chains, which are directly in contact with the LDH nanolayers, would behave differently than epoxy chains present in the bulk. It is expected that those epoxy chains present at the interface were firmly adsorbed on the surface and within the gallery of LDH, which restricted the free movement of the epoxy network. As it can be expected that the majority of the epoxy chains were adsorbed within and outside the gallery of LDH due to chemical interactions, hence, the Tg value of the composites was shifted toward higher temperature. Tg values for composites (0.25 wt %) increased to 62 °C against 56 °C of the pure epoxy, signifying that the cross-linking density of the epoxy network significantly improved on incorporation of APTES-DS-LDH. The shifting of Tg values toward higher temperatures indicated that functionalized LDH accelerated curing reaction of epoxy. As the APTES-DS-LDH possesses amine functionalities, so it was expected that the curing reaction between epoxy and hardener would be hindered. However, the results defy this assumption. The reason for this phenomenon may be the presence of plenty of −OH groups on LDH surfaces, which might have autocatalyzed the oxirane ring opening reaction, as −OH groups possess autocatalytic property. The Tg values achieved from the midpoint of the endothermic heat flow in DSC measurement were found to be less than the Tg derived from DMA. It could be due to the difference in analysis method, as the sample size and

Figure 10. DSC plots for pure epoxy and epoxy composites with various weight fractions of APTES-DS-LDH.

samples displayed a single exotherm peak suggesting the epoxy-amine ring opening reaction during the course of the curing reaction. It was observed that, for pure epoxy, the peak temperature was found to be at ∼53.3 °C, which was shifted to ∼57 °C with the addition of 0.25 wt % of APTES-DS-LDH. The increase in peak temperature could be possibly due to the increase in cross-linking density in epoxy composites associated with fine dispersion of functionalized LDH particles. The finely dispersed functionalized LDH must have acted as a physical interlock points in the epoxy matrix, thereby impeding the movement of the molecular chain of epoxy. To further confirm the probable role of APTES-DS-LDH in enhancing the cross-linking density, swelling test was performed, which has been discussed in the next section. 3.7. Cross-Link Density. A swelling test was performed to measure the cross-linking density of neat epoxy and its composites. The cured network structure of the epoxy resists absorption of most of the organic solvents. The solvent cannot penetrate the cured structure but can cause swelling. The extent of swelling can be correlated with the degree of crosslinking density in the cured epoxy resin or composites. Table 1 Table 1. Cross-Link Density of Neat Epoxy and Its Composites Obtained from Swelling in Toluene sample code

sample weight (g)

swollen weight (g)

Vr

cross-linking density

pure epoxy 0.25 wt % 0.5 wt % 1 wt %

1.483 1.401 1.130 1.315

1.694 1.574 1.293 1.467

0.843 0.754 0.659 1.694

0.0194 0.0269 0.0343 0.0086

shows the cross-linking densities of epoxy and the composite samples derived from swelling test measurements in toluene. The cross-linking densities for the APTES-DS-LDH/epoxy composites were determined following the observation of Kudus et al. studied for epoxy/graphene composite system.21 The cross-linking densities of the composites was found to increase against pure epoxy except 1 wt % APTES-DS-LDH, 8737

DOI: 10.1021/acs.iecr.8b01399 Ind. Eng. Chem. Res. 2018, 57, 8729−8739

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Industrial & Engineering Chemistry Research

Figure 11. (a) Variation of storage modulus (E′) with temperature and (b) variation of tan δ with temperature of epoxy composites at different weight percentage of APTES-DS-LDH.

epoxy and 0.25 wt % composites were found to be ∼176 and 202 °C, respectively, corresponding to ∼26 °C improvement, while ∼189 and 192 °C were recorded for 0.5 and 1 wt % composites, respectively. The significant improvement in onset degradation temperature may be due to the obstruction rendered by lamellar LDH particles to the oxygen molecules from being diffused into the epoxy matrix, which delayed further oxidation of epoxy matrix. The T10 was found to improve remarkably for the studied composites. The T50 for the 0.5 wt % composite was found to be comparable to that of pure epoxy, which is likely due to the dilution effect caused by residual solvents. Hence, it can be concluded that firmly interconnected functionalized LDH enabled epoxy matrix to sustain longer during thermal degradation.

