Unusual thermoprecipitation behavior of Poly(N,N-diethylacrylamide) from aqueous solution in the presence of anionic surfactants

Surfactants (charged amphiphiles) are usually found to promote the solubili ty of reversibly water-soluble neutral polymers at concentrations above the critical aggregation concentration. Below that concentration or when no ag gregates/micelles are formed, solubility is reduced presumably due to a sim ple salting out effect. In this paper, the effect of linear n-alkyl sulfate s with chain length between 1 and 12 on the solubility of three oligomeric poly-N-alkylacrylamides, namely a poly(N-isopropylacrylamide), N1, and a po ly(N,N-diethylacrylamide), D1, prepared by chain transfer polymerization, a nd a poly(N,N-diethylacrylamide), Al, prepared by anionic polymerization, a re used to investigate these assumptions. The cloud point temperatures (CPT ) of the oligomers are determined spectrophotometrically, while a fluoresce nce probe (1 mum pyrene) is used to follow the alkyl sulfate aggregation (m icelle formation) in solution. N1 and D1 are found to be very similar in th eir behavior. Alkyl sulfates with n less than or equal to 4 (no micelle for mation) show a simple salting out effect. In the presence of alkyl sulfates with chain lengths of 5, 6, and 8, the CPT of the solution passes through a minimum and then reaches a plateau/maximum at a level above the CPT obser ved in pure water. If the alkyl sulfate concentration is increased beyond t he plateau region, the CPT decreases again. For alkyl chain length of n > 9 , the CPT rises steadily with increasing alkyl sulfate concentration. Despi te their much smaller size (factor 70) both NI and D1 hence show a solubili ty that corresponds to that observed by Schild and Tirrell (Langmuir 1991, 7, 665) for poly(N-isopropylacrylamide) polymers. The solubility of the pre dominantly isotactic oligomer Al, on the other hand, deviated considerably from this behavior (no CPT minimum, no salting out effect for butyl sulfate ) and thus seems to challenge some of the general assumptions concerning th e effect of surfactants at low concentration.