The reverse behavior or negatively and positively charged poly(N-isopropylacrylamide) latex dispersions in alcohol-water mixtures at temperatures below their phase transition

Results of size control experiments on both negatively and positively charged poly(N-isopropylacrylamide) (PNIPAAM) latex dispersions are reported. In alcohol-water mixtures containing low weight fractions of alcohols higher than methanol, negatively and positively charged PNIPAAM particles exhibit the opposite swelling and shrinking behavior at 20 degrees C. Negatively charged particles shrink due to cononsolvency. Conversely, positively charged particles swell dramatically. Above 40 wt% alcohol, this opposite behavior is cancelled out. At 40 degrees C, the negatively charged particles collapse and shaw a similar behavior to those with a positive charge. The general idea that positively charged particles are more hydrophobic is also underlined by other clues.