Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
Near-Model Amphiphilic Polymer Conetworks Based on Four-Arm Stars of Poly(vinylidene fluoride) and Poly(ethylene glycol): Synthesis and Characterization Demetris E. Apostolides,† Costas S. Patrickios,*,† Takamasa Sakai,‡ Marc Guerre,§ Gérald Lopez,§ Bruno Améduri,§ Vincent Ladmiral,§ Miriam Simon,∥ Michael Gradzielski,∥ Daniel Clemens,⊥ Christian Krumm,# Joerg C. Tiller,# Bruno Ernould,¶ and Jean-François Gohy¶ †
Department of Chemistry, University of Cyprus, 1 University Avenue, 2109 Aglanjia, Cyprus Department of Bioengineering, Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan § Institut Charles Gerhardt, Ingénierie et Architectures Macromoléculaires, UMR 5253 CNRS, UM, ENSCM, Place Eugène Bataillon, UM, Cedex 5 34095 Montpellier, France ∥ Institut für Chemie, Stranski-Laboratorium für Physikalische und Theoretische Chemie, Technische Universität Berlin, Strasse des 17. Juni 124, Sekr. TC7, D-10623 Berlin, Germany ⊥ Institut für Weiche Materie und Funktionale Materialien (EM-ISFM), Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Hahn-Meitner-Platz 1, D-14109 Berlin, Germany # Department of Biochemical and Chemical Engineering, Technische Universität Dortmund, Emil-Figge-Strasse 66, D-44227 Dortmund, Germany ¶ Institute for Condensed Matter and Nanosciences (IMCN), Bio- and Soft Matter (BSMA), Université catholique de Louvain (UCL), Place Pasteur 1, 1348 Louvain-la-Neuve, Belgium ‡
S Supporting Information *
ABSTRACT: Amphiphilic polymer conetworks (APCN) were prepared in N,N-dimethylformamide (DMF) by the interconnection of four-arm star poly(vinylidene fluoride) (PVDF, Mn = 8800 Da) end-functionalized with benzaldehyde groups and four-arm star poly(ethylene glycol) (PEG, Mn = 10 kDa) end-functionalized with benzaacylhydrazide groups. The PVDF stars were prepared via the reversible addition−fragmentation chain transfer polymerization of vinylidene fluoride using a tetraxanthate chain transfer agent. Equilibrium swelling of the APCNs in various solvents was dependent on the compatibility of the APCN components with the solvent, with the degrees of swelling (DS) varying from 22 in DMF (a good solvent for both PEG and PVDF), down to 8 in water (a good and selective solvent for PEG), and even down to 3 in diethyl ether (a nonsolvent for both polymers). Characterization of the conetworks in D2O using small-angle neutron scattering (SANS) indicated phase separation at the nanoscale, as evidenced by a (broad) correlation peak, consistent with a 19 nm spacing between the formed PVDF-based hydrophobic clusters of ∼10 nm diameter and an aggregation number of ca. 50 (growing in size with PVDF content). This behavior was independent of temperature from 25 to 70 °C and slightly dependent on deviations (±ca. 50 mol %) from the PVDF: PEG stoichiometry. Conetwork characterization in the bulk using atomic force microscopy (AFM) revealed a domain spacing of 14 ± 6 nm, in good agreement with the spacing of 11 nm calculated from the SANS results above (19 nm) but also taking into account the DS in D2O (5.5). Annealing the conetworks at 200 °C, a temperature above the melting point of PVDF, did not improve the morphological order in the AFM images. Finally, APCNs prepared in the room temperature ionic liquid binary mixture lithium bis(trifluoromethanesulfonyl)imide:1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (1:9 molar ratio) exhibited an electrochemical stability up to 4.3 V and a good room temperature ion conductivity of 0.6 mS cm−1.
