Ti3C2Tz MXene Nanocomposites Synthesized by in Situ Ring

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Applications of Polymer, Composite, and Coating Materials 3

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Nylon-6/TiCT MXene Nanocomposites Synthesized by In Situ Ring Opening Polymerization of #-caprolactam and their Water Transport Properties Michael Carey, Zachary Hinton, Maxim Sokol, Nicolas Javier Alvarez, and Michel W. Barsoum ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b05027 • Publication Date (Web): 09 May 2019 Downloaded from http://pubs.acs.org on May 10, 2019

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ACS Applied Materials & Interfaces

Article

Nylon-6/Ti3C2Tz MXene Nanocomposites Synthesized by In Situ Ring Opening Polymerization of -caprolactam and their Water Transport Properties Michael Carey [a], Zachary Hinton [b], Maxim Sokol[a], Nicolas Alvarez[b], Michel Barsoum*[a] [a] Department of Materials Science and Engineering Drexel University Philadelphia, PA 19104, USA; E-mail: [email protected] [b] Department of Chemical and Biological Engineering Drexel University Philadelphia, PA 19104, USA Keywords: MXene; Polymer nanocomposites; water permeation; polyamides; water diffusion coefficients

Abstract Clay-reinforced nylon-6 nanocomposites (NCs) - characterized by the full exfoliation of the nanoreinforcement – were introduced in the marketplace in the 1990s. Herein we demonstrate, for the first time, that Ti3C2Tz MXene can be incorporated into nylon-6 to synthesize melt-processable nanocomposites with excellent barrier properties (94% reduction in water vapor permeation). To intercalate the -caprolactam monomer between the MXene multilayers they were first treated with 12aminolauric acid, a low-cost, non-toxic, biodegradable, and long shelf life compound. Upon heating to 250 °C, in the presence of 6-aminocaproic acid, in situ polymerization occurred, yielding meltprocessable nylon-6/MXene NCs that were, in turn, studied by thermogravimetric analysis, differential scanning calorimetry, X-ray diffraction, scanning and transmission electron microscopy, infrared spectroscopy, and dynamic vapor sorption analysis. Using the latter, moisture-sorption isotherms of a neat and a 1.9 vol. % NC, at 60 °C, were fit to the Guggenheim, Anderson and de Boer (GAB) equation. Solubility, permeation and diffusion coefficients of water through the NCs were measured as a function of temperature and found to be the lowest ever reported for nylon-6, despite the fact that, at ≈1.9 and 5.0 vol. %, the MXene loads were relatively low. This record low diffusivity is ascribed to the very large aspect ratios – 500 to 1000 - of Ti3C2Tz flakes and their dispersion. The water permeation rate is a factor of 5 lower than the best reported in the much more mature nylon/clay field, suggesting lower values can be achieved with further optimization. Lastly infrared spectroscopy spectra of neat and NC samples suggest the surface terminations of the 12-Ti3C2Tz flakes bind with nylon-6, limiting water adsorption sites, resulting in reduced solubility of NC films.

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1. Introduction Inorganic polymer composites can be categorized in two general categories depending on the dispersion and size of the reinforcements. In conventional composites, the reinforcement is blended with a polymer to form the composite. Examples are many and include glass- and carbon-fiber reinforced composites that have been extremely successful in the marketplace and are used extensively in many industries and consumer products, however at > 25 wt. %, the reinforcement loading is relatively high. In the second type, nanocomposites (NC), the reinforcement is either a 1D or 2D nanomaterial that is thoroughly and fully exfoliated in the polymer matrix. Larger surface area to volume ratios lend themselves to greater enhancements in a variety of material properties, at much lower filler loads. A schematic of such a composite is shown in Figure 1a. Note here the NC is not defined by the nature of the reinforcement, but rather by its dispersion. It is only when most of the individual reinforcement particles are exfoliated and evenly dispersed in the matrix (right schematic in Figure 1a) that the composite can be properly described as a NC. In 1987, Okada and Usuki, reinforced nylon-6 with smectite type clays, particularly montmorillonite (MMT).1 Here, 1 nm thick silicate layers (Figure 1b), with cross sectional areas of ≈ 100 nm2, were used. MMT is particularly useful, being a ubiquitous, inexpensive clay mineral that undergoes intercalation and swelling in the presence of water and organic cations (see Figure 1a).2 The Toyota researcher’s breakthrough came about when they realized that if, before exposing the clay MLs to the monomer, they first ion exchanged the interlayer cations with 12-aminolauric acid, the nylon-6 monomer - -caprolactam - readily intercalated the interlayer space. Subsequent polymerization of the monomer resulted in clay/nylon NCs (see Figure 1a). The intercalation of the interlayer space with 12aminolauric acid changed the otherwise hydrophilic interlayer space to one that was organophillic and thus more amenable to the intercalation of the monomer.

Figure 1: Schematic of a) steps required to produce MXene NCs where lithium intercalated multilayer Ti3C2Tz MXene is treated with 12-aminolauric acid to form 12-Ti3C2Tz to be used for the in-situ ring opening polymerization of -caprolactam. b) structure of montmorillonite, a 2:1 clay, and c) multilayer Ti3C2Tz MXene. Coincidentally, the thickness of the silicate and Ti3C2Tx blocks are both ≈ 1 nm. In both ACS Paragon Plus Environment

