Tuning the Viscoelastic Properties of Poly(n-butyl acrylate) Ionomer

Nov 30, 2017 - Tuning the Viscoelastic Properties of Poly(n-butyl acrylate) Ionomer Networks through the Use of Ion-Pair Comonomers ... In the ionomer...
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Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

Tuning the Viscoelastic Properties of Poly(n‑butyl acrylate) Ionomer Networks through the Use of Ion-Pair Comonomers Guodong Deng and Kevin A. Cavicchi* Department of Polymer Engineering, University of Akron, Akron, Ohio 44325, United States S Supporting Information *

ABSTRACT: An organic ion-pair comonomer (IPC) based on anionic and cationic styrenic monomers was synthesized and copolymerized with n-butyl acrylate (BA) by reversible addition−fragmentation chain transfer (RAFT) polymerization to generate physically cross-linked polyampholyte ionomer networks. Evidence of microphase separation of the ion pairs to produce ion-rich domains was found by rheological and atomic force microscopy measurements. Comparison of these polymers to chemically similar cationic and anionic ionomers with only one type of ion covalently bound to the polymer backbone demonstrated that the connectivity of the ions to the polymer backbone had a strong effect on the viscoelastic properties. Characterization of the corresponding polyelectrolytes showed a ca. 125 °C increase in the glass transition temperature (Tg) from the cationic to the polyampholytic polyelectrolyte. In the ionomers, this elevated Tg allowed the vitrification of the ion-rich domains at ambient temperatures in the polyampholyte networks over a range of ion-pair concentrations. This produces long-lived physical cross-links at room temperature. The weak microphase separation of the neutral and ionic segments resulted in the increase of the effective volume fraction of the ion-rich domains, increasing the resulting modulus of the ionomers and plasticization of the ion-rich domains with the low Tg BA segments. This plasticization allowed ion hopping at accessible temperatures to enable thermoplastic processing at 150−200 °C. More generally, this work demonstrates that variation of the connectivity of the ion pairs is a facile method to tune the thermomechanical behavior of ionomers with nonmetal ion pairs.

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neutral, hydrophobic monomers.23−25 As an alternative, anionic ionomers with organic, cationic counterions and cationic ionomers with halogen or bulkier, organic, anionic counterions, such as tetrafluoroborate, have been prepared by direct polymerization.23,26,27 In addition to tuning the ionic interactions, direct copolymerization of metal-free ionic and nonionic monomers allows for more precise placement of the ionic groups and more complex polymer architectures (e.g., block copolymer and gradient copolymer).23,27 However, these materials typically form inferior dynamic networks compared to metal neutralized ionomers. The introduction of large, organic ions tends to weaken ionic interactions leading to faster bond exchange and more transient network properties unless high ion concentrations are used.26,28 A potential approach to overcome this limitation is to directly form ion-pair cross-links by using polyionic or polymer-bound counterions analogous to multivalent metal counterions in anionic ionomers.19,22 There are numerous examples of blending ionomers, polyionic small molecules, and polyelectrolytes with their oppositely charged analogue or cross-pairings of oppositely charged building blocks (e.g., polyeletrolyte + polyionic small molecule).29−38 A less

INTRODUCTION Dynamically bonded networks, which can break and re-form under external stimuli (e.g., heat, mechanical stress, light, etc.), have emerged as a useful approach to fabricate stimuliresponsive polymers,1−3 such as thermoplastic and self-healing elastomers,4−8 shape memory polymers,9−11 and tough elastomers.12,13 A general route to introduce dynamic bonds in a polymer network is through noncovalent, supramolecular interactions (e.g., hydrogen bonding, π−π stacking, metal− ligand interactions, ionic interactions).1,3 Ionic interactions have a broad utility due to the wide range of bond strengths and structures from isolated ion pairs to larger multiplets and aggregates.14,15 The most widely used ionic polymers to build dynamic networks are ionomers, where the ionic groups are randomly, covalently bound to the polymer backbone at up to 15 mol %.16 In anionic, metal neutralized ionomers, the ion pairs tend to aggregate in a low-polarity matrix, thus forming multiplets or larger clusters, allowing chains with pendant, monovalent counterions (e.g., Na+, K+, Rb+, etc.) to physically cross-link.17,18 For ionomers with multivalent metal counterions, direct cross-linking of the ion pair also occurs and can produce stronger and/or longer-lived cross-links.19−22 However, the preparation of these types of ionomers generally requires post-polymerization modification as it is difficult to directly copolymerize metal neutralized monomers with © XXXX American Chemical Society

