Light-Responsive Polymer Networks

Debondable Two-Component Structural Adhesives

Cross-linked polymer networks that can be disrupted through application of an external stimulus are highly relevant, e.g., as adhesives with debond-on-demand properties for manufacturing (semiconductor, automotive, aerospace, construction, packaging, sportswear), healthcare applications (wound dressing, transdermal patches), and numerous other domains, and they can significantly contribute to the sustainable use of materials (repairing, reworking, recycling). In all cases, there is a technological need for effective and environmentally benign solutions that provide secure adhesion during use, while also permitting for a simple and clean separation of bonded parts “on command” without the need for additional complex process steps.

In this context, a new technology has been developed based on the unprecedented use of azo compounds as the stimuli-responsive components in adhesive formulations. The stimulus triggering the de-bonding of the adhesive, which can be either UV irradiation (380 nm) or heat (>100°C), induces the selective cleavage of the azo moiety consequently causing the degradation of the polymer. As additional benefit, the nitrogen gas evolved during the photodecomposition of the azo group causes the foaming of the polymer matrix facilitating the separation of the glued components. In this contest, novel azo compounds functionalized with a variety of reactive groups have been synthesized in our group and tested as curing agents for the preparation of two-component glues. A combination of an epoxy prepolymer and an azo compound functionalized with a secondary amine (azo-NHR) was found to be the most promising formulation in terms of curing time, strength, and de-bonding ability. The mechanical properties of the azo-epoxy glue are comparable with those measured for commercial epoxy adhesives (strain at break ~10 MPa).

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