Study of the conformational change of poly(<i>N</i>‐isopropylacrylamide)‐grafted chains in water with neutron reflection: Molecular weight dependence at high grafting density

Abstract

Abstract The temperature‐dependent conformational change of poly( N ‐isopropylacrylamide) (PNIPAM) brushes in D 2 O was investigated as a function of the molecular weight ( M ) at a high grafting density with neutron reflection. PNIPAM chains with three different M values were grafted at the same high surface density from a gold surface by atom transfer radical polymerization. A significant change in the segment concentration profile was observed for all three samples as the temperature passed through the lower critical solution temperature (∼30 °C), in contrast to previous results obtained for samples with much lower surface density. Somewhat surprisingly, the fractional change in the first moment of the segment concentration profile (〈 z 〉) from 20 to 41 °C was weaker with increasing M . This is contrary to the trend for systems involving only van der Waals (VDW) interactions, in which higher M chains experience larger conformational changes with change in solvent quality. Indeed, the M dependence of the first moment of the segment concentration profile for the grafted PNIPAM chains at 20 °C was much weaker than has been reported previously for dense brushes involving only VDW interactions under good solvent conditions. At 20 °C, the form of the segment concentration profile varied systematically with M . A single‐layer profile resulted for the highest M , but the profiles became more bilayer in character with decreasing M . At 41 °C, the profiles for all three samples were adequately described by a single dense layer with a smooth transition region to bulk D 2 O. The weak dependence of 〈 z 〉 on M at 20 °C and the trend from a bilayer profile at lower M to a single‐layer profile at higher M appear to be related. These results are interpreted in terms of concentration‐dependent segment–segment interactions that result in a weak attraction for high segment densities at 20 °C. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 3302–3310, 2004