Received: July 18, 2017 Revised: November 12, 2017

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Scheme 1. (a) Synthetic Route for the Ion-Pair Comonomer VBTOP-SS through Quaternization and Following Metathesis IonExchange Reaction; (b) RAFT Copolymerization of VBTOP-SS with n-Butyl Acrylate (BA) To Afford Polyampholyte Ionomer, P-1; (c) Chemical Structures of Anionic and Cationic Ionomers P-2 and P-3

well-explored route to prepare dynamic networks with all polymer-bound ions is through the direct copolymerization of ion-pair comonomers (IPCs) with noncharged monomers, as first reported by Salamone and co-workers.39,40 This method circumvents many of the complications of blending oppositely charged species (e.g., salt removal, incomplete reaction, and stoichiometry) and would allow for the design of polyampholytes with more complex architectures (e.g., block, graft, and star copolymers) or with other reversible bonding interactions (e.g., hydrogen bonds) through terpolymerization. While the solution characterization of polyampholyte ionomers has demonstrated their basic dynamic network properties, there has been little attempt to characterize melt systems.41−43 Furthermore, the synthesis of these systems has concentrated on conventional free radical polymerization, which can produce additional features, such as covalent cross-linking through chain transfer or sample heterogeneity, which complicate the fundamental viscoelastic characterization of the resulting polymers.39,40,44 One recent attempt to prepare polyampholyte ionomers using IPC copolymerization through controlled free radical polymerization had limited conversion, likely due to the asymmetric structure and reactivity of the IPC.45 In this article, we present the synthesis of an IPC based on styrenic monomers. The copolymerization of this IPC with nbutyl acrylate (BA) by RAFT polymerization and the characterization of the physical properties of these polymers were investigated by dynamic mechanical analysis, calorimetry, and atomic force microscopy. These polymers serve as a synthetically straightforward platform to isolate the role of covalent bonding of the counterion and direct ion-pair crosslinking on their viscoelastic properties through direct comparison to the analogous cationic and anionic ionomers, where only the cation or anion is covalently bound to the

polymer chain. It is shown that using an IPC is an effective strategy to overcome the limitation of weak ion-pair interactions in organic ionomers to prepare room temperature, physically cross-linked polymer networks. The key feature of the IPC presented in this work is the substantial increase in the glass transition temperature of the resulting ampholytic polyelectrolyte from the homopolymerization of the IPC when compared to the polyelectrolyte prepared by homopolymerization of a monomer with a pendant counterion. In ionomer systems this property allows the formation of vitrified, ion-rich domains at ambient temperature, which act as both physical cross-links and filler, while retaining processability at elevated temperature. A interesting feature of these particular ionomers is weak phase separation of the neutral and ionic segments leading to much larger apparent volume fractions of the ion-rich domains enhancing their reinforcing effect and the plasticization of the ionic domains leading to the processability at elevated temperature. More broadly, this synthetic approach using IPCs has the potential to expand the material properties of ionomers and fine-tune their physical properties toward ultimately gaining full control over the viscoelastic behavior of ionically bonded networks.

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EXPERIMENTAL SECTION

Materials. Hexane (>98.5%, Sigma-Aldrich), dichloromethane (CH2Cl2, >99.8%, Sigma-Aldrich), chloroform (CHCl3, >99%, Sigma-Aldrich), chlorobenzene (Cl-Bz, >99%, Sigma-Aldrich), magnesium sulfate (>99.5%, Sigma-Aldrich), trioctylphosphine (TOP, >97%, Sigma-Aldrich), vinylbenzyl chloride (VBCl, m- and p- mixture, >88.0%, stabilized, Tokyo Chemical Industry), sodium 4vinylbenzenesulfonate (SS-Na, Alfa Aesar), hydrochloric acid solution (38 wt %, Mallinckrodt Chemicals), sodium p-toluenesulfonate (>90.0%, Tokyo Chemical Industry), and p-toluenesulfonic acid monohydrate (98%, Alfa Aesar) were used as received. n-Butyl B