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cases the interlayer space contains exchangeable cations (white/blue in (b) and blue in (c)) and water molecules (red and white). In c, the Ti, C and Tz atoms are green, black, are pink, respectively. The enhancements in mechanical and permeation properties of these NCs with loadings of < 5 vol. % were quite impressive and led to the very successful commercialization of these composites. The importance of exfoliation comes about because, for the most part, the enhanced properties exhibited arise from the large reinforcement/polymer interfacial areas obtained. Since the pioneering work of Okada and Usuki, many 2D materials have been incorporated in polymers to create NCs.1,3,4 A relatively recent addition to the growing list of 2D materials are MXenes, discovered in 2011.5 MXenes are synthesized via the wet etching of the MAX phases which are ternary carbides and nitrides with the formula Mn+1AXn, where M is an early transition metal, A is an A-group element, X is C and/or N, with n ranging from 1 to 3. These machinable layered hexagonal ternaries are nanolaminated with layers of pure A separating Mn+1Xn layers.6 By selectively etching the A group element, the 3D MAX phase is converted to its 2D counterpart, MXene. Upon the selective etching of the A-layers in typically F-containing solutions, they are replaced by surface terminations that are a mixture of -OH, -O, and -F. The proper MXene chemistry is thus Mn+1XnTz, where T stands for surface terminations. Because n can range from 1 to 3, single MXene sheets consist of 3, 5 or 7 atomic layers to yield M2XTz, M3X2Tz, and M4X3Tz, MXene respectively. Currently the MXene family includes Ti3C2Tz, Ti2CTz, Nb2CTz,, V2CTz,, (Ti0.5,Nb0.5)2CTz, (V0.5,Cr0.5)3C2Tz,, Ti3CNTz and Ta4C3Tz, among many others.7,8 Figure 1c is a schematic of the atomic structure of Ti3C2Tz used in this work. To date, several studies have reported on the synthesis and characterization of polymer/MXene composites. The first reported on the introduction of Ti3C2Tz into poly (vinyl alcohol) (PVA) and poly (diallyldimethylammonium chloride) (PDDA).9 PDDA was chosen because of its cationic nature, while PVA was used due to its solubility in water (and hence, compatibility with aqueous colloidal MXene suspensions), large concentration of backbone hydroxyl groups and extensive use in gel electrolytes and composites. The authors found that they could produce free-standing, flexible composites which remained electronically conductive. However, at 40 wt. %, the minimum MXene loading was quite high and full delamination of the Ti3C2Tz multilayers, MLs, was not achieved, as evidenced by the presence of basal peaks in x-ray diffraction (XRD) patterns. The largest increases in tensile strength, elastic modulus and strain to failure were all found in the PVA composites loaded at 40 wt. %, however the conductivity dropped significantly as the MXene loading decreased, with the 40 wt. % film having a conductivity of just 0.04 S/m, as compared to the 240,000 S/m of a pure MXene film or the 22,000 S/m in a 90 wt. % PVA/MXene sample. As these films were produced with electronic conductivity as a priority, it is not surprising that lower weight percentages were not reported. Ti3C2Tz MXene has since been incorporated into other polymer matrices, including polypyrrole (PPy), poly(vinlyidene fluoride) (PVDF), chitosan, poly (ethylene oxide) (PEO), alginate/PEO and poly (acrylic acid), and sodium alginate.10–16 To date, only one study has reported on Ti3C2Tz MXene being incorporated into a thermoplastic polymer that was extruded, however no evidence of dispersion into the linear low density polyethylene matrix was provided.13 Additionally, only the PPy study utilized an insitu polymerization method to produce Ti3C2Tz containing composites. All other composites were made starting with pre-existing polymers, rather than via the polymerization of monomer. More significantly, these synthesis methods typically rely on the use of delaminated MXene colloidal suspensions, which are limited to water or other polar organic solvents, and have relatively low concentrations of 10-20 mg/mL.17 Due to these limitations, MXene polymer composites have been restricted to small-scale synthesis and, until now, have not been appropriately developed in order to find applications on industrial scales. Methods that utilize multilayer MXene are necessary for MXene polymer nanocomposites to be industrially relevant. ACS Paragon Plus Environment

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Recently, we have reported extensively on the similarities between MXene and typical 2:1 More specifically both: i) form stable aqueous colloidal suspensions around pH ≈ 7; ii) contain exchangeable cations, iii) expand when select cations, including small organic cations, are introduced in the interlayer space, iv) have water in their interlayer space, and v) have interlayer distances that are functions of humidity.21–24 Similarities in structure and interlayer space are better appreciated when the atomic structures of a typical 2:1 clay and Ti3C2Tz are compared in Figure 1b and c, respectively. In both cases, the sheets are 1 nm thick and the interlayer space contains exchangeable cations and water. There are also important differences, however. For example, in clays the charges stem from aliovalent cations, mostly Al, substituting for the Si in the silica tetrahedra. In MXene on the other hand, the charge arises about because the average oxidation states of the Ti and C atoms are ≈ ±2.5, respectively.25 clays.18–21

