Conformation of end-tethered PNIPAM chains in water and in acetone by neutron reflectivity

Poly(N-isopropylacrylamide) is perhaps the most well-known member of the class of temperature responsive polymers. It has a lower critical solution temperature (LCST) in water at about 32 degreesC. This very sharp transition (similar to5 degreesC) is attributed to alterations in the hydrogen-bonding interactions of the amide group. In this work we investigated the conformation of end-tethered PNIPAM chains at the interface of silicon with D2O and d-acetone using neutron reflection. End-tethered PNIPAM layers were prepared utilizing the interaction between COOH-terminated PNIPAM and OH-terminated self-assembled monolayers (method A) and also by polymerizing N-isopropylacrylamide monomers from the silicon surface (method B). Reflectivity data from the protonated layers in deuterated water were obtained using a liquid cell over a range of temperature from 10 to 55 degreesC. For method A, PNIPAM molecular weights of 33K and 220K were examined. In D2O, we observed very limited change in the conformation of the tethered chains as the temperature increased through the LCST. No conformational change was detected for the lowest molecular weight PNIPAM-COOH sample (33K). Only a slight change in conformation with temperature was detected for the higher molecular weight sample (220K) and the sample from method B. The profiles indicated that the chains were well hydrated above 32 degreesC, consistent with the observation of very low receding water contact angles. On the other hand, a significant change in the segment concentration profiles occurred when D2O was replaced by d-acetone. Bilayer profiles were observed for all samples in D2O. By contrast, in d-acetone the profiles were composed of a single monotonically decaying layer. Surprisingly, the profiles were more contracted in d-acetone than in D2O despite the fact that PNIPAM dissolves in d-acetone but solutions of PNIPAM in D2O are cloudy above the LCST. This observation, as well as the lack of conformational change with temperature for these low surface density brushes in D2O, is explained on the basis of a concentration-dependent Flory X parameter.