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Macromolecules acrylate (n-BA, >98.0%, stabilized) was purified by filtering through a column of basic alumina. 2,2'-Azobis(2-methylpropionitrile) (AIBN, Sigma-Aldrich) was purified by dissolving in methanol at 45 °C and recrystallization in a freezer. Benzyl dodecyl trithiocarbonate (BDTC), a nonionic RAFT agent, was synthesized using a previously reported method.46 Monomer and Polymer Characterization Methods. 1H nuclear magnetic resonance (1H NMR) (Varian Mercury-300 MHz spectrometer) was used to characterize the synthesized materials using deuterated chloroform (CDCl3) or deuterated acetone ((CD3)2CO) as the solvent with a concentration of 10−15 mg/mL for each analyte. Thermogravimetric analysis (TGA) was used to investigate the decomposition temperature of the synthesized materials in a nitrogen atmosphere. The temperature scan was run from 40 to 800 °C with a constant heating rate of 10 °C. The degradation temperature was assigned to the temperature at which a 5% overall reduction of the original mass was recorded. Differential scanning calorimetry (DSC, TA Instruments Q200) was performed to measure the glass transition temperatures (Tg) of the synthesized polymers. The samples were sealed in hermetic pans and scanned over the temperature range from −75 to evaluated temperature with a constant heating and cooling rate of 5 °C/min. The Tg was determined by the midpoint of the inflected curve from the second heating scan. Rheology measurements were performed on a strain-controlled Advance Rheometric Expansion System (ARES), model G2 (TA Instruments, New Castle, DE). A parallel plate fixture with a diameter of 8 mm and a gap ∼1.0 mm was used for the temperature sweep experiments. Prior to each experiment, the solid samples were prepared by compression molding at elevated temperature (150−250 °C) under vacuum for around 1 h, while the fluid samples were directly loaded at 100 °C and held for 10 min in the fixture before measurement. For the temperature sweep, the samples were ramped from −20 °C to elevated temperature under nitrogen with a constant heating or cooling rate of 5 °C/min. The upper limit temperature of the experiment was set to 250 °C to ensure no degradation of the samples. Strain sweeps were conducted at each temperature range to ensure the measurements were in the linear viscoelastic region over the entire measurement. Sample Preparation and Morphological Characterization. Thin films of PBA homopolymer and ion-containing copolymers (P-1 and P-3) on silicon wafers were prepared for atomic force microscopy (AFM) study. Silicon wafers with a 300 nm thick native oxide layer were first cleaned with UV ozone (Jelight Company Inc., model no. 42) for 2 h before use. Polymer solutions with 50 mg polymer/950 mg toluene were prepared and spin-coated on clean silicon wafers at 2500 rpm for 1 min. The surface morphologies of these films were characterized by AFM (Dimension ICON, Veeco) using the tapping mode at 0.5 Hz using PPP-NCC-50 tips (Nanosensors). Small-angle X-ray scattering (SAXS) samples were prepared by thermally annealing at 150 °C under vacuum for up to 12 h and then gradually cooling to room temperature. SAXS data were collected at room temperature using a Rigaku MicroMax 002+ instrument operated at 50 kV working voltage and 0.6 mA current (0.6 mA). The recording time for each sample was 20 min with an X-ray wavelength of 0.154 nm. The instrument was calibrated using silver behenate. The SAXSGUI software (JJ X-ray Systems ApS) was used to analyze the obtained SAXS images.

exchange reaction draws the more hydrophobic 4-styrenesulfonate ions from the water phase to ensure the 1:1 stoichiometric ratio of the anion and cation in the resulting IPC. This type of metathesis ion exchange reaction has been proven useful for generating IPCs or other ionic-based monomers.45,48 For comparison, two phosphonium-containing, ionic monomers with pendant (i.e., nonpolymerizable) counterions (MPC) were also prepared. The successful synthesis of these ionic monomers was confirmed by 1H nuclear magnetic resonance (1H NMR) (Supporting Information Figures S1− S5). In addition the ion-pair monomers have high thermal stability as measured by thermogravimetric analysis where VBTOP-Cl and VBTOP-SS have 5% mass loss at 356 and 413 °C, respectively (Figure S9). Previous reports detailed similar results for ionic monomers where exchange to less basic anions enhanced their thermal stability.47 The IPC was copolymerized with a nonionic monomer, nbutyl acrylate (BA), to generate a polyampholyte ionomer (P1) through reversible addition−fragmentation chain transfer (RAFT) polymerization (Scheme 1b). In an example copolymerization VBTOP-SS and BA were copolymerized for 24 h in chlorobenzene (30 wt % of BA) with a 5:100 molar ratio of IPC to BA and a 1:200 molar ratio of RAFT agent to BA. At the end of the reaction a highly viscous solution was formed, which was considerably more viscous than the homopolymerization of BA under the same reaction conditions. This result is consistent with the generation of a high fraction of interchain ion pairs and network formation. After extraction of the unreacted BA monomer in hexane and drying under vacuum, a gravimetric yield of 97% was observed. The extraction was confirmed by 1H NMR analysis where no peaks from unreacted n-butyl acrylate were observed (Figure S6). A series of copolymers (named P-1-X-Y, where X:1 is the molar ratio of BA:RAFT agent and Y:100 is the molar ratio of IPC:BA (Y:100)) were prepared using variable IPC concentration (Y = 0, 1, 3, 5, and 10) and constant RAFT agent concentration (X = 200). This RAFT agent concentration was used to target a backbone molecular weight in the absence of ion-pair cross-links of 25 600 g/mol, slightly higher than the PBA’s entanglement molecular weight of 20 000 g/mol.11 For comparison, ionomers bearing the same general structure, but with pendant organic counterions (Scheme 1c, copolymers P-25-200, P-3-5-200, and P-3-200-10) were also synthesized using the same synthetic conditions, but substituting the monovinyl ionic monomer for the IPC (Figures S7 and S8).The results for the copolymerization of these different polymers are tabulated in Table 1. Further characterization of the molecular weight distribution of these polymers, such as by gel permeation Table 1. Molecular Characterization of Synthesized Copolymersa

