Viscoelastic and self-healing behavior of silica filled ionically modified poly(isobutylene- co-isoprene) rubber

Abstract

Rubber composites were prepared by mixing bromobutyl rubber (BIIR) with silica particles in the presence of 1-butylimidazole. In addition to pristine (precipitated) silica, silanized particles with aliphatic or imidazolium functional groups, respectively, were used as filler. The silanization was carried out either separately or in situ during compounding. The silanized particles were characterized by TGA, ¹H-²⁹Si cross polarization (CP)/MAS NMR, and Zeta potential measurements. During compounding, the bromine groups of BIIR were converted with 1-butylimidazole to ionic imidazolium groups which formed a dynamic network by ionic association. Based on DMA temperature and strain sweep measurements as well as cyclic tensile tests and stress-strain measurements it could be concluded that interactions between the ionic groups and interactions with the functional groups of the silica particles strongly influence the mechanical and viscoelastic behavior of the composites. A particularly pronounced reinforcing effect was observed for the composite with pristine silica, which was attributed to acid-base interactions between the silanol and imidazolium groups. In composites with alkyl or imidazolium functionalized silica particles, the interactions between the filler and the rubber matrix form dynamic networks with pronounced self-healing behavior and excellent tensile strength values of up to 19 MPa. This new approach in utilizing filler-matrix interactions in the formation of dynamic networks opens up new avenues in designing new kinds of particle-reinforced self-healing elastomeric materials with high technological relevance.

Scheme 1. Ionic modification of bromobutyl rubber by conversion with 1-butylimidazole.

Scheme 2. Silanization of precipitated silica (Ultrasil 7000GR).

Fig. 1. TGA traces for unmodified silica S₀ and surface-modified silicas S_1–3.

Fig. 2. ¹H–²⁹Si CP/MAS NMR spectrum of unmodified silica S₀ (a) and modified silica S₂ (b) with assignments of T and Q groups. Q₂ = geminal silanol, Q₃ = single silanol, Q₄ = siloxane bridges. Contact time τ = 2 ms.

Fig. 3. ¹H MAS NMR spectra of unmodified silica S₀ (a) and modified silica S₂ (b).

Fig. 4. Zeta potential of unmodified (S₀) and alkoxysilane modified silica fillers (S_1–3) in dependence of pH.

Fig. 5. DMA temperature sweep measurements of rubber-silica composites C_0–2 and BIIR-i (a) storage modulus plots (b) tan δ plots (c) enlarged section of the tan δ plots.

Fig. 6. DMA temperature strain sweep measurements of rubber-silica composites C_0–2. The symbols represent the measured variables. The lines are fitted according to the Kraus model.

Fig. 7. Mechanical hysteresis curves of (a) C₀, (b) C₁, and (c) C₀ in comparison to BIIR-i.

Fig. 8. Stress–strain curves of composites C_0–2 compared to BIIR-i.

Fig. 9. Stress–strain curves of (a) C₁ and (b) BIIR-i. The black (dotted) and the red (solid) curves represent the virgin and the healed samples respectively.