Stretchable, degradable and self-healing polymers through a combination of imine bonds and metal coordination
11377558 · 2022-07-05
Assignee
Inventors
Cpc classification
C08K3/11
CHEMISTRY; METALLURGY
C08G69/48
CHEMISTRY; METALLURGY
C08K3/11
CHEMISTRY; METALLURGY
C08L83/08
CHEMISTRY; METALLURGY
C08J3/24
CHEMISTRY; METALLURGY
C08F8/30
CHEMISTRY; METALLURGY
International classification
C08L83/08
CHEMISTRY; METALLURGY
C08K3/11
CHEMISTRY; METALLURGY
Abstract
In a preferred embodiment, there is provided a self-healing polymer composition preferably for use in manufacturing a dielectric layer of a capacitive pressure sensor, and which comprises a central atom and a polymer having a main chain and one or more terminal moieties coupled to the main chain, wherein said one or more terminal moieties are selected to operate as ligands to form a coordination complex with the central atom, and preferably, the ligands and the central atom are selected to form the coordination complex after being subject to an external force sufficient to remove coordinate covalent bonds therebetween.
Claims
1. A polymer composition comprising a central atom or ion and a polymer having a main chain and one or more terminal moieties coupled to the main chain, wherein said one or more terminal moieties are selected to operate as ligands to form a coordination complex with the central atom or ion, each said terminal moiety having structural formula (II): ##STR00011## wherein MC is the main chain, L is alkylene selected from the group consisting of methylene, ethylene, n-propylene, isopropylene and butylene, Z is a carbon atom or a nitrogen atom, X is 0 or 1, and R.sup.1 to R.sup.4 are independently of each other hydrogen, halo, hydroxyl, amino, alkoxy, alkyl, alkenyl, alkynyl, cycloalkyl, heterocycloalkyl, aryl, aralkyl, heteroaryl, heteroaralkyl, carbonyl, carboxyl or cyano, each of which is optionally substituted, wherein R.sup.1 and R.sup.2, together with the carbon atoms bonded thereto, R.sup.2 and R.sup.3, together with the carbon atom and the nitrogen atom bonded thereto, and/or R.sup.1 and R.sup.4, together with the carbon atom and the nitrogen atom bonded thereto, may form unsubstituted or substituted cycloalkyl, heterocycloalkyl, aryl or heteroaryl, and wherein if X is 0, then R.sup.4 is absent.
2. The polymer composition of claim 1, wherein the central atom or ion comprises a transition metal selected from the group consisting of Mn.sup.2+, Fe.sup.2+, Fe.sup.3+, Co.sup.2+, Co.sup.3+, Ni.sup.2+, Cu.sup.1+, Cu.sup.2+ and Zn.sup.2+, or the transition metal is Fe.sup.2+, Zn.sup.2+ or Co.sup.2+.
3. The polymer composition of claim 1, wherein the main chain comprises low-density polyethylene (LDPE), high-density polyethylene (HDPE), polypropylene (PP), polyvinyl chloride (PVC), polystyrene (PS), polytetrafluoroethylene (PTFE), polychlorotrifluoroethylene (PCTFE), polyurethane, polyethylene terephthalate, polyamide, polyacrylonitrile, polyimide, nylon, polyacrylamide, polyphenylene, polythiophene, polysaccharide, polyglycol, polysiloxane or a combination thereof.
4. The polymer composition of claim 1, wherein the main chain comprises polysiloxane of the chemical formula —O—[R.sub.2SiO].sub.n—, wherein n is between 3 and 10,000, inclusive, and R is alkyl, alkenyl, alkynyl, aryl, alkoxy, halo or heteroaryl, each of which is optionally substituted.
5. The polymer composition of claim 1, wherein the main chain comprises polydimethylsiloxane (PDMS) having a molecular weight between about 300 and about 3000 Da.
6. The polymer composition of claim 1, wherein the polymer comprises two said terminal moieties coupled to opposing ends of the main chain.
7. The polymer composition of claim 1, wherein Z is a carbon atom, X is 1, and R.sup.1 and R.sup.2, together with the carbon atoms bonded thereto, R.sup.2 and R.sup.3, together with the carbon atom and the nitrogen atom bonded thereto, and/or R.sup.1 and R.sup.4, together with the carbon atom and the nitrogen atom bonded thereto, form unsubstituted or substituted aryl or heteroaryl.
8. The polymer composition of claim 1, wherein X is 0, R.sup.1 is hydrogen, halo, hydroxyl, amino, alkoxy, alkyl, alkenyl, alkynyl, cycloalkyl, heterocycloalkyl, aryl, aralkyl, heteroaryl, heteroaralkyl, carbonyl, carboxyl or cyano, and R.sup.2 and R.sup.3, together with the carbon atom and the nitrogen atom bonded thereto, form unsubstituted or substituted heteroaryl.