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RESULTS AND DISCUSSION Monomer and Polymer Synthesis. The two-step synthetic route for synthesizing the IPC vinylbenzyltrioctylphosphonium 4-styrenesulfonate (VBTOP-SS) is shown in S c h e m e 1 . T h e c a t i o n ic m o n o m e r v i n y l b e n z y l trioctylphosphonium chloride (VBTOP-Cl) was first prepared by quaternization of vinylbenzyl chloride (VBCl) with trioctylphosphine (TOP).26,47 VBCl is soluble in organic solvent (e.g., CH2Cl2), and it was ion-exchanged with the water-soluble monomer sodium 4-styrenesulfonate (SS-Na) to produce the IPC VBTOP-SS. The interfacial metathesis ion-

sample

state after reaction

yield (%)

Ytarget

Ymeasured

PBA P-1-200-1 P-1-200-3 P-1-200-5 P-2-200-5 P-3-200-5 P-1-200-10

solution low-viscosity solution high-viscosity solution high-viscosity solution low-viscosity solution low-viscosity solution gel

93.3 93.5 95.9 97.1 90.0 91.0 98.3

0 1 3 5 5 5 10

0 0.9 2.9 5.0 5.0 5.0 10.1

a

Ytarget is the amount of ionic monomer in the reaction feed. Ymeasured was determined from the 1H NMR spectra. C

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with network formation being solely due to noncovalent crosslinking by the formation of interchain ion pairs. In deuterated acetone ((CD3)2CO) P-1-200-10 formed a milky, semitransparent solution (inset Figure 1d). The 1H NMR spectrum displayed no peaks in the region of the vinyl groups (5−6 ppm) consistent with complete incorporation of the IPC into the copolymer. The characteristic peak at 4.07 ppm corresponds to the −Ar−CH2−P− and −O−CH2−CH2− resonances, and the responses for the eight H atoms from the aromatic rings were assigned to the peaks at 6.5−8.0 ppm. The ratio of segments bearing ion pairs to BA segments was calculated to be 5.5 to 100, which was lower than the feed amount of Y = 10 (10:100 IPC:BA). This inconsistency was attributed to the incomplete dissolution of the ionic groups in (CD3)2CO. When a few drops of 1 M hydrochloric acid was added to P-1-200-10 solution in (CD3)2CO, the solution turned into a transparent yellowish solution (inset Figure 1e). The 1H NMR spectrum and corresponding chemical structure of materials after the ion exchange are shown in Figure 1e. Here the mole ratio of the ion pairs (−Ar−CH2−P−) to BA segments (−O−CH2−CH2−) was found to be 10 to 100, in agreement with the feed amount (Y = 10). Table 1 lists the Ymeasured for each P-#-X-Y sample where the P-1-X-Y samples were dissolved in (CD)3CO with added acid and the P-2-200-5 and P-3-200-5 were dissolved in CDCl3. The 1H NMR spectra are shown in Figures S6−S8. Physical Properties of the Ionic Polymers. All of the polymers synthesized were found to be transparent, indicating no large scale macrophase separation of the ionic and nonionic groups. The physical state at room temperature ranged from a viscous liquid (PBA) to tacky solids (P-2-200-5 and P-3-2005) to an elastic solid (P-1-200-5) showing a clear effect of the architecture of the ion pair (i.e., pendant vs polymer bound counterion) on the bulk properties (Figure 2). As shown in Figures 2d and 2e, P-1-200-5 could be compression molded at elevated temperature into a self-standing, elastic, transparent film, indicating thermoplastic properties derived from the dynamic bonding of the ion pairs. The viscoelasticity of these materials was quantitatively characterized by small-amplitude oscillatory shear rheology. Figure 3a shows isochronal temperature sweeps of the PBA homopolymer and ionic copolymers bearing the same ion-pair concentration. Similar to the results qualitatively shown in Figure 2, the rheological behavior of each type of polymer is drastically different. For the neat PBA, the loss modulus (G″) is larger than the storage modulus (G′) over the whole temperature range tested, and G′ decreases more rapidly than G″ with temperature, indicating liquid-like, terminal behavior. The incorporation of 5% pendant counterions (P-2-200-5 and P-3-200-5) delays the onset of the terminal region compared to the pure PBA, and a region with G′ ≈ G″ is observed at temperatures below ca. 30 °C. For comparison, the bonding of both the anion and cation to the polymer backbone (P-1-2005) has a dramatic influence on the viscoelastic properties. G′ is much higher than G″ over the low temperature region (−20 to 50 °C) where G′ > G″ and a minimum in G″ is indicative of solid network formation. At higher temperature G″ and G′ intersect, and G′ decreases substantially at the highest temperatures measured, which would occur at the onset of the terminal relaxation regime. There is additionally some rheological complexity in this system as there is an intermediate region where G′ ≈ G″ between the plateau and the onset of terminal relaxation similar to the P-2-200-5 and P-3-200-5 ionomers. Figure 3b also shows isochronal temperature sweeps