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their name “polymer conetworks”) rather than randomly distributed with the hydrophilic units (for which “copolymer networks”
INTRODUCTION Amphiphilic polymer conetworks (APCN) represent a relatively new macromolecular entity, compositionally more complex than simple hydrogels, comprising sequences of hydrophobic monomer repeating units in addition to hydrophilic ones.1−5 The arrangement of the hydrophobic units in segments (justifying © XXXX American Chemical Society
Received: November 22, 2017 Revised: February 15, 2018
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DOI: 10.1021/acs.macromol.7b02475 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules
Thus, the objectives of this study are the preparation of such novel heterostar-structured, well-defined APCNs via a novel, hydrazone-based,35−42 cross-linking strategy (without studying the dynamic properties of the cross-links) between four-arm PEG and four-arm PVDF stars, the structural characterization of the resulting APCNs in the bulk and in a selective solvent at swelling equilibrium, and their preliminary evaluation as matrices for fabrication of GPEs. This first report also opens the way for the evaluation of the presently developed system for other APCN applications, including drug delivery, tissue engineering, lowadsorption coatings, and phase transfer catalysis.
would be the appropriate name) creates a driving force for phase separation at the nanoscale when the material is in the bulk (dried state), but more so when placed in a solvent selective for the hydrophilic units.6−10 The morphologies obtained upon this nanophase separation become better ordered when the polymer chains constituting the APCNs are well-defined in terms of molecular weight and composition11 and the number of arms emanating from the cross-linking cores.12 The best-known application of APCNs is their use as the material for soft contact lenses,13−15 in which ill-defined bicontinuous morphologies are formed. Other applications include APCN utilization as matrices for drug delivery,16 tissue engineering,17 thermosensitive and chiral separation membranes,18,19 and phase transfer reactions.20 In 2008, one of us reported the development of tetraPEG gels which are hydrophilic model polymer networks prepared by combining two mutually reactive four-arm star poly(ethylene glycol)s (tetraPEG stars): one end-functionalized with primary amine groups and the other one with the chemically complementary succinimide groups.21 The combination of these fourarm star PEGs with well-defined hydrophobic building blocks was also investigated, leading to the formation of the first tetraPEG star-containing model APCNs.22−27 Because of their model structure, the resulting APCNs exhibited interesting properties. In particular, when the first component was a tetraPEG star and the second was the thermoresponsive, four-arm star poly(ethyl glycidyl ether) (tetraPEMGE star), the obtained APCNs displayed extremely fast shrinking and reswelling responses23,24,27 and a volume (discontinuous) phase transition temperature more akin to polyelectrolytes.22 On the other hand, when the first component was a tetraPEG star again, while the second component was the very hydrophobic linear poly(dimethylsiloxane),25 the APCNs nanophase separated to lamellae with long-range order.26 Gel polymer electrolytes (GPEs) constitute an important, safety-securing component in lithium ion batteries, eliminating the risks of corrosive products leakage and bursting, arising from release of flammable gases.28 An efficient design is based on the combination of PEG29 and poly(vinylidene fluoride) (PVDF),30 with PEG helping the solvation of lithium cations and PVDF providing high thermal and chemical resistance and inertness in several solvents and acids as well as mechanical strength arising from its crystallinity.31 The combination of these two types of polymers has been reported in blends32 and graft copolymers,33 while a rare example of block copolymers of the two has also been reported in the patent literature.34 From the above PEG−PVDF architectures, the block copolymer one, bringing the two components in interconnected segments, is the best, as it most effectively retains the individual character of both very different polymers. A PEG−PVDF architecture analogous to the block copolymer one could be a conetwork based on interconnected stars of PEG and PVDF. Within this heterostar conetwork architecture, the specific properties of each polymer would be preserved at least as efficiently as in the block copolymer architecture. Furthermore, the network structure could confer a solid-like texture and superior mechanical properties to the system without the use of filler, such as silica particles.31 Moreover, the amphiphilic character of these materials (hydrophilic four-arm PEG, hydrophobic fourarm PVDF) could lead to internal self-organization with polar PEG nanodomains, efficiently solvating the electrolyte, and the less polar crystalline PVDF nanodomains endowing the material with thermochemical resistance and mechanical stability.