9. The polymer composition of claim 1, wherein the terminal moiety comprises one of the following structural formulas: ##STR00012## wherein one or more of the carbon atoms and the nitrogen atoms in the structural formulas are optionally substituted.
10. The polymer composition of claim 1, wherein the coordination complex has a geometry selected from the group consisting of tetrahedral, square planar, trigonal bipyramidal and octahedral.
11. The polymer composition of claim 1, wherein the coordination complex is formed with the terminal moieties of two or three of said polymers, and wherein each said terminal moiety is a bidentate ligand.
12. The polymer composition of claim 1, wherein the polymer comprises structural formula (III): ##STR00013## wherein n is 20 to 30.
13. The polymer composition of claim 1, wherein the coordination complex comprises structural formula (IV): ##STR00014## wherein M(II) is the central atom or ion, the central atom or ion being Fe.sup.2+, Zn.sup.2+ or Co.sup.2+, and a wavy line represents MC, L and Z.
14. The polymer composition of claim 1, wherein the polymer composition is self-healing.
15. The polymer composition of claim 1, wherein the polymer composition is prepared by contacting the polymer and a compound comprising the central atom or ion and a counterion, wherein the central atom or ion is Fe.sup.2+, Zn.sup.2+ or Co.sup.2+, and the conunterion is tetrafluoroborate, perchlorate or trifluoromethanesulfonate.
16. A method of preparing a self-healing polymer composition, the method comprising contacting a polymer and a central atom or ion, the polymer having a main chain and a plurality of terminal moieties coupled to the main chain, and forming a coordination complex with the central atom or ion and said terminal moieties operating as ligands, wherein each said terminal moiety has one of the following structural formulas: ##STR00015## wherein MC is the main chain, L is a bond or a linker selected from the group consisting of methylene, ethylene, n-propylene, isopropylene and butylene, and one or more of the carbon atoms and the nitrogen atoms in the structural formulas are optionally substituted.
17. The method of claim 16, wherein said contacting comprises contacting the polymer with a compound having the central atom or ion and a counterion, wherein the central atom or ion comprises a transition metal selected from the group consisting of Fe.sup.2+, Zn.sup.2+ or Co.sup.2+.
18. The method of claim 17, wherein the counterion is tetrafluoroborate, perchlorate or trifluoromethanesulfonate.
19. The use method of claim 16, wherein the main chain comprises polysiloxane of the chemical formula —O—[R.sub.2SiO].sub.n—, wherein n is between 3 and 10,000, inclusive, and R is alkyl, alkenyl, alkynyl, aryl, alkoxy, halo or heteroaryl, each of which is optionally substituted.
20. The method of claim 16, wherein the coordination complex has a geometry selected from the group consisting of tetrahedral, square planar and octahedral, and the coordination complex is formed with the terminal moieties of two or three of said polymers, each said terminal moiety being a bidentate ligand.
21. The method of claim 16, wherein the polymer comprises structural formula (III): ##STR00016## wherein n is 20 to 30.
22. The method of claim 16, wherein the coordination complex comprises structural formula (IV): ##STR00017## wherein M(II) is the central atom or ion, the central atom or ion being Fe.sup.2+, Zn.sup.2+ or Co.sup.2+, and a wavy line represents MC, L and Z.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
(1) Reference may now be had to the following detailed description taken together with the accompanying drawings in which:
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DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
(49) Described herein is a preferred facile synthesis of novel silicone-based materials specifically designed to include a unique combination of dynamic imine and metal-coordination bonds. Through an easy condensation reaction, imine bonds are generated at the end-termini of a short siloxane chain. The resulting end-capped material was chemically crosslinked with different ratios of a Fe(II)-based salt to achieve a dynamically crosslinked network. By controlling the ratio of metal crosslinker utilized, the dynamic materials and new crosslinking system were shown to be highly tunable, leading to materials with ultra-high stretchability (800% strain elongation), autonomous self-healing (24 hours at room temperature), and degradability in mild acidic conditions. A preferred approach to forming a preferred self-healing polymer is shown in
(50) The new materials have been characterized chemically and mechanically before and after healing, and after degradation through a combination of techniques, including UV-Vis and infrared spectroscopy (FTIR), nuclear magnetic resonance (NMR), size-exclusion chromatography (SEC), and tensile-pull strain testing. Furthermore, the resulting self-healing polymer has been utilized to fabricate highly sensitive and self-healable capacitive pressure sensors, capable of regenerating electrical and mechanical properties after damage. The method and results presented in this work may permit new opportunities for the large-scale synthesis of autonomous self-healing materials in a cost-effective manner, without requiring expensive or external healing agents as well as without impacting the long-term environmental sustainability.
Experimental Section
(51) Materials: Commercial reactants were used without further purification unless stated otherwise. All the solvents used in these reactions were distilled prior to use. Aminopropyl-terminated polydimethylsiloxane with molecular weight of 1000 Da, and dispersity of 1.33 was purchased from Gelest (Pennsylvania, USA). Pyridine-2-carboxaldehyde and Iron(II) tetrafluoroborate hexahydrate were purchased from Sigma-Aldrich and used as received.
