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Materials relying on association/dissociation of reversible covalent bonds
Dissociative systems were reported using a variety of reversible covalent reactions, such as Diels-Alder (DA), thioesters, dynamic urea bonds, Schiff’s bases, boronic esters and boroxines.21-32 Especially the Diels-Alder reaction, a cycloaddition, which is known to be reversible under certain conditions, was studied in depth. For example, Wudl and co-workers reported a thermally remendable crosslinked polymeric material that behaves like classical epoxy resins at service temperatures, but can restore its properties when fractured by heating to elevated temperatures (Figure 1.5).32, 33 Taking advantage of the relatively mild temperature and rather fast equilibrium of this maleimide/furane DA system, the degree of “open” crosslinks at 120 °C was determined to be around 30% from solid-state nuclear magnetic resonance spectroscopy. Upon cooling, the system was shown to recover its initial crosslinking degree. Similar materials, made via a solvent free method, were reported to possess healing efficiencies of more than 80%, as determined from tensile testing experiments. Other approaches were reported by Lehn and co-workers who used a bis(dicyanofumarate) in the presence of ethylene glycol based bis(fulvenes) to generate dinamers and eventually self-healing materials by the introduction of fulvene crosslinkers.
Materials relying on associative exchange reactions of covalent bonds
Materials containing dissociative/associative reversible reactions were shown to be selfhealing and processable under certain conditions and temperatures. However, they can lose their network integrity due to a decrease in crosslink density and become soluble. Materials which take advantage of dynamic covalent exchange reactions can be self-healing, (re-) processable and malleable while remaining insoluble thanks to the associative nature of the underlying exchange reaction.
A lot of research has been conducted on exchange reactions between thiols and disulfide compounds, as well as on disulfide exchange. Odriozola and co-workers described the generation of an poly(urea-uethane) elastomer containing aromatic disulfides as crosslinker.
The present hydrogen-bonding motifs prevent creep at room temperature while the dynamic nature of disulfide links allows processing at elevated temperatures (Figure 1.9). Indeed, the system showed remarkable recyclability in tensile testing experiments with almost no alteration of parameters such as stress at break. The material could easily be molded at 150 °C for 20 minutes under pressure and pristine as well as processed samples were reported as insoluble in a good organic solvent. As expected and due to the covalent exchange reaction the stress relaxation follows temperature with almost no relaxation at 25 °C (hydrogenbonding) and full stress relaxation at 150 °C (few hydrogen bonding). Despite the highly crosslinked character of this material, its mechanical properties such as Young’s modulus are rather weak. The nature of the un-catalyzed disulfide metathesis and the presence of monofunctional catalyzing thiol species remained un-discussed and could play a very significant role as exemplified and demonstrated by Goossens and co-workers.
Kinetic studies followed by 1H-NMR spectroscopy
Imines can be hydrolyzed by water to aldehydes and amines. It is known that free amines react with imines by transimination to generate another imine and another amine.41-42 To address this issue and minimize contaminations by water, the kinetic studies of imine-imine exchange and imine-aldehyde exchange reactions were performed in anhydrous CDCl3 (new bottle, septum). With the overall aim to introduce imine/aldehydes functionalized monomers in polymeric materials, we decided to perform some of the experiments on the later used monomers.
General mixing procedure: Stock solutions of all compounds were generated in closed vials (0.25 mM) in anhydrous CDCl3 (new bottle). Via micro syringes, one of the compounds (0.1 mL of stock solution) was mixed with more CDCl3 (0.5 mL) in the NMR-tube, before the second compound was added (0.1 mL, of stock solution). The tube was closed, sealed and shaken once before analysis was started. The time between mixing and acquisition of the first spectra was 3.5 minutes. For the analysis at elevated temperature, the NMR-machine was preheated.
The temperature in the room of analysis was between 23.0-23.6 °C. Total concentration of the two reactants was 0.071 M (0.05 mmol/0.7 mL). The spectra at time tx was treated with the de-convolution tool (Mestrenova, highest resolution, 20 fitting cycles).
Kinetic studies followed by gas chromatography under air
GC analysis: GC analysis was conducted on a Shimadzu gas chromatograph GC-2014 equipped with a Zebron “inferno” or a Waxplus column and helium as carrier gas. Injection was done manually by injecting 1L sample volumes using a 10 L syringe from Hamilton (gastight 1701). Before running analysis, the entire set-up was pre-heated to 350 °C and kept at constant carrier gas flow of 5 mL/min and split ratio of 2.0 for at least 30 minutes. The GC method (Tinj, Tcol, Tdet, gas flow, split ratio) was chosen according to the nature of the studied molecules and the respective exchange reaction. The column was reconstituted regularly by heating to 350 °C as described above.
