Mechanical desorption of a multiblock copolymer chain in the isometric ensemble studied by generating functions approach

Mechanical desorption of an infinitely long macromolecule in the isometric ensemble, that is, by moving its end to a certain distance from the grafting surface, is studied theoretically. The approach is based on the generating functions (GFs) method and the calculation of the average tail length of the desorbed macromolecule as a function of the end distance from the surface which gives access to the reaction force of the macromolecule to the desorption. The cases of a homopolymer and a regular multiblock copolymer are considered. For the homopolymer, the tail length is a linear function of the end distance and the reaction force is a constant independent of the end height above the surface which value is defined by the adsorption strength. In the case of a multiblock copolymer consisting of alternating adsorption-active and inert blocks, the reaction force is a non-monotonic oscillating function of the end distance from the surface, with decaying amplitude. The shape of the force-distance curve depends on the adsorption strength, blocks' lengths, and monomer composition of the multiblock copolymer. These dependences perfectly describe the invariant part of the force-distance curves for mechanical desorption of finite multiblock copolymer chains grafted at one end to the surface calculated by using the transfer matrix approach.