Segmental orientations of trains versus loops and tails: the adsorbed polymethylmethacrylate system when the surface coverage is incomplete

We measure, for the first time, the separate surface coverage-dependent ori entations of trains and of loops/tails in the structure of an adsorbed poly mer, and show that these have different average orientations. The system wa s PMMA (atactic polymethylmethacrylate) adsorbed onto oxidized silicon from dilute solution in carbon tetrachloride at 25.0 degrees C. The method was FTIR-ATR (infrared spectroscopy in attenuated total reflection). With incre asing surface coverage, train segments were observed to maintain constant a verage anisotropy with respect to the solid surface. Free segments were pre ferentially flattened when the surface coverage was low, but with increasin g surface coverage became progressively more random in orientation. Free an d bound segments were separated according to shifts of the Vibrational freq uencies of carbonyl segments upon hydrogen-bonding to the surface silanol g roups. The dependence of segmental orientation on molecular weight of the c hain is also discussed. (C) 2000 Elsevier Science B.V. All rights reserved.