The mechanical properties of polyamide-6 are particularly dependent on the degree of sorbed water as the glass transition temperature and elastic moduli are drastically decreased with increasing water content, typically attributed to the replacement of hydrogen bonds in regions of the polymer that are accessible to water molecules.26,27 In addition to end-use mechanical properties, the moisture content of nylons is a key variable in processing conditions such as polymerization, compounding, molding and welding.28 As polyamides are used extensively in many industrial and commercial applications, it is of great interest to reduce to amount of sorbed water in order to retain consistent and reliable processing methods as well as excellent post-processing properties. One of the hallmarks of clay/nylon NCs is their low permeation for gases and moisture. Specifically, for nylon/MMT NCs, it is now fairly well established that the diffusion of gases and water in NC decreases with increasing volume fraction of nanofiller content, 𝜙𝑓.29–31 Furthermore, increasing 𝜙𝑓 reduces the solubility of water in the composite. In MMT/nylon NCs this reduction in solubility is small and is ascribed to the reduced polymer matrix volume.29–32 For these aforementioned reasons we have developed a simple and scalable method of incorporating clay-like, multilayer Ti3C2Tz MXene into nylon-6, and have studied the permeation of water vapor into these nanocomposites. 2. Materials and Experimental Methods 2.1 Preparation of Ti3AlC2 and Ti3C2Tz Materials: To make the Ti3AlC2 phase, Ti, Al and titanium carbide, TiC, powders (-325 mesh, 99.5%, Alfa Aesar, Havervill, MA) were used. For ion-exchange and subsequent polymerization, -caprolactam (99+%, Acros Organics, Hampton, NH), 12-aminolauric acid (98.0+%, TCI America, Portland, OR) and 6-aminocaproic acid (99%, Acros Organics) were employed. Ti3AlC2 Synthesis: Powders of Ti3AlC2 were produced by first ball milling TiC, Al and Ti powders in a 2:1.15:1 molar ratio with zirconia milling balls for 24 h. This mixture was separated from the milling balls, placed in an alumina boat and heated in an alumina tube furnace at a rate of 5 °C/min under continuous argon, Ar, flow to 1350 °C. After holding for 2 h at temperature, the sample was furnace cooled. The resulting sintered porous brick was milled into a fine powder with a drill press before passing through a 400 mesh sieve to achieve a particle size of < 38 µm. Ti3C2Tz Synthesis: In this work we targeted Ti3C2Tz to be 7 and 17 wt. %, corresponding to 2 and 5 vol. % in the final NC. These will henceforth be referred to as 7-NC and 17-NC. The majority of the work carried out herein is on the 7-NC films. The neat nylon will be referred to as PA6. The Ti3C2Tz MXene used in this study was prepared as follows: five grams of the sieved MAX powder was immersed in a 50 mL mixture of 10 wt. % hydrofluoric acid, HF, and 5.4 g lithium chloride, LiCl, for a LiCl/Ti3AlC2 molar ratio of 5:1. This mixture was stirred for 24 h with a PTFE coated magnetic stir bar at room temperature (RT) at 300 rpm. After this etching period, the reaction vessel was removed, and the contents were ACS Paragon Plus Environment

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divided into ten centrifuge tubes so that approximately 5 mL of the mixture occupied each tube. To separate the sediment, these tubes were centrifuged at 3500 rpm (or 2301 rcf) for 2 minutes. The supernatant was discarded and replaced with deionized water, for a total volume of about 40 mL in each centrifuge tube. The tubes were again centrifuged for 2 minutes at 3500 rpm. After decanting the supernatant and refilling with 40 mL of water, this procedure was repeated until a pH of 5-6 as measured by pH paper was obtained. The product was vacuum filtered and dried in a vacuum oven at 100 °C for 12 h, where a small amount was set aside for analysis by XRD. Treatment of Ti3C2Tz with 12-aminolauric acid: The remaining, Ti3C2Tz powder was introduced slowly into a pre-prepared solution of 12 mmol of 12-ALA, dispersed in 100 mL of water and 1.2 mL of 12 M HCl. This solution was allowed to stir for 4 h at RT and 300 rpm before vacuum filtration through a Celgard film. Following filtration, the product was dried in vacuum at 100 °C for 12 h. This resulted in a fine powder hereafter referred to as 12-MX. 2.2 Preparation and Synthesis of Neat and Composite Samples Polymerization of neat and composite samples: This treated MXene powder was then mixed with caprolactam in a mortar and pestle along with 6-aminocaproic acid in a molar ratio of 9:1 monomer to catalyst. As noted above the 12-MX content was targeted at a load of 7 and 17 wt. %, respectively. After compounding, the mixture was transferred to a three-necked separable flask. The mixture was stirred at 60 °C for 1 h in a sonicating bath under flowing Ar to degas the reaction vessel before transferring to a heating mantle. The temperature was then raised to 100 °C and held for 0.5 h, then at 250 °C for 6 h after which the mantle was shut off and the flask allowed to cool passively. After cooling, the product was removed, immersed in liquid nitrogen, N2, and cryomilled into small pellets by mechanically crushing it with a hand-held rock crusher. The resulting small pieces were washed three times with 2.5 L of boiling water to remove any unreacted monomer. After washing, the pellets were dried in vacuum at 100 °C for 12 h. Once dried, the pellets were extruded into filaments using a Filabot EX2 (Filabot Barre, VT) extruder at 215 °C. These filaments were cut by hand into small pellets. The same procedure was repeated for the neat nylon-6 samples. The reaction for the 17 wt. % sample was done at a small scale in a scintillation vial housed in a modular vial heating block, however the same degassing, reaction and washing procedure was used. Due to the small scale nature of the reaction, the obtained product was not extruded. 2.1 Characterization Methods Thermogravimetric Analysis: The amount of Ti3C2Tz in the samples was determined by thermogravimetric analysis, TGA, (TA Instruments Q50) by heating at 10 °C/min under purging Ar at 10 mL/min - to 800 °C and holding at that temperature for 0.5 h. The final mass relative to the starting mass was taken as the wt. fraction of MXene present in the original pellets. In this work we loaded the nylon with 6.9 and 17.4 wt. %. If the density of Ti3C2Tz is assumed to be 4.26 g/cm3 and the density of nylon-6 is assumed to be 1.08 g/cm3, this translates to ≈ 1.9 and 5.0 vol. %, respectively. Differential scanning calorimetry analysis: A differential scanning calorimeter, DSC, (TA Instruments Q2000) was used to determine the degree of crystallinity as well as temperatures and enthalpies of melting and crystallization for both neat and NC. Before testing, the temperature was allowed to equilibrate at -50 °C, then ramped at 10 °C/min to 300 °C and held isothermally for 60 s before ramping down to -50 °C at 10 °C/min. Two cycles were carried out to ensure that the thermal histories of the two samples are identical. Only results from the second cycle are presented.


