NEUTRON REFLECTIVITY STUDIES OF END-GRAFTED POLYMERS

Neutron reflectivity is used to measure the volume fraction profile of carboxylic acid-terminated polystyrene (PSCOOH) absorbed on silicon o xide from deuterated cyclohexane, a near-Theta solvent. Because of the affinity of the acid group for the silicon oxide surface, the PSCOOH adsorbed amount is 4 times larger than the unfunctionalized PS case at 23 degrees C. Although both PS and PSCOOH have similar surface volume fractions (ca. 0.60), the PSCOOH layer thickness is 4 times larger, s uggesting that the PSCOOH chains stretch by 4 times their unperturbed size. Upon increasing the degree of polymerization N from 67 to 780, t he PSCOOH layer thickness scales as N-1 under near-Theta solvent condi tions. Under good solvent conditions, the PSCOOH surface volume fracti on decreases by a factor of 3; whereas the layer thickness remains the same as in the near-Theta case. In both solvents, the reflectivity ca n be fit with PSCOOH profiles which are parabolic with some rounding n ear the free chain ends. Neutron reflectivity studies of DPSCOOH and P SCOOH adsorbed from cyclohexane and cyclohexane-d, respectively, sugge st that DPSCOOH adsorption is slightly stronger due to isotope effects .