A quantitative study of tethered chains in various solution conditions using Langmuir diblock copolymer monolayers

This article summarizes our investigations of tethered chain systems using Langmuir monolayers of poly(dimethylsiloxane)-polystyrene (PDMS-PS) diblock copolymers on organic liquids. In this system, the PDMS block absorbs stro ngly to the air surface while the PS block dangles into the subphase liquid . The air surface can be made either repulsive or attractive for the tether ed PS chain segments by choosing a subphase liquid which has a surface tens ion less than or greater than that of PS, respectively. The segment profile of the PS block is determined by neutron reflection as a function of the s urface density, the molecular weights of the PS and PDMS blocks, and the so lution conditions. We cover the range of reduced surface density (Sigma) ch aracteristic of the large body of data in the literature for systems of cha ins tethered onto solid surfaces from dilute solution in good or theta solv ent conditions (Sigma < 12). We emphasize quantitative comparisons with ana lytical profile forms and scaling predictions. We find that the strong-stre tching limit assumed in analytical self-consistent field calculations (SCF) and scaling theories is not valid over this Sigma range. On the other hand , over a large portion of this range (Sigma less than or equal to 5) tether ed chain profiles are well described by a renormalization group theory for weakly interacting or noninteracting chains. Simultaneous with the study of the profile form, the free energy of the tethered chains is examined throu gh the surface tension. A strong increase in the surface pressure is observ ed with increasing surface density which determines the maximum surface den sity which can be achieved. This effect is attributed to a combination of h igher order osmotic interactions and configurational constraints. This effe ct may explain several outstanding discrepancies regarding the adsorption o f end-functionalized chains and diblock copolymers onto solid surfaces.