Measurements and Characterization: Nuclear magnetic resonance (NMR) spectra were recorded on a Bruker 300 MHz spectrometer. The spectra for all polymers were obtained in deuterated chloroform at room temperature. Chemical shifts are given in parts per million (ppm). Number average molecular weight (Mn), weight average molecular weight (M), and polydispersity index (PDI) were evaluated by high-temperature size-exclusion chromatography (SEC) using 1,2,4-trichlorobenzene at 180° C. and performed on an EcoSEC HLC-8321GPC/HT (Tosoh Bioscience) equipped with a single TSKgel GPC column (GMHHR-H; 300 mm×7.8 mm) calibrated with monodisperse polystyrene standards. UV-Visible spectroscopy was performed on a Varian UV/Visible Cary 50 spectrophotometer. The surface structure of polymer film was obtained using a Multimode atomic force microscope (AFM, Digital Instruments) operated in the tapping mode at room temperature. Images were collected using Nanoscope 6 software and processed using WSxM 5.0 Develop 8.0 software. FTIR spectroscopy was performed on a Bruker ALPHA FTIR Spectrometer using a Platinum ATR sampling module.
Sample Preparation: After crosslinking with Fe(II), the polymer was solubilized in dichloromethane, filtered on 0.45 μm filter and casted into 3-cavities PTFE mold with cavity dimensions of 37.6 mmL×13.8 mmW×3 mm D (Ted Pella). The solution was filtered on 0.45 μm filter mostly as a precaution to remove any solid clusters or impurities, and to ensure the formation of smooth films. Once a gel was formed, the samples were placed in a vacuum oven at 50° C. and left to dry for 48 hours. Structuring of the dielectrics was performed by using an inverse PDMS-based mold prepared from commercially-available safety tape ribbons following a previously reported procedure (see B. Grzybowski, D. Qin, R. Haag and G. M. Whitesides, Sensors Actuators B. Chem., 2000, 81-85 and J. Pignanelli, K. Schlingman, T. B. Carmichael, S. Rondeau-Gagne and M. J. Ahamed, Sensors Actuators A. Phys., 2019, 285, 427-436, the entire contents of which are incorporated herein by reference). The mold was cured with UV-ozone for 5 minutes and was used as a stamp to produce the nanostructure on the desired materials. The polymer was then left for 24 hours in vacuum at 50° C. to dry. Once dried, the sample was slowly peeled off and directly used for the fabrication of the devices.
Device Fabrication and Characterization: Devices were fabricated by lamination of the structured dielectrics (self-healing polymer) with copper tape. Note that an overhanging end was left to be connected and tested using Agilent E4980A 2 MHz Precision LCR meter for capacitance measurements. Sensitivity upon pressure was evaluated using a home-made force linear actuator and force sensor connected to the device. The range of pressure evaluated was from 0 to 32 kPa. The devices were tested at various pressures within this range in order to construct a sensitivity curve. Stretched samples were subjected to 30% strain for 100 cycles using a custom-built stretching station. Healed samples were cut in half with a blade and pressed back together for intrinsic self-healing before being used as a dielectric.
Evaluation of self-healing properties: Self-healing was evaluated by tensile strain analysis as well as by device signal sensitivity before and after healing. For the tensile strain analysis, a flat molded sample was simply cut in half with a blade, pressed back together and left to heal for 24 or 48 hours before being characterized. Tensile-strain analysis was performed on an Instron Tensile Strain instrument with a test rate of 100 mm/min. Self-healing ability of the pressure sensing devices was characterized by testing the device before and after a healing period of 48 hours. For evaluation of the self-healing properties by atomic force microscopy, a sample was frozen in liquid nitrogen and carefully cut with a scalpel. The films analyzed were spin coated on a cleaned glass substrate, at 2000 rpm for 1 min. Several attempts were required to make a cut small enough to properly self-heal, but large enough that the full capability of the material is demonstrated. The cut sample was then left on the AFM stage on day 0 and measured after 24, 48 and 72 hours.
Evaluation of degradability: A small piece of the self-healing material was characterized in terms of degradability by placing a solid piece of the polymer in mild acid solutions (hydrochloric acid or acetic acid) and left to stir until fully dissolved, or in solution with about 1 M acetic acid and left to stir until fully dissolved (2 days). The resulting solution was then directly characterized without further treatments.