Taking into account the encountered difficulties with the experimental set-up applied for 1HNMR spectroscopy, we decided to test the exchange reactions in less hydrophilic solvents (dried TCB and dried toluene) in the presence of a water trapping agent (molecular sieves). Kinetic studies of the possible exchange reactions between imines and between imines and aldehydes were followed by GC. Compounds were analyzed individually at different concentrations and external calibration curves with compound specific response factors (concentration – area relation) were generated. To further minimize the effect of humidity, a series of experiments was performed under protective atmosphere (argon) in bulk.
External calibration curves were generated as follow. A new bottle of the (anhydrous) solvent was opened and the solvent was then stored over molecular sieves.61 Stock solutions of each compound with concentrations between 0.1 mM – 0.074 mM were generated over molecular sieves. The stock solutions were diluted subsequently by adding 10, 50 and 100 L of the stock solution to 0.1 mL of the (anhydrous) dried solvent and characterized by GC using an injection volume of 1 L. Concentrations were corrected for impurities observed by 1H-NMR analysis if present. The slope of the linear fit of the four resulting points was used as external calibration reference to obtain the concentration – area dependence for each molecule.
The GC methods were adapted to solvent, reaction and column. For exchanges in TCB we tested the less polar Zebron “inferno” column and for exchanges in toluene and bulk the Waxplus with a higher polarity (Table 2.1).
Kinetic studies followed by gas chromatography under argon
The obtained results seem to indicate that the exchange reactions between imines and aldehydes can be highly influenced by traces of water (humidity) or amines (from the synthesis). To further minimize contamination, we performed the exchange between two imines and between an imine and an aldehyde in bulk under protective atmosphere (dry argon) in oven-dried and purged Schlenk flasks. Compounds were synthesized and purified as described above, stored under argon and tested in high concentration (1.3 M) in 1H-NMR to determine the exact amount of water, free amine and free aldehyde species (Scheme 2.6 and Figure 2.15). The dissociation constant Kdiss of imine I5 was determined by adding a five-fold excess of water to its 25 mM THF-d8 solution. The 1H-NMR spectra after 2 h and 38 h of mixing at room temperature showed that only a very small amount of imines hydrolyzed under these conditions (Figure 2.16). Assuming a similar aldehyde and amine concentration and the thermodynamic dissociation constant was estimated according to equation 2.3 to be as small as 1.76 × 10-4 (Equation 2.3).
Synthesis of monomers, polymers and vitrimer formation
Chemical compounds: Chemical compounds and solvents were purchased from Sigma Aldrich, TCI Chemicals, Alfa Aesar or Acros. Solvents were used as purchased and dried over new and oven-dried molecular sieves (3 Ǻ).1 Oven dried glassware was usually used. Commercial monomers were passed over a basic alumina column to remove inhibitors or antioxidants. AIBN was recrystallized from hot methanol.
Gel permeation chromatography (GPC): GPC was performed on a Viscotek GPCmax/VE2001 connected to a Triple detection array (TDA 305) from Malvern. Obtained raw data were treated with the respective standard homopolymer calibration (PMMA or PS) in the presence of toluene which was added as internal standard to check the flow. More detailed descriptions of the machines and methods used can be found in the respective sections.
Two approaches were tested to generate vitrimers with pending aldehydes and/or imines . We designed functionalized thermoplastics bearing aldehydes or imines pending from the polymer backbone and performed crosslinking in solution or in extrusion by addition of a bisimine molecule (Figure 3.1). Another approach consisted in mixing all compounds as monomers in the presence of a polymerizable bis-imine and generate the vitrimers in one-pot during polymerization. Both approaches were tested and the generation as well as the chemical characterization of the networks are described in the following subchapters.
Table of contents :
Ackknowledgements
Abbreviations and Variables
Table of Contents
General Introduction
Chapter 1 – Literature Review
Chapter 2 – Dynamic Exchange Reactions with Imines and Aldehydes
Chapter 3 – PMMA and PS Vitrimers with Pending Imines and Aldehydes
Chapter 4 – Dynamic Exchange Reactions of Boronic Esters
Chapter 5 – PMMA, PS and HDPE Vitrimers with Pending Boronic Esters
General Conclusion
Résumé
