Train-loop-tail description

The second technique of choice is SANS. Small angle neutron scattering is a classical tool for studying separately. polymeric and colloidal systems. As first shown by Cosgrove et al. it can also be used to observe polymers at interfaces. These authors were able to measure for the first time the concentration profile of poly(oxyethylene) chains adsorbed onto polystyrene colloidal grains. Their data were in qualitative agreement with the train-loop-tail description of Scheutjens... 

Lattice models play a central role in the description of polymer solutions as well as adsorbed polymer layers. All of the adsorption models reviewed so far assume a one-to-one correspondence between lattice random-walks and polymer configurations. In particular, the general scheme was to postulate the train-loop or train-loop—tail architecture, formulate the partition function, and then calculate the equilibrium statistics, e.g., bound fraction, average loop... 

In sec. 5.2 we emphasised the fact that polymers have a large number of interned degrees of freedom. Adsorption must imply changes in this number. The usual description of conformations at an adsorbing Interface, first proposed by Jenkel and Rumbach and depicted schematically in fig. 5.1, is in terms of three types of subchains trains, which have all their segments In contact with the substrate, loops, which have no contacts with the surface and connect two trains, tind tails which are non-adsorbed chain ends. 

Adsorption of a polymer necessarily implies a change in the conformation the most common description is the loop-train-tail model (Jenkel and Rum-bach 1951) shown schematically in figure 5.10. The trains are made up of segments in direct contact with the surface, whereas loops have no direct contact with the surface but are in close proximity. Tails are non-adsorbed chain ends. Although tail segments may constitute a small proportion of all segments, they determine the hydrodynamic layer thickness of the adsorbed polymer. Many other properties of adsorbed polymers are determined by the total segment concentration profile as a function of the distance from the surface. 

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