Table 2. Storage Modulus, Glass Transition Temperature, and Height of tan δ of Neat Epoxy and Its Composites at Different Temperatures sample code

E′ (MPa) 30 °C

E′ (MPa) 60 °C

E′ (MPa) 120 °C

E′ (MPa) 180 °C

Tg (°C)

pure epoxy 0.25 wt % 0.5 wt % 1 wt %

497.52 677.16 578.79 557.34

62.08 117.17 100.64 58.40

13.09 15.39 13.60 12.00

10.09 17.45 15.74 11.11

56 62 60 58

instrument cavity in DMA testing are larger than that in DSC measurements. 3.9. Thermogravimetric Analysis. Incorporation of lamellar filler in the polymer matrix is expected to improve the thermal degradation behavior of the composites. Figure 12

4. CONCLUSIONS Herein, functionalization of SDS intercalated LDH was performed using APTES for the development of epoxy composites. The epoxy was selected as a model system, as it contains epoxide groups, which had a higher probability of interacting with the amine functionalities of grafted APTES, leading to a strong interface. The grafting of APTES on LDH was confirmed by FT-IR, XRD, and TGA analyses. The functionalized LDH showed improved dispersion in the organic solvent THF. The prepared composites showed remarkable improvement of ∼104 and 90% in flexural strength and modulus, respectively, with 0.25 wt % loading. The tensile strength and Young’s modulus of the composites, at 0.25 wt % loading, were found to increase by ∼44 and 50.8%, respectively. Rheological study was carried out to understand the impact of functionalized LDH on flow behavior and absolute viscosity of the epoxy resin. It was found that, up to a certain loading, the composites exhibited Newtonian behavior in the shear rate range of 0.1−100 s−1. It was observed that the judiciously engineered surface of LDH had a profound impact on the microscopic properties of composites, as revealed by the significantly improved mechanical properties. Further, the robust interface was reflected in improved DMA properties. The storage modulus was found to be enhanced by ∼36% and Tg by 6 °C at 0.25 wt % loading. The Tonset(T5) of the 0.25 wt % APTES-DS-LDH/epoxy composites was found to be improved by ∼25.89 °C against pure epoxy.

Figure 12. TGA curves of APTES-DS-LDH/epoxy composites.

shows the TGA curves of pure epoxy and APTES-DS-LDH/ epoxy composites. The TGA curve showed one-step degradation behavior of pure epoxy and APTES-DS-LDH/ epoxy composites, suggesting that the presence of functionalized LDH did not alter the degradation behavior of epoxy matrix significantly. The main weight loss of epoxy occurred between 320 and 383 °C owing to the thermal decomposition of the epoxy network.29 The parameters derived from TGA curves, such as, temperature at 5% weight loss (T5), temperature at 10% weight loss (T10), and temperature at 50% weight loss (T50), were investigated to characterize the thermal stability of prepared composites. The T5 for pure

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. 8738

DOI: 10.1021/acs.iecr.8b01399 Ind. Eng. Chem. Res. 2018, 57, 8729−8739

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Industrial & Engineering Chemistry Research *Tel.: +91-9647205077. Fax: 91-343-2548204. E-mail: tkuila@ gmail.com.

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ORCID

Tapas Kuila: 0000-0003-0976-3285 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are thankful to the Director of CSIR-CMERI. The authors are also thankful to the Department of Science and Technology, New Delhi, India for the financial supports GAP211412.

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DOI: 10.1021/acs.iecr.8b01399 Ind. Eng. Chem. Res. 2018, 57, 8729−8739