A study of polypyrrole/poly(N-isopropylacrylamide-co-acrylamide) dispersions: Electrically conducting polymer dispersions stabilised by copolymers with lower critical solution temperatures

Abstract

Aqueous dispersions have been studied where the particles consist of an electrically conducting polymer (polypyrrole, PPy) and anchored temperature-responsive sheaths. The sheaths consist of poly(N-isopropylacrylamide-co-acrylamide) [poly(NP-xAM) (x=35, 50 and 65)] copolymer [where x represents the mol% of acrylamide (AM) used during synthesis]. The properties of PPy/PAM (PAM=polyacrylamide) and PPy/PVA [PVA=poly(vinyl alcohol)] dispersions were also investigated for comparison. Photon correlation spectroscopy (PCS) measurements show that the PPy/poly(NP-xAM) dispersions exhibit temperature induced contraction of the sheaths over the temperature range 30–70°C. The extent of contraction increases with decreasing value of x. Dispersion stability in pure water and aqueous NaCl solution was found to be due to electrosteric and steric stabilisation, respectively. Flocculation in the presence of electrolyte (NaCl) was studied using PCS and optical density measurements. PPy/poly(NP-35AM) dispersions in aqueous 0.10 and 1.90 M NaCl solution exhibited upper critical flocculation temperatures (UCFT) of 58 and 30°C, respectively. These values were indistinguishable from the lower critical solution temperature (LCST) for poly(NP-35AM) copolymer measured under identical conditions. Flocculation occurred due to segment–segment attraction of the sheaths under worse than ϑ-solvency conditions. Theoretical calculations indicate that synthesis of stable PPy dispersions requires a minimum sheath thickness (δ) to core radius (a) ratio of δ/a ∽0.30 when ferric chloride is the oxidant. Electrical conductivity measurements of pressed pellets yielded room temperature conductivities in the range 0.0035–0.14 S cm^-1; increased levels of AM incorporation within the sheaths increased the conductivity.

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