Influence of small uncharged but amphiphilic molecules on the lower critical solution temperature of highly homogeneous N-alkylacrylamide oligomers

The influence of small alkylamines with increasing carbon chain length (les s than or equal to5) on the temperature-induced precipitation of two N-alky lacrylamide oligomers, poly-N-isopropylacrylamide [PNIPAAm; weight-average molecular weight (M-omega) = 1600 g/mol] and poly-N,N-diethylacrylamide (PD EAAm; M-omega = 4000 g/mol), from an aqueous solution was investigated. The alkylamines in question were too small to form micelles in the classical s ense but were capable of premicellar aggregation. PNIPAAm was prepared by r adical polymerization in the presence of a chain-transfer agent and, theref ore, carried a carboxylic acid end group. The structure was heterotactic. P DEAAm was prepared by anionic polymerization and, therefore, carried a buty l end group. The structure was predominately isotactic. The solubility of t he oligomers was investigated by cloud-point measurements and differential scanning calorimetry. In addition, pyrene was used as a fluorescent polarit y probe. Alkylamines up to C-2 depressed the lower critical solution temper ature (LCST) of PNIPAAm, whereas higher alkylamines first depressed the LCS T and at higher concentrations elevated it. The LCST minimum showed a clear dependence on the alkyl chain length and structure. For PDEAAm, only penty lamine addition resulted in an LCST minimum. Otherwise, the LCST was raised . When the critical self-association concentration (CSAC) of the alkylamine s in water was compared to the critical association concentration (CAC) in aqueous oligomer solutions, PDEAAm, but not PNIPAAm, stabilized mixed aggre gates (CAC < CSAC). The transition enthalpy of PNIPAAm decreased with an in creasing alkylamine concentration and became 0 above the CAC. For PDEAAm, n o transition endotherm could be recorded above an alkylamine concentration of 0.1 M. (C) 2000 John Wiley & Sons, Inc.