chromatography (GPC), was not possible as these polymers were difficult to elute from a standard GPC instrument using THF as the mobile phase even when the polymer was completely soluble in THF. This is attributed to interactions between the column and the polymer, which prevent pure size exclusion measurements.23 After drying, the copolymers displayed different solubilities in organic solvents. The anionic and cationic ionomers (P-2 and P-3) were found to be easily dissolved in excess organic solvent (e.g., toluene), while for the polyampholyte ionomers (P-1) the solubility was dependent on the ionic content. The P-1 series were difficult to be dissolve in excess toluene, a good solvent for PBA (ca. 1−2 wt % polymer solution) over short time (24 h) as shown in Figure 1a. This was attributed to the rearrangement of the ion

Figure 1. Photographs of samples (a) P-1-200-5, (b) P-1-200-10, and (c) P-1-200-20 in the excess toluene after 24 h. Photos of P-1-200-10 in excess acetone (d) before adding HCl solution and (e) after adding drops of HCl solution as well as the corresponding 1H NMR (300 MHz, (CD3)2CO) spectrum.

pairs from interchain to intrachain ion pairs during swelling to relieve chain stretching. When Y = 10, the polymer swelled rather than dissolved in toluene at long time (>24 h), consistent with the ion pairs acting as cross-links, indicating the overall chain dynamics slow with increasing ion-pair concentration (Figure 1b,c). These swollen polymers can further be dissolved by adding drops of an acidic molecule (e.g., p-toluenesulfonic acid monohydrate or HCl) dissolved in tetrahydrofuran (THF) or acetone. This result is consistent D

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Figure 2. Photographs of macroscopic appearance of (a) PBA, (b) P-2-200-5, (c) P-3-200-5, (d) P-1-200-5, and (e) P-1-200-5.

temperature before the terminal region. At Y ≥ 3, the P-1200 series curves are self-similar with a plateau in G′ at lower temperature, in which both the height and length of the plateau increase with increasing Y, indicating the formation of a solid network. For P-3-200-10 the shapes of the G′ and G″ vs T curves are qualitatively similar to the P-1-200-10 curve from T > ca. 70 °C but shifted to a lower temperature and compressed horizontally. The high moduli region with G′ > G″ around T = 5 °C in P-3-200-10 appears to be the end of a rubbery plateau, which would imply physical cross-linking in this sample. As only the cations are bound to the polymer backbone in the P-3 series, physical cross-linking would only occur by ionic aggregation,49 which would also be expected to occur in the P-1 series given the structural similarity of the ion pairs. If the ion pairs in the P-1 series did not aggregate and only acted as pairwise cross-links, according to ideal rubber elasticity theory, the plateau modulus (GN) should be given by50

Figure 3. Isochronal temperature sweep of G′ (filled symbols) and G″ (empty symbols) for (a) PBA homopolymer and P-1-200-5, P-2-2005, and P-3-200-5 and (b) P-1-200-5, −20 to 160 °C (heating cycle, red) and 160 to −20 °C (cooling cycle, black). All data were measured at 1 Hz at strains in the linear viscoelastic region.

of the heating and cooling cycle of P-1-200-5 over a temperature range of −20 to 160 °C. The cooling cycle was nearly identical to the heating cycle, indicating little thermal hysteresis and the fast thermal reversibility and stability of the polymers. Figure 4 displays the isochronal temperature sweeps of the P1 and P-3 series with different ion-pair concentrations. In both the P-1 and P-3 series increasing the concentration of ionic groups shifts the onset of terminal relaxation to higher temperatures and increases the magnitude of the storage and loss moduli at lower temperature. The curve of P-1-200-1 is similar to P-3-200-5, where G′ is close to G″ at low