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Dynamic vapor sorption analysis: A dynamic vapor sorption, (DVS), (TA Instruments Q5000SA) technique was employed to study water permeation. Special care was taken to ensure that the films used in these tests were of the same diameter and thickness. Neat and composite pellets were hot pressed at 250 °C into thin films, from which a 4.76 mm hole-punch was used to form samples that were loaded into the DVS balance. The thicknesses of the PA6, 7-NC and 17-NC films were 0.156 mm 0.152 mm and 0.152 mm, respectively. These films were first equilibrated at 80 °C and 0 % until the percent mass change was less than 0.1% for 12 h. The samples were held isothermally at 25°, 40° and 60 °C, at a relative humidity of 80 % (the 25°C test was conducted at 50 % RH to simulate ambient use), while continuously measuring their weight. The same samples were used to obtain all the results. To shed light on the thermodynamic interactions of water with the NCs, the equilibrium mass for water activities between 0.1 and 0.9, at 60 °C were measured for the PA6 and 7-NC sample. Scanning electron microscopy analysis: Images were taken using a scanning electron microscope, SEM, (Zeiss Supra 50VP) with an acceleration voltage of 5 kV. X-ray diffraction analysis: X-ray diffraction, XRD, patterns were obtained with a Rigaku SmartLab Xray diffractometer with an incident Cu Kα wavelength of 1.54 Å in step-wise mode from 2-30° with a step size of 0.02° and a dwell time of 1.5 s. Focused ion beam and transmission electron microscopy analysis: To prepare samples for viewing in a transmission electron microscope (TEM) a dual beam focused ion beam (FIB) – SEM (FEI Strata DB235) was used on a 7-NC samples. Details of the FIBed sample are described in Figure S1. TEM images were taken using a JEOL JEM2100 transmission electron microscope. FTIR-ATR Analysis: Mid infrared, IR, spectra were obtained on PA6 and 7-NC thin film DVS samples with a Thermo Nicolet Nexus 870 FT-IR Spectrometer in the range of 4000 – 400 cm-1 at room temperature. For each sample, 32 scans were taken successively with a scan average of 4 (data spacing of 0.482 cm-1). Samples were prepared by removing them from the vacuum chamber they were stored in and placing them in either deionized water or in a sealed vacuum chamber over P2O5. These films were then allowed to equilibrate over the period of seven days. Care was taken to minimize time between sample retrieval and data acquisition. In the case of the sample placed in liquid water, they were gently wiped with a Kimwipe (Kimberly-Clark, Irving TX) to remove any surface water before being scanned. 3. Results and Discussion Figure 2 plots the XRD patterns of Ti3C2Tz MLs (Figure 2a), 12-MX treated MLs (Figure 2b) the compounded pre-polymerized mixture of 12-MX, 6-aminocaproic acid and -caprolactam (Figure 2c), as well as the final polymerized mixture (6.9 wt. %, 7-NC) after washing and drying, but before extrusion (Figure 2d). After intercalation with the 12-aminolauric acid (ALA) the basal spacing of the MLs increased from 14.0 Å to 15.7 Å. Interestingly this value is almost identical to that of MMT.1 By compounding the treated MXene with the monomer and catalyst (-caprolactam and 6-aminocaproic acid), the basal spacing further increased to 16.7 Å (Figure 2c) before any heat was applied. Usuki et al. found a basal spacing of 31.5 Å after heating 12-ALA acid treated MMT to 100 °C for 24 h in the presence of -caprolactam and allowing to cool to 25 °C.33 In the final polymerized 7-NC, before extrusion, the Ti3C2Tz basal peaks are totally absent. This result indicates that the MXene flakes are well dispersed in the matrix, with no order in the basal direction. In the 17-NC (5 vol. %) samples, however, a broad peak centered 5° (pattern e in Figure 2) shows that full exfoliation was not achieved in this case,


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which has been observed in other PA6-NC systems at higher loads of filler due to filler agglomeration.30,31

Figure 2: XRD diffractograms for, a) multilayer Ti3C2Tz, b) 12-MX, c) pre-polymerization mixture of 12-MX, 6-aminocaproic acid and -caprolactam and polymerized NCs with, d) 6.9 wt.% (1.9 vol. %) and, e) 17 wt. % (5.0 vol. %). Figure 3a is an SEM micrograph of Ti3C2Tz MLs after intercalation with 12-ALA. A picture of the extruded pellets are shown in Figure 3b. The lateral size, L, of these flakes were measured using the “Measure” tool in ImageJ to be 500 ± 54 nm. From the TGA results, (Figure 4a) we conclude that the Ti3C2Tz content in the NCs were 6.9 and 17.4 wt. %. The DSC results are plotted in Figure 4b and summarized in Table 1. Figures 3c to e show TEM micrographs of FIBed NCs at various magnifications. Note that the flakes appear to be aligned along the extrusion direction (Figure 3e). Other TEM micrographs can be found in Figure S2 in supplementary information. Selected area diffraction of some flakes (not shown) confirmed that the hexagonal structure of the parent MAX phase is preserved.


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Figure 3: a) SEM micrograph of Ti3C2Tz MLs after intercalation with 12-aminolauric acid. The lateral size, L, of these flakes was measured to be 500 ± 54 nm; b) Image of extruded pellets with 2 mm diameter; c) Composite TEM micrograph of FIBed sample; d) TEM of region surrounded by square in c; e) TEM of region surrounded by rectangle in d. Long axis of the FIBed sample was parallel to the extrusion direction (see Supp. Info.). Another crucial parameter of semicrystalline polymers in general, is their crystalline/amorphous content. Increasing the MXene content from 0 to 5 vol. % reduces the crystalline content from ≈ 30 % to 21 % (Table 1). Concomitantly, a reduction in Tc of 13.8 °C and 21.1 °C was recorded with the addition of 1.9 and 5 vol. % reinforcement, respectively. The depression of Tc with increasing clay content is generally accepted to be due to the restriction of chain mobility.41 The same trend is observed in MXene nylon NCs, with the proviso that the effect of MXene in reducing c is more potent (see Figure S3). For example, increasing the MXene vol. % to ≈ 5 vol. % reduces c to about 21 %. The same volume fraction of clay, on the other hand, reduces c to about 28 %. The same trend was observed in Ti3C2Tz/PEO nanocomposites.15 This is attributed to a competing effect between heterogenous nucleation, and confinement by the 2D MXene filler. ACS Paragon Plus Environment

