Mechanical desorption of a multiblock copolymer chain end-grafted to an impenetrable flat surface

Mechanical desorption of a regular multiblock copolymer chain grafted at one end to an impenetrable, flat surface is studied theoretically. The polymer consists of alternating blocks of adsorption-active and nonactive monomer units. Two modes of desorption corresponding to two conjugate thermodynamic ensembles are considered and compared: isometric ensemble, where desorption is implemented by moving the chain free end from the surface to the distance z, and isotensional ensemble, where a constant pulling force f is applied to the free chain end. Chain confirmations are modeled as simple random walks on a cubic lattice. It is shown that in the isometric ensemble reaction force versus distance dependence exhibits a characteristic sawtooth pattern due to the blocky structure of the copolymer, reflecting sequential detachment of adsorbing blocks followed by relaxation of nonadsorbing blocks. In the isotensional ensemble, desorption occurs as an abrupt "all-or-none" transition at some critical value of applied force; near the transition point, the probability distribution of the chain end position is multimodal. The results show that the shape of the detachment curve depends on the sequence structure, composition, and length of the blocks, as well as on the choice of the end through which the chain is grafted to the surface.