Intramolecular cross-linking: From single chain to bulk materials

Polymeric materials have historically been divided into thermoplastics and thermosets, as a consequence of their thermal processability. Thermoplastics typically present a glass transition temperature (Tg), and a melting temperature (Tm) while in thermosets, covalent cross-links between chains restrict relative movement. While they still present a Tg, thermosets cannot be melted. In this research, we exploit chemistry to produce chains with intramolecular cross-links only, restricting relative movement in the chain, but allowing the chains to flow in relation to each other, making thermoplastics reinforced with covalent cross-links.

The synthetic strategy involves a two-step approach; first, a linear chain is prepared; then, intramolecular cross-linking is carried out under high dilution to inhibit intermolecular reactions. The obtained polymers are studied for their mechanical and thermomechanical properties at the single molecule level, as well as in bulk materials at both the glassy and rubbery state. Our current findings reveal that in the glassy state, the bulk polymer becomes more brittle following the increase of intramolecular cross-linking, while the elastic modulus remains unchanged.[1] However, in the rubbery state, remarkable improvements occur. The combination of entanglements and intramolecular cross-links lead to materials with improved moduli, strength, toughness and amazingly high elasticity.

The two key structural properties, strength and toughness, tend to be mutually exclusive, i.e., material enhancements that increase strength also tend to make materials more brittle and thereby decrease toughness. W use structural approach to remove entanglements completely using intramolecular cross-linked architectures, and then functionalize the nanoparticles’ surfaces to tune the thermomechanical properties of the polymer independently of the monomer chemistry. By utilizing covalently intramolecular bonded SCPN, we can examine the minimal entanglement hypothesis without potential artifacts from SCPN unfolding.

[1] Galant, O., Bae, S., Wang, F., Levy, A., Silberstein, M. N., Diesendruck, C. E., Macromolecules 2017, 50 (17), 6415–6420