Tough Materials Through Ionic Interactions

This article introduces butyl acrylate-based materials that are toughened with dynamic crosslinkers. These dynamic crosslinkers are salts where both the anion and cation polymerize. The ion pairs between the polymerized anions and cations form dynamic crosslinks that break and reform under deformation. Chemical crosslinker was used to bring shape stability. The extent of dynamic and chemical crosslinking was related to the mechanical and thermal properties of the materials. Furthermore, the dependence of the material properties on different dynamic crosslinkers—tributyl-(4-vinylbenzyl)ammonium sulfopropyl acrylate (C4ASA) and trihexyl-(4-vinylbenzyl)ammonium sulfopropyl acrylate (C6ASA)—was studied. The materials’ mechanical and thermal properties were characterized by means of tensile tests, dynamic mechanical analysis, differential scanning calorimetry, and thermogravimetric analysis. The dynamic crosslinks strengthened the materials considerably. Chemical crosslinks decreased the elasticity of the materials but did not significantly affect their strength. Comparison of the two ionic crosslinkers revealed that changing the crosslinker from C4ASA to C6ASA results in more elastic, but slightly weaker materials. In conclusion, dynamic crosslinks provide substantial enhancement of mechanical properties of the materials. This is a unique approach that is utilizable for a wide variety of polymer materials.


Thermal stability of the materials
The decomposition temperatures of the films were determined through thermogravimetric analysis and the obtained curves are presented in Supplementary Figures 4-7. The decomposition temperature gives information about the thermal stability of the materials. This information was used to determine a suitable temperature range for DMA measurements. The figures show that with increasing content of ionic monomer, the TGA curve becomes increasingly close to the curve of the corresponding homopolymer, i.e. the C4ASA-films' curves near the curve of polymerised C4ASA and C6ASA-films' curves near the TGA curve of PC6ASA.

Supplementary
Supplementary Figure 8 is a depiction of the degradation onset temperatures of C4ASA-films. The films with 5% crosslinker are the most stable. High crosslink density hinders the diffusion of volatiles, hence delaying the degradation. With low concentrations of ionic monomer, the films with 2% crosslinker are thermally the least stable. However, it is worth noting that the thermal stability of the ionomers is lower than the thermal stability of butyl acrylate without an ionic comonomer. For example, butyl acrylate that has been crosslinked with 2% BudMA starts to degrade at 310 °C.
Supplementary Figure 8. The degradation onsets of the C4ASA-films with 1% (blue), 2% (black), and 5% (red) of chemical crosslinker BudMA as a function of ion content. The degradation onset was defined as the temperature wherestarting from 150 °C -2% of the material had degraded. The percentages (ion content and crosslinker content) are the concentrations of the respective monomers in the feed.
Comparison of the TGA curves of C6ASA-, C4ASA-, and PC6ASA-films reveals subtle differences in the onsets of degradation (Supplementary Figure 6). As the ionic content of the material increases, the degradation temperature increases ever so slightly, nearing the degradation temperature of corresponding pure polyelectrolyte. The film that contained 5% of PC6ASA was the most stable of the ionic films as its degradation temperature was close to that of pure PC6ASA. C4ASA-films are slightly more thermally stable than the films with C6ASA.
Supplementary Figure 9. The degradation onset temperatures of films with 2% chemical crosslinker and varying amounts of C4ASA (■), C6ASA (□), and PC6ASA (△). The points at 100% ionic monomer concentration are an exception as they belong to pure polyelectrolytes, not crosslinked films. The degradation onset was defined as the temperature wherestarting from 150 °C -2% of the material had degraded.
Supplementary Figure 13. The tan δ peak heights of C4ASA-films as a function of ion content. The films are crosslinked with 1% BudMA. The matrix peak heights are given in black, and the cluster peaks are given in red.