Effects of polymer architecture and composition on the interfacial properties of temperature‐responsive hydrophobically‐modified poly(N‐isopropylacrylamides)

Abstract

AbstractThe relationship between polymer structure and activity at the air/water interface was investigated as a function of temperature for various hydrophobically‐modified poly(N‐isopropylacrylamides) (HM‐PNIPAM). To examine the importance of several structural parameters, an unmodified PNIPAM was used as a reference polymer and two sets of HM‐PNIPAM were used (i) bearing an n‐alkyl chain at one or both terminal positions of the polymer backbone, and (ii) carrying n‐alkyl or n‐perfluoroalkyl chains randomly along the polymer backbone. The interfacial properties of HM‐PNIPAM, including the formation and the compression/expansion reversibility of the monolayers, at different subphase temperatures were studied by using the Langmuir film balance technique. The stability and dynamic nature of the HM‐PNIPAM monolayers were further studied by the time‐dependent surface pressure measurements. Within the studied temperature range (12–28°C), all polymers formed monolayers on the water surface, with the surface activity of polymers enhanced with increasing subphase temperature. Decreasing the monolayer compression/expansion rate enhanced the hysteresis between the compression and expansion isotherms. All results suggested a compression‐promoted temperature‐ and rate‐dependent conformational rearrangement of the polymer on the water surface. Increasing the level of hydrophobic modifications progressively improved the monolayer compressibility and stability, and reduced the hysteresis. Samples of HM‐PNIPAM bearing fluorocarbon chains formed more stable and compressible monolayers, comparing to their hydrocarbon analogs, and showed a less pronounced hysteresis. Copyright © 2006 John Wiley & Sons, Ltd.