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EXPERIMENTAL SECTION
Materials. N,N-Dimethylformamide (DMF) was purchased from Scharlau, Spain (99.8%), or Sigma-Aldrich, France (reagent grade). Methanol (99.8%), N,N′-dicyclohexylcarbodiimide (99%), 4-formylbenzoic acid (97%), glacial acetic acid, and 4-(dimethylamino)pyridine (≥99%) were purchased from Aldrich, Germany. Deuterated dimethyl sulfoxide (d6-DMSO, 99.8%) was purchased from Merck, Germany. 1,1-Difluoroethylene (vinylidene fluoride, VDF) was kindly supplied by Arkema (Pierre-Bénite, France). tert-Amyl peroxy-2-ethylhexanoate (95%, Trigonox 121) was purchased from AkzoNobel (Chalons-surMarne, France). The tetrafunctional (tetraxanthate) chain transfer agent (CTA) was synthesized according to a previously reported two-step procedure.43 ReagentPlus grade (>99%) dimethyl carbonate (DMC), 2-hydroxyethyl acrylate (HEA), triethylamine (TEA), dimethylphenylphosphine (DMPP), and laboratory reagent grade hexane were purchased from Sigma-Aldrich, France. The room temperature ionic liquid (RTIL) electrolyte mixture comprising lithium bis(trifluoromethanesulfonyl)imide and 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide in a 1:9 molar ratio (LiTFSI:EMIM-TFSI) was purchased from Solvionic (Toulouse, France). Preparation of Tetrabenzaldehyde-Terminated Four-Arm Star Poly(vinylidene fluoride) (TetraPVDF-Bz). Tetrabenzaldehyde-terminated four-arm star poly(vinylidene fluoride) (tetraPVDF-Bz) of Mn = 8800 g mol−1 (overall molecular weight; the molecular weight of each arm was approximately a quarter of the overall molecular weight) was synthesized in a three-step procedure. First, reversible addition− fragmentation chain-transfer (RAFT) polymerization of VDF was achieved using the above-mentioned tetraxanthate CTA to produce tetraPVDF-XA (where XA denotes the xanthate terminal groups). Second, end-group modification using a one-pot aminolysis/thia Michael addition described previously44,45 was performed to obtain tetraPVDF-OH. In the third step, esterification of the terminal hydroxyl end-groups using 4-formylbenzoic acid resulted in tetraPVDF-Bz. A typical RAFT polymerization of VDF, using a 100 mL Hastelloy Parr autoclave system (HC 276), equipped with a mechanical Hastelloy stirring system, a rupture disk (3000 psi), inlet and outlet valves, and a Parr electronic controller to regulate the stirring speed and the heating, is detailed in the following.45 Prior to the reaction, the autoclave was pressurized with 30 bar of nitrogen to check for leaks. The autoclave was then put under vacuum (20 × 10−3 mbar) for 30 min to remove any trace of oxygen. A solution of tert-amyl peroxy-2-ethylhexanoate (Trigonox 121, 80 mg, 1.7 × 10−4 mol) and 4-arm star O-ethyl xanthate CTA (CTA-4, 1.31 g, 1.56 × 10−3 mol) in dimethylcarbonate (DMC, 60 mL) was degassed by bubbling N2 for 30 min. This homogeneous solution was introduced into the autoclave using a funnel tightly connected to the autoclave. The reactor was then cooled down in liquid nitrogen to about −80 °C, VDF gas (20.0 g, 3.12 × 10−1 mol) was transferred into the autoclave at low temperature, and the reactor was gradually heated to 73 °C under mechanical stirring. The reaction was stopped after 20 h. During the reaction, the pressure increased to a maximum of 25 bar and then decreased to 10 bar after 20 h. The autoclave was cooled down to room temperature and then in an ice bath, purged from the unreacted VDF, and opened. Then, after removing DMC under vacuum, the crude product was dissolved in 30 mL of warm THF (ca. 40 °C) and left under vigorous stirring for 30 min. This polymer solution was then precipitated from 400 mL of chilled hexane. The precipitated polymer (white B