Evans method for determining effective magnetic moment: A previously reported procedure was followed (see D. F. Evans, J. Chem. Soc., 1959, 2003-2005, the entire contents of which are incorporated herein by reference). Briefly, samples of known concentration (0.9 mM) were prepared. An NMR tube made of concentric tubes was filled with pure solvent (CDCl.sub.3, inner tube) and sample (outer tube). The magnetic susceptibility (χmass) was determined by using the formula χmass 3Δf/4πfm+χ0+χ0(d0−ds)/m, where χmass is mass susceptibility (cm.sup.3g.sup.−1), Δf is the observed frequency difference (Hz), f is spectrometer frequency (Hz), m is the mass of paramagnetic substance (g.Math.cm.sup.−3), χ.sub.0 is the mass susceptibility of solvent (cm.sup.3g.sup.−1), do is the density of solvent (g.Math.cm.sup.−3), and ds is the density of solution (g.Math.cm.sup.−3). The magnetic susceptibility was further converted to the effective magnetic moment (μ.sub.eff) following a previously reported procedure (see B. Weber and F. A. Walker, Inorg. Chem., 2007, 46, 6794-6803 and L. Zheng, X. Fang, K. Lii, H. Song, X. Xin, H.-K. Fun, K. Chinnakali and I. Abdul Razak, J. Chem. Soc., Dalt. Trans., 1999, 3, 2311-2316, the entire contents of which are incorporated herein by reference).
Preparation of model imine compounds: The synthesis of the (E)-N-butyl-1-(pyridin-2-yl)methanimine has been performed according to a previously reported procedure (see M. Zeng, L. Li and S. B. Herzon, J. Am. Chem. Soc., 2014, 136, 7058-7067, the entire contents of which are incorporated herein by reference). Following its preparation, the compound (1 eq.) was reacted with 6-aminohexan-1-ol (1.2 eq.) in tetrahydrofuran (2 M). The reaction mixture was stirred for 72 h at 30° C. The product was used without any further purification for LC-MS analysis.
Synthesis of Pre-Polymer 1 Followed by Chemical Crosslinking with Fe(II)-Based Salts:
(52) ##STR00008##
Pre-polymer 1. A round bottom flask equipped with a magnetic stir bar was charged with aminopropyl-terminated PDMS (20 g, 0.020 mol), pyrimidine-2-carboxaldehyde (4.28 g, 0.042 mol) and CHCl.sub.3 (20 mL). The reaction was left stirring for 48 hours at room temperature and CH.sub.3C1 was removed under reduced pressure. The resulting materials was diluted in hexanes and was extracted with MeCN to remove unreacted monomer and dried under vacuum to afford pre-polymer 1 as a viscous thick oil. .sup.1H NMR (300 MHz, CDCl.sub.3, 298 K): 8.62 (d, J=4.2 Hz, 2H), 8.35 (s, 2H), 7.98 (dd, 7.8 Hz, 2H), 7.72 (t, 2H), 7.29 (t, J=15.3, 2H), 3.67 (t, J=14.1, 4H), 2.29 (m, 12H), 1.79 (m, 4H), 0.584 (t, 7.5 Hz, 4H). Molecular weight estimated from high temperature GPC (1,2,4-trichlorobenzene, 200° C.): M.sub.n=839 Da, M.sub.w=2286 Da, PDI=2.737
General procedure for the preparation of Fe-coordinated polymer. The crosslinking of pre-polymer 1 was performed by dissolving a selected amount of materials in dichloromethane and stirring for 5 minutes. A selected amount of iron(II) tetrafluoroborate hexahydrate dissolved in a minimal amount of methanol was then added to the mixture. The reaction was left to stir at room temperature for 30 minutes and the resulting materials was directly molded for further analysis. Molecular weight for a 1:3 pre-polymer/Fe(II) ratio sample was estimated from high temperature GPC (1,2,4-trichlorobenzene, 200° C.): M.sub.n=1140 Da, M.sub.w=3407 Da, PDI=2.989
Results and Discussion
(53) A preferred synthesis of a preferred self-healing polymer is depicted in Scheme 1 as shown below:
(54) ##STR00009##
Starting from commercially-available aminopropyl-terminated polydimethylsiloxane, a condensation reaction is performed with 2-pyridinecarboxaldehyde to generate pre-polymer 1. Pre-polymer 1 (1000 Da) was directly used without further purification in a crosslinking reaction with a Fe(II)-based salt. In this study, Fe(II) tetrafluoroborate was used. Interestingly, as soon as the salt was added, the pre-polymer immediately gelified and turned purple, which is typical for the formation of a Fe(II) coordination complex. To confirm the presence of Fe(II) in the materials, the materials' effective magnetic moment (μ.sub.eff) was determined by nuclear magnetic resonance (NMR) in solution, using Evans method (see D. F. Evans, J. Chem. Soc., 1959, 2003-2005 and B. Weber and F. A. Walker, Inorg. Chem., 2007, 46, 6794-6803, the entire contents of which are incorporated herein by reference). The crosslinked materials was found to have a μ.sub.eff=3.08μ.sub.b, which is a value that can be assigned to a high-spin Fe(II) complex.
