Butyl rubber-based interpenetrating polymer networks with side chain crystallinity: Self-healing and shape-memory polymers with tunable thermal and mechanical properties

A self-healing and shape-memory interpenetrating polymer network (IPN) is produced by UV polymerization of n-octadecyl acrylate (C18A) monomer in a toluene solution of butyl rubber (isobutylene-isoprene rubber, IIR) using Irgacure 2959 photoinitiator at ambient temperature. IPNs containing 20-80 wt% IIR have crystalline domains formed by side-by-side packed octadecyl side chains aligned perpendicular to the poly(C18A) (PC18A) backbone. TEM images reveal that the morphology of IPNs consists of crystalline domains dispersed in a continuous amorphous matrix where the size of the dispersed phase could be adjusted between mu m and nm level by changing the amount of IIR. Calculations indicate that the effective cross-link density of IPNs is mainly determined by their crystalline domains followed by hydrophobic associations while the chemical cross-links between IIR and PC18A components are negligible. We also show that the crystalline domains acting as sacrificial bonds dissipate energy under strain leading to a significant toughness improvement. IPNs exhibit tunable melting temperature (46-50 degrees C), crystallinity (1.5-25%), Young's modulus (0.6-35 MPa), toughness (1.3-12 MPa), and a stretchability of up to 1200% by varying the amount of IIR component. What is more, they also exhibit temperature induced healing behavior with 59-77% efficiency, and an effective shape-memory function. The strategy presented here is applicable for the preparation of IPNs based on various rubbers and poly(n-alkyl (meth)acrylates) with long alkyl side chains.