Conducting polymer-coated thermally expandable microspheres

Thermally expandable microspheres (TEMs) consist of a copolymer shell encapsulating a liquid hydrocarbon core and expand irreversibly to many times their original volume on heating. In this work commercial TEMs with a mean diameter of approximately 13 µm were coated with either polypyrrole, polyaniline or poly(3,4-ethylenedioxythiophene) [PEDOT] at conducting polymer mass loadings of 0.1 to 1.5%. Laser diffraction showed that aqueous suspensions of these conducting polymer-coated TEMs were well-dispersed, indicating minimal particle aggregation. Scanning electron microscopy studies indicated that these TEMs have relatively rough surfaces both before and after coating with conducting polymer. Raman spectroscopy was very sensitive to the presence of conducting polymer and could be used to confirm the presence of polypyrrole at target mass loadings as low as 0.1 wt%. The presence of polypyrrole at the TEM surface was confirmed from the Cl/N atomic ratios determined by X-ray photoelectron spectroscopy. This technique allowed the polypyrrole-coated TEMs to be ranked correctly according to their targeted conducting polymer loadings. All the conducting polymer-coated TEMs were subjected to irradiation using a near-infrared lamp with a λmax of 1200 nm. Since conducting polymers absorb strongly in the near-infrared region, this leads to efficient localised heating of the coated TEMs. Thus the onset time required for the expansion of conducting polymer-coated TEMs under a given set of irradiation conditions is reduced significantly compared to control experiments conducted with uncoated TEMs (from 162 ± 2 seconds to 11 ± 1 seconds). For the polypyrrole-coated TEMs, systematic reduction of the target polypyrrole mass loading from 1.5 to 0.3% had surprisingly little effect on the observed onset times for volumetric expansion. Polypyrrole, polyaniline and PEDOT-coated TEMs all exhibited similar onset times (10 ± 1 seconds) when compared at the same mass loading (1.5%). However, EDOT is relatively expensive compared to pyrrole and polymerisation of the former monomer is substantially incomplete even at 50 °C. Moreover, aniline is significantly more toxic than pyrrole and its polymerisation requires a more expensive oxidant. Thus it is concluded that polypyrrole is the preferred conducting polymer for coating TEMs in order to optimise their thermo-responsive volumetric expansion behaviour.