(55) From a design point of view, a system combining dynamic imine and metal-ligand bonds has been selected for multiple reasons. First, the generation of the pre-polymer does not require any heating or catalyst, and all the initial precursors are commercially available (cost below $0.5 per gram). Furthermore, both imine and metal-coordinating bonds have been shown to be highly dynamic, and typically lead to good self-healing properties in elastomeric materials. Therefore, the combination of both types of bonds may enable the formation of a very dynamic system, ideal for self-healing and stretchability. Finally, imine bonds may undergo reverse condensation in mild conditions, which is particularly interesting to trigger a controlled degradation of the materials.
(56) To optically probe for the formation of a metal-coordination complex between the N-ligand generated after condensation reaction and Fe(II), UV-vis spectroscopy was utilized and the results are summarized in
(57) Elastic modulus and tensile resistance are two significant parameters that can be used to assess the mechanical properties of soft materials. Therefore, tensile-pull testing was performed on free-standing films of crosslinked polymers before and after healing. Results are summarized in
(58) The crosslinked materials was then cut in half with a blade and self-healed (room temperature) for 24 and 48 hours. Interestingly, the resulting self-healed materials were shown to regain 48±3% and 88±7% of their maximum elongation before fracture, after respectively 24 and 48 hours (averaged on three samples). No external stimuli or trigger was used to enable that self-healing. Moreover, a similar stretchability and self-healing properties were observed for pre-polymer 1 crosslinked with 0.33 equivalents of Fe (see
(59) To demonstrate the versatility of the new dynamic crosslinking system, pre-polymer 1 was also crosslinked with 1 equivalent of Fe.sup.2+ to allow for a lower crosslinking density. As demonstrated in
(60) AFM was also used to investigate the self-healing behavior of the new crosslinked materials in thin films, and results are summarized on
(61) It has been appreciated that both imine and metal-ligand bonds are highly dynamic. In order to get insight into the contribution of both dynamic bonds for the self-healing of the current materials, further analysis on a model compound was performed. Specifically, model compound (E)-N-butyl-1-(pyridin-2-yl)methanimine) was synthesized using previously reported procedure (see M. Zeng, L. Li and S. B. Herzon, J. Am. Chem. Soc., 2014, 136, 7058-7067, the entire contents of which are incorporated herein by reference). The compound was then reacted with 6-aminohexan-1-ol in order to probe through mass spectrometry for the formation of an exchange product through transimination. As shown in
(62) With the ongoing environmental burden caused by the accumulation of non-degradable plastics and e-wastes, there is a growing interest in the design and synthesis of degradable functional materials to address the surge in demand for flexible electronics and sensors. It has been recognized that incorporation of dynamic bonds to chemically crosslink shorter units may be a promising strategy to control and trigger the degradation of materials into shorter sub-units, non-damageable for the environment. Since pre-polymer 1 was shown to efficiently crosslink upon adding metal salts and to generate a dynamic crosslinked network, the investigation of the degradability of the materials was performed in order to determine if the resulting materials could be broken down into smaller siloxane-based units through the hydrolysis of the imine dynamic bonds.
(63) Toward that objective, the crosslinked materials (0.33 equivalent of Fe(BF.sub.4).sub.2) were submerged into an acidic aqueous solution (1M acetic acid) and the degradation reaction was probed by UV-V is spectroscopy. Results are summarized in
(64) In order to probe for the degradability of the new materials under different conditions, a similar experiment was also performed in aqueous hydrochloric acid solutions with pH ranging from 1 to 6 (see
(65) In order to demonstrate the application of the new autonomous self-healing of the siloxane-based materials, flexible and self-healable pressure sensors were fabricated, and results are summarized in
(66) According to our results, the device maintained its mechanical and electrical properties to give approximately the same sensitivity (0.33 kPa.sup.−1) over four trials where the samples were either pristine, strained 30% for 100 cycles, self-healed and self-healed followed by 30% strain for 100 cycles. The fact that the capacitive devices were able to maintain a good sensitivity after healing and 30% strain for 100 cycles confirmed the strong potential of the new tunable system for the design of wearable electronics human skin is typically known to experience a 30% strain elongation. These devices may self-heal after simply pressing the two halves back together and let rest at room temperature without any other external factors. An average change in capacitance change versus average pressure applied curve was constructed in order to demonstrate the small standard deviation between the 4 devices with different test conditions (see
(67) Dynamic sensor data was also collected to show the response ability of the sensor before versus after healing of the dielectric layer (see
(68) TABLE-US-00001 Degradability/ Materials Sensitivity Healability Stretchability Ref. Fe-crosslinked 0.33 kPa.sup.−1 Autonomous IM AcOH, — siloxane oligomers ε = 800% (this work) Calcium carbonate, 0.17 kPa.sup.−1 Autonomous ε = 55% 64 polyacrylic acid and alginate PDMS 1.0 kPa.sup.−1 No — 65 PDMS 0.28 kPa.sup.−1 No — 66 Polyurethane 1.9 kPa.sup.−1 Non- — 67 autonomous
(69) However, the new system may permit tunability, thus leading to a wide variety of properties, including self-healing, degradability and ultra-high stretchability. Moreover, the complexity of the system may be reduced, as well as the cost of preparation, which may be an important advantage for large-scale applications.
