Perfluoropolyether (PFPE)-Based Vitrimers with Ionic Conductivity

Feb 25, 2019 - The viscous flow activation energy was found to be 161 ± 23 kJ mol–1, in good correlation with the activation energy calculated by D...
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Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

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Perfluoropolyether (PFPE)-Based Vitrimers with Ionic Conductivity Gérald Lopez,*,† Lérys Granado,† Gaël Coquil,† Andrés Lárez-Sosa,† Nicolas Louvain,†,‡ and Bruno Améduri*,† †

Institut Charles Gerhardt Montpellier, Université de Montpellier, CNRS, ENSCM, Montpellier, France Réseau sur le Stockage Electrochimique de l’Energie (RS2E), FR CNRS 3459, 33 Rue Saint Leu, 80039 Amiens, France

‡

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S Supporting Information *

ABSTRACT: Ion-conducting low-Tg perfluoropolyether (PFPE)-based vitrimers were obtained via thermally initiated polyaddition and in situ N-alkylation in the presence of a fluorinated cross-linker. Both reactions were quantified by differential scanning calorimetry (DSC) employing the Vyazovkin method (74 ± 1 kJ mol−1 for the polyaddition and 140 kJ mol−1 for the N-alkylation). The viscous flow activation energy was found to be 161 ± 23 kJ mol−1, in good correlation with the activation energy calculated by DSC for the N-alkylation. The creep behavior at elevated temperature is typical of a viscoelastic liquid and the relaxation times range from 2.5 h at 170 °C to 4 min at 210 °C. The topology freezing transition temperatures found via thermal creep experiment and relaxometry were in absolute agreement (ca. 110 °C). The network is stable under acidic and basic environments and can recover its mechanical properties after two recyclings. Three samples were prepared by varying the cross-linker loading, and the most stable displays a Td5% of 293 °C under nitrogen and a water contact angle of 136°. Ionic conductivities for these nondoped materials range from 0.5 × 10−6 to 1.1 × 10−6 S cm−1 at 27 °C.

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INTRODUCTION Thermosetting materials are polymers cross-linked through covalent irreversible bonds. The static covalent cross-links endow thermosets with superior mechanical properties and chemical resistance but impede reprocessing. In contrast, thermoplastics can be reshaped and recycled. Vitrimers represent a modern class of dynamic networks that can reshuffle their topology via triggered exchange reactions, endowing the materials with properties of repairing and reprocessing.1,2 They display a plastic flow predominantly driven by the kinetics of chemical exchange reactions and follow an Arrhenian dependence with temperature.3 This results in a strong-glass-former behavior in Angell’s classification.4 Their attractive features were first shown in epoxy− acid systems by Leibler and co-workers.5 The thermomechanical properties of vitrimers diverge from those of thermoplastics, which display a viscosity change near the glass transition. To date, many types of covalent exchange reactions have been reported: transesterification,6−12 transcarbamoylation,13,14 metatheses,15−20 transamination,21,22 boronic esters23 or imine exchange,24 and transcarbonation.25 Drockenmuller et al. developed ion-conducting networks that exhibit the properties of vitrimers.26 The one-step catalystand solvent-free synthesis of these networks involves two reactions: (i) the polyaddition of azide−alkyne monomers by Huisgen 1,3-dipolar cycloaddition and (ii) the in situ curing via N-alkylation of the resulting poly(1,2,3-triazole)s by a difunctional cross-linker. The vitrimer behavior originates © XXXX American Chemical Society

