Effects of surfactants on the phase transition of poly(N-isopropylacrylamide) in water

✍ Scribed by Chi Wu; Shuiqin Zhou

• Publisher: John Wiley and Sons
• Year: 1996
• Tongue: English
• Weight: 706 KB
• Volume: 34
• Category: Article
• ISSN: 0887-6266
• DOI: 10.1002/(sici)1099-0488(19960715)34:9<1597::aid-polb8>3.0.co;2-i

✦ Synopsis

The effects of both anionic (sodium dodecyl sulfate, SDS) and cationic (dodecylpyridine bromide, DPB) surfactants on the phase transition of narrowly distributed poly(N-isopropylacrylamide) (PNIPAM) microgel particles were investigated by laser light scattering. The addition of SDS swells the particles and increases the phase transition temperature, while DPB has a much smaller effect. This difference cannot be due to an association between the surfactant hydrophobic tail and PNIPAM because DPB and SDS have an identical hydrophobic tail. The amide groups in PNIPAM are slightly protonized in deionized water (pH -5.5). Our results contradict a previous prediction that oppositely charged surfactants will collapse a polyelectrolyte gel. After adding SDS, a two-step phase transition of the PNIPAM gel is observed. This suggests that SDS forms micelles inside the microgel with the help of the immobilized counter ions on the gel network. The SDS micelles are broken into individual SDS molecules in the first step of phase transition, while in the second step individual SDS molecules are gradually expelled. Surfactant effects on the microgel particles are compared with those of individual PNIPAM chains.