Chain segment ordering in swollen polymer brushes: Deuterium NMR investigations

The dynamics of deuterated poly(dimethylsiloxane) chains grafted on a solid substrate and immersed in various solvents is investigated using deuterium NMR. It is observed that the segment reorientations are uniaxial around the normal to the grafting plane. For layers of moderate grafting density, immersed in poor solvent, the anisotropy is homogeneous and characterized by a negative order parameter S. The segment orientational fluctuations are preferentially parallel to the grafting plane through the whole layer. This anisotropy, due to the squeezing of the chains, depends on the average concentration of the polymer layer. On the contrary, in good solvent, the average order parameter is positive, indicating that the segments are preferentially oriented perpendicularly to the grafting plane. In this regime, the distribution of order P(S) is broad and the chains are nonuniformly stretched.