Geometric confinement controls stiffness, strength, extensibility, and toughness in poly(urethane-urea) copolymers

Abstract

Achieving a unique combination of stiffness, strength, extensibility, and toughness in sol-cast poly(urethane-urea) (PU) copolymer films is a challenge since these properties are-in general- mutually exclusive. Here we demonstrate that geometric confinement of the basic building blocks controls stiffness, strength, extensibility, and toughness in PU films. Our results suggest that the severity of geometric confinement can be tuned by adjusting (i) soft segment molecular weight (SSMW) and (ii) drying temperature (DT) thanks to their effects on the structure formation via microphase separation and/or (confined and/or bulk) crystallization. It is therefore possible to produce (i) soft (no notable confinement) and (ii) stiff, strong, extensible, and tough (severe confinement) materials without changing any other parameter except SSMW and DT. The former has a typical physically cross-linked network and shows a welldefined elastomeric behavior with an elastic modulus (E) of 5-20 MPa, a tensile strength (sigma(max)) of 30-35 MPa, an extensibility (epsilon) of 1000-1300%, and a toughness (W) of 90-180 MJ m(-3). The latter, on the other hand, possesses an elegant hierarchical structure containing tightly packed secondary structures (7(2)-helix, 4(1)-helix, and antiparallel beta-sheets) and displays an elastoplastic behavior with an E of 400-700 MPa, a sigma(max) of 45-55 MPa, an epsilon of 650-850%, and a W of 200-250 MJ m(-3). Hence, our findings may be of interest in designing advanced materials containing synthetic replica of the secondary structures found in protein-based materials. The structure formation in the materials with this structural hierarchy is driven by the confined crystallization of helical poly(ethylene oxide) (PEO) chains in subnanometer urea channels, which-to the best of our knowledge-is a phenomenon wellknown in host-guest systems but has not yet demonstrated in PU copolymers, and complemented by the "bulk" crystallization of PEO and/or the microphase separation.