DOI: 10.1021/acs.macromol.7b02475 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules powder) was filtered, dried under vacuum (15 × 10−3 mbar) at 50 °C for 2 h, and weighed. Assuming quantitative polymer recovery by precipitation, the polymerization yield was 30%. The peak positions in the 1 H and 19F NMR spectra of the thus-obtained tetraPVDF-XA are provided in the Supporting Information. The average arm degree of polymerization (DParm = 34) and the total degree of polymerization were estimated using 1H NMR spectroscopy and the Agilent branching module with the intrinsic viscosity (IV) as the source data and a star-branching regular model with structure parameter = 0.5 and repeating units = 103 (see Appendix 1 in the Supporting Information). Attachment of Hydroxyl End-Groups to TetraPVDF-XA. The hydroxyl terminal groups were attached to the extremity of each arm of the fourarm star PVDF by aminolysis of the xanthate end-groups and thia Michael addition of the resulting thiol end-groups onto HEA, following the one-pot protocol described by Guerre et al.44,45 To this end, tetraPVDF-XA (3.50 g, 4.38 × 10−4 mol) and HEA (0.609 g, 5.25 × 10−3 mol) were dissolved in DMF (20 mL). The solution was degassed by nitrogen bubbling for 10 min, and a degassed mixture of n-hexylamine (0.354 g, 3.50 × 10−3 mol), triethylamine (0.265 g, 2.63 × 10−3 mol), and 0.1 mL of DMPP in 1 mL of DMF was injected into the reaction mixture. Nitrogen bubbling was continued for further 10 min, and the reaction solution was then stirred at 25 °C for 16 h. The dark solution was precipitated twice from chilled methanol, and the resulting solid was filtered through a filter funnel and dried under high vacuum at 70 °C until constant weight to remove any traces of DMF; a dark brown powder was obtained (yield = 85%). The peak positions in the 1H and 19 F NMR spectra of the resulting tetraPVDF-OH are provided in the Supporting Information. Attachment of Benzaldehyde End-Groups to TetraPVDF-OH. Benzaldehyde terminal groups were attached to tetraPVDF-OH by esterification of its hydroxyl end-groups using 4-formylbenzoic acid. To this end, 1.0 g (0.12 mmol) of tetraPVDF-OH, 0.29 g (1.93 mmol) of 4-formylbenzoic acid, and 0.025 g (0.19 mmol) of 4-(dimethylamino)pyridine were transferred into a round-bottomed flask and dissolved in 6 mL of freshly distilled DMF. Afterward, 0.44 g (2.12 mmol) of N,N′-dicyclohexylcarbodiimide was dissolved in 2 mL of freshly distilled DMF, and the resulting solution was then added dropwise into the polymer solution. After 48 h, the polymer solution was precipitated from cold methanol. Most of the produced tetraPVDF-Bz was obtained as a brown precipitate. Then, the supernatant methanol solution (suspension) was centrifuged to recover the remaining tetraPVDF-Bz. The two precipitates were pooled together and dried in a vacuum oven at 50 °C for 7 h to give a brown powder (0.73 g, 0.08 mmol, 70% yield). Preparation of the Tetrabenzaacylhydrazide-Terminated Four-Arm Star Poly(ethylene glycol) (TetraPEG-BAH). The tetrabenzaacylhydrazide-terminated four-arm star poly(ethylene glycol) (tetraPEG-BAH) of Mn = 10 500 g mol−1 (overall molecular weight; the molecular weight of each arm was about a quarter of the overall molecular weight) was prepared according to our previously published procedure,41 in three steps. Briefly, the hydroxyl terminal groups of tetraPEG-OH were first activated using excess mesyl chloride. Subsequently, the attached mesyl end-groups were displaced using a benzaldehyde-protected benzaacylhydrazide phenol, resulting in the attachment of a bis-aryl substituted hydrazone at the tips of the arms of the star polymer. Finally, the hydrazone terminal groups were hydrazinolyzed