Longitudinal polymer gradient materials based on crosslinked polymers

•We prepare longitudinal polymer gradient materials, covering a large modulus range.•Thermally and photochemically curing polymer systems are employed.•We examine different gradient structures and their impact on mechanical properties.•Hard-soft-hard polymer gradient materials show a higher toughness.•The incorporated rubber-glass transition zone envisions thermoresponsive materials.

Polymer gradient materials (PGMs) are known to reduce stress concentrations and to increase fracture toughness. The first system we reported for the preparation of longitudinal PGMs was based on poly(dimethyl siloxanes), covering a relatively low Young's modulus range. In this study, we used two photochemically and one thermally curing polyaddition systems, enabling us to cover a much larger modulus range up to 1300 MPa. Three different gradient structures, hard-soft, hard-soft-hard, and soft-hard-soft, were realized and confirmed by position-dependent UV/Vis absorbance measurements. Tensile testing in dependency on the gradient structure was performed. A comparison with non-gradient samples with discrete composition revealed a significant improvement, specifically in the case of hard-soft-hard gradients. Hence, PGMs are a promising approach for the development of materials with a special mechanical property profile particularly at different temperatures, leading to novel thermoresponsive materials.

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