Tuning the Morphology of an Acrylate-Based Metallo-Supramolecular

Feb 16, 2018 - Tuning the Morphology of an Acrylate-Based Metallo-Supramolecular Network: From Vesicles to Cylinders. Alice M. Savage† , Scott D. Wa...
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Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

Tuning the Morphology of an Acrylate-Based MetalloSupramolecular Network: From Vesicles to Cylinders Alice M. Savage,† Scott D. Walck,† Robert H. Lambeth,† and Frederick L. Beyer*,† †

US Army Research Laboratory, Aberdeen Proving Ground, Maryland 21005, United States S Supporting Information *

ABSTRACT: This work investigated the morphological behavior of an acrylate-based metallo-supramolecular polymer system. RAFT (reversible addition−fragmentation chain transfer) polymerization techniques were used to synthesize low molar mass, linear prepolymers of n-butyl acrylate and a 2,6-bis(1′-methylbenzimidazolyl)pyridine−acrylate monomer (MeBIP−Ac) of varying concentration (2−10%). This synthesis incorporated a systematic increase of cross-link points (MeBIP ligands) pendent to the polymer backbone. A zinc(II) salt (Zn(ClO4)2) complexed with the pendent MeBIP ligands in a 1:2 ratio to form cross-linked polymers as free-standing films. The morphology of the neat films as well as those with added unbound MeBIP−zinc−MeBIP metal−ligand (ML) complex were characterized using transmission electron microscopy (TEM), HAADF-STEM (high angle annular dark field scanning transmission electron microscopy), energy dispersive X-ray spectroscopy (EDS), electron energy loss spectroscopy (EELS), energy-filtered TEM (EFTEM), and small-angle X-ray scattering (SAXS) techniques. A vesicle morphology in the bulk material was found for films of the neat polymer containing 2% MeBIP, while films at all other compositions exhibited a disordered microphase-separated morphology. The vesicle morphology of the 2% MeBIP films was transformed into a morphology of cylinders with the addition of unbound MeBIP−zinc−MeBIP complex.

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INTRODUCTION Advanced functional polymer membranes are the basis of large scale industrial applications in the chemical, medical, and water treatment industries.1 For example, polymer membranes are used to separate many liquids and gases through filtration processes like reverse osmosis,2 pervaporation,3 and nano-, ultra-, and microfiltration.4 Other applications of functional polymer membranes include fuel cell systems,5 lab-on-chip technologies,6 antifouling membranes,7 bulk heterojunction solar cells,8 and medical devices.9 Metallo-supramolecular polymers are a class of polymers that contain dynamic and reversible metal-coordination chemistries10 where the metal centers introduce nonconventional properties like bioactivity,11 molecular magnetism,12 and ferroelectrics.13 Metal centers are incorporated into a polymeric system during monomer and oligomer synthesis or by postpolymerization functionalization.14 They can be placed in different locations along the backbone (randomly, telechelically,15 pendently,16 in blocks, etc.), all of which produce different supramolecular polymer topologies and ultimately different polymer morphologies. Films of the resulting supramolecular materials still maintain the desirable attributes of the metal−ligand linkage like stimuli responsiveness (fully reversible noncovalent interactions), withstanding multiple “healing cycles” without substantial loss of performance, and low-viscosity melts.17 The stimuli-responsive nature offers additional pathways to tailor functional polymeric membranes before and after membrane formation, including the possibility of creating a dynamic morphology. © XXXX American Chemical Society

In many different applications, performance correlates to the morphology of the polymeric membranes.8,18−21 However, tailoring or tuning the film morphology is not always easy and may require new synthesis of monomers and polymers or comprehensive studies of polymeric blends and composites.22 Film morphology is controlled by parameters similar to those for conventional polymer membranes, like polymer topology, processing parameters (i.e., temperature, solvent, humidity, etc.), and hierarchical organization as well as the parameters that govern the self-assembly of the metal-coordination site.23 There remain few methods where new synthesis is not required to alter the morphology of a polymer membrane while still maintaining the key desirable polymer properties. Previously, Jackson et al.16,24 designed and synthesized a family of metallo-supramolecular acrylate networks utilizing the metal-coordination chemistries of 2,6-bis(1′-methylbenzimidazolyl)pyridine (MeBIP) ligand and a Zn(II) metal salt which binds in a 2:1 (ligand to metal) ratio.16,24 This metal−ligand (ML) complex is thermo-, photo-, and chemoresponsive25 but still maintains high association constants in the natural state.17 Surprisingly, the addition of unbound ML complex produced a morphology containing rods or whiskers which, for median ML loadings, served as a reinforcing phase held together by the ML complex.24 Received: November 29, 2017 Revised: January 16, 2018