GN =

2Mx ⎞ ρRT ⎛ ⎜1 − ⎟ Mx ⎝ M ⎠

(1)

where ρ is the density of material, R is the ideal gas constant, T is the absolute temperature, Mx is the molecular weight between effective cross-linked strands, M is molecular weight of the network chains in the absence of cross-linking, and 2Mx/M is the fraction of dangling ends. Figure 5 displays the predicted values of plateau modulus of the P-1 series from eq 1 compared to the measured values of the rubbery plateau modulus taken at the tan δ minimum (Figure S10). For P-1-200-1, the measured value corresponds well with the predicted value by pairwise

Figure 4. Isochronal temperature sweep of G′ (filled symbols) and G″ (empty symbols) for (a) P-1-200-Y (Y = 0, 1, 3, 5, 10) and (b) P-3200-Y (Y = 0, 5, 10). All data were measured at 1 Hz at strains in the linear viscoelastic region.

Figure 5. Measured and predicated rubbery plateau modulus (based on rubber elasticity theory) as a function of varying ion-pair concentration in P-1 series. E

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While the Tg of the ion-rich domains in copolymers was not observed in the DSC measurements, they must be above room temperature for the behavior attributed to the vitrification of ion-rich domains in Figure 5 to be observed.53 While at the same time, terminal relaxation occurs far below the Tgs of the ionic homopolymers, which indicates that they are not pure ionic domains and are plasticized with PBA segments. The volume fractions of the ion-rich domains were estimated from the enthalpy change (ΔH) of the glass transition region as show in Figure S13 and are listed in Table 2. Using the ΔH of PBA (ΔHPBA) as a reference, the volume fractions of ion-poor and ion-rich domains were calculated as ΔH/ΔHPBA and 1 − ΔH/ΔHPBA, respectively. Table 2 also lists the weight fraction of the ionic segments. The volume fraction of the ion-rich domains is always larger than the weight fraction of ionic groups in the copolymers. Assuming that the density of PBA and the ionic groups are not widely different, this indicates that the ionic domains contain a significant amount of PBA segments. Therefore, the key difference between the P-1 and P-3 series is the mobility of the ionic repeat units. Even when plasticized, the significantly higher Tg of the pure IPC homopolymer allows the ion-rich domains of the P-1 polymers at Y ≥ 3 to remain glassy above room temperature and produces long-lived physical cross-links at room temperature. The peak in G″ at the end of rubbery plateau in the P-1 series indicates a transition in the chain dynamics that is related to an increase in the rate of ion rearrangement leading to terminal relaxation at higher temperatures. A number of factors could influence the dynamics of the ion-pair rearrangement including a glass transition of the ionic domains, an order disorder transition of the microphase separated structure, and a change in the dielectric constant of the ionic domains.52,55,56 However, an order−disorder transition is not consistent with the data in Figure 3b, where a hysteresis in the heating and cooling data would be expected through the ODT. To explore if the G″ maximum corresponds to a glass transition, the G″ peak maximum in Figure 4a for each polymer in the P-1 series was assumed to correspond the Tg of the ion-rich domains and plotted vs the weight fraction of IPC in the ion-rich domains, wionic, in Figure 6. For each point, wionic was estimated by the

interaction. However, as ion-pair concentration increases, the measured values diverge from the predicted values, especially for samples with higher ion-pair concentrations (Y ≥ 3). These large modulus values at high ion-pair concentration are similar to that observed in block copolymer thermoplastic elastomers and other ionomer systems consistent with microphase separation and vitrification of the ion-rich domains.51−53 To further investigate the P-1 and P-3 series and the differences between them, their thermal behavior and that of the corresponding homopolymers of PBA, poly(vinylbenzyltrioctylphosphine p-toluenesulfonate) (PVBTOP-TS) and poly(vinylbenzyl trioctylphosphine 4-styrenesulfonate) (PVBTOP-SS), were investigated by differential scanning calorimetry (DSC). The measured Tg values are listed in Table 2. Figure S11 shows DSC curves of the PBA, PVBTOPTable 2. Molecular and Thermal Characterization of Synthesized P-1 and P-3 Series and Corresponding Homopolymers sample P-1-200-0 P-1-200-1 P-1-200-3 P-1-200-5 P-1-200-10 PVBTOP-SS P-3-200-5 PVBTOPTS

ionic weight fraction

Tg 1a (°C)

0 0.045 0.132 0.208 0.346 1 0.205 1

−51.7 −46.4 −47.9 −47.7 −44.5 189.2 −47.7 65.1

Tg 2b (°C)

22.9 37.9 65.4 191.6

ΔH (W/g)

φPBA‑rich = ΔH/ ΔHPBA

φion‑rich

0.0313 0.0273 0.0220 0.0185 0.0106

1 0.871 0.701 0.590 0.338

0 0.129 0.299 0.410 0.662

65.5

a

The Tg1 values were measured from the second heating scans of DSC curves. bThe Tg2 values were determined from the maxima of G″ between the low temperature minimum and G′ ≈ G″ region.