from N-alkylation exchange reactions of the 1,2,3-triazolium cross-links. In contrast to other vitrimers, a dissociative equilibrium between 1,2,3-triazoles and 1,2,3-triazoliums seems to drive the dynamic properties.27 Membranes with good ion-conducting properties and gas separation performances were prepared by A2 + B2 Huisgen thermal polyaddition of poly(trimethylene ether glycol)-based monomers and 1,10diiododecane as a cross-linker.28 Du Prez et al.29 explored an alternative chemistry platform for transalkylating vitrimer materials based on a classical SN2type reaction of sulfonium salts with thioether nucleophiles. Very recently, the same group also pioneered the introduction of reversible covalent bonds into a fluorinated polymer matrix via the vinylogous urethane chemistry.30 A dual viscosity profile was highlighted for the first time for that class of materials. The polymer matrix was based on perfluoropolyethers (PFPE)s, which are known to possess remarkable properties such as high thermal stability, chemical inertness, low surface energy, and antifouling characteristics, among others.31 Building on our work dealing with PFPE-based thermoplastics32 and elastomers,33 and inspired by the elegant synthetic approach reported by Drockenmuller et al.,26 we report herein ion-conducting PFPE-based vitrimers. Indeed, Received: November 21, 2018 Revised: January 15, 2019

A

DOI: 10.1021/acs.macromol.8b02493 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

software. A temperature ramp was performed from 30 to 150 °C at a rate of 3 °C min−1. Strain was monitored over time. Stress Relaxation. Stress relaxation experiments were performed with an Ares II rheometer (Rheometrics) using 25 mm plate−plate geometries on 1.5−2 mm thick samples. After waiting for sufficient mechanical equilibration time, a 1% strain step was applied, and the evolution of stress with time was monitored. A normal force ranging from 1 to 3 N (depending on the experiment time) was applied to maintain the sample within the parallel plates and avoid it sliding. The relaxation modulus G(t) was normalized by the initial relaxation modulus G0 and plotted versus time. The characteristic relaxation times (τ*) were obtained by fitting the curves with a Maxwell model for viscoelastic fluids without any empirical corrections (OriginLab Software). Thermal Curing. Differential scanning calorimetry (DSC) was used to study the thermal curing processes. The measurements were performed on 8−14 mg samples on a DSC-3 F200 Maia (Netzsch GmbH) instrument, under nitrogen at a flow rate of 40 mL/min using aluminum standard crucibles with aluminum piercing lids. The temperature sensor was freshly calibrated with adamantane, biphenyl, indium, bismuth, zinc, and cesium chloride standards, at 10 °C min−1. The kinetic study was performed at different heating rates (β = 5, 7.5, 10, 12.5, and 15 K/min). The total enthalpies were determined by numerical integration with a straight baseline correction. Thermokinetic Computations. The general equation of the kinetics of a thermally activated process can be written as follows:

PFPEs also display a low glass transition temperature (−60 to −100 °C) thanks to the oxygen bridges present along the backbone. Therefore, the segmental mobility imparted by flexible polyether backbones can help to enhance the ionic conductivity. Such networks could be employed as robust solid electrolytes with properties of reprocessing, thanks to the fluorinated matrix, the vitrimer behavior, and the ionconducting properties.

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EXPERIMENTAL SECTION

Materials. PFPE-diyne A2 (Mn ∼ 1800 g mol−1) and PFPE-diazide B2 (Mn ∼ 1800 g mol−1) were synthesized according to the literature. 33 1,12-Diiodo-3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10-hexadecafluorododecane C2 was supplied by DAIKIN. High-Resolution Magic Angle Spining (HR-MAS) Spectroscopy. HR-MAS NMR experiments were performed on a Varian VNMRS 600 MHz spectrometer equipped a wide bore magnet (B0 = 14.1 T), 2D lock, and Z gradients, together with a broadband 15 N−31P two-channel 4 mm Fast Nano probe optimized for 19F. The sample was then inserted into a 4 mm zirconia HR-MAS rotor along with few drops of acetone-d6. Experiments were performed at room temperature. The sample-spinning rate was 4 kHz. A single pulse sequence was used with pulse durations of 12 ms corresponding to a flip angle of 90°. A recycling delay of 1 s was used, and 646 transients were coadded. Fourier Transform Infrared Spectroscopy (FTIR). FTIR analyses were performed in attenuated total reflection mode (ATR) using a PerkinElmer Spectrum 1000, with an accuracy of ±2 cm−1. Thermogravimetric Analysis (TGA). TGA analyses were performed on 10−15 mg samples on a TGA Q50 apparatus from TA Instruments from 20 to 580 °C, in platinum pans, at a heating rate of 10 °C min−1 under nitrogen. Differential Scanning Calorimetry (DSC). DSC measurements were performed on 10−15 mg samples on a Netzsch DSC 200 F3 instrument. Two scans were recorded at a heating/cooling rate of 10 °C min−1 from −150 to 200 °C under an inert atmosphere (N2) using aluminum standard crucibles with aluminum piercing lids. The glass transition temperatures (Tg) were reported at the inflection point of the heat capacity drop during the second heating run. Water Contact Angle (WCA). Water contact angle measurements were performed at ambient temperature with Milli-Q water on a contact angle system OCA (Data Physics) using the water sessile drop method. Reported data are mean of five experiments. Swelling Ratio and Soluble Fraction. Insoluble fraction measurements were performed after immersing ca. 100 mg of each sample for 24 h at temperatures ranging from 25 to 110 °C. The swollen networks were weighed then dried overnight under vacuum at 50 °C. Soluble fraction and swelling ratio were calculated from eqs 1 and 2, respectively.