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DOI: 10.1021/acs.macromol.7b02536 Macromolecules XXXX, XXX, XXX−XXX

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Figure 1. Synthetic scheme for MeBIP−Ac prepolymers with 2−10% MeBIP−Ac monomer and 90−98% n-butyl acrylate monomer feed ratio followed by the supramolecular polymerization of the prepolymers with the addition of Zn(ClO4)2. Prepolymers were synthesized using DCE, AIBN, and CTA at 70 °C, and supramolecular polymerization was done in a mixture of acetonitrile and chloroform.

In the present study, new polymer topologies were synthesized by varying the monomer feed ratio of the linear prepolymers which resulted in new architectures of the supramolecular polymer networks ranging from highly crosslinked networks to lightly cross-linked networks (Figure 1). The morphological changes of these linear prepolymers were probed as the concentration of the bound MeBIP ligand was systematically increased. The effect of cross-link density (varying monomer feed ratio) on the mechanical properties of these networks was evaluated using dynamic mechanical analysis (DMA). In addition, the morphological effects of the systematic addition of a nanoscale filler in these networks was investigated. The impact of changing both variables on morphology was probed using electron microscopy techniques like HAADF-STEM, EDS, EFTEM, EELS, and small-angle Xray scattering (SAXS).

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Table 1. Summary of MeBIP-Ac-co-NBA Polymer Properties MeBIP− Ac (%)

cross-link densitya (mmol g−1)

Mnb (g mol−1)

Đ

2 3 5 10

0.19 0.23 0.27 0.55

22000 17000 14000 19000

1.57 1.59 1.80 2.00

Tgc (°C)

G′d (MPa)

−28 −23 −19 −7

1.97 2.74 7.91 50.1

± ± ± ±

1 2 1 1

± ± ± ±

0.2 0.2 2 10

a Measured by UV/vis titration.14,15,28 bNumber-average molar mass was measured using THF SEC. cGlass transition temperature was measured using DMA. dStorage modulus (G′) measured at 50 °C using DMA.

technique previously described,15,16,27 the supramolecular polymer films were formed using a ratio of Zn(ClO4)2 to MeBIP ligand of 1:2 (Figure 1). To a 100 mL jar, 0.25 g of 2% MeBIP−Ac prepolymer was added and dissolved in 20 mL of chloroform using sonication. 8.75 mg (0.023 mmol) of Zn(ClO4)2 was added to a 20 mL scintillation vial and dissolved in 12 mL of acetonitrile. The Zn(ClO4)2/acetonitrile solution was added to the prepolymer solution and mixed for 30 min. The solvent was allowed to evaporate overnight. The supramolecular polymer was then redissolved in 10 mL of chloroform and 5 mL of acetonitrile using sonication and cast onto a 25 mL Teflon dish. The solvent was slowly evaporated overnight and then dried in a vacuum oven for 24 h at 50 °C. To fabricate supramolecular films with excess molar equivalents of ML complex (Figure 2), the same procedure for the neat films was used but with the addition of MeBIP−OH and extra Zn(ClO4)2 to maintain a M:L ratio of 1:2. For the 2% MeBIP−Ac films, 4 mg of MeBIP−OH (0.012 mmol) and 2.19 mg of Zn(ClO4)2 (0.006 mmol) were added to the assembly procedure for the 25% excess MeBIP film, 8 mg of MeBIP−OH (0.024 mmol) and 4.40 mg of Zn(ClO4)2 (0.012 mmol) for the 50% excess MeBIP film, and 16 mg of MeBIP−OH (0.048 mmol) and 8.80 mg of Zn(ClO4)2 (0.024 mmol) for the 100% excess MeBIP film. In the 100% excess molar equivalents of ML complex film, there was one unbound MeBIP ligand for every bound MeBIP ligand (pendent chain), or there are equal molar equivalents of unbound and bound ML complexes. The MeBIP−OH ligand was dissolved in 5 mL of chloroform, and the excess Zn(ClO4)2 was added to the original Zn(ClO4)2/acetonitrile solution. After all of the separate components were dissolved in their respective solvents, they were added together at the same time and stirred for 30 min. The solvent was allowed to evaporate overnight, redissolved in 10 mL of

