High-throughput discovery of structure-mechanical property relationships for segmented poly(urethane-urea)s

A novel high-throughput method was used to determine the effect of chain extender composition on the mechanical properties of segmented poly(urethane-urea)s. Combinatorial libraries with continuous gradients in chain extender composition (60 < phi < 160 mol % stoichiometry), cure temperature (70 < T < 110 degreesC), or both were synthesized. Stress vs strain data were obtained at numerous library positions (corresponding to different 0 values) using a unique high-throughput mechanical characterization (HTMECH) apparatus developed by the authors. These mechanical measurements were related to the morphology, hydrogen bonding, and degree of phase separation using AFM, FTIR, and SC. Optimum strength and percent elongation were observed at a chain extender composition of phi = 85 mol %, corresponding also with the finest phase-separated morphology, indicated by an even dispersion of uniform hard domains (dimensions 110-130 nm). DSC measurements indicated increased mixing of soft and hard segments when phi exceeded 85%, which was correlated to decreased urea-urea hydrogen bonding (from FTIR). SEM analysis of the library fracture surfaces suggested a transition from brittle to ductile failure with increased phi, in agreement with the increased soft-hard segment mixing and disruption of the hydrogen-bonded network. These results validate the HTMECH approach as an accurate and effective screening tool for developing mechanical property-structure relationships in combinatorial polymer libraries.