soluble fraction = (minitial − mdry )/minitial

(1)

swelling ratio = (mswollen − mdry )/mdry

(2)

dα = k(T )f (α) dt

(3)

where dα/dt is the reaction rate, k is the rate constant, and f is a sum function of the conversion α. In this study, the rate constant was assumed to follow the Arrhenius empirical equation: k(T ) = A e−E / RT

(4)

where A is the pre-exponential factor, E the activation energy, and R the gas constant. Isoconversional methods are powerful tools to determine the activation energy of a thermally activated reaction. These methods do not require to identify the reaction model f(α) and are therefore usually called model-free kinetic (MFK) methods. These methods rely on the following hypothesis: “the reaction rate at constant extent of conversion is only a function of temperature”.34 This hypothesis (H0) is fulfilled with our experimental data set (ΔHTOTAL = constant). Because f(α) is constant, α = constant and the isoconversional assumption leads to the following fundamental expression ÄÅ É ÅÅ ∂ ln(dα /dt ) ÑÑÑ ÅÅ ÑÑ = − Eα Ñ ÅÅÅÇ R ∂T −1 ÑÑÖα (5) where Eα is the apparent activation energy, which can vary throughout the monitored process. The Vyazovkin method is a numerical integral method (i.e., readily applicable on α versus T curves) and relies on the following reasoning: Small ranges of conversion (Δα = 0.01) are considered for the numerical integration, for a best accuracy. By introducing the g integral sum function

where minitial, mswollen, and mdry stand for initial, swollen, and dry masses, respectively. Dynamic Mechanical Analysis (DMA). DMA was performed on a Metravib DMA 25 with the Dynatest 6.8 software. A temperature ramp was performed from −130 to 200 °C at a rate of 3 °C min−1, with an oscillating strain of 0.1% and an angular frequency of 1 Hz. The glass transition temperature was calculated from the maximum value of the loss modulus E″. Creep Recovery. Creep experiments were performed on rectangular samples using a Metravib DMA 25 with the Dynatest 6.8 software. At isothermal temperatures, a constant stress of 50000 N/m2 was applied for 500 s and then released. The sample was allowed to recover for 500 s. Strain was monitored over time. Thermal Creep. Thermal creep experiments were performed on rectangular samples using a Metravib DMA 25 with the Dynatest 6.8

α

g (α) − g (α − Δα) =

∫α−Δα

dα = Aα f (α)

∫t

tα

e−Eα / RTα(t ) dt

α −Δα

(6) Considering the principle of isoconversional analysis

g (α) − g (α − Δα) = Aα J1(Eα , Tα ,1) = ... = Aα Jn (Eα , Tα , n) (7) where Ji(Eα,Tα,i) is the time integral of the ith temperature program Ti(t): Ji (Eα , Tα , i) =

∫t

tα

α −Δα

B

e−Eα / RTα ,i(t ) dt

(8) DOI: 10.1021/acs.macromol.8b02493 Macromolecules XXXX, XXX, XXX−XXX

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Figure 1. Preparation of the network by thermally initiated polyaddition of PFPE-diyne A2 and PFPE-diazide B2 in combination with in situ Nalkylation in the presence of 1,12-diiodo-3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10-hexadecafluorododecane C2.

