Polymeric thin films with a stimuli-responsive surface layer have many potential applications. Surface photografting is a simple and versatile method for introducing grafted chains onto polymeric substrates. However, photografted layers usually have very low swelling ratios, swelling and stimuli-response rates. This work demonstrates that photografting of acrylic acid onto polyethylene films carried out with films on the top or at the bottom of a solution leads to huge differences in the grafting reactions and the final properties of the grafted layers. The photografting reaction on the top surface of a solution is much faster, and the grafted poly(acrylic acid) layers have ultra-high equilibrium swelling ratios (>100) and very high swelling and pH response rates (in seconds). (C) 2011 Elsevier Ltd. All rights reserved.