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 D 2O 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 2O. 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.
Yim, H; Kent, MS; Satija, S; Mendez, S; Balamurugan, SS; Balamurugan, S; Lopez, GP
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