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

Aqueous dispersions have been studied where the particles consist of an ele ctrically conducting polymer (polypyrrole, PPB) and anchored temperature-re sponsive sheaths. The sheaths consist of poly(N-isopropylacrylamide-co-acry lamide) [poly(NP-xAM) (x = 35, 50 and 65)] copolymer [where x represents th e 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 (PC S) measurements show that the PPy/poly(NP-xAM) dispersions exhibit temperat ure induced contraction of the sheaths over the temperature range 30-70 deg rees C. The extent of contraction increases with decreasing value of x. Dis persion stability in pure water and aqueous NaCl solution was found to be d ue to electrosteric and steric stabilisation, respectively. Flocculation in the presence of electrolyte (NaCl) was studied using PCS and optical densi ty measurements. PPy/poly(NP-35AM) dispersions in aqueous 0.10 and 1.90 M N aCl solution exhibited upper critical flocculation temperatures (UCFT) of 5 8 and 30 degrees 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-s egment attraction of the sheaths under worse than theta-solvency conditions . Theoretical calculations indicate that synthesis of stable PPy dispersion s requires a minimum sheath thickness (delta) to core radius (a) ratio of d elta/a similar to 0.30 when ferric chloride is the oxidant. Electrical cond uctivity measurements of pressed pellets yielded room temperature conductiv ities in the range 0.0035-0.14 S cm(-1); increased levels of AM incorporati on within the sheaths increased the conductivity.