When a polymer molecule is subjected to a force (such as a tensile force) it responds and this response gives information about the thermodynamics and structural properties of the polymer. In recent years there have been a number of experimental developments, such as atomic force microscopy and optical tweezers, that allow individual polymer molecules to be pulled in various ways. This has resulted in a renewed theoretical interest in how polymers respond to applied forces. This review will focus on some particular aspects of this field. We shall be primarily interested in tensile forces and consider various scenarios, such as pulling an adsorbed polymer from a surface and pulling a polymer from one phase to another. In order to make theoretical progress one needs a model of the polymer and we shall focus on lattice models. Our emphasis will be on exactly solvable models such as Dyck and Motzkin paths, and on rigorous results for self-avoiding walk models and some relatives, though we shall also discuss scaling theories and some selected numerical results.

We prove some theorems about self-avoiding walks attached to an impenetrable surface (i.e. positive walks) and subject to a force. Specifically we show the force dependence of the free energy is identical when the force is applied at the last vertex or at the top (confining) plane. We discuss the relevance of this result to numerical results and to a recent result about convergence rates when the walk is being pushed towards the surface.

We consider a self-avoiding walk on the simple cubic lattice, as a model of localization of a random copolymer at an interface between two immiscible liquids. The vertices of the walk are coloured A or B randomly and independently. The two liquid phases are represented by the two half-spaces z> 0 and z< 0, and the plane z = 0 corresponds to the interface between the two liquids. The energy depends on the numbers of A-vertices with positive z-coordinate and B-vertices with negative z-coordinate. In addition there is a vertex-interface interaction, irrespective of the colour of the vertex. We use exact enumeration and series analysis techniques to investigate the form of the phase diagram and how it changes as the magnitude of the vertex-interface interaction changes.