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Figure 4: Thermal properties of PA6 (black lines), 7-NC (blue lines) and 17-NC (red lines) samples. a) TGA weight loss curves, and b) DSC curves of melting (lower curves) and crystallization (higher curves). The percent crystallinity, c, of synthesized samples, as measured by differential scanning calorimetry was calculated assuming: χ𝑐 =

Δ𝐻𝑚

(1 ― 𝜙𝑓)Δ𝐻0𝑚

(1)

× 100

where Hm and Hm0 are, respectively, the latent heats of fusion of the experimental sample and of 100% crystallized nylon-6, assumed to be 230.1 J/g.34,35 No cold crystallization phenomena was noted, therefore no subtraction from the melting enthalpy was necessary and is not reflected in Equation 1. Table 1: Summary of DSC results for neat and composite specimens after second pass. Vol. % Ti3C2Tz 0 1.9 5.0

Wt. % Ti3C2Tz 0 6.9 17.4

Tm (C) 214.4 210.7 200.9

Hm

g-1)

(J -67.3 -56.6 -40.7

Tc (C) 187.2 173.4 166.1

Hc

(J g-1) 68.0 56.8 44.6

c (%)

29.2 26.4 21.4

Figure 5a plots the mass increase as a function of time, t, for both neat and NC samples at 25 °C and 50% RH. Figures 5b and c, respectively, plot the same at 40 °C and 60 °C and 80 % RH. Figure 5d plots all the results as Mt/Meq vs. t1/2. In all figures, the neat polymer, 1.9 vol. % and 5 vol. %, results are


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plotted as open, black and red symbols, respectively. All test results at 25 °C are plotted as triangles; those at 40 °C as squares and those at 60 °C are represented by circles. Table 2 compares the water saturation values obtained from Figure 5 for NC and neat nylon-6 at 25, 40, and 60 °C. In all cases, the composite films took up less water, with increasing MXene content suppressing the rate and amount of uptake. The reduction of permeation has been ascribed to fact that the diffusing species have a longer, or more tortuous path.32 In the simplest case, if one assumes that the platelets are all uniformly oriented normal to the diffusion direction, Nielsen showed that the relative permeability, R, is given by: 𝑅=

𝑃𝑓 𝑃𝑢

=

1 ― 𝜙𝑓 1+

(2)

𝐿𝜙𝑓 2Δ

where Pc, and Pu are the permeability of the composite and unfilled polymer, respectively, L is the lateral size of the flakes and ∆ is their thickness.36 While this equation is used here, it is acknowledged that it is quite simple and many more sophisticated models exist.32 At this stage however, nothing is gained in introducing complexity. A common feature of all models, however, is the dependence of R on L/∆ and 𝜙𝑓 ; increasing 𝜙𝑓 and/or L/∆, in turn, decreases R. From weight gain data, the transport of water vapor can be modeled and a water diffusion coefficient, D, can be extracted by plotting the ratio of mass at time 𝑡 (𝑀𝑡) to the equilibrium mass at equilibrium (Meq). Since that ratio is given by:37 𝑀𝑡 4 𝐷𝑡 (3) 𝑀𝑒𝑞

=

𝑑 𝜋

where 𝑑 is the sample thickness, a plot of the right-hand side vs. t1/2, should yield a straight line with slope, k.31,38 If that is the case, then D can be calculated since: 𝐷=

𝜋𝑘2𝑑2

(4)

16

Often the results of such tests are reported in terms of a permeability, P, given by:39 𝑃=𝐷⋅𝑆

(5)

where S is the solubility, expressed in g of water vapor per 100 g of sample. The relative permeability, R, can then be calculated by taking the ratio of composite’s permeability, Pc, to that of the neat permeability, Pu, or: 𝑅=

𝑃𝑐

(6)

𝑃𝑢


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Figure 5: Water kinetics in PA6 (open circles), 7-NC (filled black circles) and 17-NC (filled red circles) thin films a) Dynamic vapor sorption curves at 50 % RH and 25 °C; b) same as but at 80% RH and 40 °C; c) same as a but at 60 °C and 80% RH; d) Mt/Meq. vs. t1/2 curves obtained from the results shown in a, b, and c. Slope of initial linear region of these curves is used to calculate D from Eq. 4. In all plots, the 25 °C results are plotted as triangles, those at 40 °C by squares and those at 60 °C by circles. Table 2: Water diffusion coefficients, water saturation, S, permeability, P, and relative permeability, R, of PA6 and NCs obtained herein at various temperatures, relative humidities and loadings. Load (wt. %)

Load (vol. %)

Temp. (°C)

RH (%)

D (cm2 s-1)

S (g H2O/100 g sample)


Permeability (g H2O 100 g-1

R

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sample cm2 s-1)

0 0 0 6.9 6.9 6.9 17.4 17.4 17.4

0 0 0 1.9 1.9 1.9 5.0 5.0 5.0

25 40 60 25 40 60 25 40 60

50 80 80 50 80 80 50 80 80

10-10

8.0 x 6.3 x 10-9 3.3 x 10-8 2.1 x 10-10 1.6 x 10-9 7.8 x 10-9 1.6 x 10-10 5.6 x 10-10 3.0 x 10-9

Resin -

Comp. -

3.4 5.6 5.8 2.8 5.6 5.8

2.8 5.0 4.8 1.6 3.8 3.8

2.7 x 10-9 3.5 x 10-8 1.9 x 10-9 5.9 x 10-10 8.0 x 10-9 3.7 x 10-8 2.6 x 10-10 2.1 x 10-9 1.1 x 10-8

