Synthesis and evaluation of polypyrrole-coated thermally-expandable microspheres: an improved approach to reversible adhesion

An air-stable organic conducting polymer, polypyrrole (PPy), has been deposited from aqueous solution onto micrometer-sized Thermally Expandable Microspheres (TEMs). These polypyrrole-coated TEM particles were characterized using scanning electron microscopy, laser diffraction, FT-IR and Raman spectroscopy and also uv-visible reflectance spectroscopy. Polypyrrole has a broad absorption peak in the near infra-red at around 900–1500 nm, hence irradiation of the polypyrrole-coated TEM particles using an infra-red lamp (λmax = 1200 nm) leads to efficient localized heating that causes the TEM particles to expand to many times their original volume. Hot-stage optical microscopy studies confirm that this large volumetric change is not constrained by the conducting polymer overlayer: expansion of the polypyrrole-coated TEM particles is at least as effective as that found for uncoated TEM particles subjected to conventional bulk heating. Moreover, thermal expansion achieved via infra-red irradiation is significantly quicker and leads to cleaner delamination. Thus this approach seems to be well suited to niche applications such as the reversible adhesion of car glazing and panels, which are likely to play an important role in the future for both convenient repair of increasingly complex composite materials and also end-of-lifetime manufacturer-led recycling initiatives.