(70) Conclusion
(71) A new approach towards autonomously self-healing was developed through rational design of siloxane-based oligomers with imine and metal-coordination moieties. This unique combination of dynamic bonds was achieved by preparing soft polymers by a facile condensation reaction between amino-terminated siloxanes and pyridine carboxaldehyde. The resulting materials were crosslinked with Fe(II) salts, crosslinking that can be easily tuned by controlling the amount of metal used. Interestingly, the new polymer was shown to be ultra-highly stretchable when a 1:1 Fe/polymer ratio was used, reaching a maximum tensile strain before fracture of 800%. The samples prepared with 0.33 equivalents of Fe.sup.2+ showed autonomous intrinsic self-healing and were able to regenerate 88% of their initial mechanical properties after 48 hours without aid from any external stimuli. The materials were also shown to be degraded into small oligomeric species in acidic conditions, thus opening the way for a controlled degradability.
(72) Since the new materials are particularly interesting as a component for self-healable electronics, the new polymer was used as a dielectric in self-healable capacitive-based pressure sensors. The dielectric was structured at the microscale to enhance device sensitivity in a pressure range of 0-32 kPa. The device was completely characterized before and after self-healing, and after multiple cycles of 30% strain conditions to investigate the performance of the self-healing dielectric. Our results demonstrate no significant deviation in terms of device sensitivity of the lower range of pressure applied (0-5 kPa), with a sensitivity of ˜0.33 kPa.sup.−1. Additionally, the dynamic testing of our sensor showed good response time and accuracy. Based on their low cost, high tunability and simple preparation, it is envisioned that this dynamic polymer system may provide new opportunities for the creation of next-generation stretchable electronics that require improved material robustness, degradability, conformability and durability to improve lifespan and performance of electronics.
(73) In separate studies, other self-healing materials were prepared, based on end-capped PDMS-based oligomers with N-ligands, and were tested for rational fine-tuning of their self-healing efficiency and thermomechanical properties through the variation of the nature of the metal-ligand interactions used (see
(74) ##STR00010##
(75) These were selected to probe for the effect of coordination geometry and bond strengths on the thermomechanical properties. Moreover, through this evaluation, effect of the counter-ion on the self healing properties was also assessed. The results obtained through this investigation revealed a correlation between coordination bond strength and geometry and self-healing properties of the materials. The fine-tuning of the materials' self-healing efficiency was shown, ranging from 60% to 80% after only 2 hours for cobalt and zinc-crosslinked materials, respectively. Furthermore, the Young's modulus was also fine-tuned, reaching a minimal value of 0.23 MPa, by varying the nature of the counter-ion used in complexation. This tunable approach to self-healing materials highlights the effect of dynamic metal-ligand crosslinking toward self-healing soft polymers. Furthermore, this system may provide for a simpler, lower-cost yet effective method for obtaining a wide range of self-healing materials that can be used for various applications.
Experimental Section
(76) Materials: Commercial reactants were used without further purification unless stated otherwise. All the solvents used in these reactions were distilled prior to use. Pre-polymer P1 was prepared from pyridine-2-carboxaldehyde and aminopropyl-terminated polydimethylsiloxane with a molecular weight of 1000 Da and a dispersity of 1.33 (purchased from Gelest, Pa., USA) as previously reported (J. Pignanelli, B. Billet, M. Straeten, M. Prado, K. Schlingman, M. J. Ahamed and S. Rondeau-Gagne, Imine and metal-ligand dynamic bonds in soft polymers for autonomous self-healing capacitive-based pressure sensors, Soft Matter, 2019, 15, 7654-7662, the entire contents of which are incorporated herein by reference). Iron(II) tetrafluoroborate hexahydrate, cobalt(II) tetrafluoroborate hexahydrate, zinc tetrafluoroborate hydrate, zinc trifluoromethanesulfonate and zinc perchlorate hexahydrate were purchased from Sigma-Aldrich and used as received.
Measurements and characterization: UV-Visible spectroscopy was performed on a Varian UV/Visible Cary 50 spectrophotometer. Calorimetric studies were conducted on a TA instruments DSC2500 and thermal gravimetric analysis was performed on a TA instruments TGA5500. Nitrogen (99.999%) was used to purge the systems at a flow rate of 60 mL min.sup.−1. All samples were placed and run in aluminum crucibles. TGA samples were held at 25° C. for 30 min before being heated to 500° C. at a rate of 10° C. min.sup.−1.