using excess hydrazine, yielding the desired benzaacylhydrazide terminal groups. Synthesis of the TetraPVDF-Bz/TetraPEG-BAH Amphiphilic Polymer Conetworks. The amphiphilic polymer conetworks (APCNs) were prepared by mixing the tetraPVDF-Bz and tetraPEGBAH four-arm star polymers in stoichiometric molar ratio (and also offstoichiometry in two cases) at different polymer concentrations and in the presence of different glacial acetic acid concentrations. A typical procedure for the preparation of a APCN at a gelator concentration of 10% w/v and in the presence of 5% v/v of glacial acetic acid is provided below. First, 0.020 g of the tetraPVDF-Bz and 0.024 g of the tetraPEGBAH four-arm star polymers were dissolved separately in 210 μL of DMF each. Then, 22 μL of glacial acetic acid was added to the tetraPVDF-Bz solution. Subsequently, the tetraPVDF-Bz (also containing acetic acid)
and tetraPEG-BAH DMF solutions were thoroughly mixed together at room temperature, and the gel formation time, taken as the time when the polymer solution mixture stopped flowing (“tube inversion” experiments), was noted.41 Rheology Experiments. The gel formation time for the APCNs was more accurately determined from the intersection of the storage and loss moduli using rheology experiments. To this end, a Discovery HR2 rheometer from Thermal Analysis Instruments (TA) was used. The rheometer was operated in oscillation time mode using parallel plate geometry, with 10% angular frequency and 1% strain. The top plate had a 40 mm diameter, while the bottom one was also a Peltier element, thermostatically controlling the system at 20 °C. A typical experimental procedure followed for the rheology experiments is provided below. 13.1 mg of the tetraPVDF-Bz (1.5 μmol) and 15.8 mg of the tetraPEGBAH (1.5 μmol) four-arm star polymers, in stoichiometric molar ratio, were dissolved separately in 138 μL of DMF. Then, 14.5 μL of acetic acid (0.25 mmol) was added into the tetraPVDF-Bz solution. After that, the two polymer solutions were thoroughly mixed, and the final mixture was quickly placed onto the bottom plate of the rheometer; the top plate was immediately lowered to the appropriate separation, and data collection was initiated. This procedure was also conducted at different polymer concentrations (always in stoichiometric ratio) and in the presence of different concentrations of glacial acetic acid.41 Degrees of Swelling. The equilibrium degrees of swelling of a APCN were measured in three different solvents: DMF, diethyl ether, and water. To this end, several samples of the APCN were prepared in DMF and transferred into the appropriate solvent. Sample preparation involved, first, the preparation of solutions of tetraPVDF-Bz (34 mg in 250 μL of DMF) and tetraPEG-BAH (41 mg in also 250 μL of DMF). Then, 25 μL of glacial acetic acid was added to the tetraPVDF-Bz solution in DMF. Subsequently, the tetraPVDF-Bz and tetraPEGBAH solutions were thoroughly mixed together at room temperature (ca. 20 °C), and left to react for 3 days so that the cross-linking reaction was completed. Afterward, all prepared network samples were left to equilibrate in DMF for 48 h. Some of the samples were also left to equilibrate for 5 days in water, diethyl ether, or DMF. After the completion of the equilibration period, the networks were weighed (swollen mass), dried in a vacuum oven at 50 °C, and weighed again (dry mass). The degrees of swelling were calculated by dividing the swollen mass by the dry network mass. Small-Angle Neutron Scattering (SANS). The structure of the tetraPVDF-Bz−tetraPEG-BAH APCNs in D2O was characterized using small-angle neutron scattering (SANS). Samples of the conetworks were first prepared in DMF and