Table 1. Curing, Rheological, and Thermomechanical Properties of the Network Made from A2 + B2 + C2 (1:1:1, mol:mol:mol) Formulation thermal curing properties

rheological and thermomechanical properties

ΔHtotal (J g−1)

EA2+B2 (kJ mol−1)

Ecross‑linking (kJ mol−1)

Eη (kJ mol−1)

Tν relaxo (°C)

Tν T creep (°C)

Tg#1 DSC [Tα#1 DMA] (°C)

Tg#2 DSC [Tα#2 DMA] (°C)

Td5% (N2) (°C)

E′r DMA (MPa)

ρx (mol cm−3)

68 ± 4

74 ± 1

140

161 ± 23

110

108

−104 [−97]

−78 [−66]

275

0.36

53

σdc = L /R1S

To remove the pre-exponential factor, the terms in eq 5 are normalized between each other, leading to a double-sum function minimized using the Vyazovkin method: n

Φ(Eα ) =

∑∑ i=1 j≠i

Ji (Eα , Tα , i) Jj (Eα , Tα , j)

(10)

where L is the thickness, S is the area, R1 is the bulk resistance, and σdc is the ionic conductivity.

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= n2 − n

RESULTS AND DISCUSSION Network Synthesis. The network formation is based on the thermally initiated polyaddition of PFPE-diyne A2 and PFPE-diazide B2 by Huisgen 1,3-dipolar cycloaddition in combination with in situ N-alkylation in the presence of 1,12diiodo-3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10-hexadecafluorododecane C2 (Figure 1). A2, B2, and C2 (1:1:1, mol:mol:mol) were prepared as previously described,33 mixed under heating, and then put into a silicone mold. A freestanding rubbery sample was recovered after 24 h at 130 °C (Figure 1 and Figure S1). Network Characterization. The disappearance of propargyl and azido bands at 3300 and 2100 cm−1, respectively, and the appearance of triazole bands35 at 1600 cm−1 was confirmed by infrared spectroscopy (Figure S2). In acetone, the cured sample is insoluble while the formulation components are totally dissolved (Figure S3). Thanks to a fair swelling in acetone at 25 °C (i.e., 25%, vide inf ra), the network composition (i.e., the ratio between 1,2,3-triazoles and 1,2,3triazoliums) was determined via high-resolution magic angle spinning (HR-MAS). The signals corresponding to 1,2,3triazoles were found between 7.5 and 8.3 ppm while those belonging to 1,2,3-triazoliums appeared downfield-shifted

(9)

With an iterative process, Eα values are the solution of the minimum of Φ, which can deviate from the theoretical values due to experimental data deviation. The Vyazovkin method code was implemented using the Python software and the modules Scipy and Numpy. Integral functions were calculated using the scipy.integral.quad built-in function. This program was tested with simulated data and compared to the Friedman method, giving consistent results. Ionic Conductivities. The ionic conductivities (σdc, in S cm−1) were determined using a VSP (BioLogic) instrument. The networks of known thickness (100−200 μm) were sandwiched between two stainless steel plates in a Swagelok-type cell, and the spectra were registered from 1 MHz to 10 mHz with an ac amplitude of 10 mV at temperatures ranging from 27 to 80 °C. The bulk resistance (R1) was determined from the impedance spectrum by fitting the profile with an equivalent electrical circuit, as follows:

The ionic conductivities were calculated using eq 10: C

DOI: 10.1021/acs.macromol.8b02493 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules (Figure S4).26 The network contains 34% of 1,2,3-triazoliums including dangling chains and bridging segments, thus implying a relative low cross-link density ρx. The low rubbery storage modulus E′r = 0.36 MPa (Table 1 and Figure S5), similar to that reported for vitrimeric silicone elastomers,36 confirmed this observation and led to a calculated cross-link density of 53 mol cm−3 (Table 1). For the purpose of comparison, highly cross-linked polyether-based 1,2,3-triazolium iodide networks display a much higher ρx value (i.e., 418 mol cm−3).28 Thermal Properties. Thermal properties were investigated via thermogravimetric analysis (TGA, Figure S6), differential scanning calorimetry (DSC, Figure S7), and dynamic mechanical analysis (DMA, Figure S8). With a heating rate of 10 °C min−1, the network displays an onset of degradation at which a weight loss of 5% is achieved (Td5%) of 275 °C under N2 (Table 1). Thanks to the presence of robust carbon− fluorine bonds, this value is greater than that reported for nonfluorinated polyether-based 1,2,3-triazolium iodide networks (Td5% = 222 °C under N2 at a heating rate of 10 °C min−1)28 and in a similar range than those found for PFPEbased vitrimers (Td5% = 280−290 °C under N2 at a heating rate of 10 °C min−1).30 As expected for these materials prone to phase separation, the DSC and DMA thermograms display two glass transition temperatures (Table 1). The Tg at ca. −100 °C (DSC: −104 °C; DMA: −97 °C) is typical for soft PFPE segments in PFPE-based networks,30,33,37 while that at ca. −70 °C (DSC: −78 °C; DMA: −66 °C) arises from triazoles hydrogenated segments33 and appeared lower than that reported for PFPE-methacrylate thermosets (DMA: −55 °C)37 and PFPE-based vitrimers (DMA: −40 °C).30 Kinetic Study of the Thermal Curing. The thermal curing was studied via DSC. The DSC thermogram for the mixture of A2 and B2 (Huisgen thermal polyaddition in the absence of C2) shows one single broad peak (Figure 2). The system A2 + B2 + C2 also exhibits a similar exothermic signal in the same temperature range and an additional peak at a higher temperature that can be ascribed to N-alkylations. The threecomponent system was found to be rather resolved in contrast to previous studies.28 The reaction kinetics was investigated at different heating rates, and the conversion degrees (α) were

calculated by numerical integration and then plotted as a function of the temperature for both systems (Figure S9). The typical sigmoidal-shaped curves are shifted toward the higher temperatures when increasing the heating rate. Break-in-slopes are observed for α = 0.9 due to the two-step mechanism in the A2 + B2 + C2 system. The activation energies (Ea) were calculated by isoconversional analysis using the Vyazovkin method (Figure 3).38 The activation energy is considered as

Figure 3. Activation energies (Ea) for A2 + B2 (1:1, mol:mol, blue) and A2 + B2 + C2 (1:1:1, mol:mol:mol, green) formulations vs conversion degree as calculated by isoconversional analysis.

variable throughout the curing process as previously described.39 For 0.2 ≤ α ≤ 0.4, the value of 74 ± 1 kJ mol−1 (Table 1) is consistent with those of the literature for Huisgen 1,3-dipolar cycloaddition polyadditions.40 For α ≥ 0.86, the activation energy associated with the cross-linking reactions by N-alkylation was found to be Ea = 140 kJ mol−1 (Table 1). Network Dynamic. The dynamic of the network was investigated through elevated temperature stress relaxation analysis (a set strain is applied to a sample, and the resultant stress dissipates over time). Figure 4 displays the normalized shear modulus as a function of time at several isothermal

Figure 4. Rheological study of the stress relaxation of ion-conducting networks obtained from A2 + B2 + C2 (1:1:1, mol:mol:mol): data (solid lines) and fit (dotted lines).