Sample preparation: Pre-polymer P1 was solubilized in dichloromethane, filtered on a 0.45 mm filter, and crosslinked with Fe(BF.sub.4).sub.2, Co(BF.sub.4).sub.2, Zn(BF.sub.4).sub.2, Zn(ClO.sub.4).sub.2 or Zn(OTf).sub.2. The resulting materials were cast into a custom-made dog-bone shaped PTFE mold with dimensions in accordance to the ASTM standard for thermoplastic elastomers (ASTM D412). Once a gel was formed, the samples were placed in a vacuum oven at 50° C. and left to dry for 48 hours. Once dried, the samples were slowly peeled off and used directly for further characterization.
Evaluation of self-healing properties: Self-healing was evaluated by using a flat molded sample which was cut in half with a blade, pressed back together and left to heal for 2 hours before being characterized. Tensile-strain analysis was performed on an Instron Tensile Strain instrument with a test rate of 5 mm min.sup.−1.
Results and Discussion
(77) The synthesis of pre-polymer P1, for use toward self-healing materials, is depicted in Scheme S1 noted above. The materials were prepared through simple condensation between aminopropyl terminated polydimethylsiloxane (1000 Da) with pyridine-2-carboxaldehyde to generate N-ligands from pyridine and imine functional groups as end groups. Materials were used without further purification, and were characterized by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) (see
(78) To compare the metal-ligand coordination effect on the thermomechanical properties of the system, Co(BF.sub.4).sub.2, Fe(BF.sub.4).sub.2 and Zn(BF.sub.4).sub.2 metal salts were used. The effect of the counter-ion was probed by utilizing Zn(ClO.sub.4).sub.2, Zn(OTf).sub.2 and Zn(BF.sub.4).sub.2 salts. For all samples, 0.33 equivalents of the metal salt were used in solution to generate the metal-coordinated polymer. Metal-ligand interactions were selected as a dynamic crosslinking strategy for their possible reversibility. Upon stress, the metal-ligand interactions may dissipate energy through rupture of the coordinate bond. However, due to the kinetic lability of the bonds, the supramolecular interaction may reform upon release from strain. In general, the weaker the metal-ligand interaction (more kinetically labile chain and better ability to dissipate energy), the faster the bonds are able to reform. It has also been appreciated that imine bonds may be dynamically reversible, also playing a role in the self-healing ability of this system.
(79) UV-Visible spectroscopy was first used to confirm metal coordination and to determine the coordination stoichiometry of pre-polymer P1 and each metal center. Specifically, UV-Visible absorption spectroscopy gives access to characteristic charge-transfer bands, which can be directly correlated with the coordination. This technique was also used to gain insight into the crosslinking density, as the dynamic nature of the crosslinks (potential exchange between solvent molecules and ligands) significantly complicates the utilization of other techniques such as polymer swelling or nuclear magnetic resonance (NMR). Progressive titration experiments can then be used to determine the coordination stoichiometries. UV-Visible spectroscopy was performed for all metal sources and the results are summarized in
(80) After confirming the coordination of metals and the geometry of the resulting complexes, a detailed investigation of the mechanical properties was performed in order to unveil the effect of the coordination motif on self-healing. To perform this investigation, samples of P1 crosslinked with Fe(BF.sub.4).sub.2, Co(BF.sub.4).sub.2 or Zn(BF.sub.4).sub.2 were prepared in solution and cast into a PTFE mold. The resulting materials were dried at room temperature for 24 hours to remove any solvent residue. First, tensile-strain pull tests were performed on the soft crosslinked materials with different metals (Fe, Co and Zn) to gain insight into the maximal fracture strain, and the results are shown in
(81) A similar investigation was also performed by varying the counter-ion using Zn(BF.sub.4).sub.2, Zn(ClO.sub.4) and Zn(OTf).sub.2 for the preparation of materials. These specific counter-ions were selected for various reasons. First, tetrafluoroborate, perchlorate and triflate are non-coordinating counter-ions which promote the octahedral coordination geometry of the polymer ligands with the metal center, as confirmed by UV-Vis spectroscopy. Moreover, their size and stability differ, which may have an influence on the resulting supramolecular network and thermomechanical properties. As shown in
(82) To evaluate the self-healing efficiency of the materials, freestanding films of the different crosslinked materials were cut into two pieces and were slowly brought into physical contact. The resulting materials were left on the benchtop at room temperature for 2 hours and the healed films were evaluated by tensile-strain pull testing. No additional trigger (thermal annealing, light exposure, etc.) was used in order to ensure an autonomous self-healing mechanism. The resulting self-healed materials were then analyzed by tensile-strain pull testing and the results are summarized in
(83) TABLE-US-00002 TABLE 1 Elastic moduli and self-healing efficiences of pre-polymer P1 crosslinked with various metal(.sub.II) sources Elastic modulus Self-healing Metal salt (MPa) efficiency.sup.a (%) Co(BF.sub.4).sub.2 8.10 ± 2.80 61 Fe(BF.sub.4).sub.2 1.80 ± 0.15 71 Zn(BF.sub.4).sub.2 0.26 ± 0.09 88 Zn(ClO.sub.4).sub.2 10.2 ± 1.40 75 Zn(CF.sub.3SO.sub.3).sub.2 0.23 ± 0.04 73 .sup.aDefined as the difference between the maximum elongation before damage and after a 2-hour healing period.