0.21 0.23 0.19 0.10 0.06 0.06

The water diffusion coefficients, D, permeability, P, and relative permeability, R, calculated from Figure 5d and Equation 3 are listed in Table 2. Figure 6 superimposes a plot of Equation 2, as a function of aspect ratios, together with some typical values reported for nylons NC reinforced with MMT. The most important result of this work is the greatly reduced permeation rates in the MXene NCs samples compared to the clay-based ones. The results shown in Figure 6, clearly indicate significantly lower R values than those reported in previous work. We attribte the enhancments to the high ( ≈ 500 1000) aspect ratios, and full exfoliation of the Ti3C2Tz 2D flakes. The full exfoliation is confirmed by the XRD diffraction pattern shown in Figure 2d, as well as TEM micrographs, shown in Figure 3c-e. Assuming Equation 2 is applicable here, then based on the results plotted in Figure 6, the L/∆ should be the order of 500. Since ∆ is 1 nm for Ti3C2Tz, (Figure 1c), this implies that the lateral size of 12-Ti3C2Tz flakes should be approximately 0.5 µm. Indeed the SEM micrograph of the ML flakes after intercalation with 12-ALA (Figure 3), confirms an average lateral size of 500 ± 54 nm, with some flakes having lengths of up to 8 µm. More importantly, that at least some of these large aspect ratios survive the processing steps is clearly seen in the TEM micrographs shown in Figure 3c to 3e and Figure S1 and S2 in supporting information. In the MMT/nylon-6 literature, Lav of the clay flakes is typically between 50 and 500 nm. For example, Alix et al. report L values of 142 ± 58 nm.29 Picard et al. report L values that ranged from 88340 nm.30 It is thus reasonable to conclude here that the enhancement in permeation is due to the much larger aspect ratios. Note that the activation energy for diffusion does not change with the addition of MXene; evidence that the elementary diffusion jump does not change upon incorporation of MXene.


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Figure 6: Plot of Equation 2 as a function of aspect ratios, together with results from this work and typical literature results for clay/nylon-6 NCs. (■) Picard et al. (2007) PA6/OMMT); (■) Picard et al. (2008) PA6/OMMT; (▲) Alix et al. (2012) PA6/C30B; (●) Kojima et al. (1993) PA6/12-MMT; (♦) This work, 1.9 vol. % 12-MXene; (♦) This work, 5.0 vol. % 12-MXene. The values of R for the 7-NC (1.9 vol. %) film at 25 °C, 40 °C and 60 °C are 0.21, 0.23 and 0.20, respecively. For the 17-NC films (5.0 vol. %) at 25 °C, 40 °C and 60 °C R is 0.10, 0.06 and 0.06. The reproducibility in R is not only good, but is also a weak function of temperature. It is a reassuring sign that these values are all quite close to one another and are significantly lower than those of traditional clay-based NCs (Figure 6). These result are noteworthy and cannot be easily dissmissed since comparison is made between these initial results and a mature technology. Note that these R values are achieved despite a reduction in crystallinity (Table 1), which typically increases permeability.38 The TEM results show that the flakes, while dispersed, are not uniformly distributed, in the 1.9 vol. % sample at least. If the latter can be achieved, it should be possible to further reduce R. That the flakes are not totally dispersed at 5 vol. % suggests another avenue for improvement. The fact that the inclined lines in Figure 6 are straight implies that, i) diffusion is thermally activated and, ii) parallel implies that the Ti3C2Tz does not alter the activation energy for diffusion, which is not too surprising given that the maximum loading was only 5 vol. %. Most importantly, Figure 7a-b clearly show that the diffusivity values are the lowest ever reported for nylon-6. This was achieved with a maximum loading of 5 vol. %, making this result particularly noteworthy. Comparing the results for the 1.9 vol. % NC to those obtained in traditional clay NCs, the clay content required is significantly higher, viz. 5.8 and 7.5 vol. %.31 Additionally, D values measured for the unfilled PA6 films are in excellent agreement with previous work which is gratifying and lends credence to these results, including record low values. These results are also fully consistent with the tortuosity model where higher aspect ratios result in lower permeation rates.


13


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Figure 7: a) Arrhenius plot of D. Results from this work are depicted by diamonds. All open data points are for PA6, all with filled composites are represented in blue. The red and blue diamonds were obtained with 1.9 and 5.0 vol. % loadings, respectively. The inclined lines were drawn parallel to each other and are included as a visual guide. b) Natural logarithm of D plotted as a function of filler volume percent for results obtained herein (denoted by diamonds) and those reported in the literature for clay/nylon-6 NCs. The point represented by the white inverted triangle was measured with NMR imaging in neat nylon-6 thin films by Reuvers et al., indicating that there is good agreement between this work and others who use independent methodologies in neat nylon-6 samples.40 c) Dependece of D on relative humidity of PA6 and 7-NC films at 60 °C, dashed lines are drawn as an aid to the eye. In addition to kinetics, it is important to understand the thermodynamics, or the equilibrium water concentration in NCs. In the clay/nylon NC literature the effect of adding the clay can be, for the most part, simply accounted for by taking its mass fraction into account.31 For example, at RT the addition of 6 wt. % clay to nylon-6, at 80 % humidity, reduces Meq from ≈ 6.6% to 6.0%.31 In comparison, here the addition of 6.9 wt. % Ti3C2Tz, at 80%, humidity, reduces Meq from ≈ 5.8% to 4.8% at 60 °C and from 5.6% to 5.0% at 40 °C (see Figure 5a). As the MXene content increases to 5.0 vol. %, Meq is reduced to ACS Paragon Plus Environment

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3.8%. This last value is quite low and bodes well for reducing the amount of water sorbed by nylon-6 by the addition of MXene. Why this value is reduced is discussed below. To shed light on the thermodynamics of the system, the equilibrium water content is measured as a function of water activity, a. Following the lead of work on clay/nylon composites, the resulting isotherms are then fit with the Guggenheim, Anderson and de Boer (GAB) equation:31 𝑀(𝑎) =

𝑎𝐺𝐴𝐵𝑐𝐺𝐴𝐵𝑘𝑎 1 ― 𝑘𝑎

(7)