EXPERIMENTAL SECTION

Materials. All solvents and reagents were purchased from Aldrich Chemical Co. and used without further purification unless otherwise indicated. 2,2′-Azobis(2-methylpropionitrile) (AIBN) was recrystallized from diethyl ether. n-Butyl acrylate (NBA) was distilled at 50 °C under vacuum. MeBIP−Ac monomer was prepared according to previous literature. 16 MeBIP−OH was prepared as described previously.26 Synthesis. RAFT Copolymerization of MeBIP−Ac with NBA (2% MeBIP−Ac Prepolymer) (Figure 1). MeBIP−Ac (0.25 g, 0.45 mmol), n-butyl acrylate (NBA) (3.0 g, 23.4 mmol), cyanomethyldodecyl trithiocarbonate (chain transfer agent, CTA) (15 mg, 0.05 mmol), AIBN (5.5 mg, 0.03 mmol), and 8 mL of 1,2-dichloroethane (DCE) were added to a 40 mL vial with stir bar and septum. The vial was purged with nitrogen for 30 min and then stirred for 2 h at 70 °C. The reaction was cooled to room temperature, and then polymer solution was precipitated into methanol (50 mL) and collected by centrifugation. Precipitation was repeated three times. The polymer was dried under high vacuum overnight to yield a viscous, light yellow liquid. (Mn = 22 000 g mol−1 and Đ = 1.57). The same procedure was used for the other prepolymers but with varying ratios of MeBIP−Ac to NBA while maintaining the same number of moles of monomer (see Table 1). Supramolecular Network Film Formation. After measuring the amount of MeBIP ligand per gram of polymer using a UV/vis titration B

DOI: 10.1021/acs.macromol.7b02536 Macromolecules XXXX, XXX, XXX−XXX

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Figure 2. Components of the MeBIP−Ac metallo-supramolecular films with excess ML complex: prepolymers and 25%, 50%, or 100% excess molar equivalents of ML complex. chloroform and 5 mL of acetonitrile using sonication, and cast onto a 25 mL Teflon dish. The solvent was slowly evaporated overnight and then dried in a vacuum oven for 24 h at 50 °C. Both the neat films and films with excess ML complex were then cut into small sections less than 2.5 cm × 0.5 cm in area and placed between two metal plates backed with release ply. A 0.75 mm thick aluminum spacer with 2.5 cm by 0.5 cm holes was used to set the film thickness and shape. The apparatus was placed in a hot press. The hot press cycle was as follows: warm up to 120 °C with no pressure for 1 h, hold at 120 °C with no pressure for 30 min, hold at 120 °C with 2 tons of pressure for 1 h, hold at 2 tons of pressure, and slowly cool the apparatus for 2 h. Characterization Techniques. Morphological characterization of these soft membranes can be performed using electron microscopy techniques, such as transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), and scanning electron microscopy (SEM). They do not require staining because of the higher Z-contrast of the metal core relative to the hydrocarbonbased polymer.23,28 Small-angle X-ray scattering (SAXS) and wideangle X-ray scattering (WAXS) provide complementary morphology information from a macroscopic volume of the sample. A JEOL JEM-2100F TEM and a Gatan 806 high-angle annular dark field scanning TEM (HAADF STEM) detector were used to collect dark field TEM data from each sample. The microscope was operated at 200 kV, with a 40 μm condenser aperture that gave a convergence angle of 8.3 mrad, a HAADF STEM collection angle of 48−168 mrad, and spot size of 0.2 nm. Gatan Digital Micrograph version 1.85 with DigiScan was used to collect and analyze the data. Samples for TEM were prepared using a Leica EM UC7 ultramicrotome with a Leica EM FC7 cryostage using a Diatome 35° diamond knife. Sections were cut at −120 °C to a thickness of 50−70 nm. Energy-Dispersive X-ray Spectroscopy (EDS) and Electron Energy Loss Spectroscopy (EELS). STEM with EDS and EELS measurements were performed using a JEOL JEM-2100F TEM equipped with an Oxford INCA X-ray energy dispersive spectrometer with an ultrathin window and a takeoff angle of 13° and a Gatan Tridiem GIF (Gatan Imaging Filter). TEM samples were prepared using a Leica EM UC7 ultramicrotome with a Leica EM FC7 cryostage using a Diatome 35° diamond knife. Sections were cut at −120 °C to a thickness of 50−70 nm and placed on 3 mm, 400 mesh, and bare copper grids from PELCO. EDS collection was done using either an area scan for single spectra to minimize damaging the polymer or by collecting a spectrum image with a 1 nm spot size with a dwell time of 0.5 s. With a count rate of approximately 800 cps, this provided total counts at each point in the spectrum image of about 400. Principal component analysis (PCA)29,30 and multivariate statistical analysis (MSA)31,32 were used to analyze the EDS spectrum images. The PCA data were used to reconstruct the spectrum image to remove noise. The reconstructed spectrum images were then used to generate X-ray maps using the zinc, carbon, sulfur, oxygen, and chlorine Kα X-ray lines. EELS spectra were collected using a convergence angle of 8.3 mrad and a collection angle of 9.7 mrad with an ∼1 eV energy resolution at a dispersion of 0.2 eV per channel while in HAADF-STEM mode. The energy-filtered

