Self-healing supramolecular polymer nanocomposites

Abstract

Polyborosiloxanes (PBSs) are viscoelastic, transparent, colourless, self-healable polymer matrices, synthesized by reacting linear polydimethylsiloxanes (PDMSs) with boric acid (BA) above 150°C. BA takes part not only in cleaving the PDMS chains, but also contributes boronic-acid like moieties to the cleaved chain ends, which provide supramolecular self-healing capabilities. PBSs can be regarded as supramolecular elastomers, owing to the reversible cross links formed by the abundant hydroxyl groups. Therefore they are able to recover both the microscopic and macroscopic fractures intrinsically via hydrogen-bonds. Additionally, a glass transition temperature (Tg) as low as -90°C ensures chain mobility to PBSs molecules to heal cracks autonomously without extra heat supply. Compared to traditional carbon-based polymeric materials, PBSs provide a higher resistance to heat and light owing to the siloxane backbone. Upon increasing the degree of modification, PBSs get stronger on the one hand. However, PBSs also become less adhesive and start suffering from brittle fracture. Interestingly, the addition of nano-clay (Cloisite 30B) solves this problem. It does not make PBSs more brittle as it does in traditional thermal plastics. Instead, it renders PBSs ductile but strong. Comparing to PBSs, the PBSs/Cloisite 30B nanocomposites even exhibit an additional filler-induced mechanical relaxation at high frequency. This phenomenon implies that PBSs/Cloisite 30B nanocomposites can be applicable as self-healing shock absorber. Further investigations will be focusing on characterization of the selfhealing capacity, on strengthening the composites and on tuning the position of the filler-induced rheological response. Upon applying other (nano-sized) reinforcements, PBSs can be endowed with wide variety of targeted functionalities, for instance, stiffness by nano-clay; thermal conductivity by graphite or boron nitride; piezoelectric properties by lead zirconate titanate (PZT), etc. The presence of particles is also advantageous for the adhesive properties of the composites, because particles lower the number of hydrogen-bonds by, either isolating molecules or forming particle-molecule interaction.