Interactions between surfaces bearing end-adsorbed chains in a good solvent

Abstract

We have measured the force-distance profiles between two curved mica sheets immersed in toluene and in xylene, both in the pure solvents and following addition of polystyrene (PS), end-functionalized polystyrene of different molecular weights Af, PS-X(AÍ), and polystyrene-poly (ethylene oxide) diblock copolymers (PS-PEO) with a short PEO block. Our results show that PS does not adsorb from the (good solvents) toluene and xylene but that once PS-X or PS-PEO are added to the solution the surface is rapidly covered, showing that the nonadsorbing PS tails are anchored at one end only. The force profiles following surface coverage are monotonically repulsive, and the range 2L0 for onset of interaction is roughly twice that of the corresponding adsorbed chains. There is no evidence of bridging attraction at low surface coverage or of finite relaxation times following strong compression, both of which are characteristic of adsorbed chains. The qualitative behavior with both PS-X and PS-PEO is very similar. For the PS-X(ilf), we find La Af0•6, and we are also able to estimate the mean interanchor spacing s and find it to increase markedly with M. These features are in accord with equilibrium expectations for a fixed anchoring energy of the polystyrene on the mica. We find that our data are well fitted quantitatively both by scaling and by mean-field models.

Probing Collective Motions of Terminally Anchored Polymers

“…To reveal the N dependence of ⌫, the comparison among the different samples should be made at the same grafting density and local mobility. The w() ϰ Ϫ1 dependence assumed in the simulation study (21), together with the blob picture of semidilute solutions (s Ϸ The equilibrium structure of terminally anchored PS layers (9,10) conforms to the predictions based on non-uniform chain stretching that yield L 0 Ϸ 1/3 N. The study of the dynamic structure of the same system reveals the presence of long-lived thermally induced layer fluctuations with wavelength 2/q* Х 1.3L 0 and indicates strong surface coverage effects on their thermal decay rate. The identification of the precise mechanism of the layer fluctuations would highly benefit from a theoretical calculation of the full structure factor S(q, t) and would promote fundamental studies of polymers near surfaces and provide information on chain relaxation and brush friction, which are relevant to technological applications.…”

“…Whereas polar solvents (such as acetonitrile) inhibit the reaction, smooth aerobic oxidation takes place in apolar solvents such as benzene. More significant from a practical viewpoint, toluene and trifluoromethylbenzene are superior solvents to benzene and lead to an important increase in reaction rates (9).…”

“…This stretched configuration of tethered chains is used as a model for a wide variety of confined polymer systems (3). Most theoretical and experimental studies (3)(4)(5)(6)(7)(8)(9)(10) have addressed the static structure of brushes, such as the density profile above the surface. Although theoretical predictions have been made of their dynamical structure as well (4,5,11,12), direct relevant experimental observation, although much needed (7), is difficult because of the small size of the brush and its low scattering power.…”

“…The PEO-PS model system in toluene was selected on account of the extensive information on adsorbed amount, kinetics of adsorption, and segment density profiles obtained from ellipsometry, neutron reflectivity, and surface-balance experiments (9,10). The characteristics of the PEO-PS diblocks and the brushlike grafted layers are given in Table 1.…”

“…The L 0 for the grafted PS monolayer corresponds to a parabolic density profile normal to the surface (1,10). The adsorption conditions of (9,10) were reproduced, and a copolymer concentration greater than the threshold value for maximum adsorbance was used. The adsorption process took place for 12 hours; equilibrium was confirmed by the stability of the scattered intensity.…”