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

Abstract

The influence of small alkylamines with increasing carbon chain length (≤5) on the temperature‐induced precipitation of two N‐alkylacrylamide oligomers, poly‐N‐isopropylacrylamide [PNIPAAm; weight‐average molecular weight (Mw ) = 1600 g/mol] and poly‐N,N‐diethylacrylamide (PDEAAm; Mw = 4000 g/mol), from an aqueous solution was investigated. The alkylamines in question were too small to form micelles in the classical sense but were capable of premicellar aggregation. PNIPAAm was prepared by radical polymerization in the presence of a chain‐transfer agent and, therefore, carried a carboxylic acid end group. The structure was heterotactic. PDEAAm was prepared by anionic polymerization and, therefore, carried a butyl end group. The structure was predominately isotactic. The solubility of the oligomers was investigated by cloud‐point measurements and differential scanning calorimetry. In addition, pyrene was used as a fluorescent polarity probe. Alkylamines up to C2 depressed the lower critical solution temperature (LCST) of PNIPAAm, whereas higher alkylamines first depressed the LCST and at higher concentrations elevated it. The LCST minimum showed a clear dependence on the alkyl chain length and structure. For PDEAAm, only pentylamine addition resulted in an LCST minimum. Otherwise, the LCST was raised. When the critical self‐association concentration (CSAC) of the alkylamines in water was compared to the critical association concentration (CAC) in aqueous oligomer solutions, PDEAAm, but not PNIPAAm, stabilized mixed aggregates (CAC < CSAC). The transition enthalpy of PNIPAAm decreased with an increasing alkylamine concentration and became 0 above the CAC. For PDEAAm, no transition endotherm could be recorded above an alkylamine concentration of 0.1 M. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 4218–4229, 2000

Synthesis and characterisation of thermo-responsive poly(N,N′-diethylacrylamide) microgels

“…In this extended state, which is inter alia characterised by the presence of polymer chains protruding from the particles, the microgels may be able to interact with the cell membrane lipids in a manner similar to the interaction of the corresponding linear oligomers with charged and uncharged surfactants [55,56]. Such a preferential interaction of the membrane compounds with the polymer chains will destabilise the cell membrane structure, putatively leading to the observed leakage of LDH.…”

Energetics of Cooperative Transitions of N‐Vinylcaprolactam Polymers in Aqueous Solutions

“…This estimate seems to be plausible as it is close to the data for PNIPA. [40] An increase in the transition temperature simultaneously with the decrease in the transition enthalpy shows that the insertion of charges in the PNVCL chain leads to a decrease in the transition entropy. It can be due to the negentropic effect of counterion concentrating generally accompanying polyelectrolyte precipitation.…”

The effect of salt and pH on the phase-transition behaviors of temperature-sensitive copolymers based on N-isopropylacrylamide

“…On the other hand, the presence of NaCl will undoubtedly increase the polarity of aqueous media thus enhance the hydrophobic-hydrophobic interactions. The stronger hydrophobic-hydrophobic interaction indicates the stronger tendency for the polymers to selfaggregate; a feature that will result in a less solubility of the polymer in water hence decreases its LCST [20]. For the two hydrophilic polymers, we believe that the first mechanism explains the decreases in their LCSTs;…”