TEM (EFTEM) jump ratio and elemental map images were acquired from Zn-L, Cl-L, N-K, O-K, and C-K edges using Gatan’s suggested energy windows. Small-Angle X-ray Scattering (SAXS). SAXS data were collected using a Rigaku S-MAXS3000 instrument. X-rays were generated using a MicroMax-007HFM rotating Cu anode source and then focused and monochromated using a Confocal Max-Flux double-focusing optic. The wavelength, λ, was 1.542 Å. Samples were characterized over a range of momentum transfer vector magnitude, q, from 0.008 to 0.7 Å−1, where q = 4π sin(θ)/λ and 2θ is the scattering angle. Distance and beam center calibration were performed with Ag behenate.33 Background corrections and data analysis were performed using Igor Pro 7 and procedures developed at Argonne National Laboratory.34,35 Size Exclusion Chromatography (SEC). The molecular weight of the polymers was estimated by SEC. The measurements were performed with Wyatt Optilab rEX concentration detector coupled to an Agilent 1260 Infinity HPLC system which contained an isocratic pump, autosampler, and thermostatically controlled column compartment. The polymers were eluted using DMF containing 50 mM LiBR at 70 °C at a flow rate of 0.8 mL/min through two Agilent Laboratories PLgel 5 μm mixed D columns. Polymer molecular weights are reported versus PMMA standards. Dynamic Mechanical Analysis (DMA). Rectangular film sections 5.0 mm wide × 10 mm long × 1 mm thick were tested in thin film tension mode on a TA Instruments Q800 DMA. Samples were clamped in the grips with 9 in.-lb torque and subjected to a temperature sweep from −100 to 200 °C while an oscillatory strain of 1% was applied at 1 Hz with minimum force set to 0.01 N. Each DMA experiment was repeated three times. UV/vis spectra of the supramolecular polymers were collected at a wavelength range of 250−400 nm using a Beckman Coulter DU 800 spectrophotometer.

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RESULTS AND DISCUSSION RAFT polymerization techniques were used to synthesize linear prepolymers of MeBIP−Ac and NBA with varying MeBIP−AC monomer feed ratio (Figure 1). The SEC results revealed low molar mass prepolymers (Table 1). Because of the probable radical scavenging nature of the MeBIP ligand,36 an increased concentration of AIBN (1:2, AIBN:CTA) was used in the RAFT polymerizations compared to typical initiator to CTA ratios. Along with the use of a chlorinated solvent to dissolve the MeBIP−Ac monomer,37 this resulted in higher polydispersity indices (Đ) (>1.2) for controlled polymerizations but produced the low molar mass prepolymers desired for increased polymer chain mobility. The final metallo-supramolecular films were fabricated by the addition of 1 mol of Zn(ClO4)2 for every 2 mol of MeBIP ligand in a mixture of chloroform and acetonitrile (Figure 1). The zinc metal cation was chosen due to the rapid complexation with the MeBIP C

DOI: 10.1021/acs.macromol.7b02536 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules ligand. This ratio was maintained for all metallo-supramolecular films and confirmed with UV/vis titration.15,16,27 The neat films (having no unbound ML complex) appeared as clear, off-white, free-standing films. Prepolymers with MeBIP−Ac content less than 2% were synthesized but remained in a viscous liquid form even after the addition of Zn(ClO4)2. To evaluate the morphology of the neat 2%, 3%, 5%, and 10% MeBIP films, STEM, EDS, EELS, EFTEM, and SAXS were used, revealing microphase-separated morphologies for all samples. Vesicle structures were present only in the 2% MeBIP films (Figures 3 and 4).