SS, and PVBTOP-TS homopolymers. The glass transition temperature of the ionic homopolymers can also be determined mechanically from the isochronal temperature sweep of G′ and G″ at a constant frequency (Figure S12). The glass transition temperature was taken as the temperature at the maximum magnitude of G″ before the terminal relaxation. Good agreement was found between the Tgs measured for the PVBTOP-TS and PVBTOP-SS by DSC and DMA. The Tg of the IPC homopolymer, PVBTOP-SS, was found to be much higher than the Tg of PVBTOP-TS. For all of the copolymers, a subambient Tg was observed by DSC, and no evidence of a higher temperature glass transition was observed. The Tgs of the copolymers were all higher than an equivalent PBA homopolymer, and only a small increase in the Tg was observed (ca. 7 °C). This behavior is consistent with the microphase separation of the ion pairs. For example, in a previous investigation of PBA-based cationic and zwitterionic ionomers the zwitterionic ionomers exhibited a rubbery plateau, and the Tg increased slightly with ionic concentration (7 °C increase at 9% ionic content).54 In contrast, the cationic analogues exhibited no rubbery plateaus and had higher overall Tgs, which steadily increased with increasing charge content (e.g., 21 °C increase at 10% ionic content). This was attributed to the greater microphase separation of the zwitterionic ions forming more distinct PBA and zwitterionic domains with unique glass transitions, while the cationic analogues showed poor microphase separation leading to an averaging of the component Tgs.

Figure 6. Measured (black solid) Tg from rheology and predicted Tg as a function of varying ion-pair concentration in P-1 series based on the rule of mixtures (blue dashed) and Fox equation (red dashed).

ratio of the weight fraction of ionic groups in the copolymer to the volume fraction of the ion-rich domains (wionic= (ionic weight fraction)/(φion‑rich)). This estimate is based on the simplifying assumptions that, first, the densities of the ionic and BA segments are the same, such that the weight fraction of ionic groups and the volume fraction of the ionic domain can be directly compared, and, second, that the ion pairs are F

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Figure 7. AFM phase images of IPC cross-linked copolymers (P-1) and their cationic analogues (P-3) with different ion-pair concentration: (a) pure PBA homopolymer, (b) P-1-200-1, (c) P-1-200-3, (d) P-1-200-5, (e) P-3-200-5, and (f) P-3-200-10. All phase images have dimensions of 1.0 μm × 1.0 μm, and the light phases represent the higher modulus ion-rich domains.

completely segregated in the ion-rich domains. For comparison the Tg of the ion-rich domains vs wionic was calculated using the Tgs of the PBA and PVBTOP-SS homopolymers by a rule of mixtures, Tg = (1 − wionic)Tg,PBA + wionicTg,PVBTOP‑SS, and the Fox equation, 1/Tg = (1 − wionic)/Tg,PBA + wionic/Tg,PVBTOP‑SS, and plotted as dashed lines in Figure 6. The measured Tgs are in between the two predicted curves supporting the assumption that the drop in modulus at the end of the rubbery plateau is due to a glass transition. This does not rule out other factors contributing the variation in chain dynamics, such as a variation in the dielectric constant with temperature, but implies that the vitrification of the ion-rich domains is a dominant factor in this particular ionomer system. The rheological complexity at temperatures above the Tg is therefore due to the convolution of the temperature variation of the segmental chain dynamics with ion hopping and is qualitatively similar to the behavior observed in other ionomers.49,53,56,57 Atomic force microscopy (AFM) was performed to characterize the surface morphologies of thin films cast on silicon wafers, as shown in Figure 7. For the pure PBA and P-1200-1, the surfaces of the thin films appear very smooth, and almost no nanostructures were observed. At higher ion-pair concentrations (Y ≥ 3), nanostructured morphologies with phase contrast were observed, indicating nanoscale microphase separation. This trend from AFM results is consistent with the rheological behavior where both the low temperature storage modulus and the phase contrast increase with increasing ion concentration. The large area of the bright, high modulus domains is also consistent with the large volume fraction of ionrich domains estimated from the calorimetry data. In comparison, the surface of the cationic ionomer analogues shows weaker phase contrast. For P-3-200-10 the domain size appears larger than the P-1 series, which could be due to either

