Specular neutron reflection was used to investigate the density profile of polystyrene-poly-(ethylene oxide) (PS-PEO) block copolymers adsorbed from d-toluene onto quartz. The neutron beam passed through the quartz substrate and was reflected from the quartz/d-toluene interface. The PEO block, which comprises a small fraction of the total polymer molecular weight, strongly adsorbs onto the quartz substrate, while the PS block remains in solution. Thus, the chains form a terminally attached polymer "brush". The reflectivity profiles are well described by a parabolic or error function polymer density profile normal to the interface, but the data cannot be fitted to exponential or power law decay profiles. The layer thickness values are in good agreement with the results of interlayer force measurements for the same polymersolvent system adsorbing onto mica. The molecular weight dependence of the layer thickness and adsorbance obtained from the data obey scaling laws in accord with the theory of semidilute polymer brushes.
We have used STM to study a range of biological molecules. The necessity of immobilizing the molecules on the surface was recognized and achieved by the formation of ordered layers or by the addition of chemical groups which strongly adsorbed to the graphite substrate. Clear images of the helical structure of the synthetic polypeptide poly γ-benzyl-L-glutamate were obtained. Images of protein molecules isolated from wheat were found to be consistent with the rare β-spiral conformation. The binding of the mutagen ethidium bromide to DNA was also studied and images of the cyclic glucan β-cyclodextrin, tosylated to improve adsorption of the molecule to graphite, were also obtained. Although the contrast mechanism is unclear in some of these examples, these results suggest that STM can be applied successfully to a wide range of biomolecules.
In this paper, we summarize our cumulative work on neutron reflectivity studies of polystyrene-poly(viny1-2-pyridine) (PS-PVP) and polystyrenepolyethylene oxide (PS-PEO) adsorbed at a quartz-solvent interface. Deuterated toluene was chosen as the solvent since it is a good solvent for PS and a poor one for either of the other two blocks. In this case, the polystyrene dangles into the solvent while the other block acts as an anchor. The neutron reflectivity studies reveal that the form of the polymer density profile normal to the substrate may be varied from an extended "brush" to a condensed "mushroom" conformation by manipulating the ratio of the molecular weights of the two blocks. In addition, we present new data on the PS-PEO system in a poor solvent, deuterated cyclohexane, under conditions of shear flow in Poiseuille geometry. We find that when the PS-PEO diblock is absorbed from cyclohexane and is allowed to relax, the PS chain takes on a "mushroom" conformation. However, when the shear is applied, the layer shear thickens due to the PS chains extended to nearly twice their original lengths.
ACKNOWLEDGMENTS
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