Figure 4. SAXS of 2%, 3%, 5%, and 10% MeBIP−Ac metallosupramolecular neat films.

five elements: zinc, carbon, chlorine, sulfur, and oxygen (Figure 5) in the aggregates for both the 2% and 3% MeBIP−Ac films.

Figure 3. HAADF-STEM micrographs of the 2% (a), 3% (b), 5% (c), and 10% (d) MeBIP−Ac metallo-supramolecular neat films. (b−d) Micrographs were nearly identical with very few spherical aggregates in a disordered microphase-separated polymer matrix.

As shown in Figure 3a, the HAADF-STEM micrographs revealed rings in the 2% MeBIP films. The rings ranged in diameter from 10 to 20 nm within a disordered, microphaseseparated matrix. At 3% MeBIP−Ac, the rings were not observed, and only a few small spherical aggregates were observed in each micrograph. In films with higher MeBIP−Ac content (5−10%), only a disordered microphase-separated phase remained with very few aggregates. The aggregates were also observed in traditional bright-field TEM, where they appeared as dark features. The brightness of the aggregates and vesicles observed in the HAADF-STEM images arises from the Z-contrast between the zinc-rich ML phase and the hydrocarbon, acrylate polymer backbone. Based on these microscopy results, the rings were theorized to be vesicles having a hollow core surrounded by a corona comprising the ML complex.38−41 The composition variations between aggregates or vesicles and the polymer matrix were characterized using EELS, EDS, and EFTEM. Principal component analysis and multivariate statistical analysis of EDS spectrum images and EFTEM imaging revealed only two main components: the polymer matrix and the aggregates (which appear as bright regions in the HAADFSTEM micrograph). The EDS data indicated the presence of

Figure 5. EDS X-ray maps (zinc, sulfur, carbon, oxygen, and chlorine) for 3% MeBIP−Ac metallo-supramolecular neat film.

Only the Zn-L, Cl-L, and S-L elemental maps exhibited contrast in EFTEM images (Figure 6). The O-K core loss edge could not be observed in EFTEM because of poor signal-to-noise, while poor contrast between phases inhibited elemental mapping in the case of the C-K edge (Figure S4). For the 2% MeBIP−Ac films, the EFTEM jump ratio and elemental images shows the highest concentration of zinc and chlorine in the corona of the vesicle (Figure 6) and a striking absence of zinc and chlorine in the vesicle core. This differs from the 3% MeBIP−Ac film aggregates, where EDS X-ray maps indicate the aggregates contain the elements zinc, chlorine, oxygen, sulfur, and carbon (Figure 5) and that the zinc is present throughout the aggregate. The zinc, oxygen, and chlorine resulted from the ML complex and counterion used to assemble the networked films. Not surprisingly, carbon appeared in both components due to the high carbon D

DOI: 10.1021/acs.macromol.7b02536 Macromolecules XXXX, XXX, XXX−XXX

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Figure 6. EFTEM elemental maps of a vesicle 2% MeBIP−Ac metallo-supramolecular neat film: (a) zero energy-loss BF-TEM, (b) zinc jump ratio image, (c) chlorine jump ratio image, and (d) sulfur jump ratio image. All scale bars remain the same between images. (d) is a jump ratio map of different vesicles within the same sample.

concentration gradients and aggregation during film formation.51 This unexpected formation of vesicles probably results from a combination of low molar mass acrylate chains, low concentration of bound MeBIP ligand, and solvent effects. The polymerization conditions affords low-molar-mass, linear prepolymer (copolymer) chains at or below the entanglement molar mass of n-butyl acrylate (24 000 g mol−1).52,53 The degree of polymerization for these prepolymers remains low (