High Refractive Index Aromatic and Ether-Containing Polythioureas: Improving Transparency and Mechanical Properties via Reciprocal Hydrogen Bonds

A series of aromatic-aliphatic polythiourea copolymers bearing highly polarizable poly(4,4′-phenylsulfide thiourea) and hydrogen bond (H-bond) acceptor poly(ether thiourea) units are synthesized. The copolymers are highly refractive (n_D = 1.78 at maximum) as a result of high sulfur content and lack of crystallinity and exhibit improved visible transparency (over 95%T in 1 µm thickness) compared to all-aromatic polythioureas. Further, the copolymers display higher fracture strength upon introducing 20–25 mol% of the poly(ether thiourea) unit, despite the inherent flexibility of ether groups. A detailed spectroscopic investigation indicates that this unique toughening behavior is attributed to the reciprocal (well compatible) intermolecular H-bonds intersected between aromatic and aliphatic thiourea moieties, as well as dynamic H-bond slipping/swapping between ether units and thiourea counterparts during the stretching process. These findings provide a rational strategy for simultaneously satisfying superior optical and mechanical properties, broadly applicable to various H-bond containing high refractive index polymers.