1

1 + (𝑐𝐺𝐴𝐵 ― 1)𝑘𝑎

where a is the activity of water, cGAB is the Guggenheim constant that describes the difference in free enthalpy of water molecules in the pure liquid state and in the monolayer, aGAB is the water content corresponding to saturation of all primary adsorption sites by one water molecule, and 𝑘 is a temperature dependent constant. Note the two parameter Brunauer-Emmett-Teller (BET) equation is a special case of the GAB equation when k = 1. The observed large increase of water by hydrophilic polymers as the activity of water increases is generally attributed to a clustering effect. Recognizing that this behavior could not be adequately described by the early Flory-Huggins-Guggenheim model of athermal polymer solutions, Zimm and Lundberg quantitatively described clustering phenomenon by the enhancement number; the ratio of the cluster integral, G to the molecular volume of water,.42

𝑁𝑒 =

𝐺11 𝑣1

()

∂

𝑎 𝜙

(8)

[ ]

= ― (1 ― 𝜙)

―1

∂𝑎

𝑃,𝑇

where ϕ, and a are the volume fraction of water and the water activity, respectively. If a solution is ideal, no clustering occurs and a linear uptake is observed. In the ideal case, Ne is equal to -1. The mean number of solute molecules in a cluster Nc is then defined as:43 (9)

𝑁𝑐 = 𝜙 ⋅ 𝑁𝑒 + 1

This can then be expressed in terms of the GAB parameters, as first described by Zhang, Britt and Tung as:44 𝜙 (10) 𝑁 𝑐 = ― (1 ― 𝜙 ) ( ―2𝑐𝐺𝐴𝐵𝑘𝑎 + 2𝑘𝑎 + 𝑐𝐺𝐴𝐵 ― 2) + 1

[

𝑎𝐺𝐴𝐵𝑐𝐺𝐴𝐵

]

It is thus possible to estimate Nc from the GAB parameters. Figure 8a plots water sorption isotherms at 60 °C, for PA6 and 7-NC films. The GAB parameters obtained from fitting the results shown in Figure 8a by the GAB equation (Eq. 7) are listed in Table 3. Using these parameters Nc in Eq. 10, can be approximated and plotted as a function of water activity for the neat PA6 and 7-NC in Figure 8b. Referring to Figure 8a, initially the relationship is linear suggesting Henrian behavior. As the water activity increases both lines curve upwards, attributed to either the ingress of water exposing more binding sites – i.e. a plasticizing effect - or the formation of water clusters. The results shown in Figure 8b, indicate that it is a plasticizing effect since even at the highest humidity the cluster size is < 2. These results are in total agreement with previous work on neat nylon and those reinforced with clays.31 This plasticizing effect is also manifested in Figure 7c, where a clear break in D


15


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occurs at ≈ 0.5 in the PA6 and at ≈ 0.7 for the 7-NC. It follows that by reducing the number of active sites, the Ti3C2Tz flakes delay the onset of plasticization to higher activity of water. Table 3: GAB parameters (Eq. 7) obtained from fitting the data points in Figure 8a. Composition PA6 6.9 wt. % 12-Ti3C2Tz

aGAB 0.046 0.042

cGAB

k

2.68 2.81

0.673 0.646

R2 0.9998 0.9996

Figure 8: a) Water vapor sorption isotherms – normalized by fraction of amorphous content, viz. assuming both MXene and the crystalline regions are impervious to water - of PA6 (open white circles) and 7-NC films (solid red circles). Dashed lines represent the GAB (Eq. 7) fit for each dataset. b) Average number of water molecules in a cluster, Nc, as a function of water activity for PA6 and 7-NC. Lastly, we consider what changes are occurring at the atomic level. The FTIR-ATR spectra of water saturated neat and NC films are shown in Figure 9. The spectra for the dried films are compared in Figure S5. Tables 4 and S2 list the peak positions and general band assignments for the water saturated and dry films, respectively. The most salient feature, and one that is of crucial important herein, is the presence of a shoulder around 3700-3375 cm-1 (to the left of the 3296 cm-1 hydrogen bonded NH bending peak) that is more pronounced in the neat PA6 film than in the NC ones and is attributed to the free NH stretching mode.45 This is especially true if the background, due to the presence of the MXene flakes, is subtracted from the spectra. The broadening of this shoulder has been shown to correlate with increasing water activity in aromatic polyamides and nylon-6,6,46,47 and indicates the presence of a larger degree of sorbed water in the neat samples. Note this shoulder is absent in the dry samples (Figure S4). When this important conclusion is combined with the fact that Nc for both the PA6 and the NC films are comparable (Figure 8b), it is reasonable to conclude that the MXene flakes bond with, and thus eliminate, water adsorption sites in the NCs. Notable redshifts related to bending and stretching modes of the NH bond are seen in both the dry (Figure S4 and Table S2) and water saturated spectra of the NC films (Table 4). This suggests that bonding between the amide group of nylon-6 and MXene are occurring. Most likely these are hydrogen


16

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bonds between the hydroxyl surface terminations of Ti3C3Tz which may limit the available sites for nylon-water hydrogen bond formation.

Figure 9: FTIR-ATR spectra of water saturated thin films of PA6 (black line) and 6.9 wt. % NC (red line). The amide II (C-N stretch, N-H bend) shift is minor in both cases, with shifts upon hydration from 1538 cm-1 to 1539.4 cm-1 in PA6 films and 1539.4 cm-1 to 1540.9 cm-1 in the NC films. Surprisingly, in the latter, there exists little evidence of an amide V (out of plane N-H bend) peak in the region of 725690 cm-1 while strong amide V peaks are present in the neat PA6 at 687 cm-1 and 685.6 cm-1 for the respective dry and saturated spectra. This lack of a signature amide V, coupled with the lower intensities of N-H stretching bands around 3300 cm-1 as well as amide I and II bands suggests bonding between the amide groups of the nylon-6 and surface hydroxyl groups in Ti3C2Tz which suppress the formation of hydrogen bonds with water. Table 4: FTIR-ATR- frequencies of PA6 and 7-NC water saturated films. PA6– 100% RH Wavenumber Intensity

