The chemical modification of a thermoplastic polyurethane (TPU) has been realized by the grafting of polyisocyanates onto urethane groups through allophanate bonds to prepare self-crosslinkable but still thermoplastic materials. Because of the lateral isocyanate groups, the resulting polymer can be easily crosslinked by the ambient moisture. However, because of the highly viscous character of the TPU even at a high temperature, this chemical reaction must be performed with a twin-screw extruder above 190 degrees C. The ability of linear, segmented TPUs to undergo physical (micro)structuration that strongly depends on their thermal history has been widely described in the literature, and these transformations usually occur in the same temperature range as that used for performing our chemical reactions. Moreover, the thermally reversible character of urethane and allophanate bonds above - 170 degrees C is also well-known. Therefore, the precise characterization of the thermomechanical properties of the initial and modified materials is quite delicate because microstructural phenomena and chemical rearrangements can compete and always have to be considered simultaneously. Considering these restrictions, we have nevertheless demonstrated that the structural transformation is faster than crosslinking for a partially crosslinked TPU, but microphase separation is no longer possible for a TPU with a gel fraction higher than 50%. The chemical crosslinks of the modified TPU are reversible, but they still allow an important increase in the thermomechanical properties at a high temperature. (C) 2007 Wiley Periodicals, Inc.