thermodynamic differences in the self-assembly or kinetic trapping of the morphology during vitrification. Small-angle X-ray scattering (SAXS) was used to characterize the bulk morphology of the copolymers. Azimuthally averaged SAXS intensity profiles of these copolymers are shown in Figure S14. No peaks were observed at domain spacings (ca. 20 nm) characteristic of the feature sizes from the AFM measurements in Figure 7. Previous investigation of poly(vinyl benzyltrioctylphosphonium chloride)-block-poly(n-butyl acrylate)-block-poly(vinylbenzyltrioctylphosphonium chloride) block copolymers showed poor scattering, which was attributed to low electron density contrast factor between the PBA and ionic blocks (Δρ = 2.61 × 1019 cm−4).58 In addition, SAXS patterns of poly(vinylbenzyltrioctylphosphonium chloride)random-poly(n-butyl acrylate) (PVBTOP-Cl-r-PBA) random copolymers showed no characteristic ionomer peak, while the rheology was characteristic of a microphase-separated polymer, similar to the results for the P-1 series. To calculate the scattering contrast of PVBTOP-SS-r-PBA (P-1), the mass density of PVBTOP-SS was measured as 1.0506 g/cm3 based on the weight of a compression-molded sample of PVBTOP-SS of known volume. The X-ray scattering contrast factor (Δρ2) between PVBTOP-SS and PBA was calculated as 0.5 × 1019 cm−4 using the online scattering contrast calculator from the National Institute of Standards and Technology.59 This an order of magnitude lower than Δρ2 for PVBTOP-Cl-r-PBA. In addition, the lower degree of long-range ordering and phase mixing in the IPC copolymers would further contribute to low scattering intensity from microphase-separated domains, limiting the utility of SAXS to characterize these polymer systems.

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CONCLUSIONS A new organic ion-pair comonomer (IPC), vinylbenzyltrioctylphosphonium 4-styrenesulfonate (VBTOP-SS), was syntheG

DOI: 10.1021/acs.macromol.7b01529 Macromolecules XXXX, XXX, XXX−XXX

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ACKNOWLEDGMENTS This material is based upon work supported by or in part by the U.S. Army Research Laboratory and the U.S. Army Research Office under Contract/Grant W911 NF-14-1-0239. The authors also acknowledge the financial support of the W.M. Keck Foundation. Acknowledgement is given to Longhe Zhang for providing the RAFT agent used in this work, Yanfeng Xia for assistance with AFM measurements, Zhiwei Lin for assistance with SAXS measurements, and Chongwen Huang and Chao Wang for helpful discussions about rheological measurements.

sized. This monomer was copolymerized with n-butyl acrylate (BA) via RAFT polymerization to prepare polyampholyte ionomers. The copolymers were physically cross-linked at room temperature but thermally processable by molding at elevated temperature. Comparison to analogous cationic and anionic ionomers with pendant counterions demonstrates the strong impact of direct ion-pair cross-linking on the material properties. The key effect of simply bonding both the anion and cation in the ion-pair to the polymer backbone was a substantial increase in the glass transition temperature of the ionic polymer. In the particular copolymers studied, shifting from pendant to bridged ion pairs substantially increased the glass transition of microphase-separated, ion-rich domains to allow them to act as long-lived physical cross-links similar to metal neutralized ionomers. Therefore, the connectivity of the ion pair (i.e., pendant vs bridged) provides a route to counterbalance the lower ionic interaction strength of organic ion pairs, which typically form transient networks in ionomer systems. This variation in ion-pair connectivity was directly achieved through the free radical polymerization of IPCs. Given the chemical flexibility to synthesize different IPCs, and directly copolymerize these with a range of neutral monomers to vary both the chain chemistry and associated physical properties (e.g., Tg, interaction parameter) and generate different chain architectures (e.g., random and block), IPCs should be a valuable addition to chemical toolbox for tuning the viscoelastic properties of polymers through ionic interactions. An unexpected result in these ionomers was the weak phase separation of the ionic and neutral groups compared to more traditional metal neutralized ionomers, which is directly related to the combination of physical cross-linking at room temperature and processability at elevated temperature. Therefore, future research should more deeply study the morphological behavior of these systems as a function of their viscoelastic and thermodynamic parameters to determine the thermodynamic and kinetic effects governing the structure−property relationships of these systems.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b01529. (1) Experimental procedures for all ionic monomers and copolymers; (2)1H NMR spectra of ionic monomers and copolymers; (3) thermal analysis of monomers or polymers measured by TGA or DSC; (4) rheological data (isochronal temperature sweep) of ionic homopolymers; (5) SAXS results of P-1 series (PDF)

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Article

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Tel 330-972-8368; Fax (315) 4439175 (K.A.C.). ORCID

Kevin A. Cavicchi: 0000-0002-6267-7899 Notes

The authors declare no competing financial interest. H

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Article

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