Study of the Conformational Change of Poly(N-isopropyl- acrylamide)-Grafted Chains in Water with Neutron Reflection: Molecular Weight Dependence at High Grafting Density

The temperature-dependent conformational change of poly(N-isopropylacrylamide) (PNIPAM) brushes in D2O 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 increasingM. This is contrary to the trend for systems involving only van derWaals (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 D2O. The weak dependence of z onM 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