Mimicking the velcro system from nature for an alternative rubber/silica coupling system

Rafal Anyszka1,2, Wilma Dierkes1, Dariusz M. Bielinski2, Essi Sarlin3, Karolina Beton2, Anke Blume1

1 University of Twente, Faculty of Engineering Technology, Department of Mechanics of Solids, Surfaces & Systems (MS3), Chair of Elastomer Technology & Engineering, Enschede, The Netherlands – [email protected] 2 Lodz University of Technology, Faculty of Chemistry, Institute of Polymer and Dye Technology, Lodz, Poland – [email protected] 3 Tampere University of Technology, Department of Materials Science, Tampere, Finland

Supramolecular velcro – an ability of molecular systems for full or partial Introduction: recovery of their cohesive (single-component system) or adhesive (two-component system) strength at room temperature by means of supramolecular interactions. Beside carbon black, silica becomes the most important filler for high-performance rubber products. However, The differences and similarities between microscopic and supramolecular unlike carbon black its surface properties are not favorable for providing satisfactory rubber/filler interactions. A velcro systems significant amount of silanol groups present on the silica surface results in a considerably polar character, whereas most of rubbers exhibit a non-polar or relatively low-polar character. Because of this, various Microscopic Supramolecular approaches towards different surface treatments of silica were made. velcro velcro Superior fatigue resistence The aim of this research is to introduce an alternative type of rubber/silica coupling inspired by the velcro system Superior reconnectability performance existing in nature. The bio-mimicking mechanism is based on physical entanglements and steric hindrance Good mechanical properties between mutually soluble (based on calculated solubility parameters1): cross-linked and entangled rubber macromolecules – acting as molecular loops - and oligomer brushes grafted onto the silica surface – acting as Good ageing resistance molecular hooks. This results in the formation of supramolecular hooks-and-loops, a rubber/silica interphase Hooks and loops materials chemical compatibility Not relevant Very relevant fastener exhibiting reconnectable performance (Figure 1). Molecular mobility Not relevant Very relevant

Supramolecular velcro mechanism Oligomer brushes/elastomer matrix entanglements2,3

Cross-linked elastomer matrix (loops)

Cross-links

Oligomer brushes (hooks)

A C D B Figure 1: Scheme of the supramolecular velco mechanism on silica filler/elastomer matrix interphase. Long oligomer hooks attached to the silica surface form anchor-like connections with cross-linked elastomer loops by good mutual miscibility, velcro entanglements and stearic hindrance (A). The interphase undergoes disconnection under external stress (B). After the stress relaxation (C) the interphase is able to full recovery into original structure by re-entanglement of mutually miscible hooks-and-loops (D).

Chemical pathway of the oligomer brush (molecular hooks) synthesis on silica surface

1. Reaction between isocyanate silane and mono-functional telechelic butadiene oligomer with high amount of vinyl groups 2. Grafting of the reaction product onto the silica surface by silanol/alcoxyl groups condensation 3. Branching the oligomer backbone with different thiols reactive towards isocyanate vinyl groups

Bands from unsaturated carbon-carbon urethane + chain: 993 cm-1, 909 cm-1

+

Figure 2: Progress of the reaction between the isocyanate silane and butadiene oligomer. Figure 3: FTIR spectra of the silica before and after grafting the oligomer backbone.

Branching efficiency – 40.0 % Branching efficiency – 42.7 % Branching efficiency – 63.2 % Solubility parameter – 16.52 J1/2/cm3/2 Solubility parameter – 16.08 J1/2/cm3/2 Solubility parameter – 16.22 J1/2/cm3/2

Figure 4: TGA analysis of the silicas modified with the oligomer brushes.

Rubber type Solubility parameter (J1/2/cm3/2) BR (100 % vinyl) 15.83 4. Hydrogenation of the remaining unsaturated bonds4 BR (100 % cis/trans) 15.67 IR 18.12 s-SBR (Buna SL 4525-0) 16.23 Solubility parameter – 16.50 J1/2/cm3/2 Solubility parameter – 16.42 J1/2/cm3/2 Solubility parameter – 17.00 J1/2/cm3/2 Table 1: Calculated solubility parameters1 for various rubbers

References: Summary and conclusions: 1. Van Krevelen D. W. (1997) Properties of Polymers 3rd Edition, Chapter 7: Cohesive Properties and Solubility: 189-225.  An alternative concept for coupling silica to polymer was presented. Mimicking the velcro 2. Gutsmann T., Hassenkam T., Cutroni J. A., Hansma P. K. (2005) Sacrificial Bonds in Polymer Brushes from Rat system from nature, a synthesis for supramolecular hooks was developed by grafting long Tail Tendon Functioning as Nanoscale Velcro, Biophysical Journal, 89(1): 536-542. branched-oligomer molecules at the silica surface, which exhibit high chemical affinity to 3. O’Connor K. P., McLeish T. C. B. (1993) „Molecular Velcro”: Dynamics of a Constrained Chain into an Elastomer commonly used non-polar rubber acting as a loops. Network. Macromolecules, 26: 7322-7325. 4. Lin F., Wu C., Cui D. (2017) Synthesis and Characterization of Crystalline Styrene-b-(Ethylene-co-Butylene)-b-  Application of this type of silica surface-treatment might allow in the future the Styrene Triblock Copolymers, Journal of Polymer Science, Part A: Polymer Chemistry, 55: 1243-1249. implementation of alternative filler/elastomer coupling featuring reconnectable interphase interactions.