subsequently allowed to swell in 5 mL of DMF for 24 h. Afterward, these samples were transferred to 5 mL of fresh DMF where they were left to equilibrate for another 24 h. Next, the samples were transferred to 5 mL of D2O, where they were allowed to equilibrate for 24 h. This procedure was repeated twice to thoroughly exchange DMF for D2O. Then, each D2O-equilibrated sample was cut in small thin pieces that were transferred into a quartz cuvette finally filled with D2O (it might be noted that in this way the precise amount of polymeric material in the beam was not known). The SANS experiments were performed on the V16 instrument46 at the Helmholtz-Zentrum Berlin (HZB) neutron facility. The wavelengths (λ) of the neutron pulses were 0.20 nm < λ < 0.70 and 0.16 nm < λ < 0.92 nm, and two sample-to-detector distances, 6 and 11 m, were employed to cover a range of scattering wave vectors, q, from 0.035 to 1.29 nm−1. Absolute scaling was obtained by comparison to the scattering of a 1 mm H2O sample (which was also used to determine the detector efficiency) and taking into account the transmissions of sample and H2O. Atomic Force Microscopy (AFM). AFM images of the tetraPVDF− tetraPEG conetwork in the dried state were recorded at room temperature using a Veeco Dimension Icon scanning probe microscope (Veeco Instruments) equipped with a Nanoscope V controller and an AVH-1000 workstation. All measurements of the cross sections were performed in tapping mode using commercial tapping mode etched silicon probe (TESPA) cantilevers of various frequencies from 230 to 420 kHz. Phase images were recorded at 5% below the fundamental resonance frequency of the cantilever, with a typical scan speed of 1−1.3 Hz and a resolution of 512 samples per line for a 500 and 1000 nm scan size. C
DOI: 10.1021/acs.macromol.7b02475 Macromolecules XXXX, XXX, XXX−XXX
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Scheme 1. Chemical Reaction between 4-Formylbenzoic Acid and TetraPVDF-OH (Arm Degree of Polymerization ≈34), Leading to the Attachment of Benzaldehyde Groups at the Termini of the Four Arms of the Hydroxyl End-Functional PVDF Star Homopolymer
The samples were frozen in liquid nitrogen, broken, and imaged. Subsequently, other samples were also frozen in liquid nitrogen, broken, annealed at 200 °C for 12 h in a vacuum oven, and then imaged. The images were processed using the software NanoScope Analysis 1.5. Differential Scanning Calorimetry (DSC). DSC was performed on a Thermal Analysis Instruments (TA) calorimeter model TA2910, calibrated with indium, within the temperature range from −30 to 250 °C. Both heating and cooling ramps were performed at a rate of 10 K min−1. Gel Polymer Electrolyte Preparation. The procedure for the preparation of the ion gel based on tetraPEG-BAH (Mn = 10 500 g mol−1) and tetraPVDF-Bz (Mn = 8800 g mol−1) is described below. The tetraPEG-BAH−tetraPVDF-Bz gel electrolyte was prepared by dissolution of each polymer separately. TetraPVDF-Bz (50 mg, 5.7 μmol) was dissolved in a mixture of DMF (500 μL) and LiTFSI:EMIM-TFSI (80 μL). TetraPEG-BAH (60.2 mg, 5.7 μmol) was dissolved in a mixture of DMF (560 μL) and acetic acid (60 μL, 5% v:v of the total gelation medium). Afterward, the two solutions were mixed together vigorously using a vortex mixer. The resulting solution was poured into a crystallizing dish (4.2 cm diameter) and subsequently covered with a glass slide. A homogeneous liquid film of the solution forming the gel was obtained on the glass slide via quick inversion of the system (Figure 6). Gel formation took place within less than 30 min. However, the gel was left to settle for 24 h. Finally, DMF and acetic acid were slowly evaporated under reduced pressure (