Figure 2. DSC thermograms for A2 + B2 (1:1, mol:mol) (blue) and A2 + B2 + C2 (1:1:1, mol:mol:mol) (green) formulations. D

DOI: 10.1021/acs.macromol.8b02493 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules temperatures. Three relaxation experiments were performed on the same sample, and nearly identical relaxation rates were repeatedly observed (Figure S10). The raw data were fitted with a Maxwell model for viscoelastic fluids without any empirical corrections. Exponential decays and near-total recovery are observed (e.g., G/G0 ∼ 0.05 at 190 °C for ca. 3 × 103 min). The curve at 210 °C suggests a plateau occurring prior to complete relaxation. This observation is likely due to a long-time exposure at high temperature that leads to some degradation. Conventionally, the relaxation time (τ*) is defined as the time required for the material to relax 63% of the initial stress. τ* range from 2.5 h at 170 °C to 4 min at 210 °C. τ* for the PFPE-based network appear to be relatively high, even though τ* for poly(1,2,3-triazolium ionic liquid)s (PTIL) networks are known to be longer for iodide- than bromide-containing networks.26 The relationship between ln(τ*) and 1/T was then successfully fitted with an Arrhenius model to assess the viscous flow activation energy Eη (Figure S11). Similarly to bromide-containing PTIL networks (Eη = 140 kJ mol−1),26 the PFPE-based counterpart displays a high viscous flow activation energy of 161 ± 23 kJ mol−1 (Table 1). Interestingly, Eη is in good agreement with the activation energy previously ascribed to the cross-linking reactions by N-alkylation from the DSC kinetics study. Du Prez et al. already reported an excellent agreement between the activation energies calculated from kinetic studies and relaxometry.21 Nevertheless, the kinetic study was performed on a low molar mass model while the stress relaxation experiments were conducted on a vitrimer network. Herein, a direct correlation is highlighted for a sole dynamic network. This finding supports that the network dynamics is driven by the reaction rate of the exchangeable bonds. The topology freezing transition temperature (Tν) was determined from relaxometry experiments by plotting viscosities (η) against reciprocal temperatures (Figure S12). Viscosities were calculated using the Maxwell equation (η = 1 /3τ*E′r).3 Tν is conventionally chosen as the temperature where the viscosity reaches 1012 Pa·s,4 which corresponds to the transition from the solid to the liquid state.5 In close agreement with that reported for dynamic poly(1,2,3triazolium) networks (Tν = 98 °C),26 Tν was found to be 110 °C (Table 1). This value was confirmed by thermal creep experiments (Figure 5). The strain curve presents a noticeable increase when it reaches 108 °C due to the thermally activated exchange reactions. The viscosity dependence with temperature was then plotted using the Angell fragility plot convention, where reciprocal temperatures were normalized by Tν (Figure 6).4 A gradual viscosity decrease similar to vitreous silica was observed for the PFPE-based PTIL network, therefore indicating the typical behavior of a vitrimer. The creep-recovery behavior of the cross-linked network was then examined (Figure 7). When a constant stress was applied for 500 s at 30 °C, no noticeable creep was observed, and the sample recovered its original dimension with no residual strain after releasing the applied stress, similarly to a covalently crosslinked elastomer.41 The creep-recovery behavior at 180 °C is quite different under the same conditions and appears proportional to time, indicating the typical behavior of a viscoelastic liquid.42 When the stress is released, the material recovers its initial elastic response only and a permanent deformation remains. At high temperature, the rearrangement of the network topology can adapt to the external forces.

Figure 5. Thermal creep experiment for the network obtained from A2 + B2 + C2 (1:1:1, mol:mol:mol).

Figure 6. Angell fragility plot showing viscosity as a function of reciprocal temperature normalized to 1 at the topology freezing transition temperature for ion-conducting network obtained from A2 + B2 + C2 (1:1:1, mol:mol:mol) and silica (dotted line).

Processing and Recyclability. Processing at elevated temperatures must be performed within seconds to prevent issues of degradation. Thus, the value of τ* must be low, which is not the case for the dynamic network described herein (vide supra). For instance, τ* = 29 min at 180 °C. Nevertheless, the network shows a low weight loss of ca. 2% at such elevated temperature in 30 min (Figure S13). In consequence, network degradation is expected to be insignificant. For the purpose of comparison, PFPE-based vitrimers obtained via catalyst-free vinylogous urethane chemistry display much lower τ* ranging from 57 to 390 s at 150 °C.30 The thermal dynamic nature of N-alkylations can facilitate the reprocessing of the cross-linked network. The sample was cut into slight pieces (