7-NC – 100% RH Wavenumber cm-1 Intensity ACS Paragon Plus Environment

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cm-1 3700-3375

s

3700-3375

m

NH bending

3296

vs

3287 red

s

Hydrogen-bonded NH stretching

3092

m

3093

m

Fermi-resonance of NH stretching with overtone of amide II

2933

vs

2922 red

s

CH2 asymmetric stretching

2860

s

2855 red

CH2 symmetric stretching

1634

vvs

1631

m vs

1539

vvs

1543

vs

m

1460 1435 1416

1475 1462 1438

1418 1371 1299 1279 1261 1238 1212

1365 1299 1276

1261

1202

m wsh wsh m m sh m

1169 1153 1120 1074 1029

m wsh m w w

1167 1153 1115 1067 -

975

m

972

1233 1212 -

Amide I, C=O stretching Amide II, coupling of NH deformation with C-N stretch45

-

w w sh w vwsh vwsh vw w wsh w w vwsh w vw -

w

CH2 scissoring

Amide III, in phase NH in-plane bending and CN stretching vibrations

CONH skeletal motion C-C stretching C-C stretching CONH skeletal motion, C-C stretching CONH in-plane

CONH in-plane, closely related to form

m CONH in-plane 930 m 833 w CH2 rocking 727 728 w vwsh NH out-of-plane deformation 686 m Amide V 624 vw Amide IV, V and VI 575 m Amide IV, V and VI ‡ sh = shoulder, v = very, vv = very very, s = strong, m = medium, w = weak. 960

4. Conclusions Herein we report on the first nylon-6 based NCs, loaded with ≈ 1.9 and 5.0 vol. % Ti3C2Tz MXene flakes. This was accomplished utilizing multilayer MXene, making this method easily scalable. By first exchanging the cations present between the Ti3C2Tz MLs, with 12-ALA MXene could be intercalated by the monomer, -caprolactam, and catalyst, 6-aminocaproic acid. Heating to 250 °C to initiated a ring


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opening polymerization of the monomer in the presence of the 12-ALA treated Ti3C2Tz, resulting in a fully exfoliated nylon-6/MXene nanocomposite. The absence of MXene basal peaks in the XRD patterns of the 1.9 vol. % NC was taken as evidence for full exfoliation of the flakes. TEM micrographs further confirmed both that the flakes were dispersed and that the aspect ratios of some of them were > 1000. When measured D and P values from this work are compared with others in the literature on clay/nylon NCs, they are found to be the lowest reported for nylon-6, especially for those at 5 vol. % Ti3C2Tz. This reduction in D, and subsequently P, is ascribed to the high aspect ratios the synthesized Ti3C2Tz flakes. These result are particularly noteworthy, as comparison is being made between initial results and a mature technology. The fact that the first generation nylon-6/MXene NCs – where no attempts whatsoever were made to minimize D – have lower D values than materials that have been reseachered for more than 30 years, bodes well that these values can be significantly reduced in the near future. It is worth noting that these D values are achieved despite a slight reduction in crystallinity (Table 1), which typically increases permeability. The fact that the 5 vol. % NC only absorbs 3.8% moisture is also noteworthy and suggests that it may be possible to reduce the effect of moisture on the mechanical properties of nylon/MXene NCs. Moisture-sorption isotherms of neat and 1.9 vol. % NC films, at 60 °C, were fit to the GAB equation. From the fits, little evidence for water clustering phenomena is shown. From FTIR-ATR spectra of water saturated PA6 and 7-NC thin films it is concluded that the surface terminations of the Ti3C2Tz flakes bind with nylon-6 reducing water adsorption sites, which in turn explain the reduced solubility in NC films. As far as we are aware, this is the first time a complete dispersion of MXene was achieved in any polyamide. It is also the first measurements of water diffusion of a MXene NC. In the clay/polymer NC literature, emphasis has been on mechanical properties, and at this time efforts to carry out tensile and bend tests are being made. Based on the results presented herein, it is reasonable to assume that – like for clay - the addition of small amounts of MXene will have a large positive impact on the mechanical properties given the demonstrated tendency to reduce permeability of water vapor. In general, with the encouraging results obtained herein, we hope this work inspires others to explore the underdeveloped area of MXene/polymer NCs as well as MXene/surfactant chemistry. The payback could be quite rewarding. Associated Content Supporting Information Overview of the FIB process for preparation of TEM samples; Additional TEM micrographs of 1.9 vol. % NC sample; Dependence of percent crystallinity on filler volume fraction for this work and others; Table of diffusion coefficients from previous work in clay/PA6 composites. FTIR-ATR spectra of dried PA6 and NC films. Table of FTIR-ATR frequencies of dried nylon-6 and NC films. Tables of raw data and fitting parameters for PA6 and 7-NC thin films used for GAB analysis. The Supporting Information is available free of charge on the ACS Publications website at DOI: XX.XXX/acsami.XXXXXXX. Author Information Corresponding Author *E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACS Paragon Plus Environment

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Funding This work was funded by the Army Research Office (69521MSII). Notes The authors declare no competing financial interest. Acknowledgments We would like to extend our gratitude to Dr. Giuseppe Palmese for allowing us to run sorption experiments on the DVS instrument and for excellent discussion regarding the obtained results.

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Nylon-6 nanocomposites reinforced with 2D Ti3C2Tz MXene were prepared by surface modification of MXene with 12-aminolauric acid, allowing for -caprolactam intercalation. Subsequent heating at 250 °C for 6 h initiates the in situ polymerization of -caprolactam in the presence of 12-Ti3C2Tz. These composites were found to have excellent relative permeability (R=0.06-0.23) at low MXene volume fractions (2 and 5 vol. %).


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Ti3C2Tz MXene Nanocomposites Synthesized by in Situ Ring

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