(84) The metal center and type of counter-ion used in this system not only have an effect on the maximal strain before fracture and self-healing efficiency, but also influence the thermomechanical properties of soft materials. Therefore, to unveil the impact of the crosslinker used to prepare the materials on the soft materials, a complete evaluation of the thermomechanical properties, including Young's moduli and maximum strain at fracture, was performed using tensile-strain pull testing. First, the Young's modulus for all systems was evaluated by tensile-strain pull testing, and the results are summarized in Table 1. Similar to the trend observed for the maximum strain at fracture, the selection of the metal used for coordination has an effect on the elastic modulus of the resulting crosslinked materials. Zn(.sub.II), with the weaker interaction, led to a polymer with a modulus of 0.26 MPa, which is one order of magnitude lower than commonly used PDMS. When Fe(.sub.II) was used, this modulus increased to 1.8 MPa. Finally, when Co(.sub.II) was used, the resulting material was more brittle, and its Young's modulus increased to 8.1 MPa. This result may be correlated with the coordination bond strength. The elastic moduli of Zn(.sub.II)-based materials with different counter-ions were then evaluated. Interestingly, when the counter-ion used is large and non-coordinating (tetrafluoroborate and triflate), the moduli remained fairly low, with values of 0.26 and 0.23 MPa, respectively. However, when perchlorate was used, the resulting material became brittle with an elastic modulus of 10.2 MPa, which is considerably higher than the commonly used PDMS-based elastomer. Nonetheless, in the current system, the Young's modulus was shown to have a reduced correlation with self-healing, as seen in Table 1.
(85) The self-healing efficiency of the materials was also evaluated with shear rheometry using a method previously reported (see Y. Wang, J. He, S. Aktas, S. A. Sukhishvili and D. M. Kalyon, Rheological behavior and self-healing of hydrogen-bonded complexes of a triblock Pluronics copolymer with a weak polyacid, J. Rheol., 2017, 61, 1103-1119, the entire contents of which are incorporated herein by reference). Briefly, a disk-shaped (8 mm diameter and approximately 1.5 mm thickness) pristine sample was first measured with a time sweep for 10 minutes using 8 mm parallel plates to establish the benchmark performance; then the sample, while kept between two plates, was split into two halves horizontally and brought into contact with a normal force of 0.5 N. Finally, another time sweep was immediately performed on the cut sample for 2 hours.
(86) Conclusion
(87) A novel approach to fine-tune the thermomechanical properties of soft, siloxane-based elastomers has been developed through the use of dynamic metal-ligand interactions. Specifically, a new imine-based siloxane-based polymer has been crosslinked with various metal centers, including with Fe(.sub.II), Zn(.sub.II) and Co(.sub.II). Interestingly, the resulting polymers were shown to undergo rapid self-healing at room temperature (two hours) without the use of any external trigger. By tuning the strength of the metal-ligand interactions, fine-tuning of the thermomechanical properties was effectuated. Samples prepared from Zn(.sub.II), Fe(.sub.II) and Co(.sub.II) demonstrated a maximum strain before fracture of 525%, 75% and 25%, and a self-healing efficiency of 88%, 71% and 61%, respectively. This can be directly attributed to the difference in bond lengths (strength of the M-L interactions) where the smaller the bond length, the stronger the association and thus the more brittle the material. In addition to the effect of the metal center, the evaluation of the effect of the counter-ion on the self-healing efficiency and thermomechanical properties was also performed. Interestingly, an effect of the counter-ion on the maximum strain at fracture after self-healing was observed and can be attributed to the large counter-ions having an increased plasticizing effect than the smaller ones. For the materials prepared from Zn(BF.sub.4).sub.2 and Zn(ClO.sub.4).sub.2, self-healing efficiencies of 88 and 75%, respectively, were observed after 2 hours. Furthermore, materials prepared from tetrafluoroborate and triflate counterions showed an almost similar maximum strain before fracture of around 515%. Finally, the fine-tuning of the metal-ligand interaction in the soft polymer allowed for the preparation of a wide variety of self-healing polymers with Young's moduli ranging from 0.23 to 10.2 MPa. This new strategy to fine-tune the thermomechanical properties and self-healing of soft, siloxane-based polymers is highly versatile and the preparation of the materials is relatively simple. Therefore, this approach is particularly promising for many applications, including soft electronics, advanced coatings and manufacturing and healthcare.
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(89) The entire contents of all references noted herein are incorporated herein by reference.
(90) While the invention has been described with reference to preferred embodiments, the invention is not or intended by the applicant to be so limited. A person skilled in the art would readily recognize and incorporate various modifications, additional elements and/or different combinations